By means of charging large thick specimens with hydrogen, we investigated the effects of specimen size on hydrogen-embrittlement cracking. Crack extension in a hydrogen-charged 3.5T-CT specimen extended over a longer duration than was the case for a 1.0T-CT specimen. However, the values of the lower-bound threshold stress intensity factor (KIH) for 1.0T-CT and 3.5T-CT specimens were similar to one another when these were determined with a short-term rising load (dK/dt=0.005 MPa•m1/2•s-1). We conducted numerical analysis on the hydrogen diffusion and accumulation around a crack tip, taking into consideration the hydrogen distribution in the specimen. This analysis demonstrated that the maximum hydrogen concentration for cracking can be reached under the conditions present during a short-term rising load test (dK/dt=0.005 MPa•m1/2/s). Thus, the results of the numerical analysis confirm that a minimum value of KIH equivalent to that of a heavy section steel can be obtained with a small fracture mechanics specimen. We also attempted to explain long-term crack extension characteristics, taking into consideration hydrogen dissipation from a specimen. The analysis predicts that when the mean hydrogen concentration falls below a certain level (e.g., about 1.6 ppm under certain assumptions), the value of KIH increases significantly. This increase in KIH occurs because when the necessary stress intensity factor for cracking increases as a result of a decrease in the mean hydrogen concentration, the gradient of the maximum hydrostatic stress distribution becomes moderate, especially when the applied stress intensity factor is more than about 48 MPa•m1/2. Finally, we propose a method for the prediction of the long-term crack extension behavior of a large thick specimen; the method takes into consideration the hydrogen dissipation curve and the effect on KIH of a decrease in the mean hydrogen concentration.
