The effect of overload on the fatigue crack growth behaviour of 304 stainless steel in hydrogen

Fatigue crack growth (FCG) behaviour and its characteristics following tens ile overloads were investigated for AISI 304 stainless steel in three diffe rent atmospheres; namely dry argon, moist air and hydrogen. The FCG tests w ere performed by MTS 810 serohydraulic machine. CT specimens were used for the tests and crack closure measurements were made using an extensometer. F CG rates of 304 stainless steel at both dry argon and moist air atmospheres have shown almost the same behaviour. In other words, the effect of moistu re on FCG of this material is very small. However, in a hydrogen atmosphere , the material showed considerably higher crack growth rate in all regimes. In general, for all environments, the initial effect of overloads was to a ccelerate the FCG rate for a short distance (less than a mm) after which re tardation occurred for a considerable amount of time. The main causes for r etardation were found as crack blunting and a long reinitiation period for the fatigue crack. Regarding the environmental effect, the overload retarda tion was lowest in a hydrogen atmosphere. This low degree of retardation wa s explained by a hydrogen embrittlement mechanism. In a general sense, hydr ogen may cause a different crack closure mechanism and hydrogen induced cra ck closure has come in to the picture. Scanning electron microscope and lig ht microscope examinations agreed well with the above results.