Fatigue crack propagation anisotropy of an Al–Zn–Mg–Cu super-thick plate

The fatigue crack propagation (FCP) rate is an important performance index in the aerospace industry for the safe lifespan evaluation. In this study, the FCP anisotropy of an Al-Zn-Mg-Cu super-thick rolled plate was analyzed. The results show that as the depth increases, the FCP anisotropy gradually becomes significant especially in the low Delta K region. The specimens tested along the transverse direction exhibit lower FCP rate than those tested along the rolling direction in the low Delta K region, which is related to the difference in the grain shape and grain orientation. More specifically, the crack is easy to deflect along the grain boundaries in the deformed grains when loading along the transverse direction. Meanwhile, more obvious grain orientation deviation on both sides of the crack can cause larger angle deflection, longer FCP path, and lower FCP rate. In the Paris region, the FCP anisotropy is not obvious regardless of thickness. The reason is that the main influencing factor is controlled by strength in this region, while the strength anisotropy is very limited leading to this similarity.