Influence of Mean Stress on Crack Closure Phenomenon and Fatigue Crack Propagation

Abstract

Measurements and observations on the closure of fatigue crack initiated from notch roots in an annealed and a quenched-and-tempered 0.54% carbon steel in tension-compression fatigue tests were made successively while increasing the stress level, in order to investigate the influence of mean stress on fatigue crack propagation and crack closure phenomenon. The following are the results obtained: (1) The crack closure phenomenon in tension-compression fatigue can be recognized more clearly in the annealed steel than in the tempered steel. (2) The crack propagation rate dl/dN can be expressed as [numerical formula] where Δσ_eff means the effective stress range under which the crack is fully open and l is the crack length. The exponent m is about 1.6 for the annealed steel and about 3 for the tempered steel. Mean stress affects Δσ_eff alone, therefore, the constant C is not related to the mean stress.