Abstract
The fracture toughness, J_<Ic>, of SCM435 was investigated in 115 MPa hydrogen gas, 0.7 MPa hydrogen gas and air at room temperature. J_<Ic>ig tests were conducted in accordance with the ASTM E1820. J_<Ic>ig in 115 MPa hydrogen gas was much smaller than in 0.7 MPa hydrogen gas and air. In air, a stretched zone was formed at the tip of the fatigue pre-crack, followed by dimples. In hydrogen gas, quasi-cleavage instead of stretched zone and dimples were formed at the pre-crack tip. The fracture toughness test shown in 0.7 MPa hydrogen gas is the same as that shown in a fatigue crack growth test in 0.7 MPa hydrogen gas at a number of cycles of n = 1 and stress ratio of R = 0; and thus J_<Ic>ig in hydrogen gas is not the real fracture toughness.
