Fracture toughness of X70 pipeline steel at different temperatures

The fracture toughness of X70 pipeline steel was investigated experimentally by use of the 3-point bending fracture toughness sample with different thicknesses and initial crack lengths at different temperatures. The crack tip fields and out-of-plane stress constraint of the specimens were analyzed using three dimensional elastic-plastic theory. It is found that the out-of-plane stress constraint affects the crack tip fields and fracture toughness significantly. Delamination cracks appear on the fracture surface, and the size and amount of delaminated cracks depend closely on the thickness of specimen. The geometry of delamination cracks relates with the initial crack length and the test temperature. At lower temperature the delamination occurs at some distance from the initial crack front with smaller size; while at higher temperature the delamination occurs at the initiation stage with larger size. Delamination decreases the effective thickness of the specimens so that increases the thoughness. Consequently, when using the test standards of plain strain toughness the couple effects of thickness of specimen, size of crack and temperature must be taken into account.