Effects of microstructural through-thickness non-uniformity and crack size on fatigue crack propagation and fracture of rolled Al-7075 alloy

Some of the fatigue tests performed using the standard compact tension (CT) and a non-standard specimen made of rolled 7075 aluminium alloy exhibit fatigue crack growth (FCG) lagging in a small region along the crack front. Through-thickness microstructural evaluation shows that material grains in this region did not flatten as much as other regions. In the non-standard specimen, surface cracks are either grown under fatigue loading or broken under monotonically increasing quasi-static loads at different crack sizes. The aforementioned lagging also exists in a narrow region of 3-D FCG for specimens with microstructural through-thickness non-uniformity. A more important feature for this type of specimen with surface crack is the deflection of fast fracture direction into the grain interfaces, namely from L-T orientation to S-L and S-T directions. It is proved that this is due to significant levels of second principal stresses near the free surface for small cracks and lower fracture toughness of the material in S-L and S-T directions.