Residual stresses and ductile fracture

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EDF are interested in starting a PhD examining the impact of residual stresses on ductilefracture, specifically on ductile tearing resistance up to about 2mm of growth.This problem has been examined using a number of different approaches in the past, so it isworth reviewing the state of the art briefly before defining a new project.Experimental evidenceHere we have the problem of the limit load. If crack initiation occurs close to Lr=1then it is difficult to identify any impact of residual stress since the predicted effectsare both small and masked by experimental scatter.Brittle fracture occurs at low values of the crack driving force, and it is relativelystraightforward to design and perform small-scale tests where fracture occurs at lowLr where there is little plasticity-induced relaxation in the residual stress field (forexample see [1]). It is also possible to test over a range of Lr to examine the effectsof plasticity.Most ductile materials of direct relevance to the nuclear sector have high fracturetoughness and relatively modest yield strength, and the crack growth resistance risesas tearing proceeds. It is difficult to design small-scale tests that do not undergoinitiation and tearing at the limit load in engineering steels (for example, see [2]).Thus, any effects of residual stress are masked by the usual experimental scatter.The STYLE project performed a single large-scale fracture test on an Esshete 1250 pipecontaining a repair weld, where the residual stresses were high enough to have ameasurable effect on crack initiation and the early stages of tearing, since initiationoccurred at Lr<0.5 [3]. This however was a single, complex, expensive test, whichperhaps should be regarded as a validation benchmark rather than a basis for ascientific investigation.Sherry and co-workers have approached the problem by using a model material: alow toughness aluminium alloy that still fails by micro-void coalescence. This class ofmaterial allows the use of small specimens and standard testing machines.The methods used to introduce residual stresses are varied. Making accurateestimates of their magnitude and distribution is often not straightforward, and caremust be taken that the introduction method does not alter the properties of thematerial being tested (or that any property changes are well understood).