EFFECT OF VELOCITY ON FLOW LOCALIZATION IN TENSION

One-dimensional dynamic numerical simulations of sheet tensile tests a nd expanding ring tests have been carried out to study the variation o f ductility over a wide range of deformation velocities where inertial effects are significant. For materials following the Hollomon-type co nstitutive law with power-law strain rate sensitivity, the results sho wed that ductility in both tests is invariant to velocity at low test velocity. Beyond certain velocities, as the test velocity increases, d uctility of a tensile specimen first increases, fluctuates with veloci ty at higher rates and finally drops rapidly. However, in ring expansi on ductility increases monotonically. This predicted behavior is quite consistent with experimental observations, suggesting that inertial e ffects are likely to be a first-order factor responsible for enhanced ductility observed in high velocity deformation. To characterize the v elocity-dependent behavior of ductility, two critical velocities in th e tensile test and one critical velocity in the expanding ring tests w ere defined and numerically determined as a function of material param eters. For rate-insensitive materials, these values can be simply esti mated in closed-form.