Effect of Shear Overloads on Crack Propagation in Al-7075 Under In-Plane Biaxial Fatigue Loading

A cruciform specimen of Al 7075-T651 is used to study crack propagation behavior in an in-plane biaxial fatigue loading subjected to shear overloads of different magnitudes, which were applied at different crack lengths. The microscale fracture features of the specimen were identified and compared for the pre-overload region, overload region, and post-overload region at two different values of crack lengths, using scanning electron microscopy (SEM). It was observed that the transient region, created by the application of the shear overload, improved the fatigue life of the specimen. The overload also displayed an instant upsurge in the fatigue crack growth rate, which was immediately followed by a sharp crack retardation. The crack growth rate was restored once it came out of the transient zone and traveled a distance equal to the value of recovery distance. Both, the magnitude of the applied shear overload and the location of overload with respect to crack length seemed to affect the size of the transient zone, fatigue life, crack growth rate, and recovery distance. Investigations made on the fracture surfaces revealed that there is no significant change in the microscale fracture features when the overload was applied at different values of crack lengths; however, a clear and significant difference in the fracture features appear when the surfaces of the pre-overload region, overload region, and post-overload region are compared.