FATIGUE AND FATIGUE-CRACK PROPAGATION IN ALSI7MG CAST ALLOYS UNDER IN-SERVICE LOADING CONDITIONS

In this study, the influence of microstructure and microporosity on fa tigue 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 microstruct ure, somewhat high microporosity and better static strength properties . The experiments were performed both at constant cyclic loads and wit h load sequences that simulated random in-service loading conditions o f automobiles. The results show that casting pores predominantly influ ence 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 calculat ion, because of premature crack initiation during high loads of the ra ndom sequence. Comparison of the number of cycles to fracture as deter mined with smooth cylindrical specimens and with pre-cracked flat spec imens shows the relatively high portion of the crack propagation time. The good fatigue crack growth properties of both AlSi7Mg alloys in th e threshold regime (which are even better for the coarser alloy) are e xplained by roughness-induced crack closure.