Why do high - strength bolts fracture?

Material defects: The bolt material has internal defects such as inclusions, pores, and segregation, which will reduce the strength and toughness of the material. As a result, cracks are likely to be initiated and propagated from these defective sites under stress, eventually leading to fracture.
Improper material selection: The appropriate material is not selected according to the service environment and working requirements. For example, the use of ordinary high - strength bolts in high - temperature environments will cause a decrease in their performance, making them prone to fracture.

Improper heat treatment: Heat treatment is a key process to improve the performance of high - strength bolts. If the heat treatment process parameters are inaccurate, such as too high or too low quenching temperature, insufficient tempering, etc., the microstructure and properties of the bolts will be non - uniform, resulting in excessive residual stress, reducing the strength and toughness of the bolts, and increasing the risk of fracture.
Poor surface processing quality: The surface roughness of the bolt does not meet the requirements, and there are defects such as tool marks and scratches. These parts will become stress - concentration sources. Cracks are likely to be initiated under stress, which will then lead to fracture.

Excessive pre - tightening force: The pre - tightening force applied during installation exceeds the bearing capacity of the bolt, causing the bolt to be in a too - high - stress state before work. When it subsequently bears the working load, it is prone to exceed the strength limit of the material and break.
Overload: The working load borne by the bolt during use exceeds its design bearing capacity. For example, abnormal impacts, vibrations, or overloads occur during the operation of the equipment, subjecting the bolt to excessive tensile or shear forces, resulting in fracture.
Fatigue failure: Under the action of repeated alternating loads, micro - cracks on the surface or inside of the bolt will gradually expand. When the cracks expand to a certain extent, the bolt will experience fatigue fracture. This situation is common in equipment subject to vibration loads, such as engines and bridges.
Corrosion: If the bolt is in a corrosive environment, such as humid air, acid - base solutions, etc., corrosion will occur, reducing the effective cross - sectional area of the bolt and lowering its strength, eventually leading to fracture.
Improper assembly: During the assembly process, if the bolt is not installed vertically or the nut is not tightened evenly, the bolt will bear additional bending stress. This additional stress will reduce the bearing capacity of the bolt, causing the bolt to fracture prematurely.