Fatigue Crack Propagation in S15C Steel

Abstract

Many studies on fatigue crack propagation have hitherto been presented. Most of them are directed in search of the relation between the crack propagation rate and both the crack length and the applied stress range, that is, the crack propagation law. In one of these laws, the crack propagation rate has been estimated so far with a function of the stress intensity factor K, as the simplest and the most appropriate single variable which represents the elastic stress field around the crack tip. This method of approach is very exact and accurate in such a case as the crack length is short and the applied stress in fatigue is adequately small in comparison with the yielding strength. However, there may be other cases where the choice of K is inappropriate. We need therefore another approach on the basis of the propagation mechanism. Considering that the crack propagation rate can be determined by the distribution of plastic deformation, we have tried to estimate the law with new variables.In this study annealed plane specimens of 0.16% C low carbon steel with a shallow single edge notch were fatigued under completely reversed in-plane bending stress. The plastic deformation near the crack tip was examined by means of an optical microscope and the back-reflection X-ray microbeam Debye technique. The obtained results are summarized as follows:(1) The slip bands zone size ahead of fatigue crack tips is much smaller than the plastic zone size predicted from Dugdale's model. The relation is given by the following equation.ξ/l=0.032{sec(π/2σ/0.77σy)-1}where ξ, 1, σ and σy represent respectively the slip bands zone size, the crack length, the applied stress and the tensile yielding strength. The residual stress induced by fatigue will reduce the size of the plastic zone at the crack tips in cyclic stressing.(2) The crack propagation rate dl/dN is uniquely related to the slip bands zone size ξ ahead of the crack tips, asdl/dN=1.9×10-8ξ1.5regardless of the stress amplitude or the crack length.(3) The excess dislocation density Dm at the fatigue crack tip increses, and the subgrain size t decreases as crack propagation rate dl/dN becomes larger and their relationship is expressed as, dl/dN=5.2×10-30Dm2.6dl/dN=1.7×10-5t-3.3(4) The crack propagation laws estimated with the stress intensity factor are slightly but inevitably dependent on the stress amplitudes. These laws are independent of them, on the other hand, provided that we choose the representative quantities of the plastic deformation, for examples, the plastic zone size, the excess dislocation density or the subgrain size ahead of the crack tip as the single variable.