Effects of thickness and delamination on fracture toughness of X60 pipeline steel

The effects of thickness and delamination on the fracture toughness of X60 pipeline steel were investigated experimentally by using of compact tension specimens with thicknesses of 3, 6, 9, 12 and 15 mm. The couple effect of delamination cracks and out-of-plane stress constraint was discovered. Delamination became serious as thickness increasing, and the three-dimensional stress constraint near the crack-tip is nearly low-constraint plane stress state during fracture. Appearance fracture toughness is independent on thickness. It is impossible to attain plane strain fracture toughness by increasing specimen thickness when delamination cracks turn up. Because the couple effect of thickness and delamination is related closely with the relative direction of crack growth and thickness, the safe assessment of pipeline by using test data from through-thickness crack is not very reliable. The plasticity modified stress intensity factor K_e and J integral were found to be equivalent in description of the cracking growth of such ductile steel.