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

Abstract

Abstract : 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.