The effects of salt and distilled water environments on fatigue-crack growth rales have been evaluated for an X-65 line-pipe steel. Tests were conducted over a frequency range between 10 and O.Ol Hz, under conditions of cathodic potential and free corrosion. A distinct pattern in the functional dependence of growth rates on AK and frequency has been found. Maximum environmental enhancement of growth rates was SO times that of air at a cathodic potential of ~1.04V, and 10 times that of air at a free-corrosion potential of -0.68V. In each case it occurred at Die lowest frequency and at relatively high values of AK. Hydrogen embrittlement and the two-stage cracking process are examined as the main mechanisms of growth acceleration.Transactions of the ASME Downloaded From: http://materialstechnology.asmedigitalcollection.asme.org/ on 05/31/2015 Terms of Use: http://asme.org/terms Transactions of the ASME Downloaded From: http://materialstechnology.asmedigitalcollection.asme.org/ on 05/31/2015 Terms of Use: http://asme.org/terms
A B S T R A C T Fatigue crack closure has been monitored in single-edge notched specimens of X70 line-pipe steel during low frequency fatigue-crack growth tests in 3.5% NaCI solution at free corrosion and cathodic potentials. The closures were measured by an electrical potential technique at stress ratios (R) of 0.05, 0.4 and 0.7, over a wide range of stress intensities (K).The measured closures, expressed in terms of Elber's effective stress ratio, U = (Pmax -Pop)/Pmax -Pmin, where Pop is the load at which the crack becomes fully open, are comparable with other published results, as measured by a potential technique in inert environments, or by compliance and acoustic techniques. U depends on both Kmax and R. At low Kmax the variation of U with Kmax appears to result from the pre-cracking stress history. In the upper Kma x range, U is almost independent of K .... The closure is affected by stress ratio only up to R = 0.4, while fatigue crack growth rates increase with R through the whole range of stress ratios. This indicates that closure alone cannot explain the crack growth rate dependence on stress ratio. 0376-9429/81/030301-09500.20/0 Int. Journ. oy Fracture, 17 (1981) 301-309
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