Investigation on Unstable Fatigue Crack Propagation of 7050-T7451 Aluminum Alloy Based on Small Time Scale Method

Abstract

Based on the small time scale method, unstable fatigue crack propagation of 7050-T7451 aluminum alloy could be observed directly from in-situ SEM by the manner of step loading. The results show that when the fatigue crack was blocked by grain boundary, the crack opening and closure stresses are increased 0.88 times and 0.58 times respectively comparing with the value in general case. The retardation of crack opening and prematurity of crack closure could be associated with the constraint of crack tip slipping imposed by the grain boundary. The stress of crack opening decreased gradually with the increase of loading cycles, which led to the acceleration of bridge linkage between grain boundary and crack. The crack presented unstable fast propagation when the crack was connected with grain boundary.