• ISSN 1001-1455  CN 51-1148/O3
  • EI、Scopus、CA、JST收录
  • 力学类中文核心期刊
  • 中国科技核心期刊、CSCD统计源期刊
Advanced search E-mail Alert
Display Method:
Abstract:
To improve the crashworthiness of thin-walled tube structures, a series of bio-inspired multi-cell thin-walled tubes with sinusoidal cells (abbreviated BST) have been designed based on the dactyl club microstructure of Odontodactylus scyllarus (O. scyllarus) using bionic design methods. By taking initial peak load, specific energy absorption and crushing force efficiency as crashworthiness indexes, the influence of cell number on the crashworthiness of the BST under different impact angles (0o, 10o, 20o and 30o) conditions were analyzed under low-velocity impact condition using nonlinear finite element (FE) method through LS-DYNA. Optimal number of bionic cells was obtained using complex proportion assessment. A complex proportional assessment (COPRAS) method was used to select the optimal number configuration under multiple loading angles. Base on the combination of weight factor values of different impact angles, four single-angle cases (Case-1, Case-2, Case-3 and Case-4) and three multi-angel cases (Case-5, Case-6 and Case-7) were set. A metamodel-based multi-objective optimization method based on polynomial regression (PR) metamodel and multi-objective particle optimization (MOPSO) algorithm were employed to optimize the dimensions of the optimal cell number configuration, where initial peak load, specific energy absorption and crushing force efficiency were taken as objectives and height-width ratio and thickness regarded as the design variables. According to the results of COPRAS method, the BST with four sinusoidal cells was determined to be the best design based on multicriteria process. The optimization results of single-
Abstract:
The bench blasting technology is widely applied in mining, transportation and civil construction excavations, in which numerical simulation plays an increasingly important role in the selection and optimization of parameters. In order to solve the problems of dense mesh and large amount of calculation in solid hole modelling, a one-dimensional axisymmetric explosive model is proposed in this paper. In this model, one-dimensional linear bar is used to describe the bore hole and explosive, and the solid element is used to describe the surrounding rock mass. Through the topological relationship between the bar nodes and the solid elements, the explosive gas pressure on the bar nodes is applied to the surrounding solid elements, and the cross-section change at the bar node is calculated according to the volumetric strain of the solid element, to realize the interaction between the explosive and the surrounding rock mass. Through numerical comparison with the entity bore hole model, when the pressure attenuation index is 1.25, the radial peak particle vibration velocity (PPV) attenuation law and vibration velocity time history curve obtained by the one-dimensional axisymmetric explosion source model are basically consistent with the entity blasthole model, which proves that the accuracy of the model in blasting simulation. Based on the blasting technology in Angang open-pit mine, a generalized three-dimensional bench blasting model with 5 rows and 50 bore holes is set up to simulate the damage and failure status in the blasting area. The numerical calculation results show that the tensile failure is the dominant in the blasting area, and the PPV and its variation with distance at monitor points except the first point near the blasting source. which proves the feasibility of the proposed model in the far-field simulation of three-dimensional bench blasting.
Abstract:
In order to design a lightweight thin-walled structure with high specific energy absorption and high stiffness, a new type of circular cross-section thin-walled tube with negative Gaussian curvature (NGC-C) is proposed and studied in this paper. The finite element analysis method verified by previous experimental data is used to simulate the axial dynamic impact, and various performance indexes such as specific energy absorption and effective crushing length are extracted. The comprehensive performance of the thin wall energy absorption structure with zero Gaussian curvature and positive Gaussian curvature is compared with the complex proportional assessment method (COPRAS). The Latin hypercube sampling method is used to extract 20 sample points from the design space and obtain the corresponding performance response values of each sample point, and the polynomial fitting method is used to establish the proxy model. Based on the agent model, the multi-objective optimization design is carried out by using the improved non dominated sorting genetic algorithm (NSGA-II). The results show that the comprehensive performance of the thin-walled circular tube with negative Gaussian curvature is better than that of all kinds of non negative Gaussian curvature thin-walled energy absorbing structures, especially in that it has the minimum effective crushing length. The goodness of fit of the established proxy models is higher than 98%, which can better reflect the relationship between structural design variables and performance response. After optimization, the specific energy absorption of thin-walled circular tubes with negative Gaussian curvature is increased by 16.47%, the effective crushing length is reduced by 12.4%, and the mass is reduced by 20.18%. To sum up: introducing the negative Gaussian curvature surface shape into the thin-walled tube configuration can reduce the structural quality and improve the crashworthiness of the thin-walled tube, provide a new idea for the design of the thin-walled energy absorbing structure, and can be applied to the energy absorbing scenarios such as the automobile energy absorbing box.
Abstract:
The physical mechanism of electrically exploding wires has caused much attention recently; fruitful experimental results have been reported by domestic researchers. Modeling and studying on the electrical metal wire explosion problems can help to understand the basic physics of Z pinches and other related magnetically driven plasma problems, and to evaluate the parameters of the equation of state and electrical conductivity. A zero-dimensional (0D) dynamical model of the underwater electrical wire explosion is developed; the single wire is modeled as a plasma cylinder undergoing self-similar radial motion with uniform density, temperature and pressure, while its velocity is varying linearly with radius. The kinetic equation and internal energy equation are derived from the hydrodynamic equations and used as the basic controlling equations. To close the 0D model, other parameters are supplemented, with the real gas quotidian equation of state(QEOS) model for pressure and internal energy, the modified Lee-More electrical conductivity model for resistivity, and an external circuit model for the current density. The boundary conditions are constructed from the shock Hugoniot relations in water, the pressure at the wire boundary is assumed to be equal to the water pressure behind the shock. The calculations are carried out from a cold start of wires with density and temperature in laboratory status. Results of the 0D model are validated by comparing with the results from simulations of one-dimensional (1D) magneto-hydrodynamic (MHD) model and experiments. Examples of electrical explosion of copper wires in water are taken in the applications, the rise time of the short-circuit current pulse is 5μs and the wires are varying from 50μm to 200μm in diameter. Results from the 0D-dynamical model are coincide well with the MHD simulation results and experimental data, typical discharging modes are achieved by varying the parameters of the wires. The 0D model can be used for parameters optimizing and data analysis in similar experiments.
Abstract:
Investigating the mechanical property of concrete structures subjected to impact loading has great significance on the design and evaluation of weapons and protective structures, while appropriate material models can more accurately predict the mechanical behavior and damage mode of concrete structures. In this paper, an improved damage-plasticity material model for concrete is proposed to describe its mechanical response subjected to impact loading. The equation of state, including elastic stage, transition stage and compacted stage is employed to describe the pressure-volume strain relationship. The strain rate effect is considered by combining the radial enhancement method and the semi-empirical equation of dynamic increase factor. A unified hardening/softening function related to the shear damage caused by microcracking and the compacted damage caused by pore collapse is introduced to describe the nonlinear ascend and descend of compressive strain-stress curves under plastic stage, and an exponential function related to the tensile damage is employed to reflect the strain softening behavior under tension. Based on the current extent of damage, the failure strength surface of this improved material model is determined through linearly interpolating between the maximum and yield strength surfaces or the maximum and residual strength surfaces, and the influence of third deviatoric stress invariant on the failure strength surface is considered for describing the reduction of shear strength during the transition from high pressure to low pressure. The fractionally-associated flow rule is used to consider the volumetric dilatancy of concrete materials under confining pressure. Then, the availability and accuracy of this improved material model are verified by the numerical simulations of single element under different loading conditions, and its performance improvement is discussed by comparing with the HJC model, RHT model, Kong-Fang model and empirical equation. Finally, the numerical simulations of projectile perforating reinforced concrete slab are conducted to further verify the feasibility and accuracy of this improved material model under impact loading, and numerical results indicate the damage mode and residual velocity predicted by this improved material model are closer to experimental results than HJC model.
Abstract:
To study the explosion mechanism and the energy conversion process of the interaction between low melting point metal tin and water, a visual experiment platform which was composed of a high-frequency melting furnace, a high-speed camera, signal collectors and other equipment was built to monitor the contact reaction processes at different mass ratios of tin to water, like 5, 10, 15 and 20. Meanwhile, high melting point metal aluminum was selected for comparative experiments under the same experimental conditions to explore the differences in reaction characteristics between low melting point metal tin and high melting point metal aluminum during the steam explosion. Some mathematical calculation models were established to quantitatively analyze the shock wave energy in line with the law of conservation and explosive shock theory. The results show that two steam explosions were triggered when molten tin reacted with water at a mass ratio of 5; and in the comparative explosion experiments between molten tin with water and molten aluminum with water under the same experimental conditions, the reaction intensity and the duration during the explosion of molten metal with water are respectively related to the degree of fragmentation and thermal diffusivity. In addition, it is calculated that about 0.45% to 10.91% of the heat energy stored in the molten tin is converted into the explosion shock wave energy throughout the steam explosions. Moreover, the shock wave energy conversion rate is affected by the mass ratio; and this effect is reflected in that the energy conversion rate of the shock wave first increases and then decreases with the increase in mass ratio; when the mass ratio is 10, the energy conversion rate is the largest. It is
Abstract:
Polymer material has the characteristics of fast forming and good expansion performance. Its composite structure with gravel and reinforcement has obvious advantages in foundation treatment and urban road void removal and reinforcement. In this paper, polymer gravel slab and reinforced polymer slab were designed and manufactured, and experimental research under contact explosion impact was carried out. The damage characteristics of the two kinds of slabs were studied through the damage size and the measured shock wave data. Based on ANSYS / AUTODYN nonlinear explicit finite element program, the damage mode and damage diameter of reinforced polymer slab were studied, and compared with the test results to verify the accuracy and applicability of the established finite element model. The sensitivity of reinforced polymer slab to explosive quantity and slab thickness was analyzed parametrically, and the prediction formula of failure diameter of the top surface and bottom surface of reinforced polymer slab was put forward by using the multi-parameter regression analysis method. The results show that under the action of air contact explosion, the damage mode of polymer gravel slab is mainly local collapse and perforation at the contact part. Under the impact load of contact explosion, there are punching and cutting explosion pits on the top surface of the slab, tensile failure and collapse area on the bottom surface, and a through failure hole is formed in the center of the slab, in addition to some damage cracks; Under the action of air contact explosion, reinforced polymer slab mainly occurs crater damage on the top surface, spalling damage on the bottom surface and central punching perforation damage. The reinforced polymer slab has a good attenuation effect on the explosion shock wave. The diffuse reflection effect of the closed bubble in the polymer structure on the shock wave can absorb more energy to alleviate the explosion shock wave, which further shows that the polymer has the potential to be applied to the field of anti-explosion shock protection.
Abstract:
As a non-contact, non-interference full-field non-destructive optical measurement technology, digital image correlation (DIC) technology can obtain the dynamic deformation information on the surface of materials and failure process. Aiming to evaluate the ballistic performance of armor steel and explore the application of high-speed three-dimension digital image correlation (3D-DIC) technology in perforation test of armor steel plate, seven shots high strength and hardness armor steel plate impact test with different thickness subjected to the 15 mm-caliber deformable projectile at various velocities were firstly conducted using hydrogen-oxygen detonation ballistic gun, in which the high-speed 3D-DIC measurement technology with frame rate of 144000/s was adopted to extract the out-of-plane displacement and strain field-time histories of target. Then, based on the calibrated and validated constitutive model parameters of armor steel in previous work, the current impact test was numerically simulated and the corresponding finite element model was validated by comparing the simulated residual projectile velocities and lengths with test data. Furthermore, by comparing the out-of-plane displacement-time histories and strain contours at the rear of target obtained by numerical simulation and test, respectively, the accuracy of results obtained by high-speed 3D-DIC was validated. Finally, the relationship between maximum out-of-plane displacement with projectile impact velocity and armor steel plate thickness was compared and analyzed. The application of high-speed 3D-DIC technology in this study can provide a reference for the related test, and the analysis result of maximum out-of-plane displacement of target can be used as the experimental basic for the analysis, verification and optimal design in protective barrier structures.
Abstract:
In order to study the influence mechanism of shaped charge liner on the perforation and damage fracturing effect of shaped charge penetrating shale reservoir, a three-dimensional model of "perforating charge-air-shale" was established. The cone angle of liner was set to 50 °, 60 °, 70 ° and 80 ° respectively, the thickness of liner was set to 0.5mm, 1.0mm and 1.5mm respectively, and the material of liner was set to copper, steel, titanium and tungsten respectively. The numerical calculation was carried out by using the ALE-Lagrange coupling method in the non-linear program (ANSYS/LS-DYNA).The ALE method was used to describe shell, explosive,liner and air , the Lagrange method was used to describe the shale reservoir. The systematic analysis was carried out from the aspects of jet velocity and shape, shale perforation effect and the fracture extension characteristics of shale. The results show that with the decrease of the cone angle of liner, the jet velocity and penetration depth increases, the pestle velocity and perforation diameter decreases. In a certain range with the decrease of the thickness of liner, the jet velocity, penetration depth and perforation inclination increases, the mass of the pestle decreases.The material of liner has a significant impact on the velocity of jet, pestle structure and shale perforation effect. Among them, the penetration depth of perforating charge with tungsten liner is the largest but the perforation diameter is the smallest, the penetration depth of perforating charge with titanium liner is the smallest but the perforation inclination is the largest, and the penetration depth of perforating charge with copper liner is slightly larger than that with steel liner, but the perforation diameter is slightly smaller. The charge with shell can significantly improve the jet velocity and penetration depth than the charge without shell. Because the detonation pressure has an obvious difference after the detonation wave is transmitted to the end of the explosive, which affects the jet velocity and penetration depth, the charge with shell has a greater jet velocity and penetration depth than the charge without shell. By comparing the fracture extension characteristics of shale in different groups, it is found that the fracture extension of shale mainly occurs in the stage of re reaming of pestle on shale. It is concluded that the material and structure of the liner have a significant influence on the shaped charge jet and its penetration effect, and then affect the formation and extention of damage fractures in shale. The fracture extension of shale can be promoted by reducing the initial perforation diameter of penetration, increasing the diameter of pestle and increasing the speed of pestle.
