Abstract
Fatigue crack growth test under constant amplitude loading was conducted on a grain-oriented 3% silicon iron using servo-hydraulic fatigue loading system. The configuration of specimen surface around fatigue crack tip was transferred to a thin replica film and examined by means of an atomic force microscope (AFM). Discrete slip lines which emanated from crack edges were observed clearly along the two preferred slip directions (±55°). It implies that two preferential slips operated to almost identical extent simultaneously and fatigue crack grew in mode I. The slip deformation during loading and unloading was observed successively and it was found that the operation of slips behind the fatigue crack tip resulted in crack closure and that at the crack tip brought about crack growth. The serface displacement during one cycle was measured quantitatively by using image processing technique. The pattern of the relationship between load and slip deformation was correlated well with that between load and crack tip opening displacement, CTOD, and the slipping distance near the crack tip corresponded to the ideal CTOD.
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