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Micromechanic theory of fatigue crack initiation by Lin and his associates is reviewed. Intrusions and extrusions have been observed in fatigue specimens. An initial stress field favorable for the growth of extrusion or intrusion can be produced by arrays of dislocation dipoles as shown by Lin (1969). These dipoles cause an initial tensile strain giving an elongation which is called the static extrusion. It has been suggested tiiat after extrusion reaches the height of static extrusion, extrusion growth terminates. Along the direction of an extrusion, tensile strain and tensile stress occurs. This tensile stress causes the resolved shear stiess to reach critical and slide in a secondary slip system. The slip causes plastic tensile strain which increases significantiy the extrusion growth. Similarly tiie same mechanism exists for the growth of intrusions. This explains why extrusion grows much more tiian the static extrusion.

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.