Fatigue Crack Propagation of Short Cracks

TLDR

Previous studies have shown that both threshold stress intensity factors and fatigue crack growth rates depend on crack size. The authors modify elastic and elastic‑plastic fracture mechanics solutions by introducing an effective crack length l0 into intensity‑factor and J‑integral analyses, allowing l0 to be determined from the threshold stress intensity factor and fatigue limit as the threshold stress approaches the material’s fatigue limit. The study finds that very short cracks grow faster than predicted by long‑crack laws, that the effective crack length l0 accurately predicts growth rates regardless of strain level, and that l0 varies linearly with grain size, indicating reduced surface grain flow resistance.

Abstract

Previous studies have shown that both threshold stress intensity factors and fatigue crack growth rates are dependent on crack size. The average growth rates for very short cracks considerably exceed those given by conventional stress intensity-crack growth laws fitted to long crack data. Elastic and elastic plastic fracture mechanics solutions are modified to predict this behavior of short cracks by introducing an effective crack length l0 into the solutions for intensity factors and the J integral method of analysis. The threshold stress at a very short crack length approaches the fatigue limit of the material, and therefore the value of l0 can be obtained once the threshold stress intensity factor and the fatigue limit are known. The accuracy of the term l0 in predicting crack growth rates for short cracks is found to be independent of the applied strain level. It varies linearly with the grain size of the material and can be considered at the surface as a measure of the reduced flow resistance of surface grains due to their lack of constraint.