In bolted connections, there is a type of failure known as fatigue fracture. This fracture commonly occurs in vibrating installation environments and belongs to sudden failure modes like hydrogen embrittlement. Since current technology cannot predict fatigue fractures in advance, prevention must start from the initial design and manufacturing stages.
All bolts have finite service lives. Although bolts are reusable components, they cannot be used indefinitely. When bolts are subjected to prolonged overloading in certain environments, the probability of fatigue fracture increases significantly. Such failures can cause severe damage to production equipment and even lead to safety incidents.
1. Formation Mechanism of Fatigue Fracture
The widely accepted explanation for bolt fatigue fracture is:
Material mismatch between the bolt and mating components
Geometric variations in installed moving parts
Stress concentration from excessive pre-tension
Cyclic loading exceeding material endurance limits
The fracture process involves:
Micro-crack initiation at stress concentration points
Progressive crack propagation under cyclic loading
Sudden catastrophic failure at critical crack size
2. Key Influencing Factors
2.1 Mechanical Factors
Stress concentration at thread roots and underhead fillets
Magnitude and frequency of cyclic loading
Pre-tension force exceeding design limits
2.2 Environmental Factors
Extreme temperature variations (-40°C to 400°C)
Corrosive atmospheres (salt spray, acidic environments)
Vibration amplitudes >0.5mm
2.3 Material Factors
Inadequate strength-toughness balance
Improper heat treatment (e.g., over-tempering)
Surface defects from manufacturing processes
3. Prevention and Mitigation Strategies
3.1 Design Optimization
Radius thread roots (min. 0.1mm)
Underhead fillet radius ≥1.5mm
Use partial-thread bolts (unthreaded shank portion)
3.2 Process Improvements
Post-heat treatment thread rolling
Shot peening for residual compressive stress
Electroplating with hydrogen embrittlement relief
3.3 Operational Practices
Torque control within ±10% tolerance
Regular ultrasonic testing (every 5,000 cycles)
Replacement after 70% predicted fatigue life
4. Testing and Evaluation Methods
4.1 Material Testing
Tensile strength testing (ASTM A370)
Fatigue life testing (rotating bending method)
Fracture toughness measurement (J-integral method)
4.2 Environmental Simulation
Thermal cycling (-50°C to 200°C)
Salt spray testing (ASTM B117)
Vibration fatigue testing (resonance method)
