CrNiMo钢缺口根部疲劳裂纹萌生的研究

本文采用单边缺口板状试样研究了四种含碳量的CrNiMo钢缺口根部的疲劳裂纹萌生寿命及其萌生机制。裂纹萌生可能有四种方式—沿驻留滑移带压入挤出、微孔聚集型切变、准解理和沿晶机制。文中论述了钢的含碳量、热处理规范及外加应力水平对裂纹形核机制的影响。研究表明:在双对数坐标中,表观循环应力范围和萌生寿命的关系可用两段不同斜率的直线来描述,两段萌生线的分界点随钢的强度水平提高而迅速向低周次范围推移,但两段的裂纹萌生均仍属于应力疲劳范畴。文中分析了微观塑性变形对缺口根部实际循环应力范围和应力循环比的影响,给出了对它们进行修正估算的方法。修正后的实际循环应力范围和萌生寿命的关系可用一条直线反映出来。在此基础上,提出了一种用光滑试样应力疲劳曲线估算缺口根部裂纹萌生寿命的新方法。

A Study of Fatigue Crack Initiation at Notch of CrNiMo Steels

Fatigue crack initiation has been studied in four CrNiMo steels with different carbon contents using single edge notched plate specimens. Scanning electron microscopic examinations revealed that four mechanisms involved in the nucleation of fatigue cracks, they were intrusions-extrusions, transgranular, shear deformation andintergranular mechanisms. The effects of heat treatments and carbon contents of steels on the mechanism of formation of individual micro-cracks were discussed. It was shown that the correlation between the nominal stresses amplitude and crack initiation lives can generally be illustrated by two straight life lines with different slopes in a log-log coordinate. An analysis was made of the effects of micro-deformation near a notch on the real cyclic stress amplitude and cyclic stress ratio at the notch, and a correction method for the nominal stress was developed. The variation of real cyclic stress amplitude with the crack initiation life was found to be well expressed by a single straight life line. A new estimate method of crack initiation life at a nocth based on the concept of micro-deformation was presented. The calculated initiation lives agreed well with the experimental results