Analysis and avoidance of fatigue fracture of bolts
Analysis of fatigue fracture of bolts
Bolt fatigue fracture is a common problem in mechanical manufacturing and installation of mechanical equipment, and its analysis can be mainly carried out from the following aspects:
1. Formation of fatigue source:
The fatigue source is usually the starting point of the fatigue failure of the parts, which is difficult to see with the naked eye, and often occurs in the stress concentration parts of the sharp corners, grooves, cross section mutations, or the internal defects such as inclusion, porosity, porosity, etc.
When the stress peak occurs in the work of the zero component, these parts will break through the intergranular bonding force, resulting in microscopic cracks, and then form a fatigue source.
2. Crack propagation:
The crack starts from the fatigue source and extends to the interior of the metal along the slip zone and the stress at a certain Angle (such as 45°) to form the expansion zone of the fatigue crack.
The crack propagation has both microscopic stage and macroscopic stage. Finally, when the crack propagation reaches the critical size, fatigue failure of the component will occur.
3. Specific factors of bolts:
The fatigue fracture of bolts usually occurs at the root of thread, because there are some unfavorable factors such as rounded corner, large stress gradient and high surface roughness.
In the thread connection, due to the machining error of the thread and the different contact state after spinning, the first button thread is often the most stressed and prone to fatigue fracture.
Measures to avoid fatigue fracture of bolts
In order to effectively avoid the fatigue fracture of bolts, the following measures can be taken:
1. Optimized design and processing:
Reasonable design of bolt structure to avoid stress concentration.
Improve the machining technology, improve the surface quality of bolts, reduce machining defects.
2. Reasonable material selection and heat treatment:
领英推荐
Select materials with high fatigue strength, such as alloy steel.
The bolts were heat treated to refine the grain and improve the fatigue resistance of the material.
3. Apply the correct preload force:
According to the specification and use requirements of the bolt, apply the appropriate pre-tightening force to ensure that the bolt will not cause additional stress due to loosening during the working process.
4. Adopt anti-loose measures:
Use spring washers, locking washers, self-locking nuts and other anti-loosening devices to limit the relative movement between thread pairs.
Check and tighten bolts regularly to prevent loosening.
5. Reduce alternating load:
Optimize the design and operation parameters of the equipment to reduce the alternating load on the bolts.
Where possible, damping devices are used to reduce the vibration and shock of bolts.
6. Strengthening maintenance and monitoring:
Regular maintenance and inspection of bolts, timely detection and treatment of cracks and other defects.
Use non-destructive testing techniques (such as ultrasonic testing, magnetic particle testing, etc.) to regularly test bolts.
7. Improve the fatigue strength of bolts:
Fatigue strength can be improved by increasing the diameter of the bolt, changing the shape of the thread, or using higher grade materials.
?
In summary, the fatigue fracture of bolts can be effectively avoided and the safety and reliability of equipment can be improved by optimizing design and processing, reasonable material selection and heat treatment, applying correct preload, adopting anti-loosening measures, reducing alternating load, strengthening maintenance and monitoring, and improving the fatigue strength of bolts.