The bolt fracture, as observed from the fracture location and its appearance, indicates a classic case of brittle fatigue failure. There is no noticeable plastic deformation at the fracture surface, and the crack initiation site is not clearly visible. The fracture occurred when the crack had grown to about 65% of the cross-sectional area, leading to sudden failure. Importantly, the fractured section is not the smallest part of the bolt, which suggests that the failure was not due to an overload condition. If the bolt had failed under excessive load, the break would have occurred at the narrowest point of the bolt. The connecting rod bolt experiences two main forces: the initial pre-tensioning force and the piston force during gas compression. This results in alternating tensile stress, with fluctuating stress levels over time. Due to the cyclic loading, the primary mode of failure for bolts is fatigue. During the manufacturing process, small imperfections such as burrs, tool marks, micro-cracks, or residual stresses can occur. These flaws act as stress concentrators and can grow into larger cracks under repeated loading, eventually leading to sudden failure. The threaded portion of the bolt is machined using a turning tool, which often introduces minor defects like burrs, scratches, sharp edges, or undercut areas. These imperfections are common and can be difficult to detect through magnetic particle or dye penetrant inspection. However, they serve as potential sites for crack initiation under alternating loads. Additionally, the load distribution along the thread is uneven—approximately one-third of the total load is carried by the first thread, while the eighth thread carries almost no load. This explains why bolts frequently fail near the first thread, and our case is no different. To further investigate the failure, we conducted hardness testing and metallographic analysis. The measured hardness was HB292, and the microstructure showed tempered sorbite ferrite with grain size ranging between 7 and 8. These findings support the conclusion that the failure was caused by fatigue rather than overload, and that the presence of machining-induced defects played a critical role in the failure process.
Stainless Steel Valves,Stainless Steel Valve ,Stainless Steel Ball Valves,Ss Ball Valve
WENZHOU DIYE VALVE&FITTINGS CO.,LTD , https://www.diye-valve.com