Demand for natural gas using LNG and pipelines to supply the world's gas markets is increasing. Under the large demand for high-strength linepipe, crack arrestability is one of the most important properties. DWTT (Drop Weight Tear Test) is the major test method for evaluating crack arrestability. Generally, a DWTT shear area of 85% or higher is required as the acceptance criteria, such as those of the API (American Petroleum Institute). In high-toughness line pipe steels, the inverse fracture frequently occurs in DWTT. Inverse fracture is defined as a cleavage fracture in the hammer side and often reduces DWTT shear area of high-toughness line pipe steels. However, the mechanism for occurrence of the inverse fracture and the effect of inverse fracture on crack arrestability have not been fully clarified.
This paper describes the effect of plastic strain on occurrence of inverse fracture during DWTT. The effect of inverse fracture on crack arrestability was also investigated. In addition, the effect of pre-strain on occurrence of brittle fracture was investigated, by measuring the fracture toughness (crack tip opening displacement) of specimens taken from static pre-bended DWTT specimens. The deformation during DWTT, which consists of deformation caused by hammer impact, bending compression, and bending tension was measured using high-speed camera. The strain caused by hammer impact was located just under the hammer impact. The strain caused by bending compression was located on the hammer impacted side of the neutral bending plane. The strain caused by bending tension was uniformly distributed through the specimen over time. The equivalent plastic strain on hammered side (inside of the bend) was larger than that on notched side (outside of the bend), due to the compressive pre-strain. CTOA (Crack Tip Opening Angle), which is one of the fracture mechanics parameters for ductile crack propagation, was also measured with a high-speed camera. CTOA slightly decreased with decrease in test temperature. However, inverse fracture did not affect CTOA. Crack tip opening displacement at maximum load was measured, using the pre-strained specimens. Crack tip opening displacement at maximum load decreased with increase in pre-strain, although no brittle fracture was observed at the temperature at which inverse fracture occurred in DWTT.
