Because the bellows is a thin-walled component and has to withstand large displacements, deformations and pressures, its reliability has received attention and attention. The reliability of the bellows is mainly caused by stiffness, strength, stability and fatigue, while the bellows has to withstand cyclic loads during operation. The stress fluctuations in its local area are often in the range of plastic stress, which is easy to be in the range of plastic stress. Fatigue failure occurs at a low number of cycles. Fatigue itself is a gradual failure phenomenon, but because of the thin wall of the bellows, the fatigue crack propagation is very short from the occurrence of fatigue, and the damage brought about is huge.
The life of the bellows is the short working period or number of cycles that can ensure normal operation when used under working conditions. With the elastic sealing system composed of bellows, the expansion joints often work under the condition of withstand more cycles of variable loads and larger displacements. Therefore, it is of great significance to determine the service life of the bellows. Because of the different functions of the bellows, the requirements for its service life are also different.
(1) When the bellows is used to compensate the position deviation caused by the installation in the piping system, its life expectancy is only a few times.
(2) The bellows is used in a constant temperature controller with a relatively high switching rate, and the life of the expansion joint must reach 10,000 times to meet the requirements of use.
(3) When the bellows is used as a vacuum switch as a vacuum seal, its service life must reach 30,000 times to ensure normal operation.
It can be seen from the above three use cases that due to different use conditions, the required service life of the bellows varies greatly. The life of the bellows is related to the fatigue characteristics of the selected material, but also depends on the size of the residual stress of the formed bellows, the stress concentration and the surface quality of the bellows. In addition, the service life is related to the working conditions of the bellows. For example: the displacement, pressure, temperature, working medium, vibration conditions, frequency range, impact conditions, etc. of the bellows when working.
