The influence of water vapor pressure of surrounding environment on fracture toughness of rock is clarified, based on the results of a series of semi-circular bend (SCB) test under various water vapor pressures. The rocks used in the test are Kumamoto andesite and Kunnum basalt, and the range of water vapor pressure is from 10-3 to 103 Pa. The results obtained in this paper are summarized as follows:
1) Elastic modulus at 60% of maximum load depends on water vapor pressure, and decreases with increasing water vapor pressure.
2) Fracture toughness depends on water vapor pressure, and decreases with increasing water vapor pressure. The tendency of influence of water vapor pressure on fracture toughness is the same as that on uniaxial compressive strength or tensile strength. The relation between fracture toughness and water vapor pressure can be represented as:
KIC=βp-m
where β is a constant and -m is the slope of the approximated line on the logarithmic graph. The value m is 0.013 for Kumamoto andesite and 0.012 for Kunnum basalt, respectively.
3) Based on above results for the SCB test, models for load-displacement curve and for relation between crack velocity and stress intensity factor are suggested.
4) The time until fracture was calculated under the assumption of the crack velocity V= αKIn and the above equation, where α is a constant and n is stress corrosion index. It is shown that the rock mass of large n and small m should be selected to assure the stability for long term in spite of change in surrounding water vapor environment.
