Investigation of orientation dependent near-threshold crack growth in textured magnesium alloy AZ31

The study investigates the near-threshold fatigue crack growth behavior of long cracks in wrought magnesium alloy AZ31, focusing on the influence of rolling texture and test methods on crack propagation. The rolled material shows a significant mechanical and microstructural anisotropy between the investigated rolling and transverse directions. Crack growth experiments were performed at frequencies of 50 Hz, 1000 Hz, and 20 kHz, respectively, and at a stress ratio of R = 0.1. The results show that the propagation behavior is not influenced by test frequency or growth direction. All fatigue crack propagation curves show data in the same scatter band. In the range of ΔKth, the curve is very steep and the crack propagates even below growth rates of 10-12 m/cycle. Above 10-9 m/cycle a plateau-like progression nearly independent of the stress intensity factors (SIF) was observed. This is attributed to the strong rolling texture and the activation of only one dominant deformation mechanism. Fracture surface analysis via SEM reveals a transition of crack growth mechanism in dependence of SIF resulting in multiple active slip systems at higher SIF. Further investigations reveal the dominant role of prismatic slip for the crack growth in the studied directions.