The bolt fracture, as observed from the fracture location and the appearance of the break, indicates a typical brittle fatigue failure. There is no visible plastic deformation at the fracture surface, and the crack initiation site is not clearly identifiable. The fracture occurred when the crack had grown to approximately 65% of the cross-sectional area, leading to sudden failure. Notably, the fractured section was not the smallest part of the bolt, which suggests that the failure was not caused by an overload condition. If the bolt had failed due to excessive loading, the break would most likely have occurred at the narrowest point of the bolt.
The connecting rod bolt is subjected to two main forces: the initial pre-tensioning force and the alternating tensile stress caused by the piston during the gas compression cycle. This means the bolt experiences cyclic loading, making it susceptible to fatigue failure. Any imperfections such as burrs, tool marks, micro-cracks, or residual stresses introduced during manufacturing can act as initiation sites for cracks under repeated stress. These flaws may grow over time and eventually lead to catastrophic failure.
The threaded portion of the bolt is machined using a lathe, and during this process, small defects like burrs, scratches, sharp edges, undercuts, and micro-cracks are common. These imperfections are often difficult to detect through magnetic particle or dye penetrant testing, yet they serve as stress concentration points. Under alternating loads, these areas are more prone to crack propagation. Additionally, the load distribution along the threads is uneven—approximately one-third of the load is carried by the first thread, while the eighth thread carries almost no load at all. This explains why bolts frequently fail near the first thread, and our case is no exception.
To further investigate the failure, we conducted hardness testing and metallographic analysis on the fractured bolt. The measured hardness was HB292, and the microstructure showed tempered sorbite ferrite with grain size ranging between 7 and 8. This confirms that the material was properly heat-treated but still failed due to fatigue-induced crack propagation originating from processing defects.
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