Numerical modeling of the energy dissipation and fragmentation of copper-bearing rock under impact load
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摘要: 为了研究冲击荷载作用下含铜矿岩的破碎块度与能量耗散关系,借助分离式霍普金森压杆试验装置,分析不同冲击荷载下含铜凝灰岩的力学特性及能量传递规律,结合分形理论构建耗散能与矿岩破碎块度之间的关系;同时,基于有限离散元方法(finite discrete element method,FDEM)模拟矿岩的裂纹扩展行为。结果表明:随着入射能的增加,透射能、耗散能、反射能三者的能量分布规律基本保持一致,即透射能、耗散能、反射能依次减小;根据耗散能的不同,碎石块度分布也呈现出明显的差异性。当耗散能由19.52 J增加至105.72 J时,矿岩的平均块度从27.98 mm降低至16.94 mm,分形维数提升了26.43%,表明耗散能越高,矿岩的宏观破碎程度越剧烈,破碎块度的数目越多,碎块粒径越小,均匀性越好;随着冲击荷载的增大,裂纹起裂时间缩短,拉伸裂纹数量占总裂纹数量的比重提高。FDEM数值计算方法的应用可为深入解析岩石断裂破坏特性提供新的思路。Abstract: To understand the relationship between fragmentation and energy dissipation in copper-bearing ore rock subjected to impact loading, a split Hopkinson pressure bar (SHPB) testing apparatus was employed to study the mechanical properties and energy transfer mechanisms of copper-bearing tuff under varying impact loads. Additionally, fractal theory was used to establish the correlation between dissipated energy and rock fragmentation. Utilizing the finite discrete element method (FDEM), numerical simulations of crack propagation within the rock were conducted. The results indicate that as the incident energy increases, the distribution patterns of the transmission energy, dissipated energy and reflection energy remain consistent, which are characterized by transmission energy, dissipated energy and reflection energy decreased successively. Furthermore, significant variations in fragment size distribution are observed with changes in dissipated energy. Specifically, as dissipated energy increases from 19.52 J to 105.72 J, the average fragment size decreases from 27.98 mm to 16.94 mm, while the fractal dimension increases by 26.43%. This suggests that higher dissipated energy results in more extensive macroscopic fragmentation, an increase in the number of fragments, smaller particle sizes and enhanced uniformity. As the impact load intensifies, the time to crack initiation decreases, and the proportion of tensile cracks relative to total cracks increases. The application of the FDEM offers new insights into the fracture and failure characteristics of rocks.