Abstract:
The dynamic response and failure of the gradient metal foam sandwich beams subjected to high-velocity impact are studied experimentally. With the identical areal mass, five different core configurations are employed for the stepwise sandwich beams which are composed of three different densities of aluminum foams. All the sandwich beams are supported with a simply clamped boundary condition. Combing with the three-point bending, the impact resistance of the gradient sandwich beams in terms of dynamic deformation and failure modes are evaluated by considering the effects of core density gradient and impulsive intensity. The results show that the density gradient effect has a significant influence on the dynamic response and failure modes. The initial failure mode plays an important role in the structural response and the predominant energy absorption mechanism. The initial failure mode of the uniform and negative gradient sandwich structures is the overall bending deformation, while the local core compression is the initial failure mode for others. As the impulsive intensity is low, the gradient sandwich beam has superior impact resistance to the uniform counterpart. With the rapid core densification of the weak layer, the gradient sandwich beam shows inferior deflection resistance to the uniform counterpart with the elevating impulsive loading. Therefore, the optimal design of the core density gradient can efficiently improve the impact resistance of the sandwich beams under the high-velocity impact, which is a valuable reference for the engineering applications.
Abstract:
To study the ignition behavior of micro-mesoscopic hot spots in the matrix of pressed PBXs under GPa, 10μs-level slow-front ramp wave loading, a ramp wave loading device driven by non-shock initiation reaction of heavily constrained pressed PBX explosive was designed. The burning rate model of laminar combustion and the self-compiled two-dimensional axisymmetric finite difference program were used to analyze the characteristics of the pressure wave output by the device, the influence of the crushing degree of the donor explosive during the combustion process and the structural parameters of the device (the thickness of the case and the partition) on the output pressure wave were discussed. The calculation results show that the combustion specific surface area formed by the crushing of the donor explosive is the key factor affecting the pressure evolution of the non-shock initiation reaction. With larger combustion specific surface area comes greater ramp wave pressure. The ramp wave pressure can reach above GPa, and the corresponding rising front of the pressure wave can be reduced from tens of milliseconds to several milliseconds. The thickness of the case of the donor explosive, that is, the strength of constraint, has a significant effect on the pressure during non-impact initiation reaction. As the thickness of the interlayer increases, the output ramp wave pressure decays approximately exponentially. According to the calculation results, the structure design of the device was completed, and the ramp wave loading experiment was carried out on the tested PBX. The pressure at the incident interface of the tested explosive measured by PVDF is 1.6GPa, and the front of ramp wave is 25μs, which preliminarily proved the feasibility of realizing GPa, 10μs-level ramp wave pressure output by using the non-shock initiation reaction of heavily constrained pressed PBX explosives.
Abstract:
Under impact, aluminum foam undergoes significant plastic deformation, and the kinetic energy of the impactor is dissipated in the process, thereby protecting the structure from damage. The failure modes of aluminum foam sandwich structures under impact are complex, involving plastic deformation, panel failure, and cracking of the bonding interface. Traditional numerical simulation methods are difficult to solve these discontinuous problems. Peridynamic is a non-local numerical method that describes the mechanical behavior of materials by solving spatial integral equations. It has unique advantages in solving crack propagation, material failure, progressive damage of composite materials, and multi-scale problems. Although the basic bond-based peridynamic cannot describe plasticity, the ordinary state-based peridynamic decouples distortion and dilation, and can easily simulate the plastic deformation of materials. Therefore, based on ordinary state-based peridynamic, the Mises yield criterion and the linear isotropic hardening model were introduced to study the factors affecting the impact resistance of aluminum sandwich structures. Two-dimensional mesoscopic models of aluminum foam structure were established by the Monte Carlo method and impact was simulated using peridynamic. The influence of the porosity of aluminum foam on the impact resistance and damage mode of the sandwich structure was analyzed. The results show that good plastic deformation ability of aluminum foam sandwich structure is the main factor for its buffering and protection, and within a certain range, the higher the porosity of aluminum foam core, the better impact resistance of the sandwich structure. When the porosity of aluminum foam is increased from 0.4 to 0.7, the kinetic energy absorption rate of aluminum foam to the impactor is increased from 90% to 99%. The simulation results are in good agreement with the experimental results, which verifies the accuracy of the simulation results and the effectiveness of the analysis conclusions. Numerical simulation predicts the crack propagation morphology of the plexiglass backplate, and the results show that improving the porosity of aluminum foam can obtain better protection effect.
Abstract:
Adiabatic shear is a common form of deformation and failure of materials under high-speed impact loading. It generally exists in high-speed deformation processes such as high-speed impact, stamping forming, projectile penetration, high-speed cutting, and explosive crushing. A TA2 pure titanium plate with a total deformation of 70% was obtained by multi-pass large strain cold rolling on a two-high mill. By heating cold rolled TA2 pure titanium plates at 500 °C and annealing at varying holding times, titanium plates with different recrystallization structures were achieved. Based on a hat-shaped specimen and a deformation control approach of limit ring, dynamic impact "freezing" experiments were carried out on the samples with different recrystallized structures on the split Hopkinson pressure bar. The microstructure changes of samples before and after impact were characterized by optical microscope and scanning electron microscope. The effect of recrystallized structure on adiabatic shear behavior of TA2 pure titanium was studied. The results show that with the increase of annealing holding time, the proportion of recrystallized grains increases gradually, and the grains change from dispersion to local aggregation. Under the same strain and strain rate, adiabatic shear bands are observed in all samples. The samples with high proportion of recrystallized grains are more likely to induce crack nucleation and propagation in adiabatic shear bands. The changes of recrystallization structure and geometric necessary dislocations before and after deformation are compared and analyzed. Combined with the analysis of the overall temperature rise in the shear zone, the recrystallized grain as the material softening point can induce the formation of shear band. The adiabatic temperature rise effect mainly occurs in the later stage of the development of shear band, which promotes the secondary recrystallization of materials in the shear band, improves the toughness of materials in the shear band and delays the formation of shear cracks.
Abstract:
In order to investigate deeply the loading transmitting features within the warhead and the corresponding structural response characteristics in the drop impact condition, and promote the explosive safety assessment and the warhead structure design, the loading characteristics and structural response of warhead during the drop impact process were analyzed based on numerical simulation and shock wave analysis, in which the deformation and damage characteristics of the explosive subassembly were emphasized , and the influences of various factors, including drop posture, explosive configuration and drop height, etc., were discussed. In the numerical simulations materials were characterized by the viscoplasticity constitutive model combined with the accumulative damage model which considers the effects of strain rate and temperature, and the thermodynamic equation of state were employed to calculate the pressure in materials during the deformation process. The loading transmitting features in the warhead and the corresponding structural response were investigated through stress wave analysis, and then the influences of various factors on the structural response were discussed in detail. Firstly, the effect of drop posture was investigated by comparative analysis among five typical cases, i.e., tail-downward vertical drop, nose-downward vertical drop, horizontal drop, tail-downward inclined drop and nose-downward inclined drop. Secondly, the influence of warhead configuration was analyzed based on three configurations, i.e., 1 explosive segment warhead, 8 explosive segment warhead, and 8 explosive segments combined with separator warhead. Finally, the effect of drop height was also discussed, and in the discussion the height ranges from 3m to 40m. Related results indicates that during the drop impact process, the deformation of explosive subassembly is dominated by the stress wave propagation, rather than the interaction between explosive subassembly and warhead shell, correspondingly, the severest damage zone in explosive subassembly is located in its internal region instead of the outside region which contacts with the warhead shell. The transmission of stress wave between explosive subassembly and warhead shell, and the reflection as well as superposition of stress wave within the structures dominate the major deformation region in the explosive subassembly as well as the degree of deformation. Furthermore, the drop posture affects significantly the response characteristics and the deformation of explosive subassembly. The most dangerous drop posture which leads to high safety risk is, in order, tail-downward vertical drop, horizontal drop, nose-downward vertical drop, and inclined drop. The explosive configuration also acts an important role, the explosive segment interface is easily to induce the increase of deformation degree, however, it has little influence on the acceleration and the distribution of deformation region. The separator usually leads to high acceleration, and it changes the location of deformation region as well as the deformation degree. Comparatively, the drop height has little influence on the distribution feature of deformation zone, and it mainly affects the loading amplitude, the degree of deformation and the size of deformation zone, etc. These factors increase with increasing the drop height. The present method, which investigates the structural response of complex warhead based on numerical simulation integrated with stress wave analysis, has built an effective bridge linking the basic theory and the engineering application.
Abstract:
In this paper, the formation process of the Yilan crater is studied based on the iSALE-2D simulation code, and the Euler algorithm is used to carry out numerical simulation, and 8 groups of working conditions are simulated. According to the scaling law, it can be determined that the projectile diameter range is: 90m to 120m, and the projectile speed is: 12km/s, 15km/s.The simulation results of the corresponding working conditions are obtained after 150s of impact, including the crater diameter, depth and crater profile curve. The optimal impact conditions of the Yilan crater were studied, and the formation and distribution of the molten layer during the cratering process were statistically analyzed. Combined with the point source cratering similarity law model, the cratering radius relationship under the strength mechanism was obtained by fitting. The research results show that according to the comparison between the simulated data and the actual exploration data, a granite asteroid with a diameter of 120 m and an impact velocity of 12 km/s hits the surface vertically to form a crater with a shape similar to the Yilan crater. A crater with a final diameter of 1840m and a crater edge depth of 263m is formed, which is in good agreement with the exploration data of the Yilan crater. Three stages of crater formation are reproduced: contact and compression, excavation, and modification. The distribution of the impact melting layer of the target plate material during the formation of the crater under the simulated conditions is revealed. The material melts completely when the peak pressure exceeds 56GPa during the impact process, and this process is completed within 20ms. Most of the melt is distributed in layers and stacks at the bottom of the crater, and a small amount of melt is deposited on the surface of the target plate with the projectile in a discrete distribution. The mass of the completely melted material is about 24 times the mass of the projectile. The relative error between the simulation results and the fitted crater radius relational results under the conditions of 120m diameter and 12km/s impact speed is 10.3%.
Abstract:
To investigate the dynamical response and collision failure behavior of prefabricated RC box girder flyover with over-height vehicle, a recent actual engineering accident was taken as a case to carry out refined FE numerical analysis, and a double mass-parallel spring (DM-PS) simplified vehicle model was proposed to effectively simulate the non-centripetal collision between the over-height vehicle and bridge superstructures. The effectiveness of the proposed DM-PS model was fully assessed via comparison with two widely employed vehicle models, including full-scale (FS) model and simple rigid (SR) model. The results of comparison show that the failure characteristics of the collision area can be obtained via using the FS model, which is basically consistent with the photos of the accident scene; the S-R model overestimates the local damage of the structure and weakens the overall structural deformation; while the DM-PS model has high accuracy for predicting structural failure. Therefore, the proposed DM-PS model provides a simple and effective analysis means for the protection design of bridge structure subjected to over-height vehicle collision. On this basis, a detailed parameter analysis of the structural behavior is carried out based on the DM-PS model, and the effects of vehicle collision velocity, mass, position, and structural form are investigated in depth. It is show that the structural sensitivity of the impact dynamic behavior to collision velocity of the vehicle is significantly greater than that of the collision mass of the vehicle; the deformation and failure modes of mid-span collision and side-span collision are quite different, and the damage of side-span collision to one side base is more serious; the box plate and reinforced plate in the box girder can effectively improve the structural impact resistance. Numerical results and conclusions can provide a reference for the crashworthiness design of bridges. The critical information of the finite element analysis process is presented in detail.
Abstract:
In order to explore the effect of shaped charge opening angle on the effective utilization rate and shaped effect of bilinear shaped charge structure explosive, the boundary equation of effective shaped charge was deduced through the theory of instantaneous detonation hypothesis, and the shaped charge with different shaped charge angles was analyzed. Effective utilization of structural explosives. Through the physical model test of cement mortar, the law of crack formation of pre-cracked holes with different energy-gathering opening angles is studied. Using the LS-DYNA numerical simulation software,numerical models of different energy-gathering opening angles were established to reveal the penetration process of the bilinear energy-gathering structure charge jet with different energy-gathering opening angles. The research results show that the effective utilization rate of the energy-forming effect of explosives is the largest when the energy-gathering opening angle is 75°. When the opening angle of the energy-concentrating groove of the energy-concentrating structure is 75°, the pre-split hole formation effect is obviously better than that of the energy-concentrating structure capsule whose opening angle is 60°, and the stress concentration effect and penetration depth along the direction of the energy-concentrating groove are the best. the rock element on the blasthole wall first reaches the stress peak. The pre-split blasting field test of different lithology was carried out for the double-line shaped energy-concentrated structure charge with the energy-gathering opening angle of 75°. Under the condition that the hole spacing increased by 20%, the double-line The pre-split blasting effect of the shaped energy is better than that of the conventional pre-split blasting.
Abstract:
To investigate the damage effect of underwater explosions with different standoff differences on gravity dams and explore whether there is an “optimal standoff distance”, a fully-coupled explosive-water-air-gravity dam numerical model was established based on centrifuge model tests. The fully-coupled numerical model was validated by comparisons with centrifuge test results. Results demonstrated that the employed numerical model can well predict the dam failures and the effect of bubble pulse. Then, a numerical scheme with the inclusion of 60 numerical calculations was designed. For different calculations, the water depth was 600mm, the explosive mass was 2.2g, and the geometrical scaling factor of the gravity dam model was 1/80. The detonation depth ranged from 50 to 250mm with five detonation depths. Each detonation depth corresponded to 12 standoff distances, and the standoff distance ranged from 10 to 200mm, with the scaled standoff distance ranging from 0.077 to 1.54 m/kg1/3. Comparisons of damage degrees of gravity dams against underwater explosions with different standoff distances were conducted. Quantitative comparisons of dam average damage, element erosion rate, stress, and strain were also presented. Results demonstrates that for the overall structural failure of the gravity dam, such as the overall structural bending-induced tensile failure, there is an “optimal standoff distance” for the damage effects of underwater explosions on gravity dams, namely, with the increase of standoff distance, the damage degree of gravity dam increases firstly and then decreases. The quantitative results also indicates that with the increase of standoff distance, the average damage of the damage area in the dam upstream face, the element erosion rate, the average value of the maximum tensile stress of the dam heel, and the average value of the maximum tensile strain of the dam heel all increases firstly and then decreases and reaches their maximum values around a standoff distance of 40mm. With identical water depth, explosive mass, and geometrical model of gravity dam, the “optimal standoff distances” for the damage effects of underwater explosions near the water free surface with five different detonation depths on gravity dams are all near 40mm, which suggests that for underwater explosions near the water free surface, the detonation depth owns limited influences on the “optimal standoff distance”.
