Micromechanics of an Extrusion in High-Cycle Fatigue With Creep

Extrusions and intrusions often are sites of fatigue cracks. The extent of extrusions is important in fatigue crack initiation. Metals often are subject to fatigue loadings at elevated temperatures. At temperatures below one half of the melting temperature slip is the main mechanism of inelastic deformation. In this study, an aluminum polycrystal loaded in this temperature range is considered. A most favorably oriented crystal located at a free surface of a f.c.c. polycrystal subject to creep under cyclic tension and compression of high-cycle fatigue is considered. An extrusion in this crystal is shown to be produced by a positive slip in one thin slice “P” and a negative slip in a closely located slice “Q”. An initial tensile strain εααI in the thin slice “R” sandwiched between P and Q causes a positive initial shear stress ταβI in P and a negative one in Q. It is shown that the extrusion growth causes a tensile strain in R, which can activate a second slip system giving a creep strain with a tensor component εαα. It has the same effect as the initial strain εααI in causing this difference in shear stresses in P and Q and gives much additional extrusion growth. The extent of intrusion and extrusion is important in this study of crack initiation.