Thermal effects on fracture toughness of cracked straight-through Brazilian disk green sandstone and granite

Cracked straight-through Brazilian disc (CSTBD) samples prepared using two rock materials were used for thermal treatment from room temperature to 700 °C. Uniaxial splitting experiments were performed using an automatic electro-hydraulic servo press to study the evolution laws of physical and fracture properties of different deep rock materials under high-temperature geological conditions. The fracture characteristics were measured using an industrial camera and digital image correlation technology to analyze the effect of high temperature on fracture properties and failure modes of the CSTBD samples after different thermal treatments. The micro-damage properties of green sandstone and granite materials were obtained using a scanning electron microscope (SEM). The following conclusions were drawn from the test results: (1) With the increasing temperature, the fracture characteristics of green sandstone and granite change from brittle fracture to plasticity fracture, the longitudinal wave velocity of granite decreases sharply at 600 °C, and the damage factor reaches 0.8748 at 700 °C. (2) The fracture toughness of green sandstone and granite decreases with increasing temperature; however, the decreasing range of granite is larger than that of green sandstone. (3) As the temperature increases, the fracture morphologies of green sandstone and granite materials become rougher, whereas thermal damage cracks of granite and intergranular fractures inside sandstone as well as pores of sandstone increase. (4) The crack tip opening displacement and peak strain corresponding to peak load increase with the temperature.