EFFECT OF LOADING DIRECTION AND INITIAL IMPERFECTIONS ON THE DEVELOPMENT OF DYNAMIC SHEAR BANDS IN A FCC SINGLE-CRYSTAL

We study plane strain dynamic thermomechanical deformations of a FCC s ingle crystal deformed at an average strain-rate of 1 000 s(-1) along the crystallographic direction [380] with the plane of deformation par allel to the plane (001) of the single crystal. Four different situati ons are studied; in the first two there is no initial imperfection ass umed in the crystal and it is either compressed or pulled, and in the other two the crystalis compressed but either the initial temperature is nonuniform or a small region around the centroid of the cross-selec tion is misoriented relative to the rest of the cross-section. In each case, all twelve slip systems are assumed to be potentially active, a nd the crystal material is presumed to exhibit strain hardening, strai n-rate hardening, and thermal softening. These effects are modelled by using a simple combined isotropic-kinematic hardening expression for the critical resolved shear stress, proposed by Weng, and modified to incorporate the effect of thermal softening of the material. It is fou nd that each one of the slip systems (111)[(1) over bar 10], (11 (1) o ver bar)[(1) over bar 10], ((1) over bar 11)[110], and ((1) over bar 1 (1) over bar)[110] contributes essentially equally to the plastic def ormations of the crystal and these slip systems become active soon aft er the load is applied. The same holds for the slip systems (111) [01 (1) over bar], (11 (1) over bar) [011],((1) over bar 11) [101], and (1 (1) over bar 1) [(1) over bar 01] except that they are active in a re gion different from that of the previous one. The remaining four slip systems either stay inactive throughout the deformation process, or be come active at late stages of the deformation. 1 Introduction