Validation of a fracture model based on mesoscale approach for spalling of hard steel and aluminium alloy

Abstract
A fracture model has been developed to determine the macroscopic stress of spalling as a function of loading time and damage level via meso-macro approach. This model is based on numerous post-mortem microscopic observations of spall specimens. The plate impact experiment was applied to estimate the macroscopic stress corresponding to the incipient stage of spalling for two different materials: hard martensitic steel Mars 190 and aluminum alloy 7020-T6. In order to verify the model, numerous scanning electron microscopy and optical microscopy pictures of the free surfaces created by spalling have been taken. The results of measurements in the form of statistical distribution of horizontal micro-segments of fractured surfaces of targets, corresponding to quasi-brittle fracture, and vertical micro-segments, corresponding to ductile and/or adiabatic shear banding, all along the entire specimen cross sections, are reported. It has been confirmed that the microstructure has a direct influence on the mechanism of nucleation, growth and coalescence of micro-cavities or micro-cracks by means of distribution of nucleation sites and decohesion between the harder particles and the softer lattice. The model allows for determination of the macroscopic normal stress necessary to obtain a specific level of damage from the modeling of fracture meso-mechanisms.