Strain-based fracture analysis for internal surface cracks of X80 pipe girth welds

Abstract

The girth weld is the weakest region of the entire pipeline. When the pipeline crosses areas with complex terrain or where natural disasters are frequent, the pipeline may be subjected to large displacement effects and large plastic deformations. In these scenarios, the traditional stress-based assessment methods will no longer be applicable. Therefore, it is of great importance to develop strain-based assessment methods for pipelines. This paper investigates the effects of crack size, strength mismatch, heat affected zone (HAZ) width and HAZ softening/hardening coefficient on the crack driving force (CDF) of weld center crack (WCC) and HAZ center crack (HAZCC) on the inner surface of X80 pipes using finite element method. The results show that the CDF is significantly affected by crack size, crack location, strength mismatch, HAZ softening coefficient but slightly affected by HAZ width when HAZ width is greater than 2 mm. The CDF of WCC is greater than that of HAZCC under large strain conditions with HAZ width of 2 mm. The distribution of the J-integral at the crack tip is then investigated and the results show that the maximum value of the J-integral does not necessarily occur at the deepest point of the crack tip. Finally, the factors influencing the failure assessment curve (FAC) of WCC are investigated and the lower bound strain-based failure assessment diagram (SB-FAD) considering strength mismatch and HAZ softening coefficient is developed with varying crack depths. This study can provide a reference for the safety assessment of crack-like defects on the inner surface of X80 pipeline girth welds.