DYNAMIC VOID COLLAPSE AND MATERIAL FAILURE MECHANISMS IN METALLIC CRYSTALS

The micromechanical failure mechanisms of compression-induced high strain-rate void collapse in monocrystalline copper are studied. A theoretical and a computational constitutive model are introduced to study the material failure mechanisms of shear-strain localization and cleavage fracture in monocrystalline fcc structures. An explicit dynamic finite-element algorithm is introduced for the integration of the numerically stiff micromechanical visco-plastic constitutive relations. The combined effects of strain hardening, strain-rate history, lattice rotation, and thermal softening on void collapse are investigated, and criteria for the formation of macroscopic shear-bands and tensile cracks formation are developed. The computed results are compared with experimental results and observations, and it is shown that the material failure mode is governed by the competition between the strengthening and the softening mechanisms of the crystalline structure. Shear bands may form, if the thermal and geometrical softening mechanisms of the crystal surmount the dynamic strain hardening and the strain-rate hardening of the crystal. An increase in the strain hardening and the strain-rate hardening of the crystal retards the formation of shear bands. Material failure, in this case, is due to the coupled effects of large stress concentrations and physically limiting values of plastic strain-rates, temperatures, resolved shear-stresses, and dislocation velocities near the tip of the collapsed void. These physically limiting values result in the homogeneous generation of dislocations and the local unloading of the stresses from compression to tension at the void tip. This opening mode of stress, at the void tip, in combination with the limiting values, can result in tensile cracks occurring normal to the compression stress axis.