Viscoplastic constitutive modeling of high strain-rate deformation, material damage, and spall fracture

The Perzyna viscoplastic constitutive theory, which contains a scalar variable for description of material damage, is used to study material behavior at high strain rates. The damage parameter for materials which undergo ductile fracture by nucleation, growth, and coalescence of microvoids, is taken to be the void volume fraction. The linear hardening law in both the constitutive equation and the derivation of the void growth rate equation has been replaced by a nonlinear hardening law that allows for the saturation of the hardening with increase of strain. The modified constitutive equations are then specialized to uniaxial deformation with multiaxial stress, which is typical of that occurring in flyer plate impact experiments. Calculations are performed showing the rate dependence of the material response and the effects of the growth of the void volume (damage). The change in the predicted response due to the modification of the hardening law is illustrated. Ductile spall fracture is modeled by considering the response to a simulated compressivetensile wave using a critical value of the void volume as the local criteria for fracture.