Relationship between three-dimensional crack morphology and macroscopic mechanical properties of hydrogen-related fracture in martensitic steel

In the present study, several parameters related to crack morphology in the case of hydrogen embrittlement were estimated by X-ray computed tomography and correlated with the macroscopic mechanical responses (J-integral and tearing modulus) obtained from the fracture mechanics tests. Even when the hydrogen content was high up to 4.00 wt ppm, unstable premature fracture did not immediately occur, and a certain crack-growth resistance could be confirmed. The three-dimensional crack morphology was not continuous with the formation of un-cracked ligaments in the uncharged specimen. In contrast, the hydrogen-related intergranular crack propagated more continuously with a smaller crack opening-displacement. The J-integral value monotonically increased with increasing estimated values of the surface area divided by the projected surface area on the macroscopic crack plane, indicating that crack meandering and branching increased the fracture energy. We defined crack-propagated thickness (standard deviation of the crack surface area at each section (parallel to the macroscopic crack plane) divided by the crack surface area) as a parameter representing crack meandering. The tearing modulus increased as the crack-propagated thickness increased, suggesting that crack meandering also increased the crack-growth resistance.