This paper presents the results of a combined experimental and numerical study aimed at quantifying the influence of self-balancing residual stresses on the fracture toughness constraint benefit of a ferritic pressure vessel steel tested in the cleavage fracture regime. Tests were performed on standard and pre-compressed, high constraint, compact-tension (CT) and low constraint, single-edge-notched tension (SENT) specimens at a temperature close to the Master Curve reference temperature T0. Pre-compression is undertaken prior to pre-cracking to establish a residual stress across the uncracked ligament, which is highly tensile at the pre-crack notch root and balanced by compressive stresses further ahead of the notch. The pre-crack is subsequently introduced into material ahead of the notch, within the tensile residual stress region, specimen by electro-discharge machining and fatigue. The tests demonstrate an influence of tensile residual stresses on the apparent fracture toughness properties for both CT and SENT specimens. The tests on low constraint specimens illustrate the constraint benefit on cleavage toughness for this material, and the influence of residual stresses in reducing this benefit. The paper shows how the observed behaviour can be quantified through using two parameter fracture mechanics. Here, the J-integral is determined by taking full account of the influence of preloading on the crack driving force. Both the elastic-T-stress and the elastic-plastic Q-stress are calculated and demonstrated as constraint indexing parameters. The results demonstrate a reduction in constraint benefit for cracks located within highly bending residual stress fields. Thus, when exploring any possible benefit in fracture toughness due to crack tip constraint, it is critical that the combined influence of the primary and secondary stresses on crack tip constraint be taken fully into account.

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