The Strain Concentration of High Strength Girth Weld Subjected to Tensile Displacement

High grade pipelines have been the majority in China since the beginning of this century. Some pipelines in mountainous area and other places experienced the ground movement because of geohazards and the disturb of construction activities. The strain capacity is important to keep pipelines subjected to tensile displacement in safe. However, the strain capacity does not depend on the pipe body but on the girth weld because the girth weld is always non-homogeneous. The strain concentration may happen where material yields in advance. Therefore, the strength matching of the girth weld towards pipe body can greatly affects strain capacity of pipelines. Generally, girth weld is designed to over-matching to prevent the strain concentration. However, in pipeline engineering, actual strength of pipe body may be much higher than the specified minimum yield stress, leading the girth weld to be under-matching in fact. In addition, even in over-matching girth weld, there may be softening zone in HAZ. In this paper, the tensile tests of X80 girth weld were performed. Local constitutive relations at the weld, pipe body and HAZ were obtained by using the whole field strain on the specimens. The experiment showed under-matching in the specimen. Based on the results of local constitutive properties of the specimen, the finite element model of X80 pipeline girth weld subjected to tensile strain and inner pressure was established. It demonstrated that strain concentration happened in weld area in under-matching girth weld and softening zone in over-matching girth weld. Inner pressure has an impact on strain concentration in a case that strain exceed the certain limit.