Interparticle contact behavior and wave propagation

The low-strain stiffness and energy dissipation in particulate materials is strongly determined by the behavior of contacts. This paper presents results of a test program designed to study the effect of contact response on the propagation of waves. Wave velocity and attenuation were measured during isotropic loading using a resonant column device at shear strains varying from γ = 10^-5 to γ = 10^-6. Elastic, viscoplastic and brittle contact behaviors were studied with steel spheres, lead shot, and silica-kaolinite pellets. All measured velocity-stress exponents were b/2 > ≈ 1/6, which is the theoretical value for spherical contacts. High-tolerance steel spheres approximated this value. Contact crushing showed the highest exponent. Theoretical analyses confirmed that several phenomena conduce to a velocity-stress exponent b/2 = 0.25: buckling of particle chains and increase in coordination number, elastoplastic behavior, and cone-plane contacts. Load and unload data for viscoplastic lead shot showed that contact deformation is the governing parameter for low-strain stiffness, regardless whether the causing mechanism was elastic deformation, creep, or yield. All measured damping-stress exponents were between κ = -0.45 for steel and κ = -0.11 for the brittle pellets, while the theoretical value for factional Mindlin contacts is κ = -2/3. Damping showed higher sensitivity than velocity to stress and time.