Low Cycle Fatigue of A356-T6 Cast Aluminum Alloy - A Round-Robin Test Program

A round-robin low cycle fatigue test program was conducted by the SAEFDE Committee using A356-T6 cast aluminum alloy. Three different microstructures representative of three solidification rates were sought, but only two significantly different secondary dendrite arm spacings, DAS, resulted. The smaller DAS had slightly greater monotonic yieid and ultimate strengths, greater permanent deformation at fracture and better low cycle fatigue resistance. Under strain-controlled axial low cycle fatigue conditions. A356-T6 was observed to cyclicaiiy strain harden and hysteresis loops were skewed toward the compressive stress from about 1 to 10 percent. Fatigue failures usually initiated at surface or near surface porosity. About 25 percent of the 173 test specimens that were considered valid failed between the strain gage knife edges and the specimen fillet radius. The inter-laboratory data for a given material satisfied the log-log linear model for Δε_p /2 - 2N_f but did not satisfy the log-log linear model Δε_e /2 - 2N_f due to a very fiat cyclic stress-strain curve in the inelastic region. Thus, the usual low cycle fatigue model is not recommended for use with these A356-T6 materials due to a non-conservative representation of the data at longer lives. Other models can provide a better representation of these data.