Quantitative Analysis of Fatigue Process—Microcracks and Slip Lines Under Cyclic Strains

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Abstract
Quantitative observations have been made on the microcracks or small surface cracks and slip lines on the surfaces of steels under cyclic strains, for the purpose of unifying two approaches to the study of fatigue, namely, that based on fracture by crack growth (fracture mechanics concept) and that of cyclic strain accumulation (low-cycle fatigue concept). Important results are: (1) initiation of cracks at the early stage of fatigue life even on unnotched smooth specimens; (2) growth of the small surface cracks below ΔKTH level; (3) higher growth rate of small surface cracks than that of a large through crack; (4) dependence of ΔK versus da/dN relation on stress level for small surface cracks; and (5) a definite relation between λ, the ratio of the number of slipped grains to the number of total grains, and Δϵ, the cyclic strain range. From these results it is suggested that the growth rate of small surface fatigue cracks can be represented as either da/dN = C (ΔKεp)m or da/dN = C (ΔKεt)m, where ΔKεp is the plastic strain-intensity factor range given by Δεpπa⋅f (a), and ΔKεt is the total strain-intensity factor range given by Δεtπa⋅f (a).

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