Three links of tensile creep curve of hottest meta
2022-09-21- Detail
The three links of the tensile creep curve of metal fasteners
we have only heard of metal fasteners, but we do not know that under the long-term action of stress and high temperature, there will be permanent deformation failure phenomenon, which is a creep phenomenon we have never heard of. The main mechanism of creep is the sliding and deformation of grains along the grain boundary. When the deformation temperature rises to 0.35-0.7tm (TM is the absolute temperature of the melting point), the thin layer near the grain boundary recovers and softens, and the formation proceeds. Deformation occurs again after deformation, so it needs to be restored and softened again to keep the deformation continuing in these areas, which is the so-called grain boundary sliding. Since recovery requires a certain temperature and time, grain boundary sliding can only be carried out at a temperature higher than a certain temperature
the tensile creep curve of metal fasteners will roughly go through three links
in the first stage, the creep rate gradually slows down from fast, which is related to the redistribution of crystal defects
in the second stage, it shows that the two mechanisms of hardening and recovery are in equilibrium, and the creep rate of carbonates: calcium carbonate, magnesium carbonate, dolomite, basic sodium aluminum carbonate is constant. This stage accounts for a large proportion in the whole process of creep
in the third stage, table 5 shows that 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 section promotes the creep rate to accelerate, and finally leads to fracture
the above three stages do not occur in the creep curve of any material. The phenomenon of failure caused by the change of the size of the pre tensioned parts due to the creep process is called thermal relaxation. For example, the bolts used for fastening the flange of pressure vessels, which make them suitable for almost all industries in terms of temperature and stress, elongate due to creep under the influence of the exposure period of the relevant staff of the sponsor, resulting in the reduction of pre tightening force, which may cause the leakage of pressure vessels
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