对压力容器与压力管道用钢Q345R在低周疲劳下微孔（?40～200μm）的裂纹萌生与扩展规律进行了研究。研究表明小裂纹的萌生主要机理为滑移带启裂，并且由剪应力起主导作用。微缺陷的尺寸、应力幅等因素对疲劳寿命均有影响显著，当应力幅值较低时，微孔的尺寸对疲劳寿命有明显影响。当应力幅值水平较高时，小孔直径对疲劳寿命的影响则不敏感。微观缺陷尺寸存在临界值，当缺陷尺寸大于临界值时，疲劳寿命下降很快。在同一应力幅水平下，裂纹萌生寿命与疲劳总寿命的比值（Nt/Nf）与微孔尺寸没有关系，本试验的低周疲劳下约为10%～25%。
The crack propagation and initiation of small crack and the determining of the size effects of small defects in lower cyclic fatigue lives of steel specimens were investigated. The specimens chosen was Q345R steel,normally for fab-ricating pressure vessels and pipes in china,with laser hole of different sizes on specimens surface,say from ?40-200 μm in diameter. The size of small defect affects the fatigue life apparently,but the effect weakens with increasing stress range level. It shows that one types of crack initiation character at the surface edge of small hole are possible. One is caused by slip bands in which the shear stress dominates. In certain stress range,there is a critical value of the size of small crack, and beyond which,the fatigue life will drop highly. The percent between crack initiation life and total life(Nt/Nf)has nothing to do with the small hole size under the same stress range and be about 10%-25%.
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