Fatigue Crack Growth in a High Tensile Strength Steel in Seawater and Several Other Environments

A study was made to explain why the fatigue crack growth (FCG) in the intermediate ΔK regime is accelerated in seawater and other liquid environments, compared with in air. Fatigue crack growth rates (FCGRs) were measured under various environmental conditions using compact type specimens cut from a 667 MPa-tensil-strength steel plate. Anodic and cathodic polarization at -0.6 and -0.8VSCE, and removal of dissolved oxygen and salts exerted minimal influence on the FCGR in synthetic seawater (SSW). The FCG-acceleration was observed also in several nonaqueous liquids, though the FCGR in a liquid containing no hydrogen atom in its molucular structure was almost the same as the FCGR in air. The set of results showed that the FCGR was increased by hydrogen formed in a process other than a corrosion reaction. At an overprotecting potential of -1.2VSCE in SSW, the amount of hydrogen absorbed into the steel was measured as 0.06ppm or less, and hydrogen-embrittlement type stress-corrosion-cracking (HE-SCC) did not take place at a stress intensity factor or 117MPa √m in a sustained load test. In such an environment, a marked FCG-acceleration was observed, and the FCGR was not influenced by stress ratio, if the crack closure effect was compensated. The observation of fracture surfaces showed little effect of environments. These results indicated that the FCG-acceleration was not caused by the superimposition of HE-SCC. The hydrogen-assisted fatigue-crack-growth (HA-FCG) model was proposed: Atomic hydrogen is dissociated from water or other environmental substances adsorbed onto steel surface, and then is absorbed into the steel to activate the FCG process.