Fracture behavior of cracked full scale steel pipes used for gas and petroleum industries
Abstract
The fracture toughness of pipeline steel grade X65 was investigated utilizing linear elastic fracture mechanics analysis, KIC testing according to non-standards techniques, as well as the elastic-plastic fracture mechanics analysis (J-Integral and crack opening displacement COD methods). The fracture toughness tests were conducted at full scale pipe used for gas and oil industry, circumferential surface crack were introduced to nine pipe specimens with different depths, crack/notch depths were selected to cover wide range of remaining material of the original pipe thickness, the original pipe thickness remain unchanged, a special fixture was manufactured to perform the experimental works, plane strain condition was confirmed by using plug insert inside the pipe specimen under the introduced circumferential surface crack.
While for elastic-plastic fracture analysis a circumferential through-wall-cracked (TWC) pipes subjected to pure bending experiments were conducted on four pipe specimens, pipe thickness was reduced to 5 mm to meet the capacity of MTS machine used to perform the experimental works, in this analysis different parameters were measured.
Tensile properties of the X65 steel as well as the chemical composition were measured and compared with that provided at inspection certificate and API standards, the results from experimental work were very close.
Beside experimental work finite element method were used for analysis. Finite element modelling was used for comparison with experimental results. ABAQUS is used for FEM calculations for stresses distribution along the crack area, the simulation results were observed to have a good agreement with the experimental results.