Analysis of Failure Forms and Causes of Threaded Fasteners(1)

1) Break the component by twisting and pulling

Tensile fracture is characterized by significant necking and elongation at the fracture site. The common reason is that the friction coefficient of the joint surface is too small; when tightening or preloading, the applied torque is too small, the sleeve and thread are from different shafts when the torque is applied, and the speed is too slow when the torque is applied, so the performance strength of the part itself is sufficient , The perpendicularity of the fastening surface and the thread center line is out of tolerance.

2) The thread is broken by shearing force

The fracture part broken by the shearing force is usually spiral, and there is no obvious necking phenomenon. The common reason that the thread is broken by the shearing force is that the thread is jammed during the tightening process, such as the deformation of the thread and the connection of different tooth shapes. , There is welding slag on the thread; the part where the bolt is screwed is resisted. If the nut is used as a blind hole, the precise groove depth is sufficient.

3) The stress dispersion part breaks after use

The fracture of the stress dispersing part after use is often manifested in the right angle between the bolt head and the screw. The main reason is that the right angle between the bolt head and the screw is too large and the fillet is too small; during cold heading, the bolt head Plastic streamlines are defective. The perpendicularity between the connecting surface and the bolt is out of tolerance.

4) Fatigue fracture

The main fracture during use after bolt connection is painful fracture. Common reasons are: insufficient pre-tightening force; excessive attenuation of clamping force; bolt size and performance will never be qualified; mutual cooperation between parts, assembly environment and use conditions It can never meet the design requirements.