A MODIFIED MODE II DYNAMIC FRACTURE TEST TECHNIQUE BASED ON SHTB
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摘要: 冲击剪切载荷作用下动态断裂韧性的测定是材料力学性能和断裂行为研究中重要组成部分.为了测定材料的II型动态断裂韧性，许多学者采用不同的试样与实验方法进行了实验，但限于实验条件，裂纹断裂模式往往是I+II复合型，而不是纯II型，因而不能准确测得材料的II型动态断裂韧性.鉴于此，本文基于分离式霍普金森拉杆（split Hopkinson tension bar，SHTB）实验技术，提出一种改进的紧凑拉伸剪切（modified compact tensionshear，MCTS）试样，通过夹具对MCTS试样施加约束，从而保证试样按照纯II型模式断裂.采用实验——数值方法对MCTS试样动态加载过程进行分析，将实验测得的波形输入有限元软件ANSYS-LSDYNA，得到了裂纹尖端应力强度因子——时间曲线，并与紧凑拉伸剪切（compact tension shear，CTS）试样进行了对比.同时采用数字图像相关法进行了实验，验证了有限元分析结果.结果表明，MCTS试样在整个加载过程中KI$\ll $KII，裂纹没有张开；而CTS试样在同样的加载过程中KI>KII，出现裂纹张开现象.这说明MCTS试样能够准确地测定材料的II型动态断裂韧性，为材料动态力学测试提供了一种有效的实验技术.Abstract: Dynamic fracture toughness under impact shear loading is an essential aspect in fracture behavior and mechanical property of material.Experiments have been done by several researchers using different specimens and test methods in order to measure mode II fracture toughness.But due to crack opening during loading process, the results obtained in these tests are not mode II but mixed mode I+II.Since crack opening are not be considered, dynamic shear fracture toughness of material can not be accurately detected.In view of this problem, a modified compact tension shear (MCTS) specimen based on split Hopkinson tension bar (SHTB) apparatus is proposed in this paper.The specimen was constrained with special designed clamp to prevent crack opening, so mode II fracture are ensured.Numerical analysis was carried out using experimental-numerical method.The incident pulse detected in test are introduced in ANSYS-LSDYNA as input pulse.Stress intensity factor at crack tip of MCTS specimen was calculated by relative displacement of corresponding nodes on crack surface in two directions.Simulation of compact tension shear (CTS) specimen was also done with same incident pulse as control.In addition, experimental study was also carried out using digital image correlation method based on SHTB apparatus to validate numerical results.Experimental results shows that during loading process, MCTS specimen ensures KI$\ll $KII and crack opening are not observed.However, for same incident pulse, the maximum mode I stress intensity factor of CTS specimen is even higher than mode II.Which indicates that dynamic shear fracture toughness of material can be measured effectively using MCTS specimen.This work provides an effective and convenient test technique for evaluating dynamic properties of a certain material.
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表 1 有限元模型材料属性
Table 1. Material properties of finite element model
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