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纺织结构复合材料是以纺织纤维体作为增强材料,用树脂固化后形成的纤维增强复合材料。二维纺织结构主要包括平纹、斜纹和缎纹织物,具有比强度高、比刚度大和材料性能可以设计等优点,在防护工程领域具有良好的应用前景[1]。P.M.Cunniff[2-3]研究了子弹侵彻叠层平纹织物时的入射速度和剩余速度的关系,得到了在不同形状子弹侵彻下结构的弹道极限以及半经验公式;顾冰芳等[4]研究了不同形状子弹冲击下Kevlar纤维叠层织物的防弹机理和性能,观测了纤维的表观破坏形态和微观损伤机理;R.Barauskas等[5]基于LS-DYNA软件通过考虑纱线滑动、子弹和纱线之间的滑动计算了二维编织物在可变形体侵彻下的破坏过程。这些研究主要关注纤维材料的弹道冲击侵彻性能,而对于复合材料整体动力响应方面的研究还较少。V.Kostopoulos等[6]使用有限元技术分析了3种不同的复合材料(碳、玻璃和Kevlar)制作的摩托车安全头盔的冲击动态响应过程,发现Kevlar配置的安全头盔防护性能要优于其他2种,指出Kevlar较低的抗剪性增强了头盔的能量吸收和压缩能力。I.Taraghi等[7]研究了常温(27 ℃)和低温(-40 ℃)下,多壁碳纳米管增强的平纹Kevlar/环氧树脂复合板的低速冲击响应,在基体内加入一定量的多壁碳纳米管能显著提高复合板的吸能和刚度。P.N.B.Reis等[8]研究了Kevlar/纳米粘土增强环氧树脂复合板的冲击响应,通过在基质内加入一定量的纳米粘土可以提高复合板的弹性恢复性能和侵彻阀值。本文中研究了钢制平头弹撞击下平纹Kevlar纤维复合板的动态响应,给出了复合板的变形失效模式。在实验的基础上,利用LS-DYNA分析钢质平头弹冲击载荷作用下平纹Kevlar纤维复合板的动力响应和纤维铺层数对结构动力响应的影响,模拟结果与实验吻合较好。
1. 实验
1.1 实验过程
实验试件为编织Kevlar/Epoxy复合材料层合板,尺寸为300 mm×300 mm。试件铺层厚度0.27 mm,共18层,经浸渍环氧树脂后加温加压形成。每层织物组织都为平纹组织,由2根经纱和2根纬纱组成织物循环,经纱和纬纱每隔1根纱线交织1次。实验采用平头钢制子弹,长度150 mm,直径为37 mm,质量为1.24 kg。冲击实验装置由空气动力枪、激光位移传感器(micro-epsilon LD1625-200,响应特性:采样率37 kHz,每秒采集185 000个点,能够实时探测到靶板中点的位移)、激光测速仪、实验夹具、超动态应变仪和高速摄像机等组成,如图 1所示。实验加载是通过空气动力枪驱动钢制子弹撞击复合板实现,子弹速度由空气动力枪气压控制,其大小由激光测速装置获得。实验支架采用钢制正方形夹具,端面平整,其外部边长400 mm,内部边长250 mm,通过螺栓固定在不可移动的平台上。实验中通过高速摄像仪对整个加载过程进行了拍摄。
1.2 实验结果
分别对试件的变形失效模式和背面中心点的挠度进行分析,实验结果如表 1所示,n为层数,h为纤维复合板的厚度,v为冲击速度,I为冲击冲量,W为残余挠度。结构在冲击载荷下主要呈现3种变形失效模式:Ⅰ型为未发生明显破坏失效,整体呈现弹性变形,如图 2(a)所示;Ⅱ型为复合板表面子弹作用区域的嵌入失效,结构呈整体塑性大变形,如图 2(b)所示;Ⅲ型为背面纤维拉伸断裂及分层失效,如图 2(c)所示。
表 1 实验数据Table 1. Experimental dataNo. n h/mm v/(m·s-1) I/(N·s) W/mm 变形失效模式 1 18 5.11 13.30 16.49 1.8 Ⅰ 2 18 4.98 24.70 30.63 8.3 Ⅱ 3 18 5.15 32.60 40.42 12.4 Ⅱ Ⅲ 4 18 5.13 36.47 45.22 13.5 Ⅱ Ⅲ 5 18 5.10 36.50 45.26 13.3 Ⅱ Ⅲ 平头弹撞击瞬间,复合板受载边界处将产生较大的剪切应力,导致表面纤维及胶层瞬时剪切失效,因此正面受冲击区域边缘发生了明显的嵌入失效;纤维良好的延展性使得复合板整体为塑性大变形,呈现穹形;纤维的正交分布导致背面纤维拉伸断裂后裂纹沿着垂直于断裂纤维方向扩展,并且出现了分层现象,因此背面发生近似方形的局部破坏(不考虑夹具的影响)。
从表 1中看出,在不同冲击速度下,复合板背面中心点的残余挠度随着冲击速度的增加逐渐增大。图 3给出了不同冲击速度下复合板背面中心点的挠度时程曲线,可以看出:在子弹冲击作用下,板背面中心点在0.8 ms左右达到最大挠度,随后发生反弹,在平衡位置附近进行振荡,最终静止;且当冲击速度v=13.30 m/s时,试件的后面板的瞬时挠度峰值是最终挠度的5.7倍,即后面板瞬时挠度有可能对被保护的人员或结构产生更大的伤害,因此在用作防护结构时不能仅考虑最终挠度。
2. 数值模拟
2.1 有限元模型
2.1.1 材料参数
纤维层采用复合材料平纹织物层合板模型(MAT_LAMINATED_COMPOSITE_FABRIC)具体材料参数见表 2,其中ρ为密度,E为弹性模量,Gab为面内剪切模量,Gca为层间剪切模量,ν为泊松比,Xt为纵向拉伸强度,Xc为纵向压缩强度,Yt为横向拉伸强度,Yc为横向压缩强度,Sc为面内剪切强度。环氧树脂层采用双线性应变强化弹塑性模型,密度为1 200 kg/m3,弹性模量为12.0 GPa,泊松比为0.34。假定冲击过程中子弹和夹具没有变形,采用刚体模型,密度为7 800 kg/m3。
