Abstract
Fatigue crack growth tests were conducted on 25mm thick compact tension specimens of Ni-Mo-V steels exposed to both laboratory air and hydrogen gas environments. The fatigue crack growth rates were determined under various frequencies and wave forms and the relation between the fatigue crack growth rate and fatigue fracture morphology was discussed by using fractographical observations.The main conclusions which characterize the fatigue behavior in the hydrogen gas environment are summarized as follows.(1) The relation between the fatigue crack growth rate and the stress intensity factor range in hydrogen gas was classified into three regions. In the regions II and III, a considerable raise in fatigue crack growth rate was observed.(2) The fatigue crack growth rate in hydrogen gas was not affected by frequency, nor by wave form.(3) The fracture morphology of fatigue failure in air showed normal transgranular fracture which includes ductile striations. However, the fracture morphology in hydrogen gas showed intergranular fracture in the region I and cleavage fracture in the region II and III. Brittle striations appeared on the cleavaged fracture surface.(4) A fair correspondence between the striation spacing and the fatigue crack growth rate was observed for ductile and brittle striations in both air and hydrogen gas.
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