Abstract
In discussing the mechanism of fatigue crack propagation in metals, there are two important problems as follows: (1) how the plastic deformation and the stress distribution at crack tips are related to the applied gross stress and crack length, and (2) how propagation rate is controlled by the plastic deformation or by the stress distribution at crack tips.In this study, as the first step on the way of investigation on the mechanism of fatigue crack growth, the plane specimens of coarse-grained pure iron with a shallow single edge notch were fatigued under completely reversed tension compression stress. The plastic deformation near the tip of the propagating cracks was examined by optical microscopy and the back-reflection X-ray microbeam Laue technique. The obtained results are summarized as follows:(1) The slip bands zone size ahead of fatigue crack tips is about 15 percent of the plastic zone size predicted from Dugdale's model. The residual stress induced by fatigue will reduce the size of the plastic zone at the crack tips in cyclic stressing.(2) The crack growth rate dl/dN is uniquely related to the slip bands zone ahead of the crack tips ξ, asdl/dN=6.0×10-5ξ1.7, regardless of the stress amplitude or the crack length. At the minimum rate for cracks to propagate, ξ takes the value of about 5μ. It is worthy of note that this result was obtained in the case of 15% cold-rolled 0.01% C iron.(3) Where the slip bands are observed in the vicinity of cracks, the value of misorientation becomes larger, while the subgrain size becomes smaller. The misorientation β at the fatigue crack tip increases as crack growth rate dl/dN becomes larger and their relationship is expressed as, dl/dN=2.2×10-1β3.5.
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