Study On Fracture Toughness Of Strain-Based Design Pipeline Girth Weld

Strain-based design (SBD) was used to ensure the service integrity of the pipeline with a large longitudinal plastic deformation. Girth weld was the weakest area of structure and property in the pipeline. It would failure at the first with the plastic deformation. So the tensile strain capacity of girth weld represented for tensile strain capacity of the pipeline. Tensile strain capacity of the pipeline could be quantified by the evaluation of girth weld fracture toughness to ensure the safety of pipelines.In this paper, the finite element technique was used to simulate analysis of stress-strain states on single-edge notched tensile (SENT) specimen and single-edge notched bending (SENB) specimen. The multi-specimen method and unloading compliance method were carried out by SENT test on the Weld metal(WM) and heat affected zone(HAZ) for strain-based design of X70 pipeline, respectively.The finite element analysis results showed that the crack tip constraint of SENB specimen was higher compared to the SENT specimen; The crack tip constraint of SENT specimens became higher with increasing crack depth ratio; The effect of crack depth ratio on SENT specimen to the plastic zone and crack tip constraint reduced with the decrease of crack depth ratio, especially at the range of a0/W from 0.2 to 0.35. Compared to the specimen of thickness ratio B/W=0.5, the crack front of B=W specimen was relatively straight.The test results showed that the grooves on both sides of the specimens could make the crack front relatively flat and were conductive to accurate measurement of crack extension. Validation of crack tip on fracture section was very important for HAZ specimens of SENT test. By using multi-specimen method, fracture resistance curve of girth weld and heat affected zone specimens were ?=1.39578(?a)0.78335 (WM);?=1.55338(?a)0.82175 (HAZ). By using unloading compliance method, fracture resistance curve of girth weld and heat affected zone specimens were ?=1.34853(?a)0.64334 (WM) ?=1.60042(?a)0.70999(HAZ).The test procedure and data processing of multi-specimen test method test were relatively simple, but the test data of fracture toughness resistance curve were less, and the test deviation came from specimens. The unloading compliance method needed less test material and had less deviation. The fracture toughness resistance curve from unloading compliance method was more accurate. But the test procedure and data processing of unloading compliance method was more complex.