Effect of small defect orientation on fatigue limit of carbon steels

Abstract

In order to clarify the effect of defect orientation on the fatigue limit of two types of steels, JIS‐S15C and JIS‐S45C, a small semi‐circular slit was introduced onto the surface of a round specimen. The slit was tilted at 0 °, 30 ° or 60 ° with respect to the plane normal to the loading axis, but all of them had the same defect size, area = 188 µm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. In all the combinations of the materials and tilt angles, a non‐propagating crack was found at or just below the fatigue limit, that is, the fatigue limit was determined by the non‐propagation condition of a crack initiated from the defect. In both steels, the fatigue limit was found to be nearly independent of the tilt angle for the same value of projected size area, which was in good agreement with the prediction by the area parameter model. In this paper, a mechanistic explanation for the insensitivity of the fatigue limit to the tilt angle is proposed.

Very-high-cycle fatigue behavior of a structural steel with and without induced surface defects

“…These results are shown in Fig. 14 [34,[42][43][44] are reasonably consistent within a scatter band described by both models. The scatter band of Fig.…”

“…Based on Eqs. (3) and (4), we calculated the values of fatigue limit (fatigue strength at 10 7 cycles) of the present case and several cases in literature [34,[42][43][44], and the calculated values were normalized by experimental data. These results are shown in Fig.…”

“…The research by Li et al [32] demonstrated that surface grinding defect induced crack initiation was the predominant failure mode of the steel in VHCF regime. A series of experimental research by Murakami [33] and Lorenzino et al [34] investigated the effect of surface defects on the fatigue strength at 10 7 cycles with rotary bending fatigue and proposed a formula for predicting the fatigue strength based on a large number of experiments:…”

Evaluation of effective stress along the border of lateral notches

“…In a recent paper, Lorenzino et al 9 considered the problem of small tilted sharp defects under fatigue loadings. They observed that, for tilted defects, the cracks did not initiate at the notch tip but at a certain distance from the tip, whereas, when the crack was perpendicular to the load direction, the nucleation zone agrees exactly at the notch tip.…”

“…U-notches of various shapes and under different load directions have been studied over recent years, 21,22 nevertheless the Brazilian disk has become one of the most useful tools for studying this kind of problem in a simple way. First of all, some experimental cases recently presented by Lorenzino et al 9 were analysed according to the implicit gradient (IG) theory. 15,25 The aim of this paper is to analyse when cracks can nucleate in a place near the notch tip in a zone where the stress is not singular.…”