R5 procedures for assessing structural integrity of components under creep and creep–fatigue conditions

Abstract

The R5 high temperature assessment procedures have been developed in the UK and address both creep-fatigue crack initiation in initially defect free components and the growth of flaws by creep and creep-fatigue mechanisms. The procedures were developed some time ago and included a number of novel features such as: the shakedown reference stress approach for structural assessment; the ductility exhaustion method for estimating creep damage; the inclusion of size effects in fatigue damage calculations to enable assessments of thin section components; and the treatment of the effects of crack closure on creep-fatigue crack growth rates. Recently, the R5 procedures have undergone a major revision leading to Issue 3 of the document. In the revision, the above novel features have been retained but other parts of the procedure have been modified. These include: a restructuring of the document with associated new step-by-step procedures to enable easier application; modifications to the ductility exhaustion model to address multiaxial stresses including the triaxial stresses which may be present in regions of high welding residual stress; additional advice for assessment of weldments including fatigue strength reduction factors based on experimental data on austenitic weldments; generalised hysteresis loop construction methods for complex non-isothermal cycles, supported by laboratory data collected under non-isothermal conditions; advice on inelastic analysis when simplified shakedown methods are inapplicable; and rules for determination of interactions between creep and fatigue crack growth. The present review first describes in outline the overall structure and content of R5 Issue 3. Then the new R5 creep-fatigue crack initiation assessment procedures are set out in more detail. This includes the recent modifications and some associated background experimental and analytical work on which these modifications are based. Finally, the paper sets out the new creep-fatigue crack growth procedures in R5, again with some background information on the recent developments.

A review of the changes of internal state related to high temperature creep of polycrystalline metals and alloys

“…To achieve this it is important to be able to predict the remanent life with high confidence. This requires that unnecessary conservatisms are removed which are included in the present assessment methodology 153 .…”

Effect of cyclic hardening on stress relaxation in SUS316HTP under creep-fatigue loading at 700ºC: experiments and simulations

“…The ductility exhaustion method proposed by Priest and Ellison (1981) and Hales (1983) is a well-known method to evaluate creep damage under creep-fatigue loading (Ainsworth, 2006;Yan et al, 2015). This method assumes that creep damage develops with the accumulation of creep or inelastic strain under strain holding.…”

In situ neutron diffraction measurement of residual stress relaxation in a welded steel pipe during heat treatment

“…For example, if the relaxation of tensile residual stresses occurs at an inappropriate rate, this can lead to a stress relief cracking during the post weld heat treatment (PWHT) or a reheat cracking during service [3,4]. A method for the prediction of creep crack initiation in the weldments due to residual stress relaxation can be found in the R5 procedure [6]. One of the inputs required in this prediction is the instantaneous creep strain rate, which is equivalent to the instantaneous relaxation rate of the residual stresses.…”