Assessment of defects under combined primary and residual stresses

Residual stresses can provide a significant contribution to the crack driving force for defects associated with non stress-relieved welds. Structural integrity assessment methods are available which provide detailed guidance for the assessment of such defects under the combined influence of primary and residual stresses. However, in some circumstances these methods may be unduly conservative due, in part, to an over-estimation of the crack driving force contribution from the residual stress, K-j(s). This paper describes a programme of experimental and supporting analytical work undertaken to better characterise the influence of residual stress levels on Kjs for cracks in a high strength, low toughness aluminium alloy. The work is based on a compact specimen in which a highly tensile residual stress field is mechanically induced through a compressive preload prior to fatigue pre-cracking. Different approaches to numerically simulate crack development within the residual stress field are explored by finite element analysis and, where possible, numerical data are compared with experimental measurements of crack opening displacement for different crack lengths. The results demonstrate that progressive crack insertion in finite element models provides a than instantaneous crack insertion. In addition, progressive crack insertion leads to maximum values K-j(s) which are approximately 30% below those derived using the instantaneous method. The implications of these results for structural integrity assessments are discussed.