SLOW DYNAMICS IN THE NONLINEAR ELASTIC RESPONSE OF BEREA SANDSTONE

A typical resonance curve-measured acceleration versus drive frequency -made on a thin bar of rock shows peak bending with a softening (nonli near) modulus as drive levels are increased. The shapes of these nonli near resonance curves were found in earlier work to depend somewhat on sweep rate; these ''slow dynamics'' are now examined and quantified. We have measured slow dynamics in a 0.3 m long, 50 mm diameter bar of Berea sandstone under ambient conditions. Peak strain levels during th e experiments ranged from 10(-11) to 10(-5) at driving frequencies nea r 4 kHz, the fundamental longitudinal resonance frequency of the bar. Slow dynamics begin to manifest themselves at strain amplitudes above 10(-6) at ambient conditions and at the onset of nonlinear peak bendin g. Strains above this value condition the rock, altering its response for minutes to hours after the drive has been turned off.