Fatigue and fatigue crack propagation in AlSi7Mg cast alloys under in-service loading conditions

摘要

In this study, the influence of microstructure and microporosity on fatigue crack initiation and propagation in AlSi7Mg cast alloy has been investigated at very high numbers of cycles (10~8-10~9). Two alloys with similar chemical composition were used; one had finer microstructure, somewhat high microporosity and better static strength properties. The experiments were performed both at constant cyclic loads and with load sequences that simulated random in-service loading conditions of automobiles. The results show that casting pores predominantly influence the number of cycles to crack initiation. For in-service loading conditions, the detrimental effect of microporosity on crack formation cannot be adequately predicted on the basis of linear damage calculation, because of premature crack initiation during high loads of the random sequence. Comparison of the number of cycles to fracture as determined with smooth cylindrical specimens and with pre-cracked flat specimens shows the relatively high portion of the crack propagation time. The good fatigue crack growth properties of both AlSi7Mg alloys in the threshold regime (which are even better for the coarser alloy) are explained by roughness-induced crack closure.