Combined effects of increasing temperature and confining pressure on the fracture toughness of clay bearing rocks

In order to understand the effects of increasing temperature and confining pressure on the fracture toughness of clay bearing rock, fracture toughness experiments were carried out at temperatures elevated from room temperature up to 200°C using single edge-notched round bar in bending (SENRBB) and semi-circular bend (SCB) specimens of Kimachi sandstone and Tage tuff. This paper describes the methodology for evaluating level 1 fracture toughness and the crack growth resistance curve. The crack growth resistance curve is shown to yield true fracture toughness even when under-sized specimens are employed. The under-sized specimens refer to those which are too small to produce valid fracture toughness value using the standard methods.The fracture toughness of Kimachi sandstone did not vary significantly with temperature up to 125°C, but above that point, it increased with temperature. SENRBB tests showed that the level 1 fracture toughness increased by approximately 40% at 200°C over its value at room temperature. The fracture toughness of sandstone and tuff was found to be significantly affected by increasing confining pressure. For example, in the arrester orientation, the fracture toughness of Kimachi sandstone increased by approximately 470% at 9 MPa confinement over its value at atmospheric pressure. A quite similar variation of fracture toughness is caused by the combined effects of temperature and confining pressure. For example, under a confining pressure of 7 MPa, the fracture toughness of sandstone decreased with temperature up to 75°C, and then increased between 75°C and 100°C.