Abstract:
In order to study the anti-penetration performance of CFST(concrete-filled steel tubular)with honeycomb structure, six experiments on anti-penetration of CFST with honeycomb structure were conducted by using 125mm smooth bore. The failure pattern and penetration depth of target under different working conditions were measured, and the typical failure modes of CFST with honeycomb structure were analyzed, The difference of failure pattern of target under different ratio coefficient of target and projectile size and the influence of impact point and steel tube wall thickness on the anti-penetration performance of CFST with honeycomb structure were compared. Uniaxial compression tests on 7 groups of hexagonal concrete-filled steel tubular column with different wall thicknesses and 3 groups of Hexagonal concrete column were carried out. The enhancement effect of the hexagonal steel tube on the strength and ductility of the core concrete under different wall thicknesses were studied, and the relationship between the strength enhancement coefficient of the core concrete and the hoop coefficient was fitted. By improving the empirical formula for calculating the penetration depth of ordinary concrete, the formula for calculating the maximum penetration depth of CFST with honeycomb structure was obtained. The results show that the wall thickness is important factor that affects penetration depth, The greater the wall thickness, the smaller the penetration depth; The influence of the location of impact point on the penetration depth is complex and discrete; The location of impact point has a great influence on the failure pattern of the target surface; The existence of steel tube can effectively increase the strength and ductility of core concrete; The improved penetration depth formula can predict the maximum penetration depth of the projectile to the CFST with honeycomb structure.
Abstract:
The concrete damage plasticity (CDP) model, which is commonly used in ABAQUS, failed to correlate damage parameters with strain rate. To accurately describe the damage of concrete under high strain rate, the modified CDP model (MCDP) considering the rate correlation of damage parameters was developed by defining a new strain rate field variable and compiling VUSDFLD subroutine. In MCDP model, tensile and compressive damage parameters were obtained by energy method, and the main solver can automatically update the damage parameters under different strain rates with the change of strain rate field variables. Under static load, the results calculated by MCDP model are in good agreement with those of CDP model. The MCDP model was used to calculate the dynamic compression performance of concrete under high strain rate. The analytical results indicate that the tensile and compressive damage parameters of concrete under different strain rates have a significant influence on its dynamic mechanical properties. The compiled VUSDFLD subroutine and MCDP model can solve the problem of correlation between damage and strain rate, investigate the dynamic response of reinforced concrete (RC) beams accurately and provide a more reliable technical way to predict the response and destruction of concrete structures under explosion impact and severe dynamic load.
Abstract:
The study on slow cook-off of composite propellant containing ammonium perchlorate (AP) is the focus of propellant safety research, and pre-ignition is a common and effective way to reduce the intensity of reaction in slow cook-off of engine. To study the effect of pre-ignition temperature on its response characteristics, the slow cook off experiment of composite propellant was designed and carried out, and the response characteristics of ignition at different temperatures were studied. The temperature distribution of propellant and the thermal damage law of propellant microstructure before ignition were investigated by simulation and thermal decomposition experiment. The results showed that: The engine spontaneously ignited with a reaction level of violent explosion, and the reaction level was burning when it was ignited at 120 ℃. The intensity of the reaction could be reduced effectively by pre-ignition when the propellant temperature was low before auto-ignition. The thermal
Abstract:
The determination of the blast loadings on the building structure is the prerequisite for the analyses of dynamic response and damage mode, as well as the blast-resistant design and the structural reinforcement. In the methods of determining the blast loadings on building structures, with the upgrading of computing hardware and software, the numerical simulation method, with low cost and high safety, attracted increasing attention of scholars. In order to improve the computing efficiency and accuracy, and balance the capacities of both the hardware and the software, by adopting the simplified calculation method, i.e., using symmetry (1D-2D-3D extension) and remapping method, the optimized sets of mesh sizes for the numerical simulation of blast wave propagating from long distance in large complex blocks are proposed. Firstly, aiming at the typical near-ground explosion scenarios, e.g., car bombs and ammunition depots, the sensitivity analyses of single-size mesh based on incident wave of air and ground explosions at the scaled distances of 0.2~5 m/kg1/3 and 0.2~39 m/kg1/3 were carried out, respectively. Secondly, considering the limitations of the software and hardware, a set of gradient mesh sizes against the scaled distances are recommended. Furthermore, based on the remapping technique and the suggested gradient mesh sizes, the incident overpressure and impulse of ground explosion were numerically calculated, and an improved method for correcting the peak overpressure with the scaled distances larger than 10 m/kg1/3 was proposed, which was verified by UFC 3-340-02. Finally, the computing accuracy and efficiency of the proposed optimized mesh sizes were verified by comparing the simulated and experimental overpressures and impulses (71 gauges) in the field explosion test on a full-scaled building. Besides, the applicability of the proposed gradient mesh size in simple reflection field is verified, which provides a reference for the subsequent proportional amplification application of gradient mesh size and the simulation application of blast loadings in more complex reflection environment.
Abstract:
The correlation characteristics of transmitted and reflected waves in the process of impact of the gas-solid interface by the gaseous detonation wave are of great engineering significance. A one-dimensional theoretical model was established to analyze the process of the detonation wave impacting the gas-solid interface. The changes were analyzed in the pressure and interface velocity on both sides of the interface after detonation waves with different initial pressures reaching the gas-solid interface. The process of gas-solid interface impacted by gas-phase detonation wave was numerically simulated. In the simulation, the space-time Conservation Element and Solution Element method(CE/SE) and the elementary reaction mechanism were used to simulate the gaseous detonation, and the Immersed Boundary Method(IBM) was used to simulate the fluid-structure interaction. The pressure distribution and rules of velocity change of partial reflection wave of gas and the waveform and velocity characteristics of stress wave transmitted into solid were analyzed. The experimental device of the impact of the piston by the gaseous detonation was built for further verification. The results show that after the gaseous detonation wave reaches the gas-solid interface the elastic wave in the exponential form is transmitted in the solid and a shock wave is reflected in the gas zone at the interface. The rarefaction wave after the detonation wave intersects with the reflected shock wave, which weakens the reflected shock wave. With the intersection process, the pressure after the reflected shock wave decreases, and the wave velocity becomes faster. The pressure in the intersection area of the original sparse wave and the reflected sparse wave remains uniform. Finally, the reflected shock wave becomes stable, and the gas-solid interface forms a constant state. Under different initial pressures of the same mixture, the ratio of the maximum pressure to the detonation pressure in the process of the impact of the detonation wave remains stable. The theoretical model is consistent with the calculated value and experimental data of related physical quantities of the feature points.
Abstract:
In order to explore the influence of hole spacing on tunnel excavation blasting effect, Based on the finite element numerical model of a roadway in Dahongshan Copper Mine, Calculation of cut blasting cavity section area under the condition of different spacing of double large diameter holes, The optimal scheme is verified on site. The results show that when the empty hole spacing dv = 25 cm, the cavity fracture area is 0.2116 m2. When dv increases to 35 cm, the fracture area increases by 15.1 %, but when dv increases to 45 cm, the fracture area decreases by 17.8 %. The field test was carried out on the hole arrangement scheme with the largest cavity section area dv = 35 cm. The measured cavity section area is 4.98 % smaller than the simulation results, and the cavity section width is 4.0 % smaller than the simulation results. The height of the cavity section is 3.4 % smaller than the simulation results. The error between the two-dimensional numerical simulation and the field test is less than 5 %. It lays a foundation for the numerical method construction of cavity volume prediction of underground tunnel cutting blasting.
2022-0003 re submission
PDF(33)
Abstract:
On the premise of good crushing effect, reducing the rock mass vibration above the bottom of the upward fan-shaped deep hole by reducing the peak pressure of the shock wave at the bottom of the hole is an effective measure to reduce the vibration and protect the superstructure. In order to determine the reasonable length of the air column at the bottom of the hole, the variation law of the length of the air column on the impact pressure of the hole wall when the air at the bottom of the hole is not coupled is studied by combining the theoretical research with the field model blasting dynamic test. Based on the theories of one-dimensional unsteady hydrodynamics and theoretical detonation physics, The action process and propagation law of shock wave in the blast hole after the explosion of bottom air interval charge column under the condition of cylindrical charge are discussed in stages. Considering the reflection and transmission of shock wave at the interface of different media, the parameters of shock wave front propagating in different directions and the initial shock pressure and action time acting on the hole wall in each stage are analyzed, Thus, the calculation formula and variation curve of pressure acting on the hole wall in each stage are obtained. In order to verify the above laws, six groups of 12 cylinder thick wall concrete models with different sizes were designed and made, and the bottom air interval blasting model tests were carried out. The air column lengths were 200mm, 400mm, 600mm, 800mm, 1000mm and 1200mm respectively. During the blasting process, the blast ultra high-speed multi-channel dynamic strain testing system was used to monitor the hole wall impact pressure, analyze the monitoring data and verify with the theoretical results, Finally, the variation curve of axial decoupling coefficient and hole wall impact pressure with time under the condition of uncoupled charge in air interval at the bottom of blast hole is obtained; Based on the dynamic compressive strength of rock, the reasonable length range of bottom axial air interval suitable for soft, medium and hard rocks is determined. In order to verify the rationality of the conclusion, the field industrial test is carried out, the charging blasting is carried out by using the air interval at the hole bottom, and the observation and photo analysis of the roof forming and blasting pile size after blasting are carried out. The research results show that the existence of air interval significantly increases the action time of impact pressure, The peak value of impact pressure decreases obviously; When k = 1.5 and the length of air column is 200mm, the attenuation ratio of peak pressure at the hole bottom is 73.4%; when k = 4 and the length of air column is 1.2m, the attenuation ratio of peak pressure at the hole bottom reaches 96.7%. When the air compartment is greater than 60cm, the area with low pressure value appears at the bottom of the blast hole. The reasonable bottom air interval length can not only ensure good blasting fragmentation, but also reduce blasting vibration by reducing the peak pressure at the hole bottom, so as to protect the stope roof and other protected objects.
Abstract:

In order to investigated the propagation process of underwater blasting shock wave in fish body and its effect on typical swim bladder fishes, the critical safety wave pressure model of typical fish was established and verified through theoretical analysis and field test. According to the transverse reflection pattern of one-dimensional elastic compression wave between different media, the relationship between critical safety wave pressure and body length of typical swim bladder fishes was clarified. The length and mechanical properties of swim bladder and fish body were measured using vernier caliper electronic and microcomputer tensile tester. Based on the measured data, the positive correlations of the length, width, wall thickness, and radial critical tensile stress of swim bladder with fish body length were determined, and the fish critical safety wave pressure model parameters were calibrated. The wave impedance ratio of water medium and swim bladder wall medium was 0.3~2. The width, wall thickness, shape, and radial critical tensile stress coefficients of swimming bladder were 0.04~0.09, 0.002, 0.6~1.1, and 60, respectively. The underwater blasting shock wave pressure and its effect on fishes were measured using blast wave tester, and the damage of fish was divided into three types: death, survival with influence, survival without influence. The fish critical safety wave pressure model was verified by the statistical results of fish damage. The results showed that the damage state of different body lengths fish under different shock pressure basically conformed with the maximum and minimum critical safety wave pressure that fish can withstand. The proposed fish critical safety wave pressure model can be used to describe the relationship between critical safety wave pressure and body length of swim bladder fishes under the action of underwater blasting shock wave. The research achievement can provide a theoretical basis for ecological protection of the fishes in the waterway regulation project of the upper reaches of the Yangtze River.

Abstract:
In order to explore the explosion flame propagation and explosion prevention in ducts of right-angle and variable sections, 3.5m long ducts were built for methane explosion experiment, of which 0-3.0m was kept in horizontal and invariant section, 3.0-3.5m was changed in horizontal, right-angle and different sections respectively. Different sections of duct included 10×10cm, 15×15cm and 20×20cm. The suppressant nozzles were installed at 2.5m and 2.6m of the duct. After explosion, the flame
Abstract:
The crimped flame arrester is a common disaster prevention and control device, most of the researches about it focus on the higher-pressure working conditions instead of the pressure lower than 0.1 MPa when it applies in special areas or environments. The quenching characteristics of different combustible gas-air mixtures passing through crimped ribbon flame arrester at different initial pressures were investigated, so that replenish the low-pressure protection test and understand the factors affecting the performance of flame arrester deeply. The experiments carried out at different initial pressure were in the DN80 circular pipe, and the crimped ribbon plate slit channel whose cross section is approximately equilateral triangle is 38 mm long and 0.8 mm high. The experimental gases are premixed propane-air with volume fraction of 4.2% and premixed ethylene-air with different concentrations which were distributed according to the partial pressure method. The ignition voltage is 10 kV. In this paper, the activity, concentration and initial pressure of combustible gas will affect the stability of flame velocity, propagation mode and quenching difficulty. The results shows that there are three modes of flame propagation: direct quenching, quenching after passing through the flame retardant unit, and quenching failure. They can explain as the flame not pass the slits, flame pass slits but extinguish before reaching the pipe end, and flame keeps spreading until pipe end. Also, the velocity oscillation occurs in the pipeline on the unprotected side, and the velocity rises incredibly when the quenching failure flame passes through the protected side. The formula of deflagration flame quenching velocity of premixed propane-air in closed pipe was based on the heat transfer effect and verified by the quenching experiment of premixed gas with concentration of 4.2%. The maximum initial pressure is defined as the limit pressure that quenching would fail at initial pressure higher than it. It is proposed to use the limit pressure to characterize the degree of quenching difficult. It is worth remarking that quenching is the most difficult at stoichiometric concentration where the limit pressure is the smallest, and the limit pressure will remain constant within a certain concentration range.
Abstract:
There are many limitations in the explosion experiments of the original scale model, such as the high cost of model making and the difficulty of matching the experimental equipment with large size specimens. Although the scaled-down model is commonly employed to predict dynamic response and damage effect of the original scale model, study of the similarity law of steel frame structure under the far-field explosion load is still very rare. As a result, the similarity law of steel frame structure under the far-field explosion load was investigated. According to the Π principle, the similarity law was proposed between the steel frame prototypes and the scaled-down models under the far-field explosion load. Based on the steel frame substructure explosion experiments, the numerical model of steel frame substructure was established by AUTODYN to verify the reliability of the fluid-structure interaction method in the structural explosion response analysis. On this basis, the accuracy and computational efficiency of the fluid-structure interaction method and the analytical blast boundary method were compared in the numerical simulation of the steel frame under the far-field
Abstract:
The corrosion and fatigue resistance of pressure vessel material Q345R steel after laser shock peening was studied. By electrochemical experiment and SEM micro-analysis, it shows that the corrosion resistance of the samples treated by LSP (Laser Shock Peening) with absorber layer and without absorber layer is increased by 5.8 times and 2.6 times respectively. The micro-experiment results show that the surface crack of corroded sample after LSP treatment is significantly less than untreated specimen. While the corrosion resistance decreases with the increase of the number of LSP. The S-N curves obtained from fatigue tests show that under the same stress conditions, the fatigue life after 1 hour and 2 hours’ corrosion is reduced by 36.8% and 56.4% compared to the original sample and the fatigue life after one and three LSP treatments times is increased by 43.8% and 198.2%. After XRD inspection tests, it is found that the laser shock peening can form a certain depth of residual compressive stress layer on the surface and effectively inhibit fatigue crack expansion.
