115 MPa水素ガス中におけるSCM435の水素誘起き裂進展下限界値<i>K</i>_I,Hを求める試験方法

<p>In order to investigate the suitable test methods for determining threshold of hydrogen-induced crack growth, <i>K</i>_I,H, <i>J</i>_Ic, none-unloading <i>J</i>_Ic, constant-load and constant-displacement tests were conducted in 115 MPa hydrogen gas at room temperature, using SCM435 low alloy steels whose tensile strengths were between 824 and 1127 MPa. Constant-load and constant-displacement tests have been standardized by ASME KD-10 for determining <i>K</i>_I,H in hydrogen gas. <i>J</i>_Ic test has been standardized by ASTM E1820 for determining fracture toughness <i>J</i>_Ic or <i>K</i>_Ic in air. Recently, <i>J</i>_Ic test was done in hydrogen gas to obtain <i>K</i>_I,H. In this study, there were many cases when <i>J</i>_Ic tests were not conducted in hydrogen gas, because CT specimens broke before the first unloading. Then, none-unloading <i>J</i>_Ic test, where unloading processes were eliminated in <i>J</i>_Ic test, was employed in hydrogen gas. All four tests were done under small scale yielding in 115 MPa hydrogen gas. <i>K</i>_I,H obtained by four tests was almost the same. KD-10 demands that constant-load and constant-displacement tests of ferritic steels such as SCM435 are done for 1000 hours. Contrary to this, none-unloading test at displacement rate of 2×10^-3 mm/s was finished within one hour. It was concluded from these results that none-unloading <i>J</i>_Ic test was the best test for determining <i>K</i>_I,H in 115 MPa hydrogen gas. On the other hand, <i>J</i>_Ic test was the only test for determining <i>K</i>_Ic in air, because <i>K</i>_Ic was obtained under large scale yielding.</p>