what are diffrent stages of creep curve what happen i tertairy statges
Answers
Hey,
creep is time-dependent and it goes through several stages:
Primary Creep. In the initial stage, or primary creep, or transient creep, the strain rate is relatively high but decreases with increasing time and strain due to the fact that the material is experiencing an increase in creep resistance or strain hardening. This is followed by secondary (or steady-state) creep in Stage II, when the creep rate is small and the strain increases very slowly with time.
Secondary Creep. For secondary creep, sometimes termed steady-state creep, the rate is constant—that is, the plot becomes nearly linear. The strain rate diminishes to a minimum and becomes near constant as the second stage begins. This is due to the balance between work hardening and annealing (thermal softening). This stage of creep is the most understood. The steady-state creep is often the stage of creep that is of the longest duration. No material strength is lost during these first two stages of creep. In materials engineering, possibly the most important parameter from a creep test is the slope of the second portion of the creep curve (ΔP/Δt). It is the engineering design parameter that is considered for long-life applications. This parameter is often called the minimum or steady-state creep rate.
Tertiary Creep. In tertiary creep, there is an acceleration of the rate and possibly ultimate failure. The strain rate exponentially increases with stress because of necking phenomena or internal cracks, cavities or voids that decrease the effective area of the specimen. These all lead to a decrease in the effective cross-sectional area and an increase in strain rate. Strength is quickly lost in this stage while the material’s shape is permanently changed. The acceleration of creep deformation in the tertiary stage eventually leads to failure, which is frequently termed rupture and results from microstructural and/or metallurgical changes.
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