Quantitative evaluation of fatigue strength of metals containing various small defects or cracks

Abstract

A geometrical parameter which controls the fatigue strength of materials containing small defects is proposed considering the fact that the fatigue limit is not the critical condition under which no crack appears but the threshold condition where cracks emanating from defects cease to propagate. The geometrical parameter as a representative dimension (length) is the square root of the area (√Varea) of a defect projected in the direction of the maximum tensile stress. It is found that the maximum stress intensity factor along a three-dimensional crack is approximately proportional to (√Varea)frcase|^1/2. In order to demonstrate the usefulness of the proposed parameter; Varea, fatigue tests of specimens containing very small holes, with diameters ranging from 40 to 500 μm, were carried out and the quantitative relationships in the form: σⁿ_w(√Varea) = C are obtained for several materials, where σ_w is fatigue strength.