Dynamic Apparatus for CTOA Measurement in Pipeline Steels

Abstract

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, the crack tip opening angle (CTOA) is used. Recent articles on the CTOA of pipeline steels at quasi-static rates with modified double cantilever beam specimens (MDCB), and at dynamic displacements rates by use of 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 MDCB 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 disc spring apparatus that is used to further accelerate the testing displacement rate. Initial results of the testing show that full slant fracture mode is observed at the highest rates tested for X65 and X100 steels. Maximum crack velocities approaching 10 m/s were recorded with highspeed 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 in the steady-state crack propagation regime. 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.