Abstract Acoustic velocity measurement and the acoustic emission tests of tension–torsion composite failure process for nylon 6 and nylon 66 standard specimens under different loading rates are carried out by using acoustic emission instrument and electronic tension–torsion tester respectively. The acoustic velocity, stress–strain curve and acoustic emission energy curve of each specimen are obtained; According to the theories of the plane stress state and strength criterion, the principal stress of point on the surface of specimen at failure under the action of combined tension–torsion load is developed. The Mises yield condition is suitable for judging the yield criterion of nylon at tension–torsion failure; Based on the least square method, fitting models of the second invariant of the stress deviator are established with the density, acoustic velocity, tensile loading rate and torsional loading rate of nylon material. The results show that: For nylon 6 material at the low tension–torsion rate, with the increase of strain the secondary yield phenomenon occurs after the initial yield and finally necking occurs until fracture; With the increase of loading rate, the tension strength and normal strain of nylon 66 decreases;According to the regression model, the second invariant value of the stress deviator can be predicted well.

中文翻译：

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考虑尼龙材料拉扭复合变形综合力学和声发射特性的屈服准则和强度条件

摘要 分别利用声发射仪和电子拉扭测试仪对尼龙6和尼龙66标准试件在不同加载速率下进行了拉扭复合材料破坏过程的声速测量和声发射测试。得到各试件的声速、应力应变曲线和声发射能量曲线；根据平面应力状态理论和强度准则，推导出试件在拉扭复合载荷作用下破坏时点的主应力。Mises屈服条件适用于判断尼龙拉扭破坏的屈服准则；基于最小二乘法，建立应力偏差第二不变量与密度的拟合模型，尼龙材料的声速、拉伸加载率和扭转加载率。结果表明：对于低拉扭率的尼龙6材料，随着应变的增加，在初始屈服后出现二次屈服现象，最后发生颈缩直至断裂；随着加载速率的增加，尼龙66的抗拉强度和法向应变减小；根据回归模型，可以很好地预测应力偏差的第二个不变量值。