You may ask: how can a bolt made of metal suffer from fatigue? In fact, after carbon steel is processed into bolts, long-term cyclic loading can create stress concentrations in local areas if the initial technical parameters and mechanical properties do not meet requirements. When such stress reaches a critical level, tiny cracks will form in the bolt - this is only the first stage of fatigue. As the number of loading cycles increases to a certain level, the cracks propagate and eventually lead to sudden fracture. This is the mechanism and result of bolt fatigue failure.
Why does fatigue occur in carbon steel bolts? Are higher-strength bolts more likely to fatigue? First, fatigue is not directly related to the strength level of the bolt. Ordinary bolts have lower strength requirements and are used in mild conditions where fatigue effects are limited. High-strength bolts, however, are applied in environments with strict tensile demands, which naturally increase the risk of fatigue. For this reason, most fatigue failures we encounter in practice occur in high-strength bolts, though this does not mean ordinary bolts never fatigue - they are simply subject to lower service requirements.
The fundamental cause of bolt fatigue is the repeated change of local stress during cyclic loading, which causes cumulative damage to weak points and eventually forms cracks. The process is as follows: stress first erodes the vulnerable areas of the bolt, microcracks gradually appear, the cracks grow over time, and once they reach a critical length, the bolt breaks suddenly. Long-term analysis shows that the stress causing fatigue does not need to be large; it can even be much lower than the yield strength of the bolt. Therefore, after fatigue fracture, the fracture surface usually shows no obvious deformation or bending caused by external force.
Based on the above analysis, we can improve the manufacturing process to enhance the fatigue resistance of bolts. Take a look at the following diagram:
Reinforced ThreadThe diagram above shows an optimized thread profile with a rounded root (R-radius). Fatigue cracks usually occur at the thread roots and under the bolt head, so modifying the basic thread manufacturing process can effectively prevent fatigue. Let's compare it with ordinary threads:
Ordinary ThreadThe thread above is a standard thread with sharp corners at the root. Such right-angle structures are highly sensitive to stress changes and prone to fatigue fracture. As mentioned earlier, the area under the bolt head is another critical location for fatigue failure, as shown in the diagram:
Bolt Fatigue ProcessUsing the same principle as the thread root radius, we can add a properly sized fillet radius at the junction between the bolt head and the shank within the allowable design range.
