On a bidirectional bending Hopkinson tension test method
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摘要: 为了实现对材料或结构的双向高应变率同步拉伸加载，基于曲杆中弹性应力波传播理论和Hopkinson杆原理，首先在对称的人字形曲杆结构中同时产生和传递两路压缩波，再经过接触转接头反射形成沿拉伸加载杆传播的双向拉伸波，实现对试样的双向动态拉伸。同时，为理解人字形曲杆几何构形对弹性压缩波传播的影响规律，对该加载装置进行了动力学分析和ABAQUS有限元模拟。研究发现，理想方波构形的压缩弹性波经过曲杆传播后，方波的平台段随着杆弯曲角度的增大出现前高后低的倾斜现象，同时大曲率杆引起的波形失真更严重。为获取常规方波或梯形波的平台段，也可采用定量优化的锥形撞击杆，产生前低后高的加载波，来抵消曲杆传递中的倾斜失真。最后，为了验证该加载系统的有效性，搭建了小型人字形曲杆高应变率双向拉伸装置进行试验测试。结果表明，该装置实现了脉宽约为54 μs的双向拉伸加载波良好的同步，两路波形起始点时间差可以控制在约2.5 μs以内，幅值差约6×10−6。同时对2024铝合金试样进行了双向拉伸试验，取得良好的试验效果。Abstract: In order to achieve bidirectional high strain rate dynamic tension of materials or structures, based on the elastic stress wave propagation theory in bending bars and the principle of the Hopkinson bar, a symmetrical herringbone bending bar was designed. The designed structure can generate and transmit two compression waves at the same time, and convert them into two-way tension waves propagating along the tension bar through the contact adapters. In order to understand the influence of the herringbone bending bar geometric configuration on the propagation of elastic compression waves, the dynamic analysis and ABAQUS finite element analysis (FEA) were carried out for the device. The study shows that after the square compression elastic wave propagates through the bending bar, the platform section of the square wave will incline in high front and low back, and as the bending angle increases, the slope is larger, and the waveform distortion caused by the large curvature rod is more serious. In order to realize the platform segment of the square wave or trapezoidal wave, the tapered impact bar is optimized so that it can be used to generate load waves with low front and high back to offset the tilt distortion in the transmission. In the end, to verify the feasibility and effect of the bidirectional dynamic tensile loading device based on the bidirectional bending Hopkinson bar, a small verification device was built. The results show that the device realized bidirectional tension loading for the pulse width of about 54 μs with good synchronization, the time difference between the starting point of the two waves was less than 2.5 μs, and the amplitude difference was less than 6×10−6. The bidirectional tensile test was carried out on the 2024 aluminum alloy samples, and the good test results were obtained. This confirms that the proposed method can be used for bidirectional dynamic tension and lays the foundation for the expansion of the device to biaxial tensile loading.
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Key words:
- bidirectional bending bar /
- high strain rate /
- synchronization /
- bidirectional tension
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图 3 不同位置处的应变、弯矩、剪切力、轴力时程曲线（l0=40 mm，d=5 mm，R=400 mm，α=90°）
Figure 3. Time-history curves of strain, bending moment, shear force and axial force at different positions (l0=40 mm, d=5 mm, R=400 mm, α=90°)
图 5 不同的约束条件（L=200 mm, R=400 mm，l0=80 mm，d=5 mm，α=90º）
Figure 5. Different constraint conditions (L=200 mm, R=400 mm, l0=80 mm, d=5 mm, α=90º)
图 6 不同约束条件下的波形对比（L=200 mm, R=400 mm，l0=80 mm，d=5 mm，α=90º）
Figure 6. Strain waveforms in the bar under different constraint conditions (L=200 mm, R=400 mm, l0=80 mm, d=5 mm, α=90º)
图 10 双向拉伸2024铝合金试验结果
Figure 10. Bidirectional tension test results of 2024 aluminum alloy
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