The Effect of Heat on Some Mechanical Properties of Igneous Rocks

Abstract

Abstract The elastic moduli of most igneous rocks are greatly reduced by heating and depend, at room temperature, on the highest temperature to which they have been previously heated. This phenomenon was investigated by measuring velocity, attenuation, bending strength, and point penetration as functions of moisture point penetration as functions of moisture content, pore pressure, confining pressure, and highest previous temperature for a series of gabbro samples from French Creek, Pa. it was concluded that heating introduces a system of macrocracks between grains that is distinct from a system of microcracks that exists within grains. Thus bending strength is reduced by the introduction of macrocracks, but resistance to indentation, which depends on the microcracks, is not. It is suggested that from the measurement of velocity and attenuation alone, the macro and microcracks can be detected and distinguished. Introduction In 1937 J. M. Ide reported on how heat affects the elastic constants of a wide variety of rocks. He found that the constants (Young's modulus and Rigidity modulus, measured at room temperature) were permanently reduced to values determined by the highest temperature to which the rock had been previously heated. He suggested that this might be explained by the development of microcracks in the framework of the rock. The microcracks would be caused by the unequal expansion of the various minerals in the rock. The greatest reduction of elasticity occurred with gabbros, which are composed of roughly equal mixtures of feldspars and pyroxenes. Almost no reduction in the constants was found for limestones. In more recent years a revival of interest in this subject has followed the development of high power laser beams. The destructive effect of laser beams has been attributed primarily to the rise in temperature of the irradiated rock. This has two effects. One is the initiation of macrocracks by the creation of large tensile stresses. The other is the reduction of elasticity by microcracks, as noted by Ide.