Study of Dynamic Loading Tests on Low-Impedance Specimens
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摘要: 研究了Hopkinson杆开展低阻抗试样高应变率加载测试的关键问题:低阻抗透射信号有效采集和应力平衡问题. 综合前期研究基础,选用异质低阻抗透射杆并辅以原位标定半导体应变片技术,实现低阻抗试样高精度测试;结合应力波加载试样的过程重构计算和分析,获得了低阻抗试样的应力平衡历程和影响因素敏感性特征,并提出了类梯形入射波,可使低阻抗试样尽早实现应力平衡和恒定应变率加载. 研究结果表明:低阻抗有机玻璃杆/管作为透射杆并辅以半导体应变片可实现载荷低至数Newton的透射信号采集;低阻抗试样应力平衡历程依赖于其弹性波速,在类梯形入射波加载下,2倍特征时间上升沿时可实现应力平衡以及其后的恒定应变率加载;其可基于试样厚度设计实现不同应变率加载下的临界有效应变一致性.Abstract: The key problems of the effective acquisition of low-impedance transmitted signals and the stress equilibrium in the Hopkinson bar loading tests on low-impedance specimens with high strain rates, were investigated. Based on the previous research fundaments, low-impedance elastic transmitted bars were introduced to replace the traditional metal transmitted bar, the in-situ calibrated semiconductor strain gauge technology was used to amplify weak transmitted signals, and the low-impedance specimens were tested with high precision. Reconstruction and calculation of the stress wave loading process were conducted to get the stress equilibrium history and the influential factors' sensitivity. The trapezoidal incident wave was proposed to realize stress equilibrium and achieve constant strain rate loading on specimens as early as possible. The results show that, low impedance polymethyl methacrylate bars/tubes as transmitted bars along with semiconductor strain gauges can acquire weak signals as low as several Newtons. The stress equilibrium history of the low-impedance specimen depends on the elastic wave velocity. Under the trapezoidal incident wave loading, the stress equilibrium and constant strain rate loading can be achieved at 2 characteristic periods. Based on the design of specimen thicknesses, the critical effective strain consistency can be achieved under different stress rate loadings.
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Key words:
- Hopkinson bar /
- low-impedance specimen /
- dynamic loading /
- stress equilibrium /
- transmitted signal /
- high strain rate
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表 1 金属栅丝应变片和半导体应变片输出电压信号对比
Table 1. Output signal comparison of the metal grid strain gauge and the semiconductor strain gauge
strain gauge input voltage /V resistance /Ω gain factor strain /με output signal /mV metal grid 30 1 000 150 100 2.74 semiconductor 30 1 000 2.10 100 213 表 2 改进型Hopkinson杆和试样尺寸与力学参数
Table 2. Dimensions and mechanic parameters of the modified SHPBs and specimens
incident bar(steel) specimen(rubber) specimen(TC4) transmitted bar(steel) transmitted bar(PMMA) diameter /mm 19 5 5 19 8 length or thickness /mm 3 100 5 5 3 100 1 000 elastic modulus /MPa 210 000 1 113 000 210 000 5 000 density /(kg/m3) 8 060 900 4 560 8 060 1 200 wave speed /(m/s) 5 104 33.33 4 978 5 104 2 041 specific impedance ratio 19 803 1 757 19 803 209 -
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