Effects of Hydrogen on Fatigue Crack Growth and Stretch Zone of 0.08mass%C Low Carbon Steel Pipe

Abstract

In order to investigate the influence of hydrogen and test frequency on fatigue crack growth rate, fatigue crack growth tests (R=0) were conducted on hydrogen charged and uncharged specimens of a 0.08 mass%C low carbon steel pipe at test frequency 0.001-10 Hz. Following the fatigue crack growth tests, the test for producing a stretch zone was carried out. Observing the morphologies of the striation and stretch zone on the fracture surface and slip bands on the specimen surface, it was revealed that uncharged specimens have no dependency of crack growth rate on test frequency and on the other hand, the crack growth rate of hydrogen charged specimens at the test frequency below 0.01 Hz is approximately 10 times faster than that of uncharged specimens. There exists an upper limit of the increase in crack growth rate of the hydrogen charged specimens at test frequency below 0.01 Hz. The degree of crack tip blunting is very different between uncharged and charged specimens. The crack tip is fully blunt and CTOD is large in uncharged specimens, while the crack tip is very sharp and CTOD is very small in charged specimens. Nevertheless, the stretch zone width of hydrogen charged specimens is approximately equal to that of uncharged specimens. Considering the increase in the crack growth rate and the morphology of the striation and stretch zone, it has been concluded that the acceleration of fatigue crack growth of hydrogen charged specimens is caused by the continuous hydrogen enhanced slip deformation ahead of crack tip (Hydrogen Enhanced Successive Fatigue Crack Growth, HESFCG).