The solution to bolt fatigue problems!
The term fatigue damage is frequently used to describe the damage to a part that has been sustained as a result of loading. Such fatigue damage is nowadays equated to crack length. In some critical applications there is a requirement that bolts are periodically crack detected using dye penetrant or even by x-rays to ensure that there are no detectable cracks present. (Cracks may be present on the microscopic scale, that is, below the detection threshold of the measurement technique.)
In the vast majority of applications, the most effective way to ensure that the bolt is fatigue resistant is to ensure that it is tightened sufficiently.
Solve Bolt Fatigue Problems
- Establish the bolt thread's endurance limit.
Fatigue cracks usually initiate in the roots of the threads, this is due to this region being highly stressed because of the local change in section (a so-called stress concentration). Thread rolling AFTER heat treatment induces localised compressive stresses in the thread roots. These compressive stresses help to prevent crack initiation and hence improve the fatigue life. Rolling the threads before heat treatment does not give the same benefit since the compressive stresses are flushed out with the heat treatment operation. Rolling after heat treatment can offer a significant improvement. Bolts with threads rolled after heat treatment can have double the fatigue strength compared with ground threads. Although rolling after heat treatment can offer significant improvements from a design aspect, it is pretty dire from a manufacturing cost standpoint. The higher hardness of the threads significantly increases wear on the rolls resulting in a lower useful life. Effectively this results in bolts with threads rolled after heat treatment being more expensive to produce. (On large diameter threads, inducing compressive stresses in the thread roots to improve fatigue strength has been achieved by shot peening.)
BOLTCALC will determine the endurance limit of the bolt thread.