Yield criteria and strength conditions considering comprehensive mechanical and acoustic emission characteristics of tension–torsion composite deformation of nylon materials

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.