Stress relaxation and elastic follow-up using a stress range-dependent constitutive model

Abstract

Despite the availability of detailed non-linear finite element analysis (FEA), some aspects of high-temperature design can still be best addressed through more simplified methods. One such simplified method relates to the problem of elastic follow-up where, typically in strain-controlled situations, elastic behaviour in one part of a structure can lead to large strain accumulation in another. Over the past 30 years, it has been shown that in regions with significant elastic follow-up, a plot of maximum stress against strain (a ‘stress-strain trajectory’) is virtually independent of the constitutive relation – a characteristic which can be used to estimate elastic follow-up for design purposes without detailed non-linear FEA. The majority of studies which have reported this independence on material behaviour have used simple constitutive models for creep strain, primarily based on power-law creep or variations. Recently, studies of the behaviour of high-temperature structures with a stress range-dependent constitutive law have begun to emerge. This article examines the problem of elastic follow-up using such a constitutive law for a classic two-bar structure and for a more complex structure using FEA. It is found that the independence of the stress–strain trajectory on constitutive equation is lost with a stress range-dependent relation.