1988 年 37 巻 417 号 p. 656-662
The growth behavior of small fatigue cracks was investigated in a low carbon steel under axial loading at the stress ratios R of -1 (tension-compression) and 0 (pulsating-tension). Crack closure was measured to evaluate the effects of stress ratio and stress level on the growth behavior of small cracks. Except for the accelerated growth behavior at the higher stress levels close to the yield stress σys of the material, i.e. at σmax=0.84σys for R=-1 and σmax=1.12σys for R=0, at R=-1 small cracks grew faster than large cracks below a certain crack length, but at R=0 the crack growth rate for small cracks was coincident with that for large cracks in the whole region of crack length investigated. The critical crack length 2cc above which the growth behavior of small cracks was similar to that of large cracks depended on stress ratio, being 1-2mm at R=-1 and less than 0.7mm at R=0. The 2cc value at R=-1 agreed with that obtained under rotating bending (R=-1). The crack growth rate for small cracks was well correlated with that for large cracks by the effective stress intensity range ΔKeff; thus ΔKeff was found to be a controlling parameter for small crack growth including the growth behavior at the higher stress levels.