The macro morphology of fatigue fracture of low speed and heavy load gear teeth can be divided into three obvious regions, namely, crack source region, low speed crack propagation region and instantaneous fracture region. Among the three regions, the area of stable propagation region accounts for a larger proportion, the macro morphology is relatively flat but contains the most information, and the macro morphology of rapid propagation region is relatively rough.
In the micro environment, it is observed that the stable expansion zone of fatigue fracture of low speed and heavy load gear after bending fatigue failure has obvious river like pattern, which is the typical micro morphology of cleavage fracture, and the fatigue striation of fracture is not very obvious. This is because 42CrMo steel belongs to body centered cubic crystal structure, which has higher stacking fault energy and more slip systems, Therefore, it is easy to slip under the action of load, and the surface of this area is relatively smooth. (a) and (b) in Fig. 1 show the initiation and propagation of secondary cracks at 150 times and 1000 times, respectively. The main crack propagation direction is from left to right, and the secondary crack propagation direction is the same as that of the main crack. It can be seen that facets are connected with facets by tearing edges, which contain cleavage steps. The facet is composed of lamellar structure, and secondary cracks appear at the edge of the lamellar, which indicates that the plum grain boundary can provide a place for crack initiation. The secondary crack is initiated at the grain boundary. At the beginning of propagation, the propagation mode is intergranular propagation. This is because the energy of the newly initiated fatigue crack is relatively small. With the accumulation and convergence of the secondary cracks, the energy of the crack increases. At this time, the propagation mode will change from intergranular propagation to transgranular propagation The aggregation and propagation promote the propagation of the main crack. In the process of secondary crack growth, not all the cracks will gather and converge to promote the main crack growth, and some of them will close after a certain distance. (c) and (d) in Fig. 1 show the closure of the secondary crack at 150 times and 1000 times, respectively. It can be seen that before the closure of the secondary crack, the propagation path changes from transgranular propagation to intergranular propagation, and finally stops with the grain obstruction.
As shown in Figure 2, the micro morphology of the fatigue fracture in the instantaneous fracture zone is particularly rough, with an obvious plastic brittle mixed fracture morphology including dimple structure and tearing edge.
After the fatigue crack enters into the rapid growth stage, the gear teeth are equivalent to the static tensile stress, and there is no longer crack opening and closing, but the crack continues to expand rapidly, which leads to the large plastic deformation in a small range of materials, and then the micro cavity is produced, which is nucleated, grown and gathered, Finally, these equiaxed dimples with different shapes and sizes are formed by interconnection, which leave traces on the fracture surface after the tooth fracture. The propagation direction of the main crack is from left to right. The rapid propagation of the main crack is accompanied by the initiation and propagation of the secondary crack. (a) and (b) in Fig. 2 show that the morphology is relatively wide from the beginning of the secondary crack initiation, and the crack width increases from intergranular growth to transgranular growth. (c) and (d) in Fig. 2 show that with the rapid growth of the main crack, there is also the closure phenomenon of the secondary crack. In the final stage of the secondary crack growth, the crack growth width gradually narrows. When encountering the obstruction of the grain, the growth mode changes from the original transgranular tearing state to intergranular growth. When encountering the obstruction of the grain again, the crack stops growing.
Under the action of cyclic load, the fatigue crack on the gear teeth causes the fracture of the gear teeth, and the secondary crack initiation and propagation will be accompanied around the fatigue crack. In order to observe the propagation of the secondary crack near the main crack and analyze its influence on the propagation of the main crack, the secondary crack around the main crack on the tooth side was observed by jsm-it100 SEM produced by jeol. The results are shown in Figure 3. Due to the position of the sample, the main crack propagation direction is from bottom to top.
(a) in Fig. 3 shows the initiation and propagation of the secondary crack. The black goose egg shaped dots shown in the figure are granular metallurgical defects with slag inclusion. The crack is derived from the defect and propagates in the same direction as the main crack. In the first 20 um, the secondary crack tends to grow away from the main crack, but the width of the crack is smaller
The degree of stress decreased gradually. At the 20th um of the original crack, the crack tends to close. A new crack starts to propagate from the end of the crack, and the propagation direction is the same as that of the main crack, but the distance between the crack and the main crack decreases gradually, and finally merge into the main crack. (b) in Fig. 3 also shows the secondary crack near the main crack, which propagates in the same direction as the main crack. At the crack growth length of about 10 um, the original crack is divided into two cracks and propagates separately. The propagation direction of one of them is still the same as that of the main crack, but the distance from the main crack increases gradually. Because of its long distance from the main crack, it consumes energy and has no way to supplement in the process of propagation, and finally stops propagation, but its end still tends to grow closer to the main crack; The propagation direction of the other crack is the same as that of the main crack. However, in the process of propagation, the distance from the main crack decreases gradually, the width of the crack increases gradually, and finally it merges into the main crack. At the point indicated by the arrow about 20 um away from the main crack, there are also impurities in the metal and fatigue cracks around the impurities. However, due to its long distance from the main crack, the energy for its initiation and propagation is insufficient, so it has not been extended.
The macro and micro morphology of the fatigue fracture of the gear tooth was observed and analyzed by using the scanning electron microscope. It was found that the crack source area, the repeated opening and closing of the fracture, made the two sides squeeze and grind each other, and the area presented a relatively bright state; Secondary cracks appear at the edge of cleavage fracture surface in the stable growth zone, which indicates that the twin boundary provides a place for crack initiation. The secondary cracks often originate at the grain boundary and propagate along the grain boundary, and the grain has a blocking effect on the secondary crack propagation. The rapid propagation zone is a plastic brittle mixed fracture with dimples and tearing spindles, which is the result of the growth and aggregation of equiaxed dimples, and finally connected with each other. The secondary crack is relatively wide, and the propagation process also experiences intergranular propagation to transgranular propagation, and finally stops at the grain barrier. The inclusion metallurgical defects affect the isotropy of the material, During the loading process, stress concentration occurs, which leads to the appearance of secondary cracks, accelerates the propagation of fatigue cracks and provides a place for the initiation and propagation of fatigue cracks.