表 2 Kevlar纤维平纹织物的材料参数Table 2. Material properties of the Kevlar composite fabricρ/(kg·m-3) E/GPa Gab/GPa Gca/GPa ν Xt/GPa Xc/GPa Yt/GPa Yc/GPa Sc/GPa 1 400 59.5 5.18 5.18 0.34 1.20 0.23 1.20 0.23 0.12 2.1.2 几何模型
图 4给出了Kevlar/Epoxy复合材料层合板在冲击载荷作用下的数值分析模型及冲击实验照片。为了实现与实验尽量一致的边界,数值模拟中同样采用了实验中的夹具形式:夹具与复合板之间定义自动面对面接触;子弹与纤维层、胶层之间定义侵蚀接触;纤维层与胶层之间共节点连接;在螺栓位置,采用弹簧单元来模拟夹具中螺栓的紧固作用。复合板为300 mm×300 mm的正方形,有效面积为250 mm×250 mm。基于LS-DYNA软件,建立了1/4计算模型。纤维层采用shell193壳单元,单元尺寸为1.875 mm×1.875 mm,每层厚度为0.27 mm。上下表面及纤维层之间建立环氧树脂层,环氧树脂层采用solid164实体单元,单元尺寸为1.875 mm×1.875 mm×0.27 mm。子弹同样采用solid164实体单元。整个模型中,纤维分为18层,共115 200个单元,胶层分为19层,共121 600个单元,经过网格敏感性验证,所选网格比较稳定,可以满足计算需要。
2.2 数值模拟验证
图 5给出了冲击速度v=36.47 m/s时,Kevlar纤维复合板受撞击变形的实验与数值模拟对比。复合板整体为塑性大变形,呈现穹形,中心受子弹冲击区域挠度最大,向边界处逐渐减小。正面子弹冲击区域边缘发生了明显的嵌入失效;背面纤维断裂呈现近似方形的破坏。表 3给出了复合板受冲击最大位移、冲击后残余挠度的模拟结果与实验结果的对比。可以看出Kevlar/Epoxy复合材料层合板的变形失效模式、残余挠度的数值模拟结果和实验结果吻合较好,误差均在20%以内。由此可见,本文中建立的有限元模型是可靠的,可以用于进一步的Kevlar纤维复合板抗冲击性能的分析。
表 3 Kevlar纤维复合板在不同速度冲击下实验与数值模拟对比Table 3. Comparison of the experimental and simulated results at different impact velocitiesv/(m·s-1) dmax/mm W/mm 实验 数值模拟 ε/% 实验 数值模拟 ε/% 13.30 10.3 9.3 -9.71 1.8 1.5 -16.67 24.70 14.2 14.8 4.23 8.7 7.9 -9.20 32.60 18.3 17.6 -3.83 12.4 11.2 -9.68 36.47 20.7 20.3 -1.93 13.5 13.6 0.71 36.50 19.9 20.3 2.01 13.3 13.6 2.21 3. 结果分析
3.1 动态响应过程
图 6给出了冲击速度v=36.47 m/s下子弹和复合板的相互作用过程,整个过程可以分为2个阶段。(1)加载阶段(0≤t≤1.1 ms):子弹发射后高速冲击复合板,板面受冲击后与子弹具有相同的速度一起运动,变形区域从中心向边界处传播,出现穹形大变形;t=0.8 ms后随着变形进一步增加,冲击区域环氧树脂发生失效破坏结构中点挠度进一步增加;t=1.1 ms后背面纤维拉伸断裂,结构中点挠度达到最大值。(2)卸载阶段(t>1.1 ms):结构贮存的弹性应变能转化为板和子弹的动能从而发生反向回弹, 结构与子弹以相同的速度开始反弹,t=2.0 ms结构与子弹分离,t=2.2 ms结构反弹至反向最大挠度后进入自由振动阶段,并最终静止。如图 4(a)所示,在复合板背面纤维单元上分别取7个测点,其中1#点位于板中心,3#点位于距离中心点18.5 mm处(即子弹边缘与板面的交界处),7#点位于边界处。图 7(a)给出了1#、3#、5#和7#点的x方向的应力时程曲线,可以看出7#点(边界处)应力正负交替出现,说明复合板在边界处沿x方向发生了弯曲变形;加载区域内纤维的应力要高于加载区域外,且加载区域边界处的应力最大,因而更容易发生纤维的拉伸断裂。图 7(b)给出了3#点截面处沿厚度方向第1、7、11和18层纤维单元x方向的应力分布,可以看出复合板所受应力由压应力逐渐转变为拉应力。因此,复合板首先在3#点截面第18层纤维单元处发生拉伸破坏。
3.2 参数分析
为了研究加载冲量及铺层数对结构响应的影响,分别计算了不同加载冲量下(12.4~47.12 N·s),不同铺层数(6、9、12、15和18层)的复合板的动态响应:分别从复合板的能量吸收规律和背面中心点的残余挠度进行了研究。研究表明复合材料层合板的抗冲击性能与其铺层数和外加载荷有密切的关系。分别将纤维复合板的残余挠度W、初始冲量I以及能量吸收Ea按下面的方法量纲一化:
ˉW=Wh,ˉI=Im′√σyρ,m′=ms,ˉE=EaEi 式中:ρ纤维的密度,m为纤维复合板的质量,s为纤维复合板的有效作用面积, σy为屈服应力,Ei为子弹的冲击能量。
