Fracture toughness and safety of fishplate
- What are the requirements for the fracture toughness index of fishplates?
The fracture toughness KIC of 45# steel fishplates should be ≥60MPa·m¹/², and that of Q345 steel fishplates should be ≥80MPa·m¹/², ensuring that cracks will not expand rapidly to cause fracture when there are small cracks. In low temperature environment (-20℃), the fracture toughness index must be increased by 10% to cope with the impact of low temperature brittleness. The fracture toughness of fishplates should be uniform, and the difference in the same batch should be ≤5%.
- What factors will lead to the decrease of fishplate fracture toughness?
Non-metallic inclusions in the material (such as sulfides and oxides) will reduce the fracture toughness, and inclusions with diameter ≥50μm can reduce the fracture toughness by 15%-20%. Improper heat treatment, such as excessive tempering temperature, will reduce the hardness and strength of the material, and the fracture toughness will also decrease by 10%-15%. Surface cracks and scratches generated during processing will become stress concentration points, leading to a significant decrease in fracture toughness.
- What safety hazards will insufficient fracture toughness of fishplates cause?
Under the action of train load, small cracks will expand rapidly, which may cause sudden fracture of fishplates in a short time, resulting in separation of rail joints and train derailment accidents. Fishplates with insufficient fracture toughness are more dangerous in low temperature environment, and may undergo brittle fracture without obvious signs, bringing serious safety risks to railway operation.
- How to test the fracture toughness of fishplates?
Three-point bending test is adopted, prefabricated fatigue cracks on the sample, then apply load until fracture, and calculate the fracture toughness KIC value. During detection, the temperature should be controlled at 20℃±2℃ to ensure the accuracy of test results. For each batch of fishplates, 5% are sampled for testing. If one is unqualified, the sample size should be doubled, and if there are still unqualified, the whole batch will be rejected.
- What are the differences in requirements for fishplate fracture toughness among different railway types?
High-speed railways and heavy-haul railways have the highest requirements for fishplate fracture toughness. The KIC of 45# steel fishplates should be ≥65MPa·m¹/², and that of Q345 steel should be ≥85MPa·m¹/² to cope with high stress caused by high speed and heavy load. For ordinary railways, 45# steel fishplates with KIC ≥60MPa·m¹/² and Q345 steel with KIC ≥80MPa·m¹/² can meet the requirements. Urban rail transit has lower requirements due to smaller load, but KIC should also be ≥55MPa·m¹/².