Effects of variable amplitude loading on fatigue crack propagation behaviour of microcracks in Al-Zn-Mg-Cu alloys

The fatigue crack propagation behavior of self-initiated microcracks was studied in air for the commercial Al-alloy 7475 and a high-purity X-7075 alloy, both in an underaged condition. A comparison was made between constant amplitude tests and tests with periodically applied tensile overloads superimposed on constant amplitude cycles at R = 0.1. The number n of intermittent baseline cycles between consecutive overloads was varied between 100 and 1000. The fatigue crack growth resistance increased significantly for both alloys with increasing numbers of n as compared to crack growth rates under constant amplitude loading. The results are explained on the microscopic variations of the fracture surface appearance as a result of overload enhanced changes in slip distribution in the plastic zone at the crack tip.