Brian P. Bonner
Chantel Aracne-Ruddle
Cindy Shew
Earth Sciences Div., Lawrence Livermore Natl. Lab., Livermore, CA 94551
Rocks with appropriate microsctructure are nonlinear with hysteresis in the strain range 10-6 to 10-3 while showing little permanent damage during deformation. Nonlinear effects alter strong motions produced by earthquakes and explosions and may lead to new methods for relating the mechanical properties of rock to microstructure and transport properties. Results of laboratory torsion experiments will be presented to demonstrate that various nonlinear responses appear when granite and sandstone samples are driven in harmonic motion at low frequencies with increasing amplitudes. Modulus reduction, amplitude-dependent attenuation and harmonic generation occur and depend on microstructural parameters, including microfracture density and pore fluid content. Friction and adhesion at internal surfaces and the deformation of fluid phases in pores all play a role in these observed phenomena. Numerical constitutive models which include micromechanics are being developed to simulate nonlinear behavior for comparison with experiment. [Sponsored by OBES geosciences and performed under the auspices of the U. S. DOE by LLNLL under Contract W-7405-ENG-48.]