Abstract:
Combined with simulation calculation and experiment, the damage mechanism of shaped charge warhead with hyperbolic liner and eccentric sub-hemisphere liner on water partitioned structure is studied. The results show that in the process of target penetrating, the penetrator formed by the combined charge liner is separated into multiple segments. The head part of the penetrator forms a cavity path to let the subsequent penetration follow with low resistance. The secondary damage performance of the penetrator formed by the combined charge liner is better than that of the hyperbolic liner. The radial size of the head of the penetrator for the hyperbolic liner increases significantly during the penetration process. Although the energy of the penetrator attenuates faster than that of the combined charge liner, the hole size to the target plate is larger than that of the combined charge liner. The research shows that because of the combined action of penetrator and explosion shock wave, the first target plate is completely broken. Cracks appear near the holes of the second target plate and the third target plate, but the length of the second crack is longer than that of the third one. Because of the significant attenuation of water medium and target plate on penetrator and shock wave, the fourth target plate only has regular circular holes and no obvious cracks.
Abstract:
Abstract: To study the protective effect of the combat helmet against traumatic brain injury induced by blast wave, an anti-explosion test was carried out for 50g TNT from the head model with and without helmet protection at 1m. The frontal, parietal and posterior cranial blast wave overpressure of the head model with and without helmet protection were compared and analyzed. The finite element model of the head with typical cranial structure was established and the blast wave loading was carried out to simulate the test conditions. The validity of the simulation model was verified by the test results. At the same time, the variation rule of the blast wave flow field under different conditions was analyzed by numerical simulation. The effect of foam liner on helmet protection capability was studied by numerical simulation. The results show that the typical combat helmet can attenuate the frontal air overpressure to 54.5% of that without protection, but can enhance the posterior cranial air overpressure to 2.19 times that without protection, which harms the protection of posterior cranial. The foam padding in helmet suspension can reduce the negative effect of the helmet on cranial posterior protection and improve the protective ability of the helmet against blast wave.
Abstract:
Aiming at the problem that there are differences in dynamic mechanical properties with different sizes of rock specimens on a large diameter split Hopkinson pressure bar system, the sandstone specimens with three different diameters (50 mm, 75 mm and 100 mm) and five kinds of length-diameter ratios (0.4, 0.5, 0.6, 0.8 and 1.0) were used for impact experiments on a pressure bar with a diameter of 100 mm. And the change rule of stress versus strain and strain rate versus time of specimens with different sizes were analyzed. The concept of superposition factor for comparing waveform alignment overlap was proposed, and together with the equilibrium factor. And, the study system of dynamic stress equilibrium was constructed, Thus, the recommended size range of specimens was determined for a large diameter split Hopkinson pressure bar tests. Also, a high-speed camera was used to test the dynamic damage of the specimens. The results show that when the length-diameter ratio of specimen is the same, the tested dynamic compressive strengths are close for the specimens with the diameter of 75 mm and 100 mm, but it is affected by more pronounced specimen length for the specimens with the diameter 50 mm. With the increase of specimen diameter, the curve of strain rate versus time changes from "single peak" to "double peak". The small-size specimen is more prone to axial splitting failure, and the large-size specimen produces larger tensile stress due to the superposition of internal stress waves, which is prone to the composite failure of spallation tension and axial splitting. When the specimen with a diameter of 75 mm and the length-diameter ratio of 0.3 ~ 0.4 is used, the coincidence degree after waveform alignment is better, sufficient stress balance time is available before initial failure, and the strain rate loading effect is better. It is helpful to reveal the size effect of rock dynamic compression mechanical properties with different sizes of specimens, and can provide a reference for the reasonable specimen size selection of large-diameter SHPB impact compression test.
Abstract:
To investigate the influence of gasoline-air mixture volume fraction, ignition position and liquid level on explosion overpressure parameters and flame development in vertical dome oil tank. The experiments of nine initial hydrocarbon volume fractions, four ignition positions and five liquid levels were carried out in a transparent simulated oil tank. Dynamic data acquisition system and high speed camera were used to detect the changes of internal and external field pressure, and to record the transformation of flame shape. The results show that: (1) 1.7% is the most dangerous gasoline-air mixture volume fraction under any working condition. The development of overpressure in the inner field can be divided into three stages: overpressure rise, overpressure release and oscillation attenuation. The formation and spatial distribution of free radicals such as CH, C2 and OH during the explosion process make the flame show different color changes at different initial volume fractions or at different explosion stages. (2) Ignition position has a great influence on explosion overpressure parameters. The lower the ignition position is, the greater the explosion power is. When the ignition position is in the center of the bottom of the tank, the average pressure boost rate of the internal and external fields reaches the maximum value, which is 0.464 MPa/s and 0.053 MPa/s, respectively. (3) The change of liquid level has a great influence on the overpressure of the internal and external field of oil and gas explosion. When the position ignition is located at the top of the side wall of the oil tank, the 50% liquid level is the most dangerous level. At any liquid level, the outfield overpressure decreases exponentially with the increase of scaled distance. The relationship among the maximum overpressure peak of the outfield shock wave of gasoline-air mixture explosion at different liquid levels, the distance and the volume of gasoline-air mixture can be expressed by a unified formula. Compared with gas space, the overpressure in liquid space has the characteristics of delay, enhancement of negative overpressure and faster oscillation attenuation frequency.
Abstract:
Based on the Kong-Fang concrete material model and the muti-material arbitrary Lagrangian Eulerian (MMALE) algorithm available in the LS-DYNA, the attenuation of stress wave in concrete subjected to explosion was numerically investigated. On the basis of comparative analysis of different material models, numerical algorithms and selection of appropriate mesh size, the proposed numerical algorithm and material models along with the corresponding parameters were firstly validated by comparing the numerically simulated spherical charge detonated in concrete target with corresponding test data in terms of peak stress and stress-time history. Then numerical investigation on attenuation of stress wave subjected to spherical charge detonated in concrete was conducted, and the radial and circumferential stress-time histories at different scaled distance were analyzed in detail to illustrate the mechanism of stress wave attenuation. The numerical results were fitted to develop an empirical formula for peak stress of free-field compression wave in concrete at the close zone with the aid of dimensional analysis. Besides, the applicability of the developed empirical formula were also discussed. The influences of charge buried depth on peak stress in concrete at different distances were also numerically investigated to develop a quantitative relationship between charge buried depth, distance and the so-called coupling factor. Numerical results demonstrated that the Kong-Fang concrete material model can be used to simulate the attenuation of explosion stress wave in concrete with good accuracy. The influence of charge buried depth and distance from charge center on the coupling factor of peak stress can be quantified by defining the mass coefficient and coupling constant. The empirical formula for peak stress of compression wave in concrete at the close zone is appropriate for varied charge buried depth, distance and concrete strength. The present numerical results are useful for blast-resistant design and can provide a reliable reference for estimating the damage degree of concrete caused by explosion.
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
Display Method:
Abstract:
In order to study the influence of bilinear resistance model on the vibration displacement of beam members under air blast loading, both the theoretical elast-plastic displacement solutions of the flexible and rigid members in forward and rebound stages were deduced, respectively. According to the relationship between blast duration and elastic duration from static position to maximum elastic displacement for members, the vibration situations could be divided into elastic forced vibration, elastic free vibration, plastic forced vibration, plastic free vibration, elastic rebound and plastic rebound. The equivalent single degree of freedom method was used to establish the vibration equations of each stage and the theoretical solutions of each stage were derived for different initial conditions. The method of the general solution plus the special solution was applied to solve each differential equation. Based on the theoretical solutions and the representative plastic strengthening coefficient, the elastoplastic vibration displacements of two types of beam members under different plastic strengthening degrees in the bilinear resistance model were verified under typical calculation cases. The corresponding complete vibration curves were finished for comparative analysis. The influence of the degree of plastic strengthening on the vibration representative value was analyzed. The results show that the displacement theoretical solution based on the bilinear resistance model has a wider range of application. With the increase of plastic strengthening coefficient of the bilinear resistance model, the maximum elastic-plastic displacement and residual deformation of the two types of beam members decrease gradually, and the reduction degree of residual deformation is higher than that of the maximum elastic-plastic displacement. When the plastic strengthening coefficient increases to a certain extent, the plastic vibration displacement will appear in the rebound stage of the beam members, further reducing the residual deformation. Compared with the bilinear resistance model, the elastic-perfectly plastic resistance overestimates the residual deformation of beam members under air blast loading.
Abstract:
The penetration resistance of concrete can be greatly improved by lateral confinement, and it would be continued to increase when pre-stress is further applied. However, the existing methods are difficult to realize the pre-stress on the confined concrete. In this paper, a relatively simple method for pre-stress confinement is proposed. Based on the principle of wedging the wedge-shaped block, a truncated cone-shaped concrete target with a cone inclination of 3° and a diameter slightly larger than the ferrule was squeezed into the matching steel ferrule, so the concrete target was pre-stressed along the radial direction by means of cone-shaped fitting and tightening, while the pre-stress was controlled by the indicators such as the pressing depth of the concrete target, the margin, and the pressing force. The feasibility of this method is then verified by simulation using LS-DYNA, and the penetration resistance of pre-stressed confined concrete is studied by the so-called restart algorithm. Numerical results demonstrate that the proposed method can provide enough radial pre-stress to the confined concrete target, and the pre-stress of the target increases approximately linearly with the increase of the pressing depth or the margin. Furthermore, within a certain range, the penetration resistance of the concrete target increases with the increase of pre-stress, while it decreases rapidly when the pre-stress is too high, which causes the damage of the concrete target. Parametric study on the parameters such as steel ferrule strength, concrete strength, steel ratio and projectile velocity, shows that reasonable matching of the steel ferrule strength with the concrete strength and selection of appropriate steel ratio of the target can effectively improve the pre-stress, penetration resistance of the target and the efficiency of steel; the higher the projectile velocity, the more obvious the effect of pre-stress on the improvement of the anti-penetration performance of the target. The proposed method for applying pre-stress provides a new approach to improve the anti-penetration capability of brittle materials such as concrete.
Abstract:
To investigate the mechanics, deformation and energy evolution characteristics of concrete under dynamic loading, impact compression tests with impact velocities of 5, 6 and 7 m/s and splitting tensile tests at 4 m/s were carried out on concrete specimens with aggregate rates of 0, 32%, 37% and 42% using the 100 mm diameter Split Hopkinson Pressure Bar (SHPB) device. The failure process of concrete specimens was acquired by a high-speed camera, and the damaged concrete fragments were collected and sorted, furthermore, the fractal dimension of fragments can be calculated by dividing the fragments into different grades through a standard sieve. The stress and strain of concrete were obtained through the corresponding calculation formulas. The relationships between specimen deformation, dynamic strength and fractal dimension with impact velocity and aggregate rate were studied, and the expression for dynamic strength with respect to impact velocity and aggregate rate was developed, in addition, the fractal dimension was used to characterize the surface roughness of concrete fragments, and the function expression between crack surface energy and fractal dimension was established, the relationship between sample absorption energy and crack surface energy was compared and analyzed. The results show that deformation hysteresis occurs when concrete specimens are destroyed and the failure is mainly in the form of splitting tensile damage. The dynamic strength increases with the increase of impact velocity and aggregate ratio, the dynamic strength of concrete can be better predicted by using the proposed dynamic strength formula. The fractal dimension of concrete breaking fragments, absorbed energy and crack surface energy all increase with the increase of impact velocity and decrease with the increase of aggregate rate, and the absorbed energy is always higher than the crack surface energy, the highest conversion rate of absorbed energy is achieved when the aggregate rate is 37%, with approximately 91% converts to crack surface energy.
Abstract:
The effects of different diminished ambient pressure, temperature and altitude from sea level on blast wave parameters (overpressure, impulse and wave front trajectory) were investigated by employing the dimensional analysis theory and the AUTODYN software. Meanwhile, the relationship equations between blast wave parameters with diminished pressure and temperature were established, which were verified by numerical simulations and experimental data. Results indicate that the equations can evaluate the blast wave parameters at diminished temperature and pressure effectively. It is noted that the blast wave overpressure and far-field (scaled distance Z>0.2 m/kg1/3) impulse decrease, but the propagation velocity increase, as the ambient pressure decreases. The blast wave impulse increases, and the propagation velocity decreases, but has little effect on the overpressure, as the ambient temperature decreases. It is shown that when the altitude increases by1000 m (the range of 0~9000 m above sea level), the overpressure and far-field impulse of blast wave decrease in average by about 3.9% and 3.2%. In addition, the blast wave propagation velocity in near-field increases, but the far-field decreases with the altitude increase. The influence of diminished pressure on blast wave overpressure and impulse is greater than diminished temperature at high altitude. The blast wave propagation velocity depends on diminished pressure in near-field, but on diminished temperature in far-field.
Abstract:
The collapse of the support part and break in the air of the reinforced concrete chimney during blasting demolition seriously affect engineering safety. Monitoring and analysis of a 180m chimney demolition were carried out to analyze the mechanism of these phenomena and distinguish them. Based on the characteristics of the stress-strain curve of concrete, the progressive failure process of the support part is analyzed. The static equilibrium equation of the cross-section is constructed, and the discrimination model for the instability and support part collapse of the chimney is proposed. By establishing the dynamic response model of the chimney above the blasting notch under the bottom impact, the propagation characteristics of the stress wave in the chimney are analyzed. The results show that the ratio of gravity moment to resisting moment can be used as a criterion of instability determination, considering the distribution characteristics of stress and strain in the cross-section of the support part. The compression failure of the concrete in the support part is almost inevitable under large eccentric compression. The necessary condition to prevent support part collapse of the chimney is that the minimum residual bearing capacity of the support part is not less than the weight of the chimney. When the chimney with a certain initial velocity impacts the foundation at the end of the support part collapse, an impact load will be generated and cause the strain in the middle of the chimney greater than the strain at the bottom. The elevation amplification effect of dynamic strain is an important reason for the chimney breaking in the air. The higher the chimney is, the shorter the impact duration is, and the more significant the dynamic strain elevation amplification effect is. As the height increases, the position of the most dangerous section of the chimney will move from the middle and lower to the middle and upper.
