In bolt connections, there is a type of fracture known as fatigue fracture. Fatigue fracture mostly occurs in long-term cyclic vibration environments. Similar to hydrogen embrittlement, its fracture is sudden, but the two are fundamentally different-fatigue fracture is the result of cumulative damage under long-term cyclic loads, while hydrogen embrittlement is brittle fracture caused by hydrogen atoms. Currently, there is no technology to predict the exact time when a bolt will experience fatigue fracture in advance. Therefore, preventive measures must be taken from the initial stages such as design, material selection, and installation.
Every bolt has a service life. Although some bolts can be reused, they cannot be used indefinitely. When a bolt is in operating conditions exceeding the designed load for a long time, the probability of fatigue fracture increases significantly. Such fractures not only cause severe damage to production equipment but may also lead to safety accidents in serious cases.
So, why do bolts experience fatigue fracture? A relatively consistent understanding in the industry is as follows: under the action of cyclic loads (such as vibration and alternating pressure), stress tends to accumulate at the bolt's stress concentration areas (e.g., thread roots and the transition between the head and shank). If the matching components have dimensional deviations or the bolt is installed with improper preload (either too tight or too loose), local stress imbalance will be further aggravated. When the accumulated stress exceeds the material's fatigue limit and the material's plasticity is insufficient to buffer this damage, microcracks will gradually form inside the bolt. As the number of cycles increases, the cracks continue to propagate; when they reach a critical point, the bolt will suddenly fracture. What we see with the naked eye as a "sudden fracture" is actually the result of long-term crack accumulation and gradual propagation. The complete process can be summarized as: cyclic stress acts on the stress concentration points of the bolt → gradually tears the bolt matrix → microcracks form → cracks propagate to the critical point → the bolt suddenly fractures.
This is one of the reasons why bolts need to undergo tensile strength testing before use. Although the tensile test takes a short time, it allows for a preliminary assessment of the bolt's basic mechanical properties by observing the fracture location (if the fracture occurs at stress concentration areas such as thread roots or the head-shank transition, fatigue risks need to be vigilant) and recording the fracture force. If the fracture force of the bolts in the test is significantly lower than the design standards, it is not recommended to use this batch of bolts.
In addition, environmental temperature changes also affect the fatigue life of bolts. If the bolt is used in an environment with excessively high or low temperatures, or frequent alternating temperature fluctuations, it will accelerate the fatigue damage of the material. When combined with the erosion of the bolt by corrosive media in the air (such as moisture and salt spray), the probability of fatigue fracture will further increase.
Most of these fracture risks are related to the adaptability of the material to the operating conditions. We can reduce the probability of fatigue fracture by optimizing production processes: when conditions permit, the bolt processing sequence can be adjusted-first, the bolt blanks undergo heat treatment (quenching and tempering), and then thread rolling is performed (the traditional process in some scenarios is thread rolling followed by heat treatment. However, for high-strength bolts, heat treatment before thread rolling can reduce the additional stress concentration caused during thread processing, thereby improving fatigue resistance). Alternatively, fully threaded bolts can be replaced with partially threaded bolts. After all, the smooth shank section of the bolt has no thread structure, resulting in more uniform stress distribution and much better fatigue resistance than the threaded section.
