A B S T R A C T Defects in structural components are often associated with welds that may contain significant levels of residual stress. Whilst the primary load acting on the component may induce low constraint conditions at the crack tip, the presence of residual stresses, e.g. due to welding, can modify this constraint level and consequently influence the observed fracture toughness behaviour. This paper presents the results of a combined experimental and numerical programme aimed at exploring this issue. Cleavage fracture toughness data for high and low constraint specimens are presented with and without residual stresses. The results indicate that under certain conditions, the constraint-induced increase in cleavage fracture toughness may be eroded by the presence of a residual stress in the vicinity of the crack. The results are quantified with respect to two-parameter fracture mechanics in which the T and Q parameters are appropriately defined. Preliminary guidance is provided for the assessment of defects when residual stresses may influence crack-tip constraint.

A B S T R A C T Whether flaws in structures containing residual (secondary) stresses will extend under particular operational (primary) loads depends on the extent to which the residual stress field affects: (a) the nature and distribution of initiators; (b) the combined (primary + secondary) stresses and strains experienced by potential initiators. This paper compares fractographic data from specimens loaded by only a primary stress with data from specimens also containing a tensile residual stress field. Three-dimensional elastic-plastic finite element calculations are used to characterize the stress-strain conditions at the initiation sites at the onset of brittle fracture. The introduction of a residual stress changes the dominant stage in fracture nucleation from microcrack extension to particle cracking. This offsets some of the decrease in fracture toughness expected when the residual stress field increases specimen constraint.CT = compact tension specimen E = Young's modulus EDX = energy-dispersive X-ray analysis of composition FE = finite element HT = heat treatment J = contour integral J c = value of J at cleavage K = stress intensity K Jc = stress intensity at specimen failure MVC = ductile microvoid coalescence PCT = pre-loaded compact tension specimen SEM = scanning electron microscope X c = separation of initiation site from pre-crack front (in direction of macroscopic crack growth) Z c = separation from initiation site from mid-line of specimen, in plane normal to loading direction ε 22elastic = elastic portion of ε 22,total ε 22,total = total (elastic+plastic) strain in loading direction, acting on plane normal to loading direction ν = Poisson's σ 22 = principal tensile stress, in loading direction, acting on plane normal to loading direction