Abstract:
The influence of assembly cushions on the fracture of an expanding metal cylindrical shell is studied. The velocity of the outer surface of the shell with or without a cushion in it was measured by a DPS array, and images with the obvious influence of an inner cushion on the fracture of the shell were recorded by the high-speed photography. These results show that compared with the area without the cushion, the outer surface of the cylindrical shell in the cushion area experienced a process of first convex and then concave movement, which makes the radial displacement of the surface repeatedly misplaced, leading to a final displacement of 0.34 mm lower. This displacement difference may lead to radial shear fracture of the cylindrical shell. Besides, in the experiment, a crack appeared on both sides of the cushion/gap interface (7.5° deviation on the cushion side and 9° deviation on the gap side). These cracks were resulted from the disturbance of two sparse stress waves, which were generated from the cushion/gap interface and then transmitted to the outer surface of the cylindrical shell. The fracture mode is different from both circumferential tensile fracture and shear fracture along 45° direction. This new fracture mode is closely related to the dynamic mechanical properties of cylindrical shell’s material. Further numerical simulation analysis shows that the influence of the assembly cushion on the fracture mechanism of the cylindrical shell includes three aspects: firstly, the additional mass effect; secondly, the amplitude change of the explosive impact loading after it passing through the cushion, and the asynchronous difference of the impact loading sequence with other parts; and thirdly, the influence of the propagation of surface waves, which originate from the interface between cushion and gap, on the subsequent development behavior of cylindrical fracture mode .
Abstract:
Aiming at the problem that the initiation mode of the explosion device is highly dependent on the gunpowder products in the simulation experiments of large-scale underground explosions in a vacuum chamber, and based on the similarity theory of underground explosions and the principle of the two-stage gas gun, a micro explosion device initiated by a synchronous launcher of marbles driven by two-stage high-pressure gas has been developed independently. A glass enclosure with compressed gas (filled by air compressor) was used to simulate the high-pressure cavity generated at the beginning of a real underground explosion. Two-stage high-pressure gas was used to drive marbles to break the glass shell synchronously, thus releasing the high-pressure gas in the spherical shell to simulate the ejection of gas products in a real underground explosion. The pressure in the launcher chamber is 4 MPa, and the residual steady-state gas pressure in the glass enclosure is about 3 kPa. The above set of the launch parameters could be used for simulation experiments of real underground explosions with an equivalent of 0-20 kt TNT. Through high-speed imaging of the air and water blasting sphericity tests, the reliability of the explosion device and the sphericity of the blasting effect were verified. When there is a difference in the internal and external pressure of the glass spherical shell, the cracks of the shell are fully developed and the fragments are evenly distributed. The applicability test shows that the blasting mechanism and blasting effect of the explosion device can meet the requirements of the simulation experiment of large-scale underground explosions in the vacuum chamber, and the device has the characteristics of high efficiency, low pollution, convenient operation, good repeatability, good controllability and low requirements for site conditions, which could provide a novel technology for the simulation experiments of large-scale underground explosions in the vacuum chamber.
Abstract:
The quasi-static and dynamic compressive mechanical properties of flexible polyurethane foam were studied using DDL-200 electronic universal testing machine and Instron 9350 drop-weight testing machine at a range of strain rates from 0.001 to 100 s−1. The stress-strain characteristics and strain rate sensitivity were analyzed, and the effect of strain rate on strain rate sensitivity index and energy absorption performance was discussed. Based on the experimental results, the strain rate-independent constitutive model was established to accurately describe the dynamic compressive mechanical behavior of the flexible polyurethane foam. The results show that the compressive stress-strain responses of flexible polyurethane foam exhibit typical “three-stage” deformation characteristics including initial elastic region, extended plateau region and finial densification region, and the characteristics of material mesostructure at different deformation regions were analyzed. In addition, the material display an obvious strain rate-strengthening effect, both the yield stress and platform stress increase with the increase of strain rate, and the strain rate sensitivity index is affected by the coupling of strain rate and compressive strain. The energy absorption, energy absorption efficiency and specific energy absorption of flexible polyurethane foam at different strain rates were compared and the material shows higher energy absorption efficiency but less energy absorption, and strain rate has little effect on maximum energy absorption efficiency and specific energy absorption under quasi-static loading. With the increase of strain rate, the maximum energy absorption efficiency significantly reduces and the specific energy absorption significantly increases under dynamic loading. Both the modified Sherwood-Frost model and the modified Avalle model considering the effect of strain rate can well characterize the static and dynamic compressive stress-strain responses of the flexible polyurethane foam, but the modified Avalle model is easier to apply in engineering due to its fewer parameters. The research results can provide a guide for the design and optimization of flexible polyurethane foam on impact-resistant structures.
Abstract:
NiTi shape memory alloy, a typical smart and functional material, has been widely applied in various engineering fields due to its excellent superelasticity and shape memory effect which originated from the reversible thermo-elastic martensite transformation. The phase field method is a powerful computational approach for modeling and predicting of mesoscale morphology and microstructure evolution of materials. It is employed to describe the microstructural evolution via a set of order parameters that are continuous in both time and space. In this study, a new non-isothermal phase field model was established based on time-dependent Ginzburg-Landau kinetic equation. In particular, an additional grain boundary energy term was introduced into the local free energy density to take into account the contribution from the grain boundary of a polycrystalline NiTi shape memory alloy system. In order to understand the underlying microscopic mechanisms for the superelastic deformation, the microstructure evolution and the overall mechanical behavior of both single-crystalline and polycrystalline NiTi shape memory alloys were numerically investigated under tensile loading and unloading at 290 K. After that, the intrinsic strain-rate sensitivity of nanocrystalline NiTi was studied with grain size of 60 nm at low strain rates (0.0005−15 s−1). The results show that the martensitic transformation in single crystalline NiTi is uniform. No austenite-martensite interface was formed during the computation. Superelastic deformation was simulated by a nanocrystalline NiTi phase field model. Such behavior is achieved through the nucleation and expansion of martensite bands during uniaxial tensile loading as well as the disappearance of martensite bands during the unloading. In comparison, the single-crystalline NiTi processes larger hysteresis area and better superelastic deformation ability than polycrystalline NiTi under the same external loading condition. Noticeable strain-rate sensitivity was exhibited in stress-strain relation of nanocrystalline NiTi shape memory alloys under low-to-medium strain-rate loadings. The phase-transformation stress increases with the rise of implemented strain rate. Such strain-rate dependence is a result of the competition in the phase field model between the speed of martensitic domain evolution and the speed of external loading.
Abstract:
A 12.7 mm projectile may remain intact or be broken during penetrating steel targets with different strength. However, previous simulations were limited to simulating a single situation. To break this limitation, the numerical simulation methods of the 12.7 mm projectile penetration into steel targets are studied, leading to a projectile-target model which is capable of simulating both the intact and broken cases. In the intact projectile case, the ballistic tests were implemented to study the dynamic behavior of 12.7 mm projectile penetrating into the 603 steel targets. Two different modeling algorithms based on the finite element method (FEM) and the smooth particle hydrodynamics particles (SPH) method, respectively, are compared with the experimental results. Then the influences of finite element and particle sizes on the numerical results are studied to establish the numerical model to simulate the intact projectile case. Furthermore, the established model is applied to simulate the broken projectile case by changing the target material and the element sizes. The numerical results are then compared with the experimental results. The numerical study shows that the projectile and target should be discretized using FEM and SPH, respectively, for simulating the intact case. Meanwhile, a large ratio between the finite element mesh size and SPH particle spacing should be used, such as 5.3. Otherwise, an abnormal numerical deformation may occur around the projectile head, which is inconsistent with the experimental result. This model can also be used to simulate the broken projectile case, as verified with the experimental results. However, the large ratio between finite element mesh size and SPH particle spacing leads to numerical problem and abort of simulations. To overcome this difficulty, a FEM/SPH coupled projectile-target model is proposed, in which the projectile is discretized using coarse mesh close to the surface and fine mesh in the core region. Numerical results show that the proposed projectile-target model can be used to model the penetration process no matter the projectile remains intact or broken.
Abstract:
The detonation position and the shape of the explosive have a significant influence on the pressure of the underwater explosion shock wave, which makes it possible to use a small charge to form a shock wave that is equivalent to a large charge in a local direction. A charge design method to adjust the amplitude and duration of shock wave pressure was established based on the slender charge structure and parameter optimization design to carry out an underwater explosion shock resistance test of ship structure or equipment using a small charge. Firstly, based on the simple wave theory, the principle of shock wave pressure control and the objective function and constraint conditions of optimal design of charge parameters are given. Then, an independently developed software is used to study the energy of underwater explosion of slender charge, and the confidence degree of numerical simulation is verified through experiments. It is found that the influence of initiation position and charge shape on the pressure peak and duration of the underwater explosion shock wave is significant. The duration of shock wave pressure of slender charge column underwater explosion can be determined by geometric approximation. Finally, to further investigate the effectiveness of the proposed method, two charge schemes equivalent to the shock wave pressure of the prototype were designed and verified by numerical simulation. The prototype is taken from the pressure-time curve of the underwater explosion shock wave with TNT equivalent to 1000 kg and a stand-off of 100 m. The comparison results show that the designed charge can form a shock wave pressure-time curve equivalent to that of the prototype on the side of the initiation end within a predetermined duration. Since the bubble pulse is not considered, the established method applies only to the middle and far-field explosion shock problem.
Abstract:
In order to investigate the temporal and spatial distribution of the stress wave in the soil produced by buried explosion, ANSYS/AUTODYN are employed for modelling and simulation, and the ground shock effect of explosion in soil is analyzed. Based on the relationship between pressure and volumetric strain of Luoyang loess obtained by predecessors, the relationship between pressure and density of the impact compaction in SAND model is modified. The numerical model is validated by the test data, which were measured from the contact explosion and semi-buried explosion test in loess. Then, a total of 22 numerical simulation conditions are examined to study the influence of the scaled buried depth of charge and the type of explosive on the ground shock subzones. The results show that as the depth of the soil medium increases, the peak of induced ground shock decreases, while the peak of direct ground shock increases, until the peak of the pressure-time curve and the peak in the curve of vertical stress vs. time finally marge into a single peak. According to the characteristics of the pressure and vertical stress at various depths, the stress wave field in soil can be divided into three subzones, i.e., surface subzone, near-surface subzone and central subzone. With the increase of the scaled buried depth of charge, the central subzone rapidly increases, the surface subzone rapidly decreases, and the near-surface subzone gradually increases from zero, when the scaled buried depth of charge ranges from −0.05 m/kg1/3 to 0.075 m/kg1/3. The distribution of ground shock subzones tends to be stable, when the scaled buried depth of charge ranges from 0.1 m/kg1/3 to 0.4 m/kg1/3. The energy of explosive coupling into the air and soil medium is affected by the type of explosive. In certain extent, the angle of the ground shock subzones is linearly related to the ratio of the air-blast overpressure impulse to the impulse of the direct ground shock stress.
Abstract:
In order to deeply explore the propagation and attenuation law of column charge blasting stress waves or seismic waves, and improve the prediction model of the blasting peak vibration velocity, a theoretical study on the blasting peak vibration velocity was carried out. First of all, based on the Heelan short-column charge theory, the concept of the equivalent radius of action is introduced, and the attenuation equation for the blasting peak vibration velocity under the action of the internal instantaneous excitation load is obtained. Then, the concepts of the equivalent action radius and equivalent blasting load were applied to the theoretical derivation of the blasting peak vibration velocity. The attenuation laws of the blast-induced vibration in cutting hole sections and non-cutting hole sections were studied separately. Finally, based on the dimensional harmony theorem, the reliability and universality of the attenuation model were verified. Combined with an example of tunnel blasting project, the attenuation law of the blasting peak vibration velocity corresponding to different segments of detonators and different types of blast holes was studied. The results show that the improved formula can well fit the peak velocities of the above two types of blasting vibrations, which can accurately reflect the transmission law of the tunnel blasting vibration. In addition, the expressions of the charge form of the improved formula under the conditions of spherical charge and columnar charge are discussed, and the prediction effects of various fitting models were compared. The comparison results show that using the equivalent radius of action as a fitting reference variable can comprehensively consider the influence of different detonator positions and different blast hole types on the blasting vibration attenuation law. The reference variables of the statistical data show that the fitting effect obtained by the improved formula is the best, which can provide a reference for similar research of tunnel blasting vibration.
Abstract:
To study the crater effect of the projectile penetrating a thick concrete target, the crater phenomenon in the penetration test is summarized, the predictive effect of the empirical formula on the crater depth, crater diameter, and crater angle is analyzed. Using the dimensional analysis method, new calculation formulas for the crater formation effect and energy consumption at the crater formation stage are established. The formula for the crater formation effect takes into account the influence of factors such as impact velocity, target strength, reinforcement ratio, projectile diameter, and projectile mass. Based on the new calculation formula, parameterized analysis of the influencing factors of pit formation effect and the energy consumption of pit formation is performed. The results show that the dimensionless crater depth is greatly affected by the strength of the concrete target, the reinforcement ratio, and the projectile mass. For reinforced concrete, with the increase of the impact velocity, the crater depth increases first, then decreases, and then increases. Within the common range of penetration speed and mass, the crater angle is about 15°~24°, and the mass has little effect on the crater angle. The energy consumption of the crater formation on the front surface accounts for 10% to 25% of the total kinetic energy of the projectile, and the reinforcement ratio and the strength of the target plate have a small effect on the proportion of the energy consumption of the crater. The proportion of the energy consumed in the crater stage increases as the mass of the projectile decreases. The calculation results of the new crater effect calculation formula for the crater depth, crater diameter, and crater angle are in good agreement with the experimental data, which can provide a reference for the design of penetrating projectiles and engineering protection.
Abstract:
In this paper, a finite element model of the granular slug launcher was constructed. Using the discrete element-finite element coupling method, the dynamic response and mechanism of mitigation and energy absorption of sandwich beams with reentrant honeycomb core of negative Poisson’s ratio subject to high velocity granular slugs were investigated. Effects of the load impulse, impact angle, core strengths and friction between the granular slug and face sheets on dynamic response of sandwich beams were analyzed. The results demonstrated that the active deformation mode of sandwich beam subject to the normal impact of the granular slug is combined local denting and overall bending. The deformation mode of the sandwich in-plane core is local denting mode due to the bending of cell walls, whilst the sandwich out-plane core is local folding mode due to buckling of the cell walls. Compared to the same areal density of the sandwich beam with the in-plane design of the ‘soft’ core, the deflections of the sandwich beam with the out-of-plane design of the ‘hard’ core are smaller but both its initial peak and level of impact force are higher and its response time is shorter. The mid-span maximum deflections of front and rear face sheets of the sandwich beam increase with the impact loading approximately log-linearly. Compared to the normal impact, the deformation mode of the sandwich beam subject to the oblique impact is asymmetrical and the local denting area reduced. The mid-span maximum deflections of the front and rear face sheets, the initial peak of impact force and the proportions of kinetic energy and momentum transferred to the sandwich beams subject to velocity granular slug with different impact angles decrease with the increase of the impact angles, while the friction between the granular slug and the face sheets has little effect on dynamic response of sandwich beams.
