Deformation and Damage Evolution in Thin Metal Sheets during Steady-State Crack Growth using Incremental Strain Field Mining

Abstract

Deformation and crack behavior is difficult to characterize in ductile metals at sub-millimeter thicknesses using conventional fracture mechanics approaches. This is because of the challenges posed by large scale plasticity, uncontained yielding, and instabilities like diffuse and transverse necking. An fundamental aspect that needs to be considered when studying cracks are the process zones ahead of the crack tip. Conventional approaches employ various techniques to estimate process zone sizes based on finding the elastic-plastic boundary in front of the crack tip. Thin ductile sheets pose a challenge since they exhibit completely yielded ligaments were the deformations are completely plastic once the cracks start growing. We observe quasi-static steady-state process zones under fully-plastic crack tip conditions that are facilitated by plane stress conditions associated with the thin sheet form factor. In this work, we utilize an incremental strain framework based on high resolution, full field digital image correlation and tracking (DICT) to examine process zone evolution during steady-state conditions. We identify the existence of zones of active plasticity (ZAP) which scale with remaining ligament and embed different types of zones including the fracture process zone (FPZ) within it. We quantify the shape and extent of these process zones and provide insights on the factors that affect them. We investigate the structure-property-processing paradigm by characterizing zones for Al, Cu, and Ti in multiple specimen orientations and study how engineering conditions such as hydrogen charging or change in temperature to induce different mechanisms affect the zones. We tie the observed zone extents in the incremental framework to strain ranges for key deformation features such as diffuse/localized necking and failure. We also show distinct differences in zone evolution for monotonic vs cycle loading conditions. This work provides an incremental strain framework which is applied across materials systems, loading and environmental conditions to provide consistent scaling relationships. We also provide key insights into the zone evolution in the steady-state regime and offer important extent information that can be used to predict deformation behavior in this class of materials.