Abstract
In order to understand the effect of elevated temperature on fracture toughness, fracture toughness experiments were carried out at increasing temperatures from room temperature to 200 °C using the single edge-notched round bar in bending (SENRBB) specimen and the semi-circular bend (SCB) specimen of Kimachi sandstone. The paper firstly describes the methodology for the evaluation of level I fracture toughness and crack growth resistance curve. Crack growth resistance curve is also shown to consider the increase of the crack growth resistance with crack growth. The experimental results showed that fracture toughness of Kimachi sandstone did not vary significantly at temperature up to 125 °C and increased with elevated temperature beyond 125 °C. The level I fracture toughness from the SENRBB testing increased by approximately 40 % at 200 °C when compared to the value at room temperature. The variation of fracture toughness can be explained in terms of thermally-induced microcracks and dehydration of interlayer water and adsorptive water included in clay material such as montmorillonite. The thermally induced microcracks, which were confirmed under microscope, lower the fracture toughness due to decrease in crack growth resistance, while the dehydration of interlayer water and adsorptive water in the rock specimen, which were confirmed by measuring weight under several temperatures, raises the fracture toughness because of increase in bonding strength between the mineral particles. Based on the strain and Young's modulus measurements under elevated temperature, it was concluded that the dehydration of interlayer and adsorptive water and the thermally induced microcracks influenced on the fracture toughness of Kimachi sandstone.
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