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. Experiments were carried out in which the collapse was arrested in two stages, in order to observe the initiation and propagation of shear bands. The occurrence of shear bands to accommodate plastic deformation in response to external tractions is a collective phenomenon, because their development is interconnected. The bands were observed to form on spiral trajectories and were periodically spaced. The spacing of the shear bands decreased with the progression of collapse, and was equal to approximately 0.6 mm in the final stage of collapse. The shear-band spacing was calculated from two existing models, based on a perturbation analysis and on momentum diffusion. Values of 0.52 and 3.3 mm were obtained 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 localization region. The initiation of localization lakes place at favorably oriented grains and becomes gradually a continuous process, leading eventually to dynamic recrystallization. (C) 1997 Acta Metallurgica Inc.