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

为了得到应力应变关系，对母材用低碳钢、常规高强钢、热控工艺钢3种钢和热处理材料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与静态加载条件下的应力应变关系不同。研究了硬化、软化、弹塑性过渡应力值、包辛格效应等响应特性。随着加载循环次数的增加而变化，在一定的加载循环次数后，应力-应变关系保持稳定的特征。研究了恒幅加载疲劳试验得到的应力应变关系参数与疲劳强度之间的关系。本研究的主要结果如下：1)本研究试验的母材试件在循环加载和随机加载条件下的极限应力均较静载条件下降低，且循环加载过程中出现软化现象。热处理试样在循环加载条件下，弹性极限应力增大，在循环加载过程中出现硬化现象。2)循环加载条件下的0.2% proof应力与疲劳强度密切相关。在循环加载条件下，可以将0.2%的证明应力作为估计材料疲劳强度的参数之一。3)可以认为，循环弹性极限应力与抗拉强度越接近，循环应变硬化指数n越小的材料，其疲劳强度越好。

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.