Under the continuous loading of cyclic loading and unloading, the slip band around the crack tip deforms, resulting in the rapid deformation of the crack. In the process of deformation, the crack surface is formed by shear debonding. It can be seen that under the premise of determining the stress ratio and microstructure in cyclic loading, Material properties play a decisive role in the micro mode of fatigue crack growth.
In the micro environment, the stable fatigue crack growth zone of 42CrMo steel compact tensile specimen under normal stress is observed. At this stage, when the stress intensity factor is at a low level, the size of the plastic zone at the crack tip is very small, compared with the size of several grains. At this time, the direction of the main slip system is the main direction of crack propagation, and the crack propagation is carried out in the form of pure shear. Slip cannot be transferred under some conditions: different grain orientations and grain slip orientations lead to the accumulation and increase of dislocations at the grain boundary, resulting in stress concentration. With the application of continuous load, dislocations continue to increase, and corresponding slip will occur at this time, and the stress concentration will be released. When dislocations move out of a grain, cracks will continue to grow, The formation has obvious cleavage fracture characteristics. The cleavage surface is (001) and (110) with the lowest binding energy in BCC. The tearing edges containing cleavage steps connect small planes and facets with high reflection ability. The small plane with high reflection ability is composed of lamellar structure, and the secondary crack occurs at the edge of the lamellar, indicating that the twin boundary can provide a place for crack initiation. The main crack propagates through the aggregation and confluence of secondary cracks.
Fatigue striation can not be clearly observed. 42CrMo steel belongs to body centered cubic crystal structure, with large stacking dislocation energy and many slip systems. Under load, slip occurs more easily. The crack propagates along the (001) and (110) slip planes with the lowest binding energy in the grain. When it meets the grain boundary, the direction changes slightly, showing flatness, smoothness Strong reflection ability. When the stress intensity factor is at a high level, the fatigue propagation enters the second stage, and the crack propagation is carried out by double slip mechanism. At this time, the number of grains crossed by the plastic zone at the crack tip is more than that at a low level. At this stage, normal stress has a significant effect on crack propagation. The most important microscopic feature at this stage is fatigue band, and the surface of this area is relatively smooth. The SEM images of the sample at different magnification in the fatigue fracture stable growth area are shown in Fig. 1. It can be seen that cleavage fracture occurs in the stable growth area, showing clear characteristics, and its appearance is river like pattern.