Abstract:
In the current design of the dynamic damage field of the fragment warheads, the central blind area effect is regarded as an essential factor affecting the warhead damage efficiency improvement. The axial enhanced warhead has become an important design means to eliminate the dynamic central blind area of the warhead, which attracts more and more attention from relevant researchers. In the present paper, based on the smoothed particle hydrodynamics (SPH) computation method, a series of numerical models for an axially reinforced warhead's shell breaking and fragment dispersion process with non-filled, polyurethane filler nylon filler and explosive filler at the end under the explosive loadings are established, and used to study the influence of the characteristics of the fillers in the front of the warhead on the dynamic response of shell. It is found from numerical simulation that the filler has a significant influence on the velocity of fragments in the front of the warhead but a minor influence on the dispersion angle of fragments. The mechanism of the influence of non-reactive filler on the fragment velocity is analyzed by comparing velocity history curves of the specific fragments. The results show that the polyurethane foam filling can significantly delay the acceleration process of the explosive shock wave to the forward fragment and reduce the explosive load to a certain extent. The nylon filler can reduce the acceleration of the forward fragment and the acceleration of the lateral fragment to a certain extent. Thus, the explosion loading is guided to be evenly distributed around the circumference of the end position. Considering the synthesis of the involved velocity of the warhead, using low-density and low-mass filler instead of head charge has the same dynamic damage effect of improving the energy utilization efficiency of the axial enhancement warhead. The numerical model established in this paper and the research finding can provide some reference for the dynamic damage field design of conventional fragment warheads.
Abstract:
To study the crater's characteristics of carbon fiber/epoxy composite targets at velocity of 3.0-6.5 km/s, experiments of some composite targets impacted by spherical aluminum projectiles were carried out by use of a two-stage light gas gun in China Aerodynamics Research and Development Center (CARDC). Targets were one kind of unidirectional braiding laminates made of carbon fiber and epoxy. The density of the targets was 1.5 g/cm3 and the size was 100 mm×100 mm×20 mm. The targets were clamped by two aluminum plates in experiments. One aluminum plate with the thickness of 2.5 mm was set 40 mm behind the targets to test fragments after the targets. The projectile diameter ranged from 1 mm to 3 mm. The damage feature of each target was obtained. A central crater surrounded with a shallow spalling region was observed in all recovered targets. Different from a semi-spherical crater, the central crater had a proximately quadrate edge, a semi-spherical bottom and a tough and rugged wall. The shallow spalling region was extremely irregular. The parameters of the crater and the shallow spalling region, such as the crater depth, the superficial area of the crater, the superficial area of the spalling region, were measured and analyzed. Moreover, the variations of the dimensionless crater depth p/dp, the dimensionless equivalent crater diameter Dh/dp and the equivalent diameter De of the spalling region with the impact velocity and energy were analyzed. Results show that the p/dp and Dh/dp depend on the density ratio ρp/ρt with a power of 0.5, and on the impact velocity vi with a power of 2/3. The results are in good agreement with NASA’s hypervelocity experiments on reinforced carbon-carbon targets. De is a power function of impact kinetic energy. The crater-shape coefficient p/Dh is slightly greater than 0.5, which means the crater depth is larger than the crater radius.
Abstract:
The blast shock wave will be transmitted to the ground through the vent as a gas explosion accident occurs in an urban shallowly-buried pipe trench, and cause serious disaster consequences. However, there are few studies on the propagating law of the explosion load outward through the explosion vent in the long and straight space. Thus, it is necessary to reveal the explosion load distribution law on the ground of such accidents. Based on the combustible gas explosion test in the long and straight venting space conducted in the previous period, the applicability of parameters and grid size in FLACS software were verified. Then the FLACS software was used to carry out numerical simulations of the gas explosion process in the urban shallow buried pipe trench. The propagation process of shock wave was divided into three stages: stable stage, Δp1 stage and Δp2 stage, and the mechanism of shock wave was analyzed by fuel, flame, flow velocity and density. The results show that the value of Δp1 is small, mainly caused by compression waves, and Δp2 is the maximum overpressure peak, mainly caused by flame waves. The characteristics of the overpressure time-history curve were studied. The results show that Δp1 has smaller differences in each direction than Δp2, and the wave propagation has obvious directionality in X and Z directions, while symmetrical in the y direction. The attenuation law of shock waves in space was studied and the attenuation formula in each direction was obtained by data fitting. The results show that Δp2 gradually decreases with the increase of the distance from the venting port, and the value of the value in each direction varies greatly, among which, it shows a symmetrical attenuation trend along the short side of the pipe trench section; Δp2 and distance roughly satisfy the exponential function relationship, and the fitting degree is above 98.8 %.
Abstract:
Concrete-filled double-skin steel tubular (CFDST) members are widely employed as load-bearing members in the ultra-high power transmission tower and offshore platform. The impact resistance of this type of members should be considered in the design stage. Based on the previous test results, in total 200 finite element (FE) models considering the coupling of axial and lateral impact loads are established with ABAQUS software, and the damage mechanism of impact resistance is analyzed. Then, the parametrical studies are carried out to investigate the influences of key factors, including the nominal steel ratio, hollow ratio, cross-sectional diameter and material strength on the impact resistance of the members for the axial load ratio ranging from 0 to 0.7. Finally, the calculation formula for the impact bearing capacity is proposed and the dynamic response at the mid-span is predicted based on the methods of dynamic increase factor and an equivalent single degree-of-freedom model. In this work, the deflection at the mid-span and the plateau impact force are taken as the key indexes to evaluate the impact resistance. Results indicate that the impact resistance of circular CFDST columns decreases with the increasing of axial load ratio. Under lateral impact, CFDST members with hollow ratio lower than 0.7 exhibit flexural failure. The interaction between external steel tube and inner concrete is stronger than that between inner steel tube and outer concrete. In addition, the nominal steel ratio, outer diameter of the cross-section, yield strength of outer tube, impact velocity and impact mass all play significant roles on the maximum deflection at the mid-span and the plateau impact force when the axial load ratio ranges from 0 to 0.7. Effects of hollow ratio and concrete strength are marginal. The proposed calculation methods can reasonably predict the impact bearing capacity and mid-span displacement response of CFDST members when subjected to an impact.
Abstract:
Shell nacre is a nature material with high strength and toughness, and the excellent performance is mainly derived from multi-scale, multi-hierarchy with “brick and mortar” structure. Inspired by the special structure of shell, a finite element model of nacre-like brick and mortar structure was created and the explosion experiment was carried out. In the experiment, the sample was destroyed catastrophically at the explosion impulse of 0.047 N·s, with the fall of the center. Additionally, shear failure existed around the clamping end of the specimen, which is in good agreement with the numerical simulation results. On this basis, the dynamic response of nacre-like brick and mortar model under explosive load was explored. Five different failure modes were analyzed, including mode Ⅰ: inelastic deformation without damage, mode Ⅱ: partial damage with damage in the back surface, mode Ⅲ: through-wall failure in the center of specimen, mode Ⅳ: through-wall failure in the center of specimen and shear failure at the clamping end, mode Ⅴ: devastating damage with large drop through in the center and shear failure. The thresholds critical of different failure modes were obtained based on the simulation results. The threshold value for the 1-layer brick-mud structure was 0.019 Ns, and this value increased to 0.047 N·s for 5-layer brick and mortar structure. When the impulse exceeds threshold value, catastrophic damage occurred. The effects of the number of stacked layers on response of brick and mortar model were analyzed. With the increase of the number of stacked layers, the failure mode of the structure changes from devastating damage to inelastic deformation. Additionally, the threshold value for brick and mortar structure under explosion load increased with the increase of the number of stacked layers. Finally, the toughening mechanism of nacre-like brick and mortar structure was given, including crack deflection and microcrack.
Abstract:
During engineering construction and service in seasonally frozen soil regions, frozen soil is often subjected to the combined action of freeze–thaw (F-T) cycles and impact loading, which changes its physical state and mechanical properties. In order to explore the effect of F-T cycles on the impact dynamic mechanical properties of frozen soil, in this paper, the typical frozen soil was taken as the research object, and the effect of F-T cycles on the impact dynamic mechanical properties of frozen soil was comprehensively studied with the help of high and low temperature F-T cycles experimental equipment and a split Hopkinson pressure bar device, through F-T cycles experiments with different F-T cycles numbers, freezing experiments at different temperatures, and impact dynamic experiments with different strain rates. The results shows that there is an F-T cycles effect in frozen soil. With the increase of the number of F-T cycles, the peak stress of frozen soil decreases to a certain extent, but after reaching the critical number of F-T cycles, the peak stress remains stable. According to the hydrostatic pressure theory, it is believed that the F-T cycles mainly changes the mechanical properties of frozen soilby changing its microstructural characteristics. Meanwhile, the frozen soil also exhibits obvious strain rate effect and temperature effect, and its peak stress increases with the increase of strain rate or the decrease of temperature. TheF-T damage factor was defined by the peak stress, and the impact damage was deduced by a statistical method that it assumes the microstructure strength of frozen soil satisfies the Weibull distribution, a damage viscoelastic constitutive model based on the Z-W-T equation was proposed. The model can better describe the impact dynamic mechanical behavior of frozen soil after F-T cycles and provide reference for the impact dynamic damageof frozen soil in seasonally frozen soil regions.
Abstract:
The impact force transducers are widely used in aerospace, national defense engineering, auto industry and other important fields involving national security and people livelihood. Those transducers usually need to be calibrated before being put into practical use. Realizing synchronous loading and developing an accurate mathematical model to describe the input-output relationship are the major challenges in the calibration of triaxial impact force transducers at this stage. In this paper, a method for the synchronous excitation of three-dimensional impact force loads was established based on a modified Hopkinson bar technique and the principle of vector decomposition. The triaxial impact force transducer being calibrated was mounted at an angle to the axis of Hopkinson bar, the one-dimensional force excited in the Hopkinson bar was then decomposed onto each sensitive axis of the transducer, thus realizing its synchronous loading. The coupling effect between the sensitive axes of the transducer was assumed to be linear. A linear decoupling calibration model of the triaxial impact force transducer was built based on a sensitivity matrix containing three main sensitivity coefficients and six transverse sensitivity coefficients. The sensitivity matrix was solved using the least squares method. The amplitude and pulse width of the impact force pulses excited in the Hopkinson bar were adjusted by varying the structure and the impact velocity of the bullet. Reference impact force pulses with varied amplitudes and pulse widths were then used to calibrate the triaxial impact force transducer. Characteristics were revealed that both the main sensitivity coefficients and the transverse sensitivity coefficients of the transducer are related to the amplitude and the pulse width of the reference impact force. The amplitude and pulse width information of the input force pulses that the transducer was subjected to can be reflected by the output voltage pulses of the transducer. Therefore, the amplitude and pulse width of the output voltages of the sensitive axes of the transducer were taken as influencing factors and added to the input layer of the artificial neural network (ANN) in form of artificial neurons. A nonlinear decoupling calibration model for the tri-axial impact force transducer was then built based on an ANN model. The calibration results show that the ANN model has higher calibration accuracy compared to the least squares model. It is feasible and valid to use ANNs to calibrate the tri-axial impact force transducers.
Abstract:
Metallic materials are widely used in automotive, aerospace, energy, national defense, and other important fields due to their excellent mechanical properties. During service periods, metallic materials are generally subjected to complex stress states. Recent researches reveal that the plastic behavior of materials is affected by the stress state. Therefore, to accurately describe the plastic flow behavior of materials under complex stress states, the influence of the stress state must be considered in the constitutive model. Under dynamic loading, however, the effects of strain rate and stress state are coupled, which makes it difficult to study the effect of stress state and to establish a stress-state-dependent constitutive model. In this work, mechanical tests were performed under various loading conditions including uniaxial compression, uniaxial tension, and simple shear using the MTS universal testing machine and the split Hopkinson bars technique. The stress-strain curves of Ti-6Al-4V were obtained over a wide range of strain rates and temperatures. It is observed that stress states have an obvious effect on the plastic flow properties and work-hardening characteristics of the material. Based on the experimental results, a new constitutive model that incorporates the influence of the stress triaxiality and the Lode angle parameter was proposed. Under tensile or compressive loading conditions, the flow stress determined by the J-C model is significantly lower than the test results, while the present model can predict the flow stress accurately. To check the applicability of the proposed model, the dynamic experiment of a specimen under the compression-shear combined load was simulated by both the J-C model and the proposed model implemented in the ABAQUS/Explicit software via the VUMAT user subroutine. The results show that the present model exhibits a higher accuracy in the prediction of the flow stress curves. Moreover, this model can predict both the transmitted pulse and the force-displacement curves more accurately. Therefore, the new model can describe the stress state effect successfully and predict the plastic behavior of the material under complex stress states more precisely.
Abstract:
To accurately calculate the hypervelocity impact of 93 tungsten alloy projectiles on Q345 steel plates, a modified constitutive model of metals is established. The GRAY three-phase equation of state is introduced to describe the phase change of the material, while the Johnson-Cook strength model is used to describe the mechanical behavior of the material in the late stage of impact process. Combined with the Feng’s damage evolution model and Johnson-Cook failure model, the tensile and shear failure behavior of materials under different stress triaxiality are represented. The fracture evolution model proposed by Cao Xiang is adopted to describe the process of stress vanishing after material failure. The applicability of the constitutive model is then verified by comparing the numerical simulation results with the experimental ones. Furthermore, the spatial distribution characteristics of fragment group in a typical process of a projectile impacting target are analyzed. The results show that based on the modified metal constitutive model, the perforation diameter of the target, the erosion length of projectile and the expansion velocity of fragment group of the hypervelocity impact are consistent with the experimental results. The GRAY three-phase equation of state can relatively accurately give the melting situation of the projectile and target materials when the projectile impacts the first layer of the target plate and the remaining projectile and fragment group impact the second layer of the target plate. The Feng’s damage evolution model can accurately judge whether spallation occurs during the hypervelocity impact. After integrating Feng’s damage evolution model, Johnson-Cook failure model and fracture evolution model proposed by Cao Xiang, the statistical curve of perforation area and cumulative number of aftereffect target plate impacted by fragment group obtained from the numerical simulation are consistent with the experiment data. The spatial distribution results of fragment group of cylindrical 93 tungsten projectile hypervelocity impacting Q345 target plate under typical conditions are obtained, and the front part of fragment group possesses high mass, high axial momentum and high transverse momentum by their absolute values.
