Experimental observations on the effect of thermal treatment on the crack speed and mode I and II fracture toughness in brittle and ductile rocks

The mechanical properties of rocks are greatly affected by thermal treatment, as this treatment can either improve or worsen these properties depending on the mineralogical composition. Therefore, the effect of thermal treatment on the (1) crack speed properties such as crack propagation speed, crack acceleration, and deceleration, and (2) fracture resistance to grow under two loading modes (pure mode I and pure mode II) was investigated for both brittle and ductile rocks. The ductile type was mudstone, which is a clay-rich rock with a high affinity for water. While the brittle type was granite, which does not contain water-active minerals such as clay or lime. Eighteen semi-circular specimens of mudstone and granite were prepared and tested at room temperature (RT), 250 and 500 degrees C. The results showed one behavioral trend for the effect of temperature on the mudstone. Due to the drying of clay minerals by increasing the temperature, the crack speed and fracture toughness improved significantly under pure mode I and II after thermal treatment. On the other hand, the thermal treatment of granite caused two behavioral trends. The first trend shows that crack speed and fracture toughness increased under pure mode I and II by increasing the temperature from RT to 250 degrees C due to crack arrest by randomly distributed thermal microcracks. In the second trend, they decreased below their normal levels under treatment from room temperature to T = 500 degrees C due to the extension and coalescence of thermal microcracks. The granite behavior changes depending on the density and extension of thermal microcracks. Under the three temperatures of RT, 250, and 500 degrees C, and for the two rock types, crack propagation speed, crack acceleration, and deceleration under pure mode II were greater than that under pure mode I. Where the fracture goes through three main stages growth, high acceleration, high deceleration, and low deceleration stages. In addition, the effect of temperature on the fracture toughness ratio (KIIC/KIC) was evaluated and the results showed that increasing the temperature increased the fracture toughness ratio. This means that after thermal treatment, the improvement rate of fracture toughness under pure mode II is better than that under pure mode I, and the crack tends to grow under pure mode I rather than mode II.