Elastic softening of sandstone due to a wideband acoustic pulse

Elastic wave propagation experiments were performed on a thin bar sample composed of Texas "moss" sandstone in order to study nonlinear elastic effects in the time domain. The present experiments utilized a pendulous hammer to produce axially propagating transient signals with strain amplitude between 15 and 130 microstrain in the mid-audio band. Particle velocity along the bar axis was measured with a laser Doppler vibrometer, focused at various locations along the bar. Nonequilibrium dynamics and nonlinear elasticity effects were observed on the propagating pulse as it reflected between the ends of the bar. The same effects were also observed at a single location along the mid-length of the bar using a continuous-wave 1 MHz probe signal, propagating transverse to the bar axis. The results demonstrate larger strain amplitudes and greater reduction in the Young's modulus than in previously reported measurements that employed narrowband excitation. Nonlinear attenuation of the axial pulse is also observed, which increases with excitation amplitude. The present results indicate significant conditioning of the sandstone, particularly "softening" of the Young's modulus of up to 20%, primarily during the tensile phase of propagation, with a "slow dynamic" memory that is similar to that reported in previous investigations.