Study of Relation between Stress-Strain Response and Fatigue Strength of Metal under Random Loading Conditions (1st Report)

Abstract

In order to improve an estimation method of fatigue strength, the fatigue strength is related to stress-strain response under cyclic loading conditions. The materials used in this study were both three kinds of base metals (mild steel, conventional high tensile steel and thermo mechanical controlled process steel) and those synthetic heat-treated metals whic are heated with simulated actual weld heating process. Static tension and axial cyclic loading tests were performed on hourglass type specimens in order to obtain the characteristics of stress-strain response such as hardening, softening, hysterisis energy, Baushinger effect and fatigue strength. The relationship between the parameters of stress-strain response and fatigue strength was examined. The main results obtained in this study are as follows. 1. The characteristics of stress-strain response vary as the number of loading cycle proceeds, and differ from those under static loading condition. 2. The elastic limit stress (yield stress) under cyclic loading conditions is lower than under satic tension test for each materials, that is to say, the softening appears during cyclic loading. 3. There is a close correlation between 0.2% proof stress under cyclic loading condition and the fatigue strength of mother metals, while the fatigue strength of synthetic heat-treated metals does not increase being proportional to the increase of 0.2% proof stress. 4.The hysterisis energy per cycle during cyclick loading can be considered to be a useful parameter for an estimation of fatigue strength.