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Three stages of fastener creep curve
Creep is the failure phenomenon of permanent deformation of metal fasteners under the long-term action of stress and high temperature. The deformation caused by the sliding of grains along the grain boundary is the main mechanism of creep. When the deformation temperature increases to 0.35~0.7Tm (Tm is the absolute temperature of the melting point), the thin layer area near the grain boundary recovers and softens, and the deformation proceeds. Distortion occurs again after deformation, so it needs to be restored and softened again to keep the deformation in these areas. This is the so-called grain boundary sliding. Since the recovery requires a certain temperature and time, the grain boundary sliding can only be carried out at a temperature higher than a certain temperature.
The tensile creep curve of metal fasteners is divided into three stages:
In the first stage, the creep rate gradually slows down from fast, which is related to the redistribution of crystal defects.
The second stage shows that the two mechanisms of hardening and recovery are in equilibrium, and the creep rate is constant. This stage occupies a larger proportion in the whole process of creep. (Guide: Introduction to Fasteners: Main Features of Self-Tapping Screws)
In the third stage, the creep rate is accelerated. At this time, the deformation hardening of the metal is not enough to prevent the deformation of the metal, and the reduction of the effective cross section promotes the acceleration of the creep rate and finally leads to fracture.
China Standard Parts Network Warm reminder: Not all the creep curves of any material appear in the above three stages. The phenomenon of failure due to changes in the size of the pre-tightened parts due to the creep process is called thermal relaxation. For example, the bolts used for the pressure vessel flange of the fastener, under the long-term action of temperature and stress, will elongate due to creep, resulting in a decrease in the pre-tightening force, which may cause leakage of the pressure vessel.
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