Abstract
Precisely matched electron fractographs of the matching fracture surfaces of two kinds of carbon steels (S 35 C and SK 5) were examined to clarify the micromechanism of fatigue crack propagation.The occurrence of pearlite boundary cracking may be explained by the fact that the active slip lines are arrested by the lamellae cementite in the neighbourhood of crack tip.Most of the tear and equiaxed dimples containing inclusions or precipitated particles are formed at the fatigue fracture surface in the steel (S 35 C) tempered at 200°C. The width-to-length ratio of dimples for a given stress intensity factor (ΔK) is about 1.3∼1.5 and the average size of dimples is correlated with ΔK. The distribution of dimple size is approximated by a logarithmic normal distribution.The discrepancy between two matching fracture surfaces is created as: (a) Two sets of active slips at the crack tip are different in their magnitude. (b) Cracks are not propagating perpendicular to the apparent maximum stress direction. (c) One of the mating fracture surfaces is plastically deformed by some physical constraints after the crack front has passed.
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