How To Prevent Fatigue in Bolted Connections

In bolted connections, a type of failure known as fatigue fracture often occurs. Fatigue fracture mostly happens under vibrating conditions. Like hydrogen embrittlement, it is a sudden failure, and current technology cannot predict in advance when a bolt will fracture due to fatigue. Therefore, we must take preventive measures from the very beginning.

Every bolt has a service life. Although bolts are reusable, they are not meant for unlimited cycles of use. When bolts are constantly subjected to heavy or alternating loads in certain environments, the probability of fatigue fracture increases significantly. Such fractures can cause serious damage to production equipment and even lead to safety accidents.

Why does fatigue fracture occur in bolts? The widely accepted explanation in the industry is: due to differences between the service environment of the bolt and the base material of the connected parts, as well as variations in the shape and size of moving components, excessive preload of the bolt can result in uneven local stress distribution. Under continuous alternating loads, and with insufficient material ductility, microcracks gradually form inside the bolt. When these invisible cracks reach a critical point, the bolt breaks suddenly. To the naked eye, it appears to be an instantaneous break, but in reality, it develops slowly over time. The process is as follows: stress concentration points are created during installation, the base material of the bolt is gradually torn, small cracks appear after a period of time, and sudden fracture occurs once the critical state is reached.

This is one reason why tensile strength tests are required before bolts are put into use. Although a tensile test does not take long, it allows a basic evaluation of the bolt based on the fracture location and breaking force. If the breaking torque is too low during testing, this batch of bolts is not recommended for use. Another important factor is temperature change in the environment. If the operating temperature is excessively high or low, or fluctuates sharply, the service life of the bolt will be greatly shortened. Combined with the corrosive effect of air, the risk of fatigue fracture becomes very high.

These failure factors are mainly caused by insufficient adaptability of the material to the environment. We can appropriately improve production processes to reduce the risk of fatigue fracture. If conditions permit, bolts can be heat-treated before thread rolling (the standard process is thread rolling first, then heat treatment). Alternatively, fully threaded bolts can be replaced with partially threaded ones, since the smooth shank section has much higher fatigue resistance than the threaded section.