Mode I steady-state crack propagation through a fully-yielded ligament in thin ductile metal foils

Abstract

Steady-state crack propagation is the advance of a self-similar crack tip with a constant driving force. Though steady-state growth is often described with analytical models, it is rarely experimentally observed or characterized under quasi-static loading conditions. Thin ductile metal sheets, such as the 25.4 μm, 50.8 μm, and 127 μm thick 1235 aluminum specimens used in this study, exhibit steady-state crack propagation but cannot be characterized by conventional linear-elastic (K) or elastic-plastic (J) crack tip parameters that require contained crack tip plastic zones. Instead we used a fully-yielded plastic crack growth resistance analysis (i.e., ligament stresses were above the tensile yield stress) to identify when cracks in thin aluminum sheet specimens reached steady-state propagation conditions. At steady-state a constant, characteristic crack growth resistance, σc was observed for each sheet thickness (σc25.4μm=60MPa,σc50.8μm=95MPa, and σc127μm=93MPa). This σc-controlled crack growth is dramatically different from conventional linear elastic and elastic plastic crack growth because the plastic zone is uncontained and extends across the remaining ligament.