Acoustical experiments on the non-linear and non-equilibrium behavior of rocks

University dissertation from Karlskrona : Blekinge Tekniska Högskola

Abstract: The study of dynamic behavior of non-perfect materials, such as rocks, is a rich topic for developing sensitive material evaluation and damage detection techniques. It has extensive applications in a variety of fields such as Physics, Geology, and Aerospace, Mechanical, Civil, Petroleum, and Material Engineering. It is experimentally known that as the level of damage in any initially 'un-damaged' materials increases, their nonlinear elasticity increases considerably and their dynamic behavior becomes more similar to heterogeneous and micro-inhomogeneous materials such as rocks. Despite the diversity of inherently non-perfect materials, they share some common features, among them are fast nonlinear dynamics, slow dynamics and conditioning.In this thesis, the fast and slow dynamics, recovery and relaxation of geomaterials subjected to external influences such as ultrasound, heat and light are investigated experimentally. The effect of light on rock is monitored using two different material state indicator tests: the measurement of deflection of a granite beam; and, the changes in sound speed. Young's modulus variations for small temperature changes in granite, steel, brass, and graphite are measured, providing us with a quantitative base for separation of softening due to the temperature and to the ultrasonic field respectively.The resonance frequency dependence on amplitude of acoustic wave at temperatures from 15-60 Celsius was investigated. The effects of each of them (the temperature and the acoustic intensity) have been known separately, but here were conducted experiments to see their combined effects. The sound speed was monitored through the resonance frequency by a series of ultrasonic frequency sweeps with successively increasing amplitudes. There was in each temperature set surprisingly one - and only one - temperature at which the sound velocity increased with the ultrasound level. At all other temperatures the sound velocity decreased.In another test, the effect of ultrasonic activation of marble in a constrained boundary condition is studied experimentally for the first time. The observed effects indicate simultaneous expansion and softening of the rock when subjected to acoustic waves