The effect of the humidity on subcritical crack growth in igneous rock in air was investigated experimentally. Rocks used in this study were Kumamoto andesite and Oshima granite. Double-Torsion method was used in this study to measure the crack velocity and the stress intensity factor. All experiments were conducted under the same temperature and different relative humidity to investigate the effect of the humidity.
It was shown that the humidity affected the crack velocity strongly. The crack velocity increased with the increase of the humidity. The crack velocity increased by 1 ∼ 4 orders of magnitude when the humidity increased threefold or fourfold. This increase is larger than that estimated from the classical stress corrosion theory. It is concluded that the humidity has the strong effect on subcritical crack growth in rock and the control of the humidity is important to prevent the weakening of rock.