Effect of Multiple Initial Imperfections on the Initiation and Growth of Adiabatic Shear Bands in Nonpolar and Dipolar Materials

Ahstraet-Simple shearing deformations of a viscoplastic block made of nonpolar and dipolar materials, and placed in a hard loading device are studied. Multiple defects in the block are modeled by perturbing the uniform temperature within the block when the material just starts deforming plastically to that given by a cosine function which assumes relative maximum values at several points in the block. It is found that for simple materials, the deformation localizes at points where the perturbed temperature has relative minima when the average applied strain-rate PO is 500 s-l and at the locations of the relative maxima of the perturbed temperature when the applied strain-rate is more than 1000 s-l. This transition occurs possibly due to different therma lengths and the time scales associated with the work-hardening in the two cases. For dipolar materials the deformation localizes near the boundaries of the block abutting the loading device when ?a = 500 s-l but at the locations of the relative maxima of the perturbed temperature when &, = 50,000 s-l. For both simple. and dipolar materials the initiation of the localization of the deformation is considerably delayed as compared to the case when the temperature perturbation has only one bump with its center coinciding with the center of the block.