Pressure derivatives of elastic wave velocities from ultrasonic interferometric measurements on jacketed polycrystals

Abstract

A buffer-rod technique is described for the measurement by ultrasonic pulse interferometry of elastic wave velocities in jacketed polycrystalline specimens as functions of pressure to 3 GPa. The technique is tested on a suite of synthetic polycrystalline specimens (alumina and forsteritic olivine) for which bounds on the pressure-dependent wave velocities may be calculated from single-crystal elasticity data. Measurements conducted at pressures below ∼1 GPa with an unbonded buffer-rod configuration are compromised by the behavior of the interface between the buffer rod and the specimen. It appears that complex reflection and transmission coefficients associated with the contact between microscopically rough surfaces give rise to significant travel-time perturbations that decrease markedly with increasing pressure. These complications can be minimized but not eliminated by insertion between the buffer rod and the specimen of a disk of gold foil that amounts to a ‘‘bond’’ with more clearly defined acoustic properties. The range of pressures within which the interface effect is negligible may be significantly increased by the use of this bonded configuration. Measured elastic wave velocities are influenced by the presence of both microcracks and pores. Very large pressure derivatives measured between 0 and 0.5 GPa on one specimen are attributed to the closure of 0.1% porosity associated with cracks with aspect ratios less than 10−3. Otherwise, pressure derivatives are normal with velocities approximately 1%, 3%, and 4%–6% below the Hashin–Shtrikman bounds throughout the 0- to 3-GPa interval for specimens of 0.3%, 2.1%, and 4.8% porosity, respectively. These modest reductions in velocity and their survival to 3 GPa indicate that essentially all of the porosity in these specimens is contributed by relatively equidimensional pores of aspect ratio ∼0.1—consistent with their origin as sintered aggregates. The measured pressure derivatives are compared with those of the Hashin–Shtrikman bounds calculated from the pressure-dependent single-crystal elastic moduli. With the sole exception of the microcracked specimen at pressure below 0.5 GPa, the measured derivatives of wave velocities reproduce within ±10% the slopes of the Hashin–Shtrikman bounds demonstrating the viability of the technique for measurement of pressure derivatives of elastic wave velocities on fine-grained polycrystals.