According to the available data, the design of the rail cross-section is flawed, with the rail web being excessively thin, which significantly contributes to bolt hole cracks. The thickness of the rail web is only 33 mm, a dimension that proves insufficient under the repeated stress from wheel-rail contact. This leads to fatigue damage over time, ultimately resulting in the formation of cracks around the bolt holes.
Another contributing factor is the wedge-type joint connection used in China’s railway system. This type of joint can induce upward tensile stress on the rails when the bolts are overly tightened. Based on stress analysis of round bolt holes, this setup increases the likelihood of oblique cracks forming between the holes, especially in the first bolt hole at the rail end. In practice, if maintenance of these joints is neglected, the bolt torque may not meet the required standards, leading to reduced joint rigidity. As a result, the two ends of the rail cannot share the load evenly, causing dynamic irregularities that accumulate over time into permanent geometric misalignments.
Under the repeated impact of train wheels, the interaction between the fishplate and the bolt hole intensifies, increasing the risk of crack propagation. Additionally, if the bolt torque is too low, excessive slippage occurs between the rail and the fishplate as trains pass through. This causes the bolts to rub against the edges of the holes, leading to plastic deformation of the hole walls and the development of stress concentrations—factors that further contribute to cracking.
In northern China, where temperatures are particularly low, the cold climate causes the rails to contract. This reduces the gap between the rail and the fishplate, increasing the force exerted on the hole walls and making cracks more likely. Moreover, there are inherent weak points in the bolt holes, such as cracks, burrs, or sharp corners. If these defects are not properly addressed during replacement, they can become focal points for stress concentration. Under continuous train loading, this can eventually lead to severe damage to the rail bolt holes.
Additionally, there are other weak areas in rail joints, including low joints, misaligned teeth, blocked rail surfaces, wear, ballast plate knots, and dark pits. These issues create unevenness on the rail head surface. When subjected to the dynamic impact of train wheels, the forces acting on these weak points increase, accelerating damage to the bolt hole walls and compromising the overall integrity of the track.
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