Effects of welding residual stresses in fitness for purpose assessment can generally be included by the use of PD 6493:1991 defect assessment methods. Where residual stresses are present, an additional adjustment ρ for the fracture parameter K r is required for level 2 and level 3 assessment in PD 6493:1991, to allow for plasticity interactions of the external and residual stresses. Previous finite element validation studies at UMIST have shown some limitations of the ρ factor calculation procedures as applied to welded structures. More recent work at UMIST has suggested that ρ factors can be established specific to particular welded structures by conducting finite element analyses. This paper presents recent finite element investigations into the effects of residual stresses on the Failure Assessment Diagrams/Curves (FADs/FACs) for welded plates and pipes. The characteristics of the FACs in the presence of residual stresses have been understood. The apparent diversity in the FACs has been successfully explained. The existing ρ factors in PD 6493:1991 and the newly proposed in R6 are validated. Simpler procedures specific to welded plates and pipes for calculation of the ρ factors are recommended.