Issues in Welding Residual Stress Model in Fitness for Service Assessment of Crack-Like Defect in Weld Area

Abstract

Current Industry Code and Standard (ICS) Fitness for Service Assessment (FFSA) procedures for crack-like defect in weld area tend to impose high level of conservatism. In addition to the necessity for using conservative Welding Residual Stress (WRS) model due to the uncertainty inherent in the WRS estimation, using the original WRS regardless of crack depth in the crack driving force calculation, like the applied operating load, is the primary reason for this solution conservatism. In addition, current ICS weld area defect assessment procedures involve ambiguities in boundary condition effects on WRS models, as well as in the fracture mode of weld area crack being treated in the context of opening mode only, even though there is no weld area geometric symmetry essential for precluding fracture modes other than mode I. To clarify these technical issues in the ICS FFSA practices rigorous numerical simulation analyses of welding process and crack growth following joint fabrication have been performed, using the finite element analysis procedure. A crack driving force calculation procedure for weld area cracks, which was developed to quantify the crack extension effects on WRS for growing crack, was used for the finite element crack growth simulation analyses. The rigorous finite element analysis results for boundary condition effects on WRS, the fracture mode of weld toe crack, and crack growth effects on crack driving force parameters caused by WRS are compared with those of current ICS solutions. These comparisons demonstrate the need for an improvement of the current ICS FFSA procedures for weld area crack-like defects. The primary objective of the present paper is to motivate the industry to improve ICS FFSA procedures by clarifying these ambiguous technical issues in weld area crack-like defect assessment parameters, as well as considering crack extension effects on WRS properly in calculating the crack driving force of growing crack to reduce undue conservatism in FFSA.

A Study on Structural Integrity Assessment of Pipeline using Weight Function Solution

“…따라서 실험을 통한 측정값이나 유한요소해석을 이용한 방법에 비해 매우 높은 값으로 나타나게 되며, 이는 보수적인 설계로 이어지게 된다는 것을 의미한다. 이러한 관점에서, 균열 성장에 따른 용접부 잔류응력의 완화효과를 고려하여 유한요소해석 방법이나 가중함수 방법을 통해 응력확대계수를 도출하는 연구들이 수행되 고 있다[6][7] .…”