Development of a Micromechanic Theory of Crack Initiation Under High-Cycle Fatigue

From the hint provided by extrusion and intrusion in fatigue specimen, a micromechanic model consisting of a thin slice R sandwiched in two thin slices P and Q is developed. P with positive initial shear stress slides forward in the forward loading, while the other Q with negative initial shear stress slides backward during the reversed loading. Micromechanic analysis shows that the positive slip in P relieves the positive shear stress not only in P, but also in Q. This helps Q to slide in the reversed loading. Similarly the negative slip in Q helps P to slide during the next forward loading. The micro stress fields generated by the alternate sliding in P and Q gives the ratchet mechanism in fatigue. Extrusion causes a tensile stress in R. This stress combining with other stress can activate a second slip systems of an aluminum single crystal. The macroscopic deformation hysteresis loop are thus computed. The computed microstructures check amazingly well with the experimental fatigue data of aluminum single crystals by Zhai et al of Oxford University.