Abstract:
This paper first reviews the development and relevant issues in relation to the strain-rate effects on the compressive strength of concrete-like materials. For different characteristics of strain-rate effects on the dynamic compressive strength of concrete-like materials under various stress states, it reveals the significant discrepancies in the measured dynamic increase factors (DIF) under different loading paths. At high strain-rate loading, the test specimen based on the initial 1D-stress state gradually evolves to a multiaxial one due to the increasing lateral confining pressure caused by the lateral inertia effect. The traditional split Hopkinson pressure bar (SHPB) test cannot obtain the genuine DIF data under real 1D-stress state at high strain-rates. The strength models based on the direct adaptation of the experimentally measured DIF using SHPB overestimate the dynamic strength of these materials. Considering the loading-path dependence of the strain-rate effect, this study extends the DIF model depending only on strain-rate to a more general DIF model depending on both the strain-rate and the stress state, which is then implemented into the Drucker-Prager strength model. Numerical SHPB tests are conducted on samples with free and constrained boundaries. The comparison between test data and numerical predications shows that the proposed DIF model can describe the stress state dependency of the strain-rate effect, and hence can predict the dynamic compressive strength of concrete-lime materials more accurately. The present study is of great significance for correctly applying SHPB technology to determine the dynamic compressive strength of brittle materials.
Abstract:
The studies of the plastic flow behaviour of metallic materials show that the plastic deformation process of metallic materials is dependent on temperature and strain rate, so the temperature and strain rate sensitivities are the most important essential properties of plastic deformation of metallic materials. It is therefore necessary to establish appropriate thermo-viscoplastic constitutive relations to accurately describe the temperature and strain rate dependences of the plastic flow behaviour of metals over a wide range of temperatures and strain rates. Advantages and disadvantages of these constitutive relationships are first reviewed in the present paper. With the increasing applications of metallic materials and the emergence of new materials, the 3rd type strain aging, K-W lock induced anomalous stress peak, and tensile-compression asymmetry are often observed in the plastic flow behaviour of metals. Due to the occurrence of those phenomena, the traditional metal thermo-viscoplastic constitutive relations may no longer be applicable. In view of the significant roles played by the 3rd type strain aging, K-W lock dislocation structure-induced anomalous stress peaks, and tensile-compression asymmetry in the plastic flow behaviour of metals, especially in high temperature loading, it is necessary to take those particular phenomena into account in the framework of the thermo-viscoplastic constitutive relationship of metals. Thus, a large variety of constitutive relation, which considers the interaction of strain, temperature and strain rate, has been established to predict the deformation behaviors of metals. In this context, this paper presents a systematic review of the thermo-viscoplastic constitutive relationships of metals, which includes the anomalous stress peaks in the flow stresses with temperature due to the 3rd type strain aging or K-W-locked dislocation structures, and the tensile-compression asymmetry. In addition, the forms of these thermo-viscoplastic constitutive relationship considering the 3rd type strain aging, K-W lock dislocation structure-induced anomalous stress peaks and tensile-compression asymmetry in the flow stress of metals, are discussed and analysed.
Abstract:
The establishment of the dynamic mechanical model of rock materials and the determination of the relevant model parameters are of great significance to the studies of rock′s dynamic mechanical properties and related simulation calculation. Taking granite in Shandong Province as the experimental object, based on the Kong-Fang fluid elastic-plastic damage material model (KF model), the model parameters are classified into three categories, and the test scheme is then correspondingly determined. The basic strength parameters of the KF model were measured by quasi-static uniaxial compression and unconfined splitting tests. The strength-surface related material parameters were fitted by the results of the conventional triaxial tests under five different confining pressure conditions. In addition, the dynamic split Hopkinson pressure bar (SHPB) tests under several strain rate conditions were carried out to determine the strain-rate related parameters, of which the effectiveness were then verified by the dynamic split Hopkinson pressure bar-Brazilian disk (SHPB-BD) tests results. According to the principle of reverse impact and the Rankine-Hugoniot equation, the plate impact experiments with different impact stress levels were conducted by using a single-stage light gas gun, the state equation parameters in the KF model were fitted according to the impact Hugoniot results of rock samples. To verify the applicability of the material model and the experimentally measured parameter values, the simulation of a penetration process is furtherly conducted. The granite penetration tests were carried out by using a ϕ30 mm caliber gun. The ϕ20 mm bullets penetrated the ϕ1200 mm×800mm rock targets vertically, which was used to characterize the semi-infinite thickness condition, at an approximately designed speed of 670m/s. To avoid accidental errors, combined with the high-speed photographic images, three effective penetrate results were obtained. The penetration depth and crater size of the target failure surface were directly measured and scanned by 3D scanner, the experimental average penetration depth, maximum and minimum diameters of the penetration craters were approximately 80.62 mm, 381.47 mm and 263.01 mm, respectively. Using the parameter values obtained from the laboratory experiments, the KF model is then implemented into LS-DYNA through a user-defined material model and used to simulate the penetration test of granite. According to the simulation result of damage distribution and cratering parameters of the target, the calculated penetration depth, maximum and minimum diameters of craters are 80.02 mm, 400 mm and 300 mm, respectively, so the errors between the calculated and the test results are less than 15%, which is acceptable in dynamic problems. The agreement between the numerical and experimental results provides a support to the application of the KF model and the relevant parameter values.
Abstract:
Different from static loading conditions, the plastic flow behavior of metallic materials under high strain rate loadings, such as impact and explosion, exhibits special rate-temperature coupling effect and deformation micro-mechanism. The design and evaluation of metallic structures used in aerospace and navigation, energy mining, nuclear industry, public safety, disaster prevention, etc. require a large number of experiments under dynamic loadings. In recent years, the rapid-developing computational mechanics can be used to analyze the structural mechanical response under complex loading, evaluate the structural safety and optimize the structural design, and can also save the experimental costs. Accurate dynamic constitutive description of materials is the basis for the reliability of structural numerical simulation. In this paper, the dynamic plastic deformation behavior and micro-mechanism of metals, as well as the origin and development of the dynamic constitutive relationship of metals are reviewed and summarized. Over wide ranges of strain rate and temperature, the metals exhibit complex rate-temperature coupling effect, such as dynamic strain aging and segmented strain rate sensitivity. The high strain rate may lead to dynamic recrystallization, deformation twinning and shock-induced phase transition. The existing constitutive models can be divided into three types: phenomenological models, physically based models and artificial neural network models. Phenomenological models refer to the constitutive models established merely by describing experimental phenomena without considering the internal physical mechanism. Physically based macro-scale continuum models can represent true physical quantities for documenting and tracking the evolution which takes place within metallic materials. Artificial neural network models are good at reproducing the plastic flow behavior as function of many factors, such as strain rate, temperature and plastic strain, without the need of identifying complex logic relationships and parameters within the system. The developments, prediction capabilities, and application scopes of the three types of dynamic constitutive models are illustrated in detail and compared horizontally. In addition, some objective suggestions for the further development of dynamic constitutive descriptions for metals are proposed. Phenomenological models are favored for their ease in application, artificial neural network models are favored for their high prediction accuracy. Recent trend has increased the focus on physically based models. This type of model extends application to a wider strain range and more clearly represents the influence mechanism of strain rate, temperature and strain.
Abstract:
Aiming at understanding the dynamic mechanical properties of titanium alloys additively manufactured by selective laser melting (SLM), quasi-static and dynamic impact experiments were carried out on selective laser melted Ti-6Al-4V alloy at different temperatures using thermal simulation material testing machine and SHPB device, respectively. Based on the experimental results, the parameters of Johnson-Cook constitutive model are fitted. Meanwhile, the mechanical behaviors of titanium alloy at high temperature and high strain rates were simulated by finite element method. The results show that the yield strength of selective laser melted Ti-6Al-4V alloy is enhanced significantly compared with those of wrought or forged counterparts, Moreover, it exhibits significant strain rate strengthening effect and thermal softening effect. The finite element simulation results are close to the experimental results and further validate the constitutive model parameters, which could provide a theoretical basis for expanding the application of selective laser melting technique and its products.
cover
cover
2022, 42(7).  
PDF (31)
Abstract:
contents
contents
2022, 42(7): 1-2.  
Read OL PDF (4)
Abstract:
Explosion Physics
Abstract:
In order to clarify the bubble pulsation process and pressure wave shock characteristics produced in the process of pulse discharge energy release in water, based on the principle of energy equivalence, the liquid-phase pulse energy was transformed into an explosion source with the same energy, and the fluid-structure coupling model of underwater explosion with needle-plate electrode structure was established in LS-DYNA software to simulate the bubble pulsation process on the upper surface of steel substrate. By comparing with the experimental physical images obtained by high-speed photography, it was found that the numerical simulation was highly consistent with the experimental results in terms of bubble morphology and time evolution scales. On this basis, the impact characteristics of the bubbles was further analyzed, and the results show that the maximum impact pressure of the shock wave on the steel base can reach 94.9 MPa when the discharge is carried out with a 4-mm gap at a voltage of 20 kV and a capacitance of 0.8 μF. Besides, the bubble radius, expansion, jet velocity, pulsation period and peak shock wave pressure enhance with the increase of the discharge energy and decrease with the rise of the hydrostatic pressure. Among them, the increase of water pressure has little effect on the bubble expansion rate. The peak value of secondary pressure wave rises from 2.89 MPa to 4.09 MPa with the increase of voltage (14−20 kV), which reaches 41.5%; and up from 5.15 MPa to 6.36 MPa with the rise of hydrostatic pressure (202.65−506.63 kPa), which reaches 23.5%. And the enhancement of discharge energy and water pressure improves the secondary pressure wave significantly. Meanwhile, with the improvement of transmission distance, the proportion of secondary pressure wave in the peak pressure of shock wave rises from 12.6% to 35.3%, and the secondary pressure wave at the far-field discharge location cannot be ignored.
Impact Dynamics
Abstract:
When studying the dynamic fracture behavior of cracked rock mass, dynamic fracture toughness is an important mechanical parameter to study the fracture characteristics of cracks, which can accurately reflect the energy required in the crack initiation and propagation stage. However, compared with the static fracture problem, it is difficult to obtain an analytical solution for dynamic fracture toughness. Therefore, many scholars measure the crack propagation speed by using crack propagation gauges, and then calculate the dynamic fracture toughness according to the universal function. In this way, the crack propagation speed plays a leading role in the calculation accuracy, but in the experiment, the crack propagation speed cannot be measured accurately due to the measuring instrument. In this paper, the fractal theory is used to correct this error. According to the fractal theory, the effects of deflected crack propagation trajectories on dynamic fracture properties of black sandstone under impact loads were studied. A traditional modified split Hopkinson pressure bar (SHPB) test device was used to conduct a dynamic impact test by using an improved single cleavage semi-circle (ISCSC) specimen, crack propagation speed and other fracture mechanics parameters were measured using crack propagation gauge (CPG). Subsequently, the fractal theory was applied to correct dynamic crack propagation speed and dynamic stress intensity factor, and the dynamic fracture toughness of black sandstone was also calculated using the experimental-numerical method. The research results indicate that the ISCSC specimen can be effectively applied to study the crack arrest behavior of rock materials. Crack propagation speed and dynamic fracture toughness after fractal correction are closer to the actual dynamic crack propagation characteristics. Comparisons between before and after the correction, the maximum error of the crack propagation speed of black sandstone material is 33.51%, and the maximum error of dynamic fracture toughness is 7.68%, indicating that it is more reasonable to use fractal theory to calculate dynamic fracture parameters such as crack propagation speed and dynamic fracture toughness.
Abstract:
Composite sandwich beams with a carbon fiber reinforced polymer (CFRP) square honeycomb core were designed and fabricated by using the interlocking method. The dynamic response and failure mechanism of fully-clamped and simply- supported sandwich beams subjected to low-velocity impact were investigated experimentally and the corresponding failure modes of the sandwich beams were obtained. Meanwhile, the damage evolvement process and the failure mechanism were analyzed in detail. Influences of the impact velocity, boundary conditions, the mass distributions of face sheets and the direction of the slots on the failure modes and load-carrying capacity of the sandwich beams were explored. The low-velocity impact experiments of composites specimens with two kinds of boundary conditions were carried out by using the drop-hammer impact test system. Three kinds of initial impact velocity were considered for the simply-supported and the fully- clamped sandwich beams sandwich beams, respectively. In the experiments, the time history curves of the impact load and the midspan deflection of the specimens were recorded by a load cell and a laser displacement sensor. Meanwhile, the deformation processes of the sandwich beams were captured by a high-speed camera. The experimental results show that the directions for the slots of the long ribs have significant influence on the failure modes of the sandwich beams. The sandwich core with the upward slots at the midspan has compression deformation whilst the cracking failure along the direction of the downward slots at the midspan is observed due to the tension, which results in the face-sheet debonding and rib fracture successively. It is found that for the same mass, the design of the thicker upper face sheet can enhance the impact resistance of the sandwich beams. The peak load and load-carrying capacity of the sandwich beams increase with increasing the impact velocity. The fully-clamped boundary conditions make the sandwich beams exhibit hardening post-failure behaviors obviously. After the initial failure at the midspan, the fully-clamped ends of the cores and the face-sheets of the sandwich beams experience the fracture failure.
Abstract:
By using a SHPB device combined with high-speed photography technology, low-velocity impact experiments of quartz glass beads with diameters of 7.90, 11.80 and 15.61 mm were carried out by means of respectively three kinds of transmission bars, i.e., steel bar, aluminum bar, and polymethyl methacrylate (PMMA) bar. According to the load-displacement curves in the breakage process of glass beads under different transmission bar conditions, combined with the load adjustment of glass beads under impact and the strain of glass beads during the experiment, the influence of stress adjustment on the breakage process of glass beads subjected to low-velocity impact is explored. The results show that under the same impact conditions, the adjustment of the material of the transmission bar will alter the load distribution in the glass bead during impact breakage, that is, the change of the wave impedance at the transmission end will change the reflected wave, which leads to the load adjustment in the process of multiple reflection loading. When the transmission bar is made of aluminum and PMMA, the load in the glass bead decreases obviously during the crushing process, and the stress adjustment duration of the glass bead becomes longer with more deformation of the cushion block during the loading process. When the transmission bar is made of steel, the strain in the glass bead is the largest at both ends, while the closer to the middle of the bead, the smaller the strain. For the glass beads loaded with aluminum and/or PMMA transmission bar, local unloading behavior is found at the transmission end of bead. By employing the PMMA transmission bar, the local stress and deformation both decrease, resulting in the glass bead being broken with larger deformation. It is further shown that glass bead breakage is controlled by local deformation and local deformation gradient.
