When a crack initiates and propagates in a pressurized pipe the only thing that might stop this high-velocity event is the release of internal pressure (decompression) resulting in a deceleration in the crack-propagation rate. This deceleration can be achieved through the use of crack arrestors, or the ability of the pipeline material to resist ductile fracture. To evaluate the resistance to crack growth, values for the crack tip opening angle (CTOA) are used to assess the pipeline fracture toughness. Recent articles written on the CTOA of pipeline steels at quasi-static rates using modified double cantilever beam specimens, and at dynamic rates using drop weight tear testing have provided data to support this need. These laboratory results from the literature, compared with results of full-scale tests, indicate that details of the fracture mode depend on the rate of fracture. To further study the dependence among the rate, fracture mode, and CTOA, a dynamic test apparatus was designed to perform CTOA testing of modified double cantilever beam specimens, so that comparisons to quasi-static and full-scale results could be made. This new apparatus consists of a 500 kN uniaxial hydraulic test machine capable of stand-alone displacement rates of 300 mm/s, and a Belleville spring apparatus that is used to further accelerate the testing displacement rate. Initial results of the testing show full slant-shear fracture surfaces are observed at the highest rates tested for X65 and X100 steels. Maximum crack speeds approaching 10 m/s were recorded using high-speed photography. CTOA measurements were typically made at a position about 30 mm ahead of the pre-fatigue crack, over a distance of about 15 mm of steady-state crack propagation. In this paper, we describe the high-speed apparatus, discuss the relationship among specimen configuration, crack speed, and CTOA, and present initial results on X65 and X100 pipeline steels.
Proceedings Title: Proceedings of the 7th International Pipeline Conference
Conference Dates: September 29-October 3, 2008
Conference Location: Calgary, CA
Conference Title: IPC 2008
Pub Type: Conferences
Belleville springs, crack tip opening angle, CTOA, dynamic testing, pipeline steel