Effect of overloads on fatigue crack growth in aluminum alloy

Abstract

The objective of this project is to investigate the fatigue crack growth behavior of 7075-T651 aluminum alloy. In many structural components that experience cyclic stresses, variable amplitude loading is more common than constant amplitude loading. Thus, fatigue tests containing periodic multiply overloads were carried out. The type of specimen used is middle-tension specimen which was designed according to the ASTM E647 standards. The effect of overload ratio on fatigue crack growth was studied. Optical methods were used to measure and calculate the fatigue crack growth rate under different overload ratios. These fatigue crack growth rates from periodic overloading were compared to constant amplitude loading. All periodic overload tests showed faster crack growth. The crack growth rate further increased when the overload ratio was increased. Microscopic features of the fracture surface such as marker bands and striations were analyzed using a scanning electron microscope. Based on the baseline striations distances found from different loading, it was predicted that the mechanism to cause crack acceleration is strain hardening at crack tip due to the overload cycles. The large ratio of overload cycles to baseline cycles is another factor that contributed to the crack growth acceleration. The stress at failure for different loadings were calculated and found to be consistent at about close to yield stress of the material.