Analysis of the influence of girth weld strength matching on pipe deformation mode and failure pattern

A theory model with two kinds of material, which are used to describe the axis stress strain relationship of the pipe segment and girth weld joints, is proposed for the pipeline axial deformation analysis, and a pipeline deformation diagram is composed for the axis deformation combination within the pipeline design limit. According to the pipeline deformation diagram, it is found that the strength of the pipe segment at the design limit ${\sigma }_b^A$ plays a key role in the strength matching discussion. The pipeline axis deformation could be classified as three categories based on the yield strength ${\sigma }_w^Y$, ultimate tensile strength ${\sigma }_w^{\mathrm{crit}}$of the weld joints and the pipe design strength ${\sigma }_b^A$, which are ${\sigma }_w^Y>{\sigma }_b^A$, ${\sigma }_w^{\mathrm{crit}}\ge {\sigma }_b^A\ge {\sigma }_w^Y$ and ${\sigma }_b^A>{\sigma }_w^{\mathrm{crit}}$. For ${\sigma }_w^Y>{\sigma }_b^A$, the axis plastic deformation will distribute along the pipe segment and this is the ideal states for the pipeline. For ${\sigma }_b^A>{\sigma }_w^{\mathrm{crit}}$, the girth weld may fail as stress rupture with tensile strain as low as 0.003. For ${\sigma }_w^{\mathrm{crit}}\ge {\sigma }_b^A\ge {\sigma }_w^Y$, the weld joint may endure the plastic deformation, and the material properties of the weld joint may change, such as the toughness may decrease. Charpy-V impact test have shown that the toughness decreases 35 % for 2 % deformation for the weld joints welded by FCAW-S. In this scenario, the girth weld may break up as brittle fracture due to the toughness decrease. This model could be used to explain the girth weld failures, optimize the strength matching and control the risk, especially for the buried high strength pipeline of the high stress locations.