Modified KT-Diagram for Stress Raiser-Involved Fatigue Strength Assessment

Abstract

The model for evaluating the fatigue strength of specimens and structure elements with sharp-edged and deep stress raisers (notches) or defects, which can be treated as initial cracks, is advanced. The model is based on the modification of known fracture mechanics approaches with employing the modified Kitagawa–Takahashi diagram. The model starts from the fact that cyclic loading of sharp-edged notch-containing specimens over the nominal stress span below the endurance limit of smooth specimens results in a crack penetrating to a certain size from the root of the notch, with its further arrest due to the two basic factors: descending gradient of local stresses ahead of the notch root and gradually growing effect of crack closure behind its tip. The crack size is dependent on the stress span and notch depth. The model permits of calculating the boundary curve of threshold stress spans and corresponding tolerable crack sizes for a sharp-edged notch of any depth, using only the characteristics of static strength and microstructure of the initial material. The model reliability was verified with experimental results taken from the literature, the calculation and experiment were in good agreement. The model need not long-term and labor-consuming fatigue and fatigue crack resistance tests to get parameters necessary for the model implementation. The model calculations would require only the data on static strength characteristics (elastic modulus, Poisson’s ratio, and proportionality limit), obtained from short-time tensile tests of standard specimens from an examined material, and microstructure characteristics (grain size, Taylor factor, and Burgers vector), determined from microstructure analysis of the initial material.