Yoshitaka Nara

Subcritical crack growth in rocks in an aqueous environment.

Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. In the present study, we investigated subcritical crack growth in rock in distilled water (pH = 5–7) and in an aqueous solution of sodium hydroxide (NaOHaq, pH = 12), comparing the results to those in air. We also investigated the effect of the pH in an aqueous environment. We used andesite and granite for all our tests. We determined the relationship between the crack velocity and the stress intensity factor using the double-torsion test under conditions of controlled temperature. We showed that crack velocities in water were higher than those in air, in agreement with other research results indicating that crack velocity increases in water. When we compared our results for NaOHaq with those for water, however, we found that the crack velocity at the same stress intensity factor did not change even though the pH of the surrounding environment was different. This result does not agree with the accepted understanding that hydroxide ions accelerate subcritical crack growth in rocks. We concluded that the pH at the crack tip influences subcritical crack growth, and not the bulk pH, which has little effect.

Effects of the Relative Humidity on the Crack Growth in Rocks

It is important to know the effect of the humidity on the fracturing in rock. However, none has reported the effect of only the relative humidity on the crack propagation under a constant temperature. In this study, the fracture toughness and subcritical crack growth in sandstone and granite were investigated in air. Especially, the effect of the relative humidity on the fracture toughness and the relation between the crack velocity and the stress intensity factor were investigated under a constant temperature. The fracture toughness decreased and the crack velocity remarkably increased in both rocks when the relative humidity was higher. The effect of the relative humidity was stronger than expected before. It is concluded that the change of the relative humidity in air has strong effects on the crack growth in rocks.

Effect of Water on Subcritical Crack Growth Index and Long-term Strength for Rock

Long-term stability is required for the structures in a rock mass. Especially, the estimation of long-term strength of rock is essential to consider the long-term stability. In this study, we investigated the time-dependent crack growth in rock. Specifically, subcritical crack growth in water and in air with different relative humidities was measured in order to investigate the effect of water on the value of subcritical crack growth index and the long-term strength for rock. It was shown that subcritical crack growth index in water was smaller than that in air. Additionally, subcritical crack growth index tended to be smaller when the relative humidity was higher. The long-term strength in water was smaller than that in air. In air, the long-term strength tended to be smaller when the relative humidity was higher. The long-term strength is larger when subcritical crack growth index is larger. For the long-term stability, it is thus important to achieve a condition where subcritical crack growth index becomes larger. It is concluded that retarding the migration of water into and through rock and keeping low humidity is important.

Effect of the Relative Humidity on Subcritical Crack Growth in Sandstone

It is important to know the effects of the environmental conditions on the crack growth in rock. However, the environmental dependence of the crack growth has not been clarified in sedimentary rocks yet. In this study, the effect of the relative humidity

Subcritical Crack Growth in Rocks Under Water Environment

Classical fracture mechanics postulates that the crack propagates dynamically when the stress intensity factor reaches a critical level, that is, fracture toughness [1]. However, the crack can propagate slowly even when the stress intensity factor is less than the critical level. This phenomenon is called subcritical crack growth, and the main mechanism of subcritical crack growth is stress corrosion [2]. Subcritical crack growth is one of the main causes of timedependent behavior in rocks. It has been shown that subcritical crack growth in rocks under air condition is facilitated by water vapour pressure [3]. In this study, subcritical crack growth in rocks under water environment is investigated.

The effects of the temperature and the humidity on the properties of the elastic wave propagation in granite

In this study, the measurement of the P-wave velocity in granite was conducted under controlled temperature and humidity. Lead-zirconate-titanate (PZT) ceramics were used for sensors that were attached directly on the surface of the rock specimen of 60 mm cube. It was found that P-wave propagation properties were affected by the environmental conditions. The relative humidity had the strong effect on the P-wave velocity. When the temperature was higher than 55 degree centigrade and the relative humidity increased, the P-wave velocity decreased due to the crack growth by stress corrosion. It can be concluded that subcritical crack growth caused by stress corrosion occurs for microcracks due to the thermal stress and the water vapor.

Dependence of Subcritical Crack Growth in Rocks on Water Vapor Pressure

Abstract It is considered that subcritical crack growth is caused by stress corrosion. In the case of rocks, stress corrosion is a weakening process of a strong siloxane bond structure, and the corrosive agent is water. In this study, the dependence of subcritical crack growth on water vapor pressure was investigated by using Double-Torsion test. Rock studied was Kumamoto andesite. During experiments, the partial pressure of water was kept constant by controlling the temperature and the humidity. When the temperature and humidity were higher, the crack growth rate became higher than that under the lower temperature and humidity. Crack growth rate in andesite was approximately proportional to the partial pressure of water vapor. The activation energy for subcritical crack growth in andesite was evaluated as 110 kJ/mol, and the crack velocity under various water vapor pressures could be estimated.