Abstract
The present investigation was undertaken to examine the relation between fatigue crack propagation characteristics and microscopic fracture mechanisms at low crack propagation rates in a mild steel. In particular fractographic aspects on the fracture surfaces were examined and discussed in connection with the knee of the crack propagation rate-stress intensity factor range curve (log(dl⁄dN)−log(ΔK) curve) and with the transition from Stage 2a to Stage 2b in crack propagation. Fatigue tests were performed under pulsating ex-plane bending using notched specimens with a small circular hole. The fracture surfaces of specimens were examined by transmission and scanning electron microscopies. The resuls are summarized as follows:(1) The value of crack propagetion rate, dl⁄dN, changes in a pulsating manner at an early stage of crack propagation in maximum stress-constant tests. In this range three types of microscopic features, that is, intergranular facets, a striation-like pattern and a shear mode pattern superimposed by the striation-like pattern are observed.(2) In the range of dl⁄dN above the knee of the log(dl⁄dN)−log(ΔK) curve, Paris’ power law is valid. In this range the plastic zone size developed at the crack tip increases with crack propagation, and the shear mode pattern disappears.(3) At the transiton point from Stage 2a to Stage 2b, the area fraction of intergranular facets takes a maximum value.(4) The striation spacing is larger than the value of dl⁄dN and takes a constant value at Stage 2a, whereas it is proportional to dl⁄dN at Stage 2b.(5) In the range of dl⁄dN below the knee, the area of the shear mode pattern becomes larger, as dl⁄dN becomes lower.
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