Xu Changxing, Liang Yilong, Yang Ming, Yu Jiabang, Peng Xiang
College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China.
Key Laboratory for Mechanical Behavior and Microstructure of Materials of Guizhou Province, Guiyang 550025, China.
Materials (Basel). 2021 May 14;14(10):2565. doi: 10.3390/ma14102565.
In this paper, the effects of the fatigue crack initiation position (FCIP) on fatigue life are discussed. Different modified gradient fields (MGFs) are prepared on the surface of 51CrV4 spring steel components by an ultrasonic assisted surface rolling process (USRP). Subsequently, the fatigue behaviour of steels with different FCIPs is systematically studied. The results show that the fatigue life of steels first exhibits an increasing tendency and then a decreasing tendency with increasing distance between an FCIP and the surface. When an FCIP shifts from the surface of the sample to the interior, the fatigue crack initiation resistance on the interior is greater than that on the surface, which leads to an increase in fatigue life. However, when the FCIP further shifts towards the centre of the specimen, the stress triaxiality experienced by the fatigue source gradually increases, which results in a peak in the curve of FCIP versus fatigue life. The magnitude of this peak fatigue life is related to the change in the stress triaxiality. Moreover, according to focused ion beam-high-resolution transmission electron microscopy (FIB-HRTEM) microstructural analysis near FCIPs, under a higher stress triaxiality, the crack tip area is subject to greater stress constraints, making the multiplication and movement of dislocations in this area more difficult, resulting in the decrease in movable dislocation density. This decrease in dislocation density leads to an increase in the stress concentration and accelerates the crack growth rate, decreasing the fatigue life. Therefore, the significant change in fatigue life is controlled by the MGF and stress triaxiality.
本文讨论了疲劳裂纹萌生位置（FCIP）对疲劳寿命的影响。通过超声辅助表面滚压工艺（USRP）在51CrV4弹簧钢构件表面制备了不同的改进梯度场（MGF）。随后，系统研究了不同FCIP位置的钢材的疲劳行为。结果表明，随着FCIP与表面之间距离的增加，钢材的疲劳寿命先呈现增加趋势，然后呈现下降趋势。当FCIP从试样表面向内部移动时，内部的疲劳裂纹萌生抗力大于表面，这导致疲劳寿命增加。然而，当FCIP进一步向试样中心移动时，疲劳源所经历的应力三轴度逐渐增加，这导致FCIP与疲劳寿命关系曲线出现峰值。该峰值疲劳寿命的大小与应力三轴度的变化有关。此外，根据FCIP附近的聚焦离子束-高分辨率透射电子显微镜（FIB-HRTEM）微观结构分析，在较高的应力三轴度下，裂纹尖端区域受到更大的应力约束，使得该区域位错的增殖和移动更加困难，导致可动位错密度降低。位错密度的这种降低导致应力集中增加，并加速裂纹扩展速率，从而降低疲劳寿命。因此，疲劳寿命的显著变化受MGF和应力三轴度控制。
