Effects of Hydrostatic Testing on the Growth of Stress-Corrosion Cracks

Two hydrostatic tests were carried out on an X-52 (Grade 359) pipe containing sixteen cracks of depths up to 55% wall thickness. Stress corrosion cracking (SCC) growth rates were measured in full-scale tests performed before and after the first hydrotest in order to demonstrate the effects of hydrostatic testing on subsequent crack growth rates. The effects on crack tip deformation were investigated by metallographic examination of crack cross-sections immediately following the second hydrotest. The SCC tests were performed using a saw-tooth type load spectrum with the maximum stress set at 95% of the actual (as opposed to specified minimum) yield strength of the linepipe and R = 0.8. During the first hydrotest, the maximum applied stress was 108% of the yield stress; the total hoop strain in the pipe body reached about 0.2%, which is less than would have been reached in a uniaxial tensile test at this stress level because of the effect of the biaxial stress state in the pipe. The highest SCC growth rate measured before the first hydrotest was about 0.88 mm per year (2.4 * 10−3 mm/day), and the growth rate of the same crack after this hydrotest was about 0.37 mm per year (0.79 * 10−3 mm/day). The other cracks all showed varying degrees of reduction in growth rate. Post-mortem examination indicated that the hydrotests did not cause significant crack blunting. The beneficial effects of hydrotests are attributed primarily to the presence of compressive residual stresses in the heavily deformed region in front of the crack tip. The majority of cracks showed some growth during the first hydrotest. SCC growth and growth during the hydrotests are associated with distinctly different microscopic features on the fracture surface.Copyright © 1998 by Minister of Natural Resources, Canada