抄録
In this study, the initiation and the propagation behaviours of fatigue cracks emanating from a circular hole were investigated under cyclic tensile loading for 18% Ni maraging steel sheets. Fracture surfaces were observed directly by a scanning electron microscope, while the crack features on specimen surfaces were observed by an optical microscope.
The main results obtained were summarized as follows.
(1) Slopes of S-N curves were very steep and the ratio of the number of cycles to crack initiation to the total number of cycles to failure was approximately equal to or greater than 0.5 in the region of finite life. This value was fairly large comparing with the values obtained for various carbon steels and aluminum alloys.
(2) In the range of stable propagation after the crack length exceeds 0.3mm, the following exponential relation exists between the crack propagation rate and the stress intensity factor.
dL/dN=C(ΔK)m
Parameters C and m do not depend on the stress level, and m is approximately equal to 4.
(3) In the range of stable propagation, a few small cracks were observed near the main crack tip in the surface layer, and these cracks grew individually until joined each other. However in the inner region, no small crack preceding the main crack was observed and it was thought that the fatigue damage was accumulated in a limited small region near the crack tip during a certain number of stress cycles, and this region was fractured instantaneously when the damage exceeded a certain value.