Abstract:
Four kinds of carbon-polypropylene hybrid fiber reinforced coral sand cement-based composites with different fiber content were obtained by mixing carbon fiber and polypropylene fiber into coral sand cement-based composites prepared by artificial seawater. Impact compression tests of this material under five strain rates were carried out with a 100-mm diameter split Hopkinson pressure bar. The parameters of Holmquist-Johnson-Cook model are determined by experimental data and parameter debugging. Based on Holmquist-Johnson-Cook model, LS-DYNA is used to simulate the impact compression of this material. By analyzing the failure mode, stress-strain curve and energy dissipation of the test blocks, the impact compression mechanical properties of carbon-polypropylene hybrid fiber reinforced coral sand cement-based composites are studied. The results are as follows. (1) The critical value of test strain rate is 200 s−1; when the test strain rate is greater than 200 s−1, the fiber network formed by hybrid carbon fiber and polypropylene fiber strengthens the toughening effect of the test block. (2) The peak stress of carbon-polypropylene hybrid fiber reinforced coral sand cement-based composites exhibits obvious strain rate effect, and the dynamic increase factor is highly sensitive to the strain rate. (3) The use of fine aggregate of coral sand results in more defects such as micro-cracks and micro-voids in the test block; after mixing carbon fiber and polypropylene fiber into the coral sand cement-based composites, the improvement of the impact compressive strength of the test block is limited, but the impact toughness of the coral sand cement-based composites is significantly enhanced. (4) LS-DYNA is used to numerically simulate the impact compression test process of hybrid carbon fiber (15.75 kg/m3) and polypropylene fiber (1.82 kg/m3), while the error between the simulation results of peak stress and the test results is within 5.97 %. The study is of great significance for the preparation of high performance coral sand cement-based composites and the emergency repair of offshore islands and reefs.
Abstract:
Sapphire (Al2O3) transparent ceramic glass has excellent light transmittance and retains the excellent mechanical properties comparing with traditional ceramics. In order to understand the relationship between strength and strain rate of sapphire transparent ceramic glass and its failure process, the electronic tensile machine and split Hopkinson bar equipment were used to load the specimen at different strain rates (10−4, 10−3, 10−2, 850, 1 100, 1 300, 1 450 s−1). The quasi-static and dynamic compression failure processes of specimen were recorded by high-speed camera. The experimental results show sapphire transparent ceramic glass is a typical brittle material with strain rate effect. With the increase of strain rate, the compressive strength of the sapphire transparent ceramic glass will also increase. The failure cycle of sapphire is long under quasi-static compression, and the crack will expand along the path with the weakest bearing capacity. In addition, the strength curve of sapphire will decline briefly and then continue to rise, which is caused by the increase and propagation of the number of cracks. In the process of dynamic compression, the sapphire reaches the cracking strength in many places, forming more crack sources, and then the crack forms and expands to split the sapphire. When the sapphire transparent ceramic glass is subjected to compression, cracks will appear in the region with the weakest bearing capacity in the process of loading; soon after the cracks take shape and expand along the loading direction, the cracks interlace to reach a saturation state; and finally reach the compressive strength failure. Under dynamic compression, however, due to the loading rate is much higher than the propagation of the crack, several crack sources appear in the sapphirine transparent ceramic glass within a very short period of time, which requires more energy to make the crack forming and extending, exhibiting as the strain rate effect on its macro-scale performance.
Abstract:
In order to reduce the cost for the impact test of the full-size airframe structures, an incomplete similar model was established by the similarity theory. Based on dimensional analysis, the correction relation for the Johnson-Cook linear strain-rate function was formulated. Due to the limitation of manufacturing technology, the effect of the incomplete similar model with distorted thickness on similarity behaviors should be taken into account, so an exponential function was adopted to establish the correction formula for the distorted thickness model. The validity of the simulation model was then verified by comparisons relevant to the deformation on the fuselage, the strain-time curves of target plates and the final deformation profile. In addition, the influences of fragment angle, material property, distortion thickness and light weight on the deformation behavior of the fuselage structure were analyzed. The following main results were obtained. (1) Under the impact velocity of 150 m/s, the most severe impact conditions appear at the impact angle of 90° and the fragment attitude of 180º; by considering various factors, the 3.5-mm-thickness titanium alloy plate is regarded as the best choice for fuselage structures, and it is used as a full-size prototype to verify the similar method. Besides, it’s worth noting that an unconventional phenomenon takes place at the impact angle of 30º, while a reasonable explanation is given. (2) The effect of strain rate on the impact of tire fragments on the fuselage structure is not notable, so the incomplete similar model is in good agreement with the prototype results. (3) The incomplete scaled-down model corrected by this method can effectively predict the deformation behavior of prototype fuselage subjected to the impact of tyre fragments. Although there is a certain deviation between the model and the prototype on the time scale, on the spatial dimensions, the incomplete scaled-down model can effectively correct the prediction error for the maximum center deformation caused by the distortion thickness, and the corrected maximum error is less than 5.1%, indicating that the method can effectively guide the design for airframe structures.
Abstract:
A shaped-charge jet compresses the target axially and radially simultaneously when the jet penetrates into a thick target, and then the axial penetration and radial crater growth occur. The research on axial penetration is abundant, but the research on radial crater growth is less and there is a certain error between theoretical prediction and experimental results. The radial crater growth equation of the shaped-charge jet was derived by considering the compressibility of the jet and target materials based on the compressible model of shaped-charge jet penetration and the Szendrei-Held equation. The main changes of equations are the stagnation pressure adopted value of the compressible model and the density changed with jet velocity. An approximate solution of the compressible model was given based on the Murnaghan equation of state in order to simplify the tedious calculation process of the complete compressible model, i.e., the calculation processes of stagnation pressure and density change were simplified. The prediction by this model is better than that by the Szendrei-Held equation compared with the experimental study of the shaped-charge jet crater growth in water. The main factors affecting the radial crater growth by the shaped-charge jet include jet radius, stagnation point pressure, target strength, target density at the stagnation point and shaped-charge jet velocity. This model can more accurately predict the crater growth of the shaped-charge jets penetrating into the compressible targets. It may be helpful to study the interference of shaped-charge jet penetration with liquid-confined structures.
Experimental Techniques & Numerical Methods
Abstract:
Explosive-driven magnetic flux compression generator is a device that converts the chemical energy of explosives into electromagnetic energy. It has attracted great attention in the field of high energy density physics due to its wide application and important development prospect in magnetic field compression and material high pressure loading. CAEP has conducted a lot of research on CJ-100 device, which can stably generate an axial magnetic field of about 700 T. In order to investigate the loading capacity of CJ-100 device, the loading process and the effects of various device parameters are discussed by using the one-dimensional magnetohydrodynamics program SSS-MHD. The results show that the peak magnetic field that can be reached by the device is inversely proportional to the initial magnetic field, and the size of sample target has a great influence on the loading pressure. A sample target of iron/copper layered structure was designed for quasi-isentropic loading experiment of pure iron. The initial inner radius of the sample target was 3 mm, and the thickness of both iron and copper layer was 1 mm. The experiment was carried out on CJ-100 device with an initial magnetic field of 5.5 T, atmospheric pressure of several hundred Pa and ambient temperature. The free surface velocity of the sample target of about 6.43 km/s was measured with Photonic Doppler Velocimetry probes. SSS-MHD program with proper material models provided curve of velocity versus time that agree well with the experimental measurement. Simulation then shows that a quasi-isentropic loading pressure of 206 GPa is obtained in DT4 iron. The p-v curve of iron material is basically coincided with the theoretical isentropic line, indicating that the loading process of CJ-100 has a high isentropic degree.
Applied Explosion Mechanics
Abstract:
To effectively protect the underground structures subjected to ground shock, a new protective component made of foam concrete was proposed. Different from the mechanism of the solid foam concrete layer protection, under the action of ground shock, the proposed components firstly exhibited brittlely fracture, and the fractured parts underwent recontact and compaction, in which the ground shock truncation, load transferred reduction and load form modification on the structures were achieved with the response of the designed components. A field experiment was conducted and the comparison of the dynamic response of the structure (with different protection scenarios, i.e. without protection, with a solid foam concrete layer protection and with the proposed component layer protection) suggested that the superior protective performance was achieved with the fracture, recontact, compaction of the new component. Due to the brittle fracture, the load transfer could be significantly reduced under a relatively low ground shock level, with which the negative protection effect using solid foam concrete layer could be avoided. Subjected to a relatively strong ground shock, the proposed component layer tended to compaction, and its protection effect gradually approached that with the solid foam concrete layer.
Abstract:
In order to study the blast-resistance characteristics of polyurea sprayed reinforced brick infill walls, a prototype explosion test of polyurea sprayed reinforced frame infill walls was carried out based on an improved large-scale explosion test device. This test device eliminates the influence of the sparse wave formed by the air shock wave at the edge of the wall and the diffraction behind the wall on the real blast resistance test dynamic response of the wall, and significantly improves the accuracy of the blast resistance test of brick infill walls strengthened with polyuria. The dynamic response characteristics, failure process and mode of reinforced brick walls under explosion load were analyzed, and the failure mechanism was revealed. The results show that under small deformation conditions, polyurea reinforcement can improve the blast-resistance of infilled wall members. Under large deformation conditions, polyurea reinforcement can increase the ductility of filled wall members. The system stiffness of reinforced brick wall changes continuously during forced vibration, and the maximum difference is 133%. With the decrease of the proportional distance, the failure mode of the reinforced brick wall gradually changes from bending failure to shear failure. The polyurea thickness of more than 6 mm can effectively limit the local shear failure phenomenon. The theoretical calculation model based on the resistance function of brick wall and polyurea coating can accurately predict the forward displacement response process of two-way brick wall reinforced by back blasting surface under explosion.
Abstract:
To identify the anti-explosion performance, dynamic response and typical failure mode of a reinforced concrete beam-slab composite structure, the explosion experiment was conducted by the shock tube, which was used to simulate the long-lasting long-distance explosion shock wave. The failure form of the reinforced concrete beam-slab composite structure, the shock wave variation curve and the displacement change at the center point of the backside surface were obtained through the experiment. The dynamic response process of the reinforced concrete beam-slab composite structure is numerically simulated by finite element software. Compared with the experimental results, it is found that the simulated failure phenomenon is similar to the experimentally observed one, and the peak displacement at the center point of the backside surface is also close to the experimental one. Both of these have verified the accuracy and applicability of the numerical model adopted. On this basis, the dynamic response and failure process of the beam-slab composite structure under the simplified triangular explosion shock wave load are analyzed. The simplified triangular explosion shock wave used in numerical simulation has the same impulse as that in experiment but different peak values and durations. According to the deflection-span ratio a and the failure form, the failure patterns are classified into four modes as light failure, moderate failure, severe failure and complete failure. The results show that the cracks are firstly distributed along the diagonal of the backside surface of the reinforced concrete beam-slab composite structure. Under the same impulse, with the increase of the peak value of the explosion load, the damage degree of the beam-slab composite members gradually deepens. Meanwhile, the failure mode changes from a bending failure to a combined bending-shear failure, and finally appears as a punching failure. The failure of the plate part of composite members occurs earlier than the cross-beam part, while the former’s damage degree is greater than the latter’s.
Abstract:
To study the effects of magnetic fields on the gas explosion, considering equivalent acetylene premixed combustible gas as the research object, the effects of different magnetic field intensities on acetylene explosion characteristics were studied experimentally. The explosion pressure and flame propagation velocity were measured simultaneously by transient pressure sensors and a detonation velocity instrument, respectively. The results show that the magnetic fields reduce the explosion pressure and the pressure rise rate of acetylene. With increasing magnetic field intensity, the suppression effect is more significant. Along the direction of flame propagation, the magnetic fields first promote and then suppress the explosion flame propagation velocity of acetylene, and the inhibition effect is stronger than the promotion effect. In these experimental conditions, the average propagation velocity of the explosion flame decreased by 38.94% under lower magnetic fields intensity, and at higher magnetic fields intensity, it decreased by 49.62%. To further study the impact mechanism of magnetic fields on premixed combustible gas explosion, the acetylene explosion free radicals reaction process was simulated numerically by Chemkin-Pro software. The chain reactions, rate of products, and sensitivity are analyzed. And the key radical and reaction paths of acetylene explosion are obtained. Combined with the force analysis of magnetic fields on free radicals, it is deduced that magnetic fields change the reaction paths of acetylene to produce carbon dioxide and water, which is the main internal reason for the decrease in explosion parameters. The different free radicals have different molar masses and magnetization. Lorentz force and gradient magnetic field force have stronger effects on small molecular weight free radicals than on large molecular weight free radicals. The calculation shows that the magnetic fields change the trajectory of the free radicals, cause the aggregation of free radicals with the same small molecular weight, and produce a wall effect, which reduces collisions between key free radicals and the rate of elementary reactions, resulting in a decrease of explosion intensity.
Abstract:
The pyrolysis and oxidation characteristics and flame propagation characteristics in the semi-enclosed vertical pipe of lauric acid dust and stearic acid dust were studied by using the synchrotron thermal analyzer, improved Hartmann explosive test device and high-speed photography system, the pyrolysis kinetics was analyzed by Coats-Redfern method to obtain the kinetic parameters, and the influence of pyrolysis and oxidation characteristics on the law of flame propagation during the explosion and combustion of lauric acid and stearic acid dust was analyzed and discussed. The results show that, when the dust cloud concentration is 125 g/m3, the flame front structure of lauric acid dust cloud is smoother than stearic acid dust, but the flame propagation speed of stearic acid dust is significantly greater than that of lauric acid dust; with the increase of dust cloud concentration, the flame front structure of lauric acid dust and stearic acid dust gradually becomes discrete, and the flame propagation speed gradually increases, but the speed difference gradually decreases. The average flame propagation speed of lauric acid dust is higher than that of stearic acid dust at a dust cloud concentration of 750 g/m3, and the flame structure continuity is significantly reduced. The difference in flame propagation between lauric acid dust and stearic acid dust at low concentrations is mainly determined by the oxidation exothermic characteristics of the fast pyrolysis stage. The larger the pre-exponential factor, the more active sites involved in the pyrolysis and oxidation reactions, the larger the oxidation exothermic heat, the faster the exothermic rate, the faster the flame propagation speed, and the faster the flame frontal structure transition from smooth continuous to discrete complex. And with the increase of dust cloud concentration, the flame propagation difference is gradually controlled by the activation energy and the mass transport process of oxygen in the preheating zone of the flame front. The greater the activation energy, the greater the oxygen consumption, the faster the oxygen consumption rate, the easier it will lead to the decrease of flame propagation speed, the more complex the flame front, and the decrease of flame structure continuity.

Founded in 1981    monthly

Sponsored byChinese Society of Theoretical and Applied Mechanics
Institude of Fluid Physics, CAEP

Editor-in-ChiefCangli Liu

Industry InformationMore