1. Fracture Failure
1. Overload Fracture: Preload exceeds the bolt's yield strength or the external load is excessive, causing "necking" in the threaded portion. Typical examples include excessive installation torque and excessive lubrication that reduces the friction coefficient.
2. Fatigue Fracture: Under long-term vibration or alternating stress, insufficient preload or weakened clamping force can cause crack propagation in the bolt, often resulting in a brittle fracture.
3. Hydrogen Embrittlement Fracture: High-strength bolts (e.g., grade 10.9 and above) absorb hydrogen during the electroplating or pickling process, leading to delayed fracture in stress concentration areas such as the under-head fillet.
2. Thread Stripping and Loosening
1. Stripping: Thread wear or misaligned installation leads to engagement failure, commonly caused by insufficient lubrication or repeated assembly and disassembly.
2. Torque Fade: Material embedding or vibration in flexible joints (such as shock-absorbing cotton washers) can reduce clamping force, requiring step-by-step tightening or speed control. III. Corrosion and Wear
1. Chemical Corrosion: Galvanic corrosion can occur in humid environments or in contact with dissimilar metals, such as corrosion of automobile chassis bolts by deicing agents.
2. Mechanical Wear: Insufficient thread surface roughness or a small bearing surface can lead to plastic deformation or crushing.
4. Processing Defects
1. Excessively Small Radius Under the Head: This can easily cause stress concentration cracks during quenching, requiring standardized die radius transitions.
2. Material Defects: Inclusions in cold-headed steel or improper heat treatment (such as insufficient tempering) can reduce strength.
Preventative Recommendations
Material Selection: Select a strength grade (e.g., 8.8 or 10.9) based on the load.
Processing: Control the radius of the cold-heading die corners and optimize the quenching and tempering process (e.g., 42CrMoA steel requires quenching at 820°C and tempering at 560°C).
Installation: Use a torque wrench to ensure preload, and install lock washers in vibrating environments.
