This paper considers finite element analyses that have been performed to support a fatigue endurance testing programme. This programme is aimed at understanding the influence of Light Water Reactor (LWR) environment on the fatigue life of austenitic steels under thermo-mechanical loading. Testing has typically been performed on membrane loaded fatigue specimens under isothermal conditions. However, a new test facility at Amec Foster Wheeler has been developed to enable hollow specimens to be subjected to thermal and mechanical loading for a range of thermal cycles. This work has provided a theoretical underpinning of the observed difference in lifetimes between pressurised and unpressurised specimens, and therefore provides a means of mapping data from hollow specimen testing back to solid specimen data. The parameter used to quantify these differences was the Von Mises equivalent strain. This accounts for the additional contributions from radial and hoop strains (which come about due to the internal pressure) and therefore gives enhanced strain amplitude which is fed into the S–N curve. By comparing unpressurised and pressurised lifetimes this way, a direct comparison could be made with test results. This serves two purposes; one is to provide mechanistic understanding of the difference in lifetimes. Secondly the approach will develop an assessment methodology to treat hollow specimens so a direct comparison can be made to bar specimen S–N curves. To provide further confidence in this mechanism being the dominant factor behind this difference, an independent calculation was carried out using multi-axial fatigue life models, namely Brown-Miller and Fatemi-Soci. Good agreement was observed with these models indicating that the Von Mises strain parameter was a valid parameter to characterise the multi-axial strain behaviour at the initiation site. The increase in Von Mises strain between pressurised and unpressurised specimens was found to be a factor of 1.20 on average. This was slightly less than the experimentally derived strain differences of about 1.25. However, there was another aspect of the test results that required investigation. It has been observed from tests that the hollow specimens had a tendency to fail at a region just after the shoulder, which is where the specimen increases in thickness. Upon inspection of the plastic strains from FEA, it became clear that there was a small increase in strain on the inside surface due to this geometric feature of the specimen. The strain at the high strain region was a factor of 1.025 higher than the centre of the specimen after shakedown. Therefore, when the two effects of geometry and incremental plasticity are combined, the agreement between experiment and FEA is better. There is still a slight difference between the observed factor and the predicted factor, and reasons for this discrepancy are discussed in the paper.

A paper was presented at the 2009 ASME PVP Conference on evaluating, by finite element techniques, crack opening area (COA) and stress intensity factor, KI, values for through-wall cracks located in the region where an attachment is welded to a plate geometry. Both membrane and bend loads were considered. In addition, based on the stress profile in the un-cracked complex geometry over the region where the cracks would be introduced, COA and KI values were evaluated for the same crack sizes located in a simple plate geometry. This enabled information to be established on the conservatism, or otherwise, of using simple plate solutions to evaluate COA and KI for cracks in the complex geometry. The present paper reports on further studies that have been undertaken to investigate the effect on the previous COA and KI results of considering (i) large displacement theory which may be important for combined membrane and bend loading, and (ii) contact elements in the finite element models since in the previous studies, the mesh was allowed to “overlap on itself” when crack closure was evident due to compressive stresses during bend loading.

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This paper presents the latest results of a finite element study undertaken to evaluate crack opening areas (COA) and stress intensity factors (KI) for through-wall cracks located in the region where an attachment is welded to a plate geometry. Both membrane and bend loads have been considered. In addition, COAs and stress intensity factors have been evaluated for the same crack sizes located in a simple plate geometry. These values have been determined by applying both membrane and bend stresses to the plain plate, the magnitudes of which correspond to those for the stress profile in the un-cracked complex geometry in the vicinity of where the cracks would be introduced. This has enabled information to be established on the conservatism or otherwise of using simple plate solutions to evaluate COAs and stress intensity factors for cracks in the complex geometry.

Within the UK nuclear industry the assessment of fracture in pressurised components is often carried out using procedures to calculate the margin of safety between a lower-bound fracture toughness and the crack driving force. Determination of the crack driving force usually requires the calculation of elastic stress intensity factor solutions for primary loads and secondary loads arising from weld residual stresses and/or thermal stresses. Within established UK assessment procedures weight function solutions are available which allow the stress intensity factors to be calculated from the through-wall opening-mode stress distribution in an uncracked component. These weight-function solutions are generally based on models where either no boundary condition is applied, or where one is applied at a distance either side of the crack plane that is very long compared with the crack size and wall thickness. Such solutions do not take into account any reduction in the stress field that might occur as the distance from the crack faces increases. Weld residual stress fields may often be expected to reduce in this manner. A separate, earlier study has shown that the stress intensity factor for a cracked plate loaded in displacement control decreases substantially as the loading plane is moved closer to the crack plane. It would therefore be expected that a similar reduction in stress intensity factor would be obtained for a residual stress analysis when displacement boundary conditions are imposed at a distance relatively close to the crack plane. This paper describes an investigation of the differences, particularly in terms of a reduction in calculated stress intensity factor, which may arise from application of displacement controlled stress intensity factor solutions, as compared with load controlled solutions, when considering weld residual stresses. Consideration is also given as to how new displacement controlled stress intensity factor solutions could be developed by modification of existing load controlled solutions.