![]() ![]() Residual stress measurements made using neutron diffraction techniques show data well below the Level 1 and Level 2 profiles, in this case never exceeding 100N/mm 2. A Level 2 profile, estimated from the characteristics of the weld procedure and the pipe properties, shows an area toward the inside of the pipe wall where the residual stress is lower (but still tensile – around 200N/mm 2). Based purely on the yield strength of the pipe material, a Level 1 estimate of residual stress would be 490N/mm 2, acting as a uniform membrane stress across the section. ![]() This shows various ways of estimating/measuring residual stresses perpendicular to a 28mm thick girth weld in a large-diameter pipe spool, ie stresses parallel to the pipe axis. ‘Level 3’ assessment is potentially the most accurate approach, and typically involves direct measurement of residual stresses in a mock-up of the actual joint to be analysed.Īn example to illustrate the use of these three levels of residual stress profile is shown in Figure 1, taken from work carried out by Eren et al 6. ‘Level 2’ assessment makes use of idealised upper-bound residual stress profiles, based on a range of experimental data for particular types of joint. A ‘Level 1’ treatment assumes, in the absence of any better information, that the residual stress is uniform across the thickness of the welded joint. In R6 terminology, there are three possible ‘Levels’ of treatment of welding residual stress. Consequently, the treatment of welding residual stress in both R6 and BS 7910 is, like that of fracture in general, subject to a hierarchical approach tailored to the amount of information available to the user. Welding residual stress is typically treated as a secondary stress, and is notoriously difficult to determine reliably, whether by measurement or modelling. One of the aspects they have in common is the use of a hierarchical approach to fracture assessment, whereby the analysis becomes more accurate and less conservative as the user moves through the available ‘Options’, from Option 1 to Option 3 (older versions of BS 7910 and PD 6493 used ‘Levels’ 1-3, where the concept is similar, but not identical) 3,4,5.īoth BS 7910 and R6 emphasise the importance of distinguishing between primary stresses (which contribute to failure of cracked bodies by both fracture and plastic collapse) and secondary stresses, which contribute to fracture only. The procedures are similar in many respects, although they are developed and maintained by different user groups. There are two major fracture mechanics assessment procedures used in the UK for pressure equipment operating at ambient and low temperatures: the R6 procedure 1 (principally applied to nuclear plant) and BS 7910 2 (previously PD 6493), which is used mainly in non-nuclear applications, including pipelines, pressure vessels and pressure piping (it is also used for non-pressurised structures such as bridges and offshore structures). The approach proposed in BS 7910 combines ‘global’ relaxation of residual stress (Q m) under high mechanical load with ‘local’ enhancement of crack tip driving force through the adoption of a simplified primary/secondary stress interaction factor, ρ. A considerable programme of work was carried out by TWI at the time to justify and validate the clause, but the full underlying details of the work have not hitherto been available in the public domain, and are described in a separate companion paper. The residual stress relaxation clauses of BS 7910:2013 date back to the 1991 edition of PD 6493 and have not changed substantially since then. The aim of this study was therefore to review the basis of the BS 7910 clauses on stress relaxation with a view to harmonising the BS 7910 and R6 rules for cases in which the original welding residual stress distribution is not known. Conversely, the UK structural assessment code for nuclear structures, R6, contains a warning on the ‘limited validation’ of the BS 7910 approaches for stress relaxation and suggests that they should be used ‘with caution’. The UK flaw assessment procedure BS 7910:2013 also assumes that mechanical loading (either as a result of proof testing or during the initial loading of an as-welded structure) will bring about a relaxation in residual stress. In as-welded structures, a typical assumption is that the magnitude of welding residual stress is bounded by the room temperature yield strength of the parent material. Presented at Proceedings of the ASME 2014 Pressure Vessels & Piping Conference (PVP2014), July 20-24, 2014, Anaheim, California, USA Abstractįailure of welded structures due to the presence of flaws is typically driven by a mixture of applied and residual stresses, yet in most cases only the former are known accurately. National Structural Integrity Research Centre.Structural Integrity Research Foundation. ![]()
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