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

为了得到母材和热处理材料的应力应变关系，对三种钢(低碳钢、常规高强度钢、热机械控制工艺钢)进行了轴向静载和循环加载试验。循环加载和随机加载条件下的应力应变关系不同于静态加载条件下的应力应变关系，硬化、软化、弹塑性转变应力值、包辛格效应等响应特征随着加载循环次数的增加而变化，经过一定次数的加载后，应力应变关系保持稳定。研究了恒幅加载疲劳试验的应力应变关系参数与疲劳强度之间的关系，得到的主要结果如下：1)对于本试验的母材试件，在循环加载和随机加载条件下的弹性极限应力比在静态加载条件下的要小，在循环加载过程中出现软化现象。对于热处理试件，在循环加载条件下，弹性极限应力增加，在循环加载试验中出现硬化现象。2)循环加载条件下0.2%的保证应力与疲劳强度密切相关。3)循环弹性极限应力越接近拉伸强度，循环应变硬化指数n越小，材料的疲劳强度越好。

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.