Very High Cycle Fatigue of a Cast Aluminum Alloy: Size Effect and Crack Initiation

Fatigue properties of an Al-Si-Cu cast alloy were investigated with life up to very high cycle fatigue regime. Particular attention was given to specimen size effect and fatigue crack initiation mechanism. Uniaxial fatigue tests with constant amplitudes were carried out by using ultrasonic fatigue testing machine operating at 20kHz, compared to the results obtained by using conventional hydraulic fatigue machine at 35Hz. In order to evaluate the size effect, two sets of specimens with different dimensions were used. The results show that the fatigue strength of the studied alloy decreases with the increase in specimen size. Scanning electron microscopy observation of fracture surfaces revealed that most fatigue cracks initiated from microstructural defects such as porosity located on specimen surface or in subsurface. Weibull statistical analysis of fatigue data accounting for size effect was performed to predict the fatigue life of the Al-Si-Cu alloy in the very high cycle regime. Good agreement was found between the fatigue life prediction and the experimental data.