Kevlar纤维复合材料层合板用于工程防护结构时,一般将其背面的残余变形作为抗冲击性能的主要参数。图 8给出了铺层数不同的复合板背面中心点量纲一残余挠度随量纲一冲量变化的规律,可以看出:铺层数相同的复合板背面中心点的残余挠度随着冲量的增大逐渐增大;当I=0.8时,12层的复合板挠度最小,表现出最好的抗冲击性能;I≤0.75时,15和18层的复合板挠度差异很小;当I>0.8时,18层的复合板挠度最小。因此在本文中研究的冲量范围内,随着冲量的变化,并不是纤维层数越多,复合板的挠度越小。
冲击能量Ei代表了结构可以转化的最大能量即子弹的动能,反之吸收能量Ea为结构实际转化的能量。从图 9可以看出,纤维复合板的吸能效率E随着加载冲量和复合板铺层数的增加逐渐增大。冲量相同的情况下,吸能效率的提高随铺层数的增加呈现递减趋势;当I>0.6时,15和18层铺层的复合板的吸能效率差异很小。虚线对应的点的横坐标为各结构发生侵彻破坏的阀值。
4. 结论
针对Kevlar/Epoxy复合材料层合板在钢制平头弹冲击下的动态响应开展了实验研究和数值模拟,分析了结构在不同冲量下的变形失效模式以及抗冲击性能,主要结论如下:
(1) 编织Kevlar纤维层合板的冲击失效模式与结构配置和载荷强度有关,主要表现为弹性变形、复合板表面嵌入失效及整体塑性大变形和背面纤维拉伸断裂及分层失效。
(2) 数值模拟表明,子弹撞击区域边界处纤维应力沿厚度方向由压应力逐渐变为拉应力,且最大拉伸应力出现在背面几层。
(3) 在一定的冲量范围内,数值模拟结果表明复合板的动力响应与铺层数和加载冲量密切相关;通过对复合板铺层数的优化,能够有效地减小后面板挠度,提高结构的能量吸收效率,增强结构的抗冲击性能。
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表 1 含铜矿岩基本物理力学参数
Table 1. Basic physical and mechanical parameters of copper-bearing rock specimen
编号 密度/
(g·cm−3)纵波波速/
(m·s−1)弹性模量/
GPa泊松比 抗压强度/
MPaJ-1 3.10 3 549 93.35 0.33 59.23 表 2 含铜矿岩的冲击实验数据
Table 2. SHPB test data of copper-bearing rock samples
编号 冲击气压/
MPa平均应
变率/s−1峰值应力/
MPaWi/J Wr/J Wt/J Wd/J A-3 0.5 30.68 108.03 63.34 7.24 45.13 10.62 B-2 0.6 35.71 119.72 81.67 5.73 55.75 19.51 C-4 0.7 44.25 141.35 105.92 7.32 66.10 31.57 D-1 0.8 50.93 163.19 130.76 8.05 74.19 47.75 E-3 0.9 53.62 189.55 168.28 23.94 83.45 60.60 F-2 1.0 59.15 200.93 203.33 37.88 89.99 75.12 G-4 1.1 64.81 249.80 222.91 43.46 93.01 85.52 H-1 1.2 77.39 265.90 267.09 62.21 99.06 105.72 表 3 含铜矿岩破碎块度筛分试验结果
Table 3. Test screening results of crushed copper-bearing rock fragments
编号 Wd/J 各等级粒径岩矿碎块的质量/g 平均块度/
mm<0.3 mm <0.5 mm <1.0 mm <2.0 mm <4.0 mm <9.5 mm <16.0 mm <19.0 mm <26.5 mm <37.5 mm B2-0.6 19.52 0.04 0.18 0.13 0.31 0.16 1.35 2.95 12.8 36.44 123.42 27.98 C4-0.7 31.58 0.09 0.13 0.25 0.55 0.64 3.03 4.50 18.16 91.39 51.51 23.29 D1-0.8 47.75 0.11 1.52 2.46 3.74 3.39 12.10 18.87 24.19 47.12 53.92 20.54 E3-0.9 60.61 0.07 0.12 0.32 0.79 0.78 9.06 25.52 33.49 55.18 20.42 19.62 F2-1 75.13 0.10 0.24 0.54 1.33 1.25 9.85 40.75 39.61 22.46 20.47 18.28 G4-1.1 85.53 0.15 0.37 0.75 1.55 1.38 16.84 44.16 30.27 69.63 0 16.92 H3-1.2 105.72 0.27 0.68 1.20 2.60 1.92 20.17 51.22 32.99 46.95 12.74 16.94 -
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Engineering Failure Analysis, 2024, 163: 108584. DOI: 10.1016/j.engfailanal.2024.108584. -