Crack Growth Sensitivity to the Magnitude and Frequency of Load Fluctuation in Stage 1b of High-pH Stress Corrosion Cracking

Abstract

Pipelines undergo sequential stages before failure caused by high-pH stress corrosion cracking. These sequential stages are the incubation stage, intergranular crack initiation (Stage 1a), crack evolution to provide the condition for mechanically driven crack growth (Stage 1b), sustainable mechanically driven crack propagation (Stage 2), and rapid crack propagation to failure (Stage 3). The crack propagation mechanisms in Stage 1b are composed of the nucleation and growth of secondary cracks on the free surface and crack coalescence of secondary cracks with one another and the primary crack. These mechanisms continue until the stress intensity factor (K) at the crack tip reaches a critical value, known as KISCC. This investigation took a novel approach to study Stage 1b in using precracked compact tension (CT) specimens. Using precracked specimens and maintaining K at less than KISCC provided an opportunity to study crack initiation on the surface of the specimen under plane stress conditions in the presence of a pre-existing crack. In the present work, the effects of cyclic loading characteristics on crack growth behavior during Stage 1b were studied. It was observed that the pre-existing cracks during Stage 1b led to the initiation of secondary cracks. The initiation of the secondary cracks at the crack tip depended on loading characteristics, i.e., the amplitude and frequency of load fluctuations. The secondary cracks at the crack tip can be classified into four categories based on their positions with respect to the primary crack. Low R-ratio cycles generated an evident cyclic plastic zone, where high density of intergranular cracks were formed. The higher the frequency of the low R-ratio cycles, the higher the density of the intergranular cracks forming in the cyclic plastic zone. The crack growth rate increased with an increase in either the amplitude or the frequency of the load fluctuations. The minimum and maximum crack growth rates were 8 × 10−9 mm/s and 4.2 × 10−7 mm/s, respectively, with the R-ratio varying between 0.2 and 0.9, frequency varying between 10−4 Hz and 5 × 10−2 Hz, and at a fixed stress intensity factor of 15 MPa√m. It was found that avoiding rapid and large load fluctuations slowed down crack geometry evolution and delayed the onset of Stage 2. The implication of these results for pipeline operators is that reducing internal pressure fluctuations by reducing the frequency and/or amplitude of the fluctuations can expand Stage 1 and increase the reliable lifetime of operating pipelines.