Transition from Flat to Slant Fracture in Ductile Materials

We investigate ductile fracture in an aluminum alloy which exhibits an interesting switch in mode from flat fracture to slant fracture. While this transition is typically considered to be triggered by a change in crack tip constraint with specimen thickness, we explore this transition in rolled sheet material simply by controlling the strain hardening behavior of the material. Specifically, experiments are performed on two different heat-treatments of the same alloy, resulting in two materials that differ only in their strain-hardening behavior. Based on an examination of the microscopic and macroscopic strain evolution, we conclude that the transition arises because of strain localization that precedes fracture.