Creep modelling for long-term low strain regime including the limit of negligible creep

The challenges in successfully predicting long term creep behaviour of common engineering materials, for example to design high temperature equipment for long term base load or cyclic spectrum service, include the need to consider the behaviour within a regime for which there is little support from testing data. The challenge is already present for established as-new parent materials and becomes even greater for new or ex-service materials, welding consumables and other situations with limited availability of relevant data. Numerous suggested approaches aim to overcome the challenge, for example by using various types of life fraction rules and parametric or comparable creep models for predicting long-term creep and creep rupture. The issue extends to the limit of negligible creep, or conditions where design for creep is no longer necessary. Unfortunately, there is no general agreement on how to model for creep and creep rupture within these long-term, low strain regimes and how to establish such limits. In this paper, we use the Wilshire model and available data on C-Mn, P22, 1CMV and P91 steels to evaluate a possible route for the purpose. The approach appears to show more consistent limits when these are set with strain-based rather than time-based rules.