Digital image analysis of ASB-assisted failure of impacted structural materials

Abstract

Experimental analysis of fracture mechanisms is a key point to understand and further reproduce physical phenomena involved in structural material failure. Actually, crack propagation under high loading rate is a strongly coupled thermo-mechanical problem involving large deformation, high strain rate and (quasi) adiabatic conditions. The work presented herein aims at analyzing the successive steps of the dynamic deterioration of a high strength structural material, namely the ARMOX500T armor steel, leading to the ultimate fracture. The Kalthoff and Winkler impact test, consisting in impacting the edge of a double notched plate, was retained for that purpose. Images are captured at high frame rate (1M frame/s) during the impact loading then analyzed according to a line tracking method. Three stages are accordingly observed: first the progressive development of a weak localization of the deformation within a wide zone behind the notch tips, second the propagation of a narrow band of strong localization, and finally the propagation of a crack. The propagation of the white band is associated to the development of an adiabatic shear band (ASB), as a precursor of the crack. Corresponding displacement fields are identified thanks to the adopted line tracking method, allowing for determining the evolution of mechanical quantities, including notably notch tip shear displacement and velocity, ASB-related white band velocity, global and local shear strain and shear strain rate magnitudes, in view of being implemented into numerical models.