Unconsolidated granular materials exhibit softening behavior due to external perturbations such as acoustic waves; namely, the wave speed and elastic modulus decrease upon increasing the strain amplitude. Recovery of the elastic modulus after the cessation of acoustic wave disturbance is logarithmic with time, over a span of hours or even years. One example of this class of behavior in nature is the elastic wave speed anomaly following the 2004 M6.0 Parkfield earthquake. In this talk, I describe a theoretical model for such behavior.
The model is based on the idea that shear transformation zones (STZs) -- clusters of grains that are loose and susceptible to contact changes, rearrangement and nonaffine displacement -- are responsible for plastic strain and softening of the material. We apply the theory to experiments on glass bead packs, and demonstrate that the theory predicts nonlinear resonance shifts and reduction of the P-wave modulus, in agreement with experiments. We also show that the theory produces the logarithmic aging behavior seen in glass bead experiments, from which we can extract a characteristic time scale associated with the dynamics of the STZ plasticity carriers. The theory thus offers insights on the implications of structural heterogeneities for damage, failure, and healing in materials.
Charles Lieou is currently a postdoctoral research associate at the Los Alamos National Laboratory, where his primary collaborators include Paul Johnson (Earth and Environmental Sciences Division), Curt Bronkhorst (Theoretical Division) and Irene Beyerlein (then at LANL's Theoretical Division before moving to UCSB), among many others. He has a broad interest in plasticity and deformation in a wide range of materials ranging from granular materials to fabricated nanomaterials. He is intent on pursuing a statistical-physics description of materials driven away from equilibrium by external forces. Charles Lieou is a UCSB alumnus; he received his PhD in physics in 2015, under the supervision of Jean Carlson and James Langer.