Evaluation on strain capacity of girth welds in high-grade pipelines

In this paper, the finite element model of pipeline girth weld based on material damage theory was built to evaluate the strain capacity of girth welds in high-grade pipelines. The stress distribution and tearing process at the crack tip were accurately simulated by optimizing the grid size at the crack tip. The material properties of high-grade pipeline were characterized by using yield strength and uniform elongation. The crack driving force curve of the pipeline girth weld was calculated and analyzed. Two failure criteria were developed, i.e., crack growth instability criterion and material fracture toughness criterion. The effects of pipe material performance, crack length and internal pressure on the crack driving force were investigated. And the relationships between these influential factors and pipeline strain capacity were quantitatively analyzed. It is shown that crack length has more effect on the pipeline strain capacity than the inner pressure, and the uniform elongation of pipeline material has more effect on crack growth driving force than the yield strength. It is recommended that in the pipelines designed on the basis of strain, girth welds should satisfy the requirements of plastic capacity indicators (e.g. uniform elongation). The research results provide the technical basis for the design and evaluation of the strain based high-grade pipelines.