SELF-ORGANIZATION IN THE INITIATION OF ADIABATIC SHEAR BANDS

The radial collapse of a thick-walled cylinder under high-strain-rate deformation (similar to 10(4) s(-1)) was used for the investigation of shear-band initiation and pattern development in titatium. Experiment s were carried out in which the collapse was arrested in two stages, i n order to observe the initiation and propagation of shear bands. The occurrence of shear bands to accommodate plastic deformation in respon se to external tractions is a collective phenomenon, because their dev elopment is interconnected. The bands were observed to form on spiral trajectories and were periodically spaced. The spacing of the shear ba nds decreased with the progression of collapse, and was equal to appro ximately 0.6 mm in the final stage of collapse. The shear-band spacing was calculated from two existing models, based on a perturbation anal ysis and on momentum diffusion. Values of 0.52 and 3.3 mm were obtaine d with material parameters from quasi-static and dynamic experiments. The predictions are found to give a reasonable first estimate for the actual spacings. The detailed characterization of the shear-band front leads to an assessment of the softening mechanisms inside a shear loc alization region. The initiation of localization lakes place at favora bly oriented grains and becomes gradually a continuous process, leadin g eventually to dynamic recrystallization. (C) 1997 Acta Metallurgica Inc.