Zenji Ando

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments

Abstract Fatigue tests under rotating bending and reversed torsion were carried out in air, distilled water and 3% saltwater, using smooth specimens of high-strength low alloy steel (Cr-Mo steel). The initiation and growth behavior of small fatigue cracks in each environment were evaluated based on detailed observations, and the effects of corrosive environment were also discussed. The fatigue strength decreased with increasing aggressiveness of test environment. The decreases in corrosive environment were due to earlier fatigue crack initiation. From the observed locations at which small fatigue cracks began, it was considered that the crack initiation was primarily governed by hydrogen embrittlement in distilled water and also affected by corrosive dissolution in 3% saltwater. The validity of the application of linear fracture mechanics for small fatigue cracks was established. The growth rates of small fatigue cracks were higher than for large through cracks, and not accelerated by the corrosive environment. Moreover, fatigue life in the corrosive environment was estimated by using the crack growth characteristics in air.

The fatigue crack retardation of a low alloy steel in saltwater.

Fatigue crack retardation behaviour was investigated in saltwater using two materials of a low alloy steel, SCM435, whose strength or susceptibility to hydrogen was changed by heat treatment, and was compared with the data in laboratory air. The results show that there exists a significant difference between the overload affected zone sizes in saltwater and in air, depending on both the strength of the material and the baseline stress intensity level. The decreased overload affected zone size in lower strength material is related to a rapid resumption of the crack propagation rate after the minima, due to hydrogen embrittlement (HE) and corrosive dissolution. In higher strength material, in addition to a rapid resumption of the crack propagation rate due to the extreme HE, the overload affected zone size is also affected by the existence of SCC crack propagation caused by the overload application because of overload stress intensities above the KI SCC value of this material.

A static and a dynamic stress intensity factor of a lug with cracks.

A simple procedure for obtaining a stress intensity factor using FEM proposed by the authors was applied to analyze the problem of a lug with cracks. First, to confirm its usefulness for the present method, a problem for a rectangular plate with an edge crack subjected to three point bending was calculated by the present method. The present results were compared with present photoelastic experimental results or usual results. Next, a dynamic calculation for the same problem and a problem for a lug with cracks were carried out by the present method, and the results were compared with the present experimental results using a strain gaga method. Consequently, as the usefulness of the present method was recognized, the static and the dynamic stress intensity factor of the lug with cracks was investigated by the present method varying a pin hole diameter or a crack length systematically. Furthermore, the relation between the static stress intensity factor and the dynamic one was presented.