The fracture of the bolt, from the fracture position and the shape of the fracture, can be seen that the fracture of the bolt is a typical fatigue brittle fracture. There is no obvious plastic deformation of the fracture, and the source of the crack is not obvious. From the fracture, the bolt first expands to 65% of the entire section and the bolt suddenly breaks. The fracture part is not the smallest section of the bolt, so it can be concluded that the bolt is not subjected to excessive load and the fracture is caused. If the bolt is broken due to excessive load of the bolt, the broken part of the bolt should be the smallest section of the bolt. The force received by the connecting rod bolt is the initial pre-tightening force of the bolt and the piston force of the connecting rod during the gas compression process, that is, the load on the connecting rod bolt is the alternating tensile stress, and the stress moment is changing. Since the bolt is subjected to an alternating load, its damage is mainly fatigue damage. When there are defects such as burrs, knife marks, micro-cracks, internal stresses, etc. during the processing of the bolts, these defects will develop into cracks under the alternating load, and will expand rapidly, and eventually break suddenly due to unbearable load. The bolt thread is made of a turning tool. Micro-defects such as burrs, scratches, scratches, sharp corners, undercuts and micro-cracks are inevitable in the threaded parts during processing.
These defects are not easily found or neglected in magnetic powder or stain detection. These defects are stress concentration points, and it may develop into cracks when the bolt is subjected to alternating loads. The load on the threaded part of the bolt is not uniform. The first thread of the thread and the nut is about 1/3 of the load, and there is almost no load on the eighth ring. It can be seen why the bolts are often broken near the first ring and our bolts are also broken near the first ring. For the bolts after the fracture, we also made hardness analysis and metallographic analysis, the hardness was HB292; the metallographic structure was tempered sorbite ferrite, and the crystal grain thickness was 7-8.
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