Reliability Control of Fishplate and Bolt Connection
What type of steel is usually used for fish plates? Why is "synergy between toughness and strength" emphasized?
Fish plates are mostly made of high-strength alloy steel, such as steel containing manganese, silicon and other elements, and their tensile strength needs to match the rail base metal, usually not less than 880MPa. Emphasizing "synergy between toughness and strength" is because fish plates need to bear tensile, compressive and bending loads transmitted by rails simultaneously. High strength ensures their load-bearing capacity, while high toughness prevents brittle fracture under impact loads. The instantaneous impact generated when trains pass through joints requires toughness to absorb energy and prevent cracks. If only strength is emphasized while toughness is insufficient, fish plates are prone to fatigue cracks under repeated loads, leading to joint failure. This synergistic performance is the core principle for fish plate material selection.
What is the main failure mode of bolt fracture? What factors can cause this failure mode?
The main failure mode of bolt fracture is brittle delayed fracture, which is characterized by slow crack initiation and final fracture under stress lower than the yield strength. Improper control of heat treatment process leading to excessively high surface hardness is an important factor causing this failure, which will significantly reduce the crack resistance of bolts. Poor cleanliness of raw materials with defects such as inclusions will become crack initiation points and accelerate the fracture process. Cracks in the under-head fillet processing or incomplete metal flow lines will cause stress concentration and promote fracture. In addition, residual stress caused by excessive torque during installation will also increase the risk of brittle delayed fracture.
What are the requirements for the fitting accuracy between fish plates and rails? What consequences will poor fitting cause?
The flatness error of the fitting surface between fish plates and rails must be ≤ 0.5mm to ensure uniform stress on the contact surface; the position deviation of bolt holes must be controlled within ± 0.3mm to ensure symmetrical stress after bolt installation. Poor fitting will cause stress concentration at the joint, making the fish plate bear excessive local load and accelerating wear and fatigue. When trains pass, the fitting gap will generate additional vibration and impact, intensifying damage to wheel-rail and joint components. Long-term poor fitting will also lead to gauge expansion, affecting the stability of track geometric position and increasing driving safety hazards. In addition, impurities and moisture are easy to accumulate in the gap, accelerating component corrosion and shortening service life.
How to control the preload of bolts through installation process? What are the hazards of insufficient preload?
During installation, a torque wrench must be used to set the standard torque value according to the bolt specification, and the preload must be accurately controlled by controlling the torque to ensure consistent preload of each bolt. The symmetrical tightening sequence must be followed to avoid uneven preload caused by one-sided stress. Bolts can be lubricated before installation to reduce the impact of frictional resistance on preload. Insufficient preload will cause loose fitting between fish plates and rails, loose joints, and inability to effectively transmit loads. When trains pass, joints produce relative displacement, intensifying component wear and even causing bolts to loosen and fall off. Insufficient preload will also reduce the overall stiffness of the joint, affect track smoothness, and generate abnormal noise and vibration.
What special requirements does heavy-haul railways have for the corrosion resistance of fish plate bolts? How to achieve it?
Heavy-haul railways require that the surface of fish plate bolts has no red rust after salt spray test, and the anti-corrosion coating has strong adhesion and is not easy to peel off to resist erosion from harsh environments such as humidity and dust. Galvanizing process is usually adopted to form an anti-corrosion coating with thickness ≥ 50μm on the bolt surface, isolating water from oxygen contact. Some high-standard scenarios will use stainless steel composite materials to fundamentally improve corrosion resistance. Anti-corrosion grease can be applied to the bolt threads, which not only enhances the anti-corrosion effect but also facilitates subsequent maintenance and disassembly. In addition, the uniformity of the anti-corrosion coating must be controlled to avoid local thin spots becoming corrosion breakthroughs. These measures can significantly extend the service life of bolts in heavy-haul environments.