Study of Stress-Strain Relation and Fatigue Strength of Steel under Random Loading Conditions

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Inorder to obtain the stress-strain relation, axial static and cyclic loading tests were performed for three kinds of steels (mild steel, conventional high strength steel, thermo-mechanical controlled process steel) for mother materials and heat-treated materials. The stress-strain ralation under cyclic-loading conditions and random loading conditions differs from the one under static loading condition.The characteristics of the response such as hardeing, softening, elastic-plastic transition stress value, Bauschinger effect, ets.vary as the number of loading cycles proceeds, and after a certain number of loading cycles, the stress-strain relation preserves stable characteristics. The relationship between parameters of the stress-strain relation and the fatigue strength obtained from constant amplitude loading fatigue tests was investigated.The main resuls obtained in this research are as follows.1) For mother material specimens tested in this research, the clastic limit stress under cyclick-loading condition and random loading conditions decreases as compared with one under ststic-loading condition, and softening appears during cyclic-loading tests. For heat-treated specimens, the elastic limit stress increases under cyclic-loading condition, and hardening apppears during cyclic-loading tests.2) There is a close correlstion between 0.2% proof stress under cyclic-loading condition and the fatigue strength. We can regared 0.2% proof stress under cyclic-loading condition as one of parameters which estimate the fatigue strength of material.3) It can be considered that materials, of which cyclic elastic limit stress is closer th the tensil strength and cyclic strain-hardening exponent n is smaller, are good for fatigue strength.