Numerical study of spalling in an aluminum alloy 7020-T6

Planar impact experiment is frequently used to investigate dynamic fracture of materials, particularly the spall phenomenon. Spalling is caused by the superposition of rarefaction waves reflected from free surfaces and the spall zone is found in the interior of the target. Behavior of materials in this kind of experiment is strongly affected by the stress level, time of loading and temperature. The rate and temperature effects are closely related to the thermally activated micromechanical processes 1]. Thus, in a stressed body the creation of new fracture surfaces frequently occurs with the assistance of thermal activation. For a more detailed study, it is therefore necessary to take into account the physical aspects of spalling, including dynamic plasticity and temperature coupling. This paper reports the numerical analysis performed using a finite element FE code by implementation of a cumulative fracture criterion proposed in 2] where the apparent energy of activation for spalling depends on stress, temperature and load history. Initially, a series of calculations have been run for the purely elastic case to analyze the minimum critical impact velocity needed to obtain the spall stress and it has been determined to be a function of the critical time of loading. Such analysis is of great value in designing experiments that are relatively expensive. Next, a viscoplastic constitutive relation together with the cumulative criterion, and the equation of heat conduction have been implemented in a FE code. The set of relations takes into account strain hardening, strain rate sensitivity and temperature. This series of FE calculations have been performed in order to take into account, changes of temperature due to volume dilatation as well as conversion of plastic work into heat. In addition to spalling, the free surface velocity–time profiles have been calculated for a number of impact velocities. Specific variations of the free surface velocity indicates the creation of a new fracture surface inside the target plate. The two sets of FE calculations reported in this paper led to some discussion on the influence of physical parameters on spall mechanics.