No abstract

Since all residual stress measurement methods have inherent limitations, it is normally impractical to completely characterise a three-dimensional residual stress field by experimental means. This lack of complete information makes it difficult to incorporate measured residual stress data into the analysis of elastic-plastic fracture without resorting to simplified methods such as the Failure Assessment Diagram (FAD) approach. We propose a technique in which the complete residual stress field is reconstructed from measurements and used in finite element analysis of the fracture process. Residual elastic strains and stresses in three-point bend fracture specimens were measured using neutron diffraction and an iterative method was used to generate a self-consistent estimate of the complete residual stress field. This enabled calculation of the J contour integral for a specimen acted on by both residual stress and an externally-applied load, allowing the interaction between residual and applied stress to be observed in detail.

For nuclear welded components the complex nature of the residual stresses involved means it is often advantageous to produce mock-ups in order that the structural integrity and performance may be assessed. The weight and size of these components can make the production of mock-ups prohibitively expensive, and so the use of scaled models is considered here. Numerical analysis and finite element simulations have been carried out to investigate the scaling laws encountered affecting the applied loads, stress fields and crack driving forces that are of interest in the full sized component. To illustrate the effects of scaling we consider the introduction of a residual stress through prior plastic deformation in rectangular beams of different sizes. A simple scaling law provides the loads required to introduce the same magnitude and distribution of residual stresses in different sized specimens. This is pertinent to uncracked beams. In contrast, if a crack is introduced this scaling law is no longer applicable and the stress intensity factor associated with residual and applied stresses differ for different sized specimens. Alternatively, to create the same crack driving force in different sized specimens different initial residual stress fields are required. The implications of these findings are discussed in the context of future work.

Pressurised Thermal Shock (PTS) is one potential risk to the integrity of the reactor pressure vessel in a pressurised water reactor. It has been postulated that PTS could occur as a result of various initiating events such as loss-of-coolant accidents with subsequent re-pressurisation. Experimental studies of PTS are typically very difficult and expensive to perform because both a severe thermal shock and a primary load must be applied to the test specimen, while the specimen itself must be very large to imitate the behaviour of the RPV wall. We investigated the feasibility of using scaled-down PTS test specimens based on the spinning-cylinder concept. The use of scaled-down specimens could greatly reduce the difficulty and cost of experimental PTS testing. To explore this concept, we used a particularly well-characterised spinning-cylinder PTS test: the NESC-1 test which was performed in the late 1990s. A large parametric set of elastic-plastic finite element models was used determine a combination of specimen dimensions and test conditions that would very closely mimic the crack tip conditions which occurred during NESC-1. Specifically, the modelling demonstrated that it was indeed possible to replicate the KJ vs. temperature trajectory, and crack tip constraint, at a critical point on the crack tip line from which tearing initiated during the actual NESC-1 test. The reduced-size specimen must be carefully designed: it cannot be a simple linear scale-down due to the inherent non-linearity of both the thermal and mechanical processes which occur during PTS.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. AbstractPressurised thermal shock tests that simulate fracture conditions relevant to nuclear reactor pressure vessels are difficult to perform and require very large specimens. This study examines the feasibility of scaled-down specimens to allow easier testing. A series of finite element models was used to design scaled-down specimens that produce a crack-driving force, crack tip temperature trajectory and constraint conditions very similar to those which occurred in a previous large-scale spinning cylinder experiment known as NESC-1. It is shown that equivalent conditions can be achieved in much smaller specimens than NESC-1 using a practical set of testing parameters.Keywords: Thermal shock, finite element analysis, pressure vessel, spinning cylinder, J-integral Graphical abstractHighlights  Thermal shock tests for nuclear reactor pressure vessels can be miniaturised.  Models show that scaled-down tests can give equivalent fracture initiation conditions.  Specimens equivalent to the NESC-1 thermal shock test have been designed.