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[en] Procedures for structural integrity assessment normally contain criteria to predict the significance of the interaction between neighbouring defects in a structure. Here, the elastic interaction between coplanar semi-elliptical surface cracks is examined in detail by considering a large number of dissimilar crack pairs with different depths and aspect ratios. Surface defect interaction criteria from several assessment procedures are critically assessed and found to be satisfactory for cracks loaded in uniform tension. The criterion used in the R6 Rev. 4 and BS 7910:2013 procedures is the least inherently conservative of those considered here. However, the amount by which interaction exacerbates the most severe crack front loading state can depend strongly on the distribution of stress applied to the cracks. This means that the loading mode should be taken into consideration when judging whether the interaction between surface defects is significant. - Highlights: • Interactions between dissimilar semi-elliptical surface defects are studied. • Several interaction criteria used in current assessment procedures are validated. • The through-thickness stress distribution can affect the level of interaction.
[en] Closely-spaced surface cracks in structures interact with each other when subjected to load. The degree of interaction depends strongly on the distribution of stress that is applied. In pressure boundary components, thermal shock, residual stress and global bending can all cause load distributions that are non-uniform through the wall thickness. A wide range of crack pairs subject to various non-uniform stress distributions have been modelled using finite element analysis. Cracks sometimes interact more strongly under non-uniform loading than when loaded in uniform tension. Consequently, interaction criteria developed by considering uniform tension may not be inherently conservative for all loading conditions. A simple weight function method for determining the interaction of twin cracks under an arbitrary through-wall stress is presented, and weight function coefficients for a wide range of crack sizes and aspect ratios are given. - Highlights: • Non-uniform loading is shown to affect the interaction of surface cracks. • Interaction factors have been determined for a range of crack geometries and loads. • A weight function method for calculating surface crack interaction is presented.
[en] Residual and thermal stress fields in engineering components can act on cracks and structural flaws, promoting or inhibiting fracture. However, these stresses are limited in magnitude by the ability of materials to sustain them elastically. As a consequence, the stress intensity factor which can be applied to a given defect by a self-equilibrating stress field is also limited. We propose a simple weight function method for determining the maximum stress intensity factor which can occur for a given crack or defect in a one-dimensional self-equilibrating stress field, i.e. an upper bound for the residual stress contribution to K_I. This can be used for analysing structures containing defects and subject to residual stress without any information about the actual stress field which exists in the structure being analysed. A number of examples are given, including long radial cracks and fully-circumferential cracks in thick-walled hollow cylinders containing self-equilibrating stresses. - Highlights: • An upper limit to the contribution of residual stress to stress intensity factor. • The maximum K_I for self-equilibrating stresses in several geometries is calculated. • A weight function method can determine this maximum for 1-dimensional stress fields. • Simple MATLAB scripts for calculating maximum K_I provided as supplementary material.
[en] Highlights: • The initiation of fracture at adjacent flaws can be affected by flaw interaction. • Under elastic-plastic conditions, increases in flaw interaction occur. • Inelastic behaviour should be considered when formulating interaction criteria. - Abstract: Closely-spaced cracks in structures can interact with each other; the presence of one crack can change the strain energy release rate at another crack nearby. Since this interaction is enhanced by the onset of plasticity, elastic analysis alone should not be used for judging whether interaction between cracks will have a significant effect on the integrity of a structure.
[en] Highlights: • Residual stresses in a CRDM nozzle susceptible to PWSCC were measured. • Strongly tensile stresses are observed in the cladding and nozzle attachment weld. • Results support the use of bounding stress distributions for integrity analysis. - Abstract: Residual stress in the welds that attach Control Rod Drive Mechanism nozzles into the upper head of a PWR reactor vessel can influence the vessel’s structural integrity and initiate Primary Water Stress Corrosion Cracking. PWSCC at Alloy 600 CRDM nozzles has caused primary coolant leakage in operating PWRs. We have used Deep Hole Drilling to characterise residual stresses in a PWR vessel head. Measurements of the internal cladding and nozzle attachment weld showed that although modest tensile stresses occur in the cladding, the attachment weld contains tensile residual stresses of yield magnitude. Despite the large dispersion of residual stress data for nozzle attachments of this type, all available data suggest that assuming a residual stress profile bounded by the weld material’s yield stress would be conservative for assessment purposes.