A Model to Account for the Effects of Load Ratio and Hydrogen Pressure on the Fatigue Crack Growth Behavior of Pressure Vessel Steels.

A phenomenological model for estimating the effects of load ratio and hydrogen pressure PH2 on the hydrogen-assisted fatigue crack growth rate (HA-FCGR) behavior in the transient and steady-state regimes of pressure vessel steels is described. The "transient regime" is identified with crack growth within a severely embrittled zone of intense plasticity at the crack tip. The "steady-state" behavior is associated with the crack growing into a region of comparatively lower hydrogen concentration located further away from the crack tip. The model treats the effects of and PH2 as being functionally separable. In the transient regime, the effects of the hydrogen pressure on the HA-FCGR behavior were negligible but were significant in the steady-state regime. The hydrogen concentration in the steady-state region is modeled as being dependent on the kinetics of lattice diffusion, which is sensitive to pressure. Experimental HA-FCGR data from the literature were used to validate the model. The new model was shown to be valid over a wide range of conditions that ranged between -1≤R≤0.8 and 0.02≤PH2≤103 MPa for pressure vessel steels.