When Do Bolts Loosen and Fail Due to Fatigue?

In recent years, high-strength bolts have been increasingly used to enhance the strength of bolted joints. High-strength bolt connections are also widely used in subway vehicle system)s, especially the bolted joints between braking units and bogies, which are crucial for the reliability of vehicle operation. Therefore, research on bolt failure is of great significance.

Bolt failure research primarily focuses on bolt loosening, bolt fatigue, fatigue in the bolt area, and the competitive mechanism between bolt loosening and fatigue. This article will comprehensively review and study bolt failure from these four aspects, aiming to discuss the theories and methods for assessing bolt failure and to provide a reference for engineering applications.

Bolt Loosening

Under dynamic loads such as vibration, impact, and cyclic loading, bolt loosening often occurs. Bolt loosening is a major failure mode of bolted connections, which can not only cause mechanical noises or medium leaks but may also lead to serious industrial accidents. Due to the complex mechanisms of loosening and the difficulty in monitoring the loosening process, a unified and comprehensive understanding has not yet been formed. This section will discuss the theory of bolt loosening, loosening tests, loosening simulation, and anti-loosening measures.

1.1 Theory of Bolt Loosening

The loosening process of bolts is generally nonlinear, often starting with microscopic sliding at the contact areas of the threads, which gradually transforms into a macroscopic loosening phenomenon. Precise measurement of bolt loosening and research on the mechanism of bolt loosening are prerequisites for predicting bolt loosening. However, due to limitations in technical conditions and the difficulty of observation, no effective direct method for monitoring bolt loosening has been developed to date. The mechanism of bolt loosening has also been based on assumptions without solid theoretical support.

1.2 Bolt Loosening Tests

To better understand the phenomenon of bolt loosening, our company has conducted extensive bolt loosening tests. These tests include vibration tests, temperature change tests, and cyclic loading tests that simulate actual working environments. Through these tests, we have collected a significant amount of data and analyzed the loosening behavior of bolts under different conditions, providing experimental evidence for the development of bolt loosening theory. Additionally, we have studied the impact of different pre-tightening forces, materials, and coatings on bolt loosening, offering valuable data support for engineering applications.

1.3 Bolt Loosening Simulation

With the advancement of computer technology, bolt loosening simulation has become an important means of studying bolt loosening issues. Our company has used advanced simulation software to simulate the loosening process of bolted connections. Through simulation analysis, we can more intuitively understand the loosening mechanism of bolts under dynamic loads, providing theoretical guidance for bolt design and anti-loosening measures. Meanwhile, simulation technology also helps us optimize bolt structures, improving the reliability of bolted connections.

1.4 Anti-loosening Measures

In response to the issue of bolt loosening, our company has developed a series of anti-loosening measures, including the use of special thread coating materials, the application of pre-stress locking technology, and the optimization of thread design. These measures have achieved significant results in practical applications, effectively reducing the risk of failure caused by bolt loosening.

Bolt Fatigue

Bolt fatigue refers to the process of damage accumulation in bolts under cyclic loading until fracture. Bolt fatigue is another common form of bolt failure, especially prominent in dynamic and alternating load environments.

2.1 Theory of Bolt Fatigue

The theory of bolt fatigue mainly studies the rules of damage evolution, crack growth mechanisms, and fracture criteria in bolts under cyclic loading. Our company has researched prediction models for the fatigue life of bolts, providing a theoretical basis for bolt design and maintenance.

2.2 Bolt Fatigue Tests

Our company has conducted bolt fatigue tests, studying the impact of different load levels, stress amplitudes, and frequencies on the fatigue life of bolts. The analysis of test data has provided important references for the design of our company's bolt products.

Fatigue in the Bolt Area

Fatigue in the bolt area refers to the fatigue damage of the bolt and its surrounding materials. Fatigue in the bolt area not only affects the bolt itself but may also severely impact the safety of the connected structure.

3.1 Research on Fatigue in the Bolt Area

Our company has conducted in-depth research on fatigue in the bolt area, analyzing the interaction between the bolt and the connected component, stress distribution, and the evolution process of fatigue damage. This research helps us optimize the design of bolted connections and improve the fatigue life of the structure.

Competitive Mechanism Between Bolt Loosening and Fatigue

Bolt loosening and fatigue are two main competitive mechanisms for bolt failure. In practical work, bolts may be simultaneously affected by loosening and fatigue, increasing the risk of failure.

4.1 Research on the Competitive Mechanism

Our company has researched the competitive mechanism between bolt loosening and fatigue, analyzing their interaction and impact under different working conditions. Through this research, we have proposed a series of preventive and control measures to reduce the risk of bolt failure.

Research on bolt failure is of great importance for improving the reliability and safety of mechanical connection structures. Our company will continue to conduct in-depth research on bolt failure, continuously improving the design, manufacturing, and testing technologies of bolts to provide higher-quality products and services for our customers. Through the organization of this article, we hope to provide a certain reference for engineering applications and jointly promote the development of the mechanical industry. In the future, we will continue to follow the research trends in the field of bolt failure, contributing to the innovation and development of China's mechanical industry.