Limitation of Linear Elastic Fracture Mechanics for Growing Small Fatigue Cracks and its Dependence on Microstructure

Abstract

The characteristics of small fatigue crack growth were investigated under rotating bending on two specimens of a low alloy steel having prior austenite grain sizes of d=15μm (fine grain) and d=91μm (coarse grain). The influences of grain boundary on the crack growth rate and the aspect ratio of small fatigue cracks were clarified, and the critical crack length above which linear elastic fracture mechanics (LEFM) was applicable, was evaluated for growing small fatigue cracks.
When the surface crack length was shorter than 3d, the crack growth rate decreased near grain boundary, and the aspect ratio varied widely by the effect of microstructure. Cracks longer than 3d had no influence of the microstructure, but they grew faster than would be expected based on LEFM until the length reached to 3d+150μm. This behaviour may be attributed to the difference in crack closure between small cracks and large ones. If the contribution of the crack closure to the small fatigue crack growth were to be established experimentally or analytically, the critical crack length would be 3d. However, it is now difficult to evaluate the crack closure and therefore 3d+150μm is regarded as the critical crack length for engineering application at present.