An Improved Method of Evaluating the Stress Intensity Factor for a Penetrating Circumferential Defect in a Self-Balancing Residual Stress Field in a Weld

摘要

It is required to determine the stress intensity factor (SIF) contributed from a through-wall residual stress distribution when assessing the structural integrity of a welded joint containing flaws. By decomposing the through-wall residual stress distribution into a membrane stress component (σ_m), bending stress component (σ_b) and self-balancing stress component (σ_(sb)), the total SIF from the through-wall residual stress distribution (K_(total)) comprises K_m (due to σ_m), K_b (due to σ_b) and K_(sb) (due to σ_(sb)). K_m and K_b can be relatively easy to determine as there are standard solutions available for common geometries and flaw types. However, it is not straightforward to calculate K_(sb) owing to the arbitrary distribution of the self-balancing stress component. In particular, no SIF solutions are available for a through-wall penetrating defect in a plate or a cylinder subjected to an arbitrary through-wall self-balancing stress distribution other than for three special distributions (cosine, triangular and square distributions). Neglecting the contribution of σ_(sb) to the K_(total) could significantly underestimate the crack driving force, thus leading to a non-conservative assessment of limiting defect size. Therefore, the calculation of K_(sb) for a though-wall penetrating defect in a plate or a cylinder under an arbitrary stress distribution is of the primary concern in this work. Understandably, finite element analysis (FEA) can be used to calculate the K_(sb) in these situations, but it is costly to perform such analysis. In this work, a simple method is proposed for estimating K_(sb) due to the self-balancing component which has a different distribution from the cosine, triangular and square distributions. This method is an extension of the approach adopted by Annex Q, BS 7910:2013 in dealing with the calculations of K_(sb) resulted from the σ_(sb) profiles which are decomposed from the proposed upper bound through-wall welding residual stress profiles. Some typical through-wall welding residual stress distributions are investigated with the proposed method in estimation of the K_(sb) for a through-wall penetrating defect in a plate or a cylinder. Discussion and highlights are given in the aspects of the effects of welding residual stress profiles on SIFs, the implications for limiting defect sizes, and the likelihood of underestimating the K_(sb) when using the equation established in R6 Revision 4 with a cosine distribution for any other distributions.