Fig. 1 shows 150, 300, 500 and 1000 times SEM images of fatigue fracture in the instantaneous fracture zone. The plastic brittle mixed fracture morphology of dimple structure and tearing edge can be clearly seen. Initial dimples can be seen on the fracture surface of the material. Dimples gather together after growth. When the material breaks, they will appear. After the fatigue crack enters the rapid growth stage, the sample is equivalent to the action of static tensile stress, there is no crack opening and closing, and the crack continues to grow rapidly.
Fatigue is easy to occur at the defect on the surface of the specimen or at the place subject to the maximum stress. Repeated slip must occur. It will not be disturbed by the presence or absence of defects. It will be accompanied by slip in the process of crack initiation. The sliding process consumes much time without defects.
Figure 2 (a) shows the initiation and propagation of secondary cracks. The black goose egg shaped dots shown in the figure are granular slag inclusion metallurgical defects. The crack is derived from the defect and then propagates in the same direction as the main crack. In the first 20 um of propagation, the secondary crack tends to grow away from the main crack, but the crack width decreases gradually. At the 20um of the original crack, the crack tends to close. A new crack starts again from the end of the crack. The propagation direction is the same as that of the main crack, but the distance from the main crack gradually decreases and finally merges into the main crack. Fig. 2 (b) shows the secondary crack near the main crack, which propagates in the same direction as the main crack. The crack propagation length is about 10 μ At m, the original crack is divided into two cracks and propagates respectively. One of the propagation directions is still the same as that of the main crack, but the distance from the main crack increases gradually. Because it is far away from the main crack, it consumes energy and can not be supplemented in the process of propagation, and finally stops propagation, but its end still tends to expand close to the main crack; The propagation direction of the other is the same as that of the main crack. However, in the process of propagation, the distance from the main crack gradually decreases, the width of the crack gradually increases, and finally into the main crack. The distance from the main crack is about 20 μ At the point indicated by the arrow of M, 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 used to provide its initiation and propagation is insufficient, so it has not been expanded.
It can be seen from the secondary crack originated at the inclusion. The inclusion will lead to stress concentration due to the following reasons: inconsistent thermal shrinkage coefficient with the matrix, mismatch of elastic modulus with the matrix, and interaction of residual stress. Secondary crack is the fracture caused by the instability of the inclusion itself under the action of external force, which affects the fatigue life of the structure.