Acoustic velocities in heated block of granite subjected to uniaxial stress

Abstract

Acoustic velocities have been measured within a 38 cm cubic block of granite isolated from the surrounding rock. The block was subjected to uniaxial stresses in the range 0‐24 Mpa and to heating in the range 14° to 98°C. An increase in temperature from 14° to 72°C in the unstressed block resulted in an increase in velocities in directions parallel and perpendicular to the applied stress. A further increase in temperature to 98°C in the unstressed block resulted in a decrease in velocities. Application of uniaxial stress with the block at a temperature of 98°C resulted first in uniform increases in velocity to an applied stress of MPa and then sharp decreases from 10 to 14 MPa, followed by uniform increases in velocity to a maximum stress of 24 MPa. A reduction in temperature to 16°C resulted in a sharp reduction in velocity, 25 percent less than originally measured at ambient temperature. It is concluded that, at temperatures rising to 72°C, anisotropic thermal expansion of the crystals comprising the granite causes the closure of microcracks present in the unheated rock. Between 72° and 98°C, particularly when stress was applied, irreversible mechanical changes occurred in the block.