Materials that flow soften exhibit plastic instability during compressive deformation when Considere's criterion is satisfied, i.e., when there is no increase in the force required to cause deformation. Further, theories developed by Hart and Jonas et al. show that the flow localization can occur during plastic instability. The strains at which the onset and termination of flow localization occur depend on the strain-rate sensitivity of the material. Flow localization initiates earlier and terminates later for a strain-rate sensitive material in comparison to a rate insensitive material. However, in an apparent contradiction, the deformation is more homogeneous in a strain-rate sensitive material.

This paper presents the results of an investigation, using computer simulation, to visualize the deformation during constant strain-rate compression of flow softening materials with different strain-rate sensitivity. The effect of strain-rate sensitivity on the initiation and termination of plastic instability and flow localization due to mechanical damage, geometric nonuniformities, and friction were studied. Parameters developed to describe the extent of inhomogeneity during deformation are used to explain the conclusions made by Jonas et al.