Residual stresses and ductile fracture

EDF are interested in starting a PhD examining the impact of residual stresses on ductile
fracture, 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 is
worth reviewing the state of the art briefly before defining a new project.
Experimental evidence
Here we have the problem of the limit load. If crack initiation occurs close to Lr=1
then it is difficult to identify any impact of residual stress since the predicted effects
are both small and masked by experimental scatter.
Brittle fracture occurs at low values of the crack driving force, and it is relatively
straightforward to design and perform small-scale tests where fracture occurs at low
Lr where there is little plasticity-induced relaxation in the residual stress field (for
example see [1]). It is also possible to test over a range of Lr to examine the effects
of plasticity.
Most ductile materials of direct relevance to the nuclear sector have high fracture
toughness and relatively modest yield strength, and the crack growth resistance rises
as tearing proceeds. It is difficult to design small-scale tests that do not undergo
initiation 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 pipe
containing a repair weld, where the residual stresses were high enough to have a
measurable effect on crack initiation and the early stages of tearing, since initiation
occurred at Lr<0.5 [3]. This however was a single, complex, expensive test, which
perhaps should be regarded as a validation benchmark rather than a basis for a
scientific investigation.
Sherry and co-workers have approached the problem by using a model material: a
low toughness aluminium alloy that still fails by micro-void coalescence. This class of
material allows the use of small specimens and standard testing machines.
The methods used to introduce residual stresses are varied. Making accurate
estimates of their magnitude and distribution is often not straightforward, and care
must be taken that the introduction method does not alter the properties of the
material being tested (or that any property changes are well understood).

In GREEN we envisage there are potentially Impacts in several domains: the nuclear Sector; the wider Clean Growth Agenda; Government Policy & Strategy; and the Wider Public.

The two major outputs from Green will be Human Capital and Knowledge:

Human Capital: The GREEN CDT will deliver a pipeline of approximately 90 highly skilled entrants to the nuclear sector, with a broad understanding of wider sector challenges (formed through the training element of the programme) and deep subject matter expertise (developed through their research project). As evidenced by our letters of support, our CDT graduates are in high demand by the sector. Indeed, our technical and skills development programme has been co-created with key sector employers, to ensure that it delivers graduates who will meet their future requirements, with the creativity, ambition, and relational skills to think critically & independently and grow as subject matter experts. Our graduates are therefore a primary conduit to delivering impact via outcomes of research projects (generally co-created and co-produced with end users); as intelligent and effective agents of change, through employment in the sector; and strong professional networks.

Knowledge: The research outcomes from GREEN will be disseminated by students as open access peer reviewed publications in appropriate quality titles (with a target of 2 per student, 180 in total) and at respected conferences. Data & codes will be managed & archived for open access in accordance with institutional policies, consistent with UKRI guidelines. We will collaborate with our counterpart CDTs in fission and fusion to deliver a national student conference as a focus for dissemination of research, professional networking, and development of wider peer networks.

There are three major areas where GREEN will provide impact: the nuclear sector; clean growth; Policy and Strategy and Outreach.

the nuclear sector: One of our most significant impacts will be to create the next generation of nuclear research leaders. We will achieve this by carefully matching student experience with user needs.

clean growth - The proposed GREEN CDT, as a provider of highly skilled entrants to the profession, is therefore a critical enabler in supporting delivery of both the Clean Growth agenda, Nuclear Industry Strategy, and Nuclear Sector Deal, as evidenced by the employment rate of our graduates (85% into the sector industry) and the attached letters of support.

Policy and Strategy: The GREEN leadership and supervisory team provide input and expert advice across all UK Governments, and also to the key actors in the nuclear industry (see Track Records, Sections 3.3 & 5.1, CfS). Thus, we are well positioned to inculcate an understanding of the rapidly changing nuclear strategy and policy landscape which will shape their future careers.

Outreach to the wider public: Building on our track record of high quality, and acclaimed activities, delivered in NGN, GREEN will deliver an active programme of public engagement which we will coordinate with activities of other nuclear CDTs. Our training programme provides skills based training in public and media communication, enabling our students to act as effective and authoritative communicators and ambassadors. Examples of such activities delivered during NGN include: The Big Bang Fair, Birmingham 2014 - 2017; British Science Week, 2013 - 2017; ScienceX, Manchester; 2016 - 2018; and The Infinity Festival, Cumbria, 2017.