Factors affecting the fatigue life of bolts and measures to improve them
- What are the main factors affecting the fatigue life of bolts?
Stress amplitude is the core factor. When the stress amplitude increases by 10MPa, the fatigue life may be shortened by 30%~40%. The stress amplitude of heavy-duty railway bolts must be controlled within 80MPa, and that of ordinary railways must be ≤100MPa to avoid premature fracture. Surface roughness has a significant impact. When Ra increases from 1.6μm to 6.3μm, the fatigue life decreases by 20%~30%. The bolt surface needs to be rolled to reduce the roughness to Ra≤0.8μm, forming residual compressive stress and improving fatigue resistance. If the fillet radius of the thread root is too small (<0.3mm), the stress concentration factor will increase to 2.0~2.5, and the fatigue life will be shortened by 50%~60%. The fillet radius needs to be increased to ≥0.5mm, and the requirements for high-speed railway bolts are more stringent (≥0.8mm). Insufficient material purity and non-metallic inclusions (diameter > 50μm) will become the source of fatigue cracks, reducing fatigue life by 15%~20%. High-quality steel (such as 40CrNiMoA) should be selected to control the inclusion content.
- What is the influence of preload on fatigue life of bolts?
Insufficient preload (<70% of the design value) will cause the bolt to bear additional lateral force, increase stress amplitude by 20%~30%, and shorten fatigue life by 40%~50%. The preload of ordinary railway bolts must be ≥80% of the design value, and ≥90% for high-speed railways. Excessive preload (>110% of the design value) will increase the risk of bolt yield. After 1 million cycles, the preload attenuation reaches 25%~30%, which will cause plastic deformation of the thread and reduce fatigue life by 15%~20%. The preload should be controlled at 85%~105% of the design value. Preload fluctuations exceeding ±10% will cause the fatigue life of the same batch of bolts to differ by 30%~40%. A torque wrench is required for precise control. After installation, 10% of the bolts should be sampled to ensure uniform preload. Properly increasing the preload (90%~100% of the design value) can reduce the stress amplitude by 10%~15% and extend the fatigue life by 20%~30%. This is the "preload strengthening effect", and this strategy is often used for heavy-duty railway bolts.
- What is the effect of bolt surface treatment on fatigue life?
Carburizing treatment can make the surface hardness of the bolt reach HRC55~60, form residual compressive stress (-200~-300MPa), and extend the fatigue life by 50%~60%. However, excessive carburizing layer thickness (>0.5mm) will increase brittleness and easily break in low temperature environments. The thickness needs to be controlled at 0.2~0.4mm. The fatigue life of galvanized bolts (thickness 8~12μm) is 10%~15% lower than that of untreated bolts. Because the zinc layer may produce microcracks, a hydrogen-free galvanizing process is required to reduce the risk of hydrogen embrittlement. Bolts in coastal areas must be galvanized for corrosion protection. Shot peening forms residual compressive stress by impacting the surface with projectiles, which increases fatigue life by 30%~40%. The projectile diameter is 0.2~0.3mm and the strength is 0.2~0.3mmA (Almen test piece). Ordinary railway bolts can be treated in this way. Phosphating can improve thread lubrication, reduce surface damage during installation, and indirectly increase fatigue life by 10%~15%. The thickness of the phosphating film is 5~10μm and needs to evenly cover the thread surface.
- How to detect the fatigue performance of bolts?
The rotating bending fatigue test is a standard method. The bolt specimen is subjected to alternating stress (stress ratio R=0.1) on a fatigue testing machine, and the number of cycles at fracture is recorded. A 10.9-grade bolt must pass 2 million cycles without fracture, otherwise it is judged as unqualified. The axial tensile fatigue test simulates the actual stress state, applies axial alternating load (stress ratio R=0.5), and measures the fatigue limit. The fatigue limit of 8.8-grade bolts must be ≥350MPa. Bolts below this value cannot be used for heavy-duty railways. Ultrasonic flaw detection detects internal defects. If inclusions or cracks with a diameter of ＞0.2mm are found, the bolts must be removed to avoid becoming fatigue fracture sources. High-speed rail bolts require 100% flaw detection. On-site tracking statistics are conducted to record the actual service life of the bolts and compare them with the design life. If the average service life is less than 80% of the design value, the reasons need to be analyzed and the production process needs to be improved.
- What are the specific measures to improve the fatigue life of bolts?
Optimize the thread design, use a large fillet root (radius ≥0.5mm) and a fine thread (pitch 2mm), reduce stress concentration, and increase fatigue life by 20%~30%. This design is commonly used in high-speed rail bolts. Surface strengthening treatment, rolling + shot peening composite treatment of the bolts, the surface roughness is reduced to Ra≤0.4μm, and residual compressive stress is formed at the same time, the fatigue life is extended by 50%~60%, the cost increases by 10%~15%, but the cost performance is high. Control the accuracy of the preload force, use an intelligent torque wrench (accuracy ±3%) for installation, ensure that the preload force deviation is ≤5%, and the fatigue life is increased by 15%~20% compared with ordinary wrenches. It must be used in key parts of heavy-duty railways. High-strength alloy steel (such as 10.9 grade 42CrMo) is selected, and its fatigue strength is 40%~50% higher than that of ordinary carbon steel. With strict heat treatment process, the fatigue life of the bolts can reach more than 3 million times, meeting the long-term use requirements of high-speed railways.