A comparison of extrapolation techniques for long-term creep strain and creep life prediction based on equations designed to represent creep curve shape

Abstract

High-precision creep data obtained for 1 2 Cr 1 2 Mo 1 4 V steel at temperatures from 808 K to 868 K in tests having a maximum duration of only about 1000 h have been used to predict the full creep strain and creep life properties expected under conditions giving lives up to 100 000 h and more. Excellent agreement with published long-term behaviour was achieved using an extrapolation procedure, referred to as the θ Projection Concept, which envisages normal creep curves as the sum of a decaying primary and an accelerating tertiary component. With this approach, the increase in creep strain ε c with time t can be described as ε c = θ 1 (1 − e −θ 2t ) + θ 3 ( e θ 4t − 1) where θ 1 and θ 3 define the strains and the rate constants θ 2 and θ 4 quantify the curvatures of the primary and tertiary stages respectively. In contrast, a significantly poorer correlation between actual and predicted long-term properties was found when the curves were described using the equation ε c = A (1 − e −αt) + B ( e αt − 1) where the same rate constant, α, is used to define the curvatures of the primary and tertiary stages.