Measurement and analysis of crack tip opening angle in pipeline steels

The quasi-static tearing tests were conducted to analyze the crack propagation process and fracture mechanism in X70 and X80 pipeline steels. The crack tip opening angle (CTOA) was measured by a single specimen method using modified double-cantilever beam (MDCB) specimen. Some influence factors of CTOA toughness were discussed including specimen thickness, precrack types, loading rate and crack tunneling. Experimental results showed that critical CTOA values increase with increasing specimen thickness, which indicated an opposite trend in comparison with the earlier aerospace aluminum alloy studies. The crack tunneling transformed from “triangular shape” to “parabolic shape”, as specimen thickness increased.