低周疲劳下16MnR微孔的裂纹萌生与扩展

Crack Initiation and Propagation of Small Defect in Low Alloy Steel

采用宏观与微观结合的方法,在疲劳试验机上进行疲劳试验,在光学显微下观察裂纹的起裂与扩展。研究了压力容器用钢16MnR在低周疲劳下微孔（Ф40-200μm）的裂纹萌生与扩展规律。研究表明裂纹的萌生机制：滑移带启裂和疏松带启裂,前者由剪应力起主要作用,后者由正应力起主要作用。而滑移带的局部性和裂纹开叉是低周疲劳下微裂纹的两大典型现象。微观缺陷尺寸、应力水平对疲劳寿命有显著影响,当应力水平较低时,微孔尺寸对寿命的影响明显。而当应力水平较高时（超过屈服极限）,孔径对寿命的影响不敏感。在同一应力水平下,微缺陷尺寸存在临界值dt,当d〉dt时,疲劳寿命下降很多。

Crack initiation and growth from small hole and the determining of the size effects of small defects in low cyclic fatigue lives of steel bar were investigated. The specimens chosen was 16MnR steel, normally for fabricating pressure vessels in china, with drilled hole of different sizes on surface, say from 40, 80, 100, 200μm in diameter. It has been observed that one types of crack initiation mechanism at the edge of surface small hole are possible. One is caused by slip bands in which the shear stress dominates, while the other is caused by so-called dispersing initiation in which the normal stress dominates. The size of small defect affects the fatigue life apparently, but the effect weakens with increasing stress level. In certain stress level, there is a critical value of the size of small defect, and beyond which, the fatigue life will drop greatly.