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Results of experiments indicate that crack growth from a notch root in a 1080 pearlitic (rail) steel exhibits anomalous behavior when analyzed in terms of linear elastic fracture mechanics (LEFM). (Cracks as long as 0.4 cm exhibit anomalous behavior.) It is shown that the length of crack exhibiting anomalous behavior corresponds with the size of the plastic zone at the notch root. A model is presented which characterizes the crack growth driving force in terms of the displacement (strain) control condition that this inelastic notch field is postulated to impose on cracks contained within it. It is shown that the model reasonably predicts the growth rate behavior within the inelastic notch field. Notched component life predictions, based on this model, coupled with a nonlinear scheme to predict the formation of a 125 μm crack and LEFM analyses to continue growth beyond the inelastic field, are presented. It is shown that, for the pearlitic steel examined, the life of the components is equally divided between initiation and growth within the inelastic notch field at higher stresses. As the applied cyclic stress applied decreases, initiation becomes increasingly dominant. The portion of life controlled by LEFM behavior is negligible except within a narrow window of cyclic stress. The implications of this observation are discussed with respect to other materials and notch severities.
1080 pearlitic steel, linear elastic fracture mechanics (LEFM), short (small) cracks, inelastic notch field, model, prediction, fatigue life, crack initiation, crack propagation, plasticity
Senior Scientist, Battelle Columbus Laboratories, Columbus, Ohio