| This work seeks to examine the dynamic growth of voids in an elastic-viscoplastic medium through analytical and numerical approaches, with a view towards addressing a number of problems that arise during the dynamic failure of metals. Particular attention is paid to the instability of void growth, and to the effects of inertia, rate-dependence, thermal softening, heat conduction, plastic strain gradient and (less completely) void-void interactions on void growth.; A critical stress is known to exist for the unstable growth of voids. The dependence of this critical stress on material properties (such as strain hardening, thermal softening and yield strain) is examined, and this critical stress is demonstrated to correspond to the lower limit for the ductile spall strength in many metals. Once the concept of the critical stress is adopted, an asymptotic solution is then obtained for the dynamic growth of voids under supercritical loading in elastic-viscoplastic materials. The asymptotic solution shows that voids of all sizes will eventually achieve the same growth rate for a given steady applied loading. Consequently, voids will grow rapidly towards similar sizes regardless of the differences in their initial sizes, explaining the void size distributions typically found on ductile fracture surfaces. Under an extremely high loading, the void growth rate is increased significantly, therefore, the time for voids to grow to the full sizes observed on fracture surfaces will be much shorter, e.g., a few nanosecond in laser spallation. Both rate-dependence and heat conduction reduce the rate of void growth and have stronger effects on smaller voids. As the voids start rapid growth, however, the effects of rate dependence and heat conduction on void growth decrease, so that they eventually give way to the dominating effects of inertia. In addition to the stabilizing effects of inertia, which have been traditionally recognized, our results show that for sustained subcritical loading the effect of inertia first impedes but later facilitates the growth of voids in the long term.; We have also developed a gradient plasticity theory incorporating temperature effects. A strong size-dependence is introduced into the dynamic growth of voids through gradient plasticity, so that a cut-off size is set by the stress level of the applied loading. Only those voids that are initially larger than the cut-off size can grow rapidly. Once the voids start rapid growth, however, the influence of strain gradients will decrease. Therefore, the rate of dynamic void growth predicted by strain gradient plasticity approaches that predicted by classical plasticity theories. |
