Micromechanics of Compressive and Tensile Forces in Partially Bonded Granular Materials

Bulk material properties such as strength and stiffness are highly sensitive to changes in the degree of interparticle cohesion or the introduction of elongated particles, effects that are both associated with an increase in the average number of constraints per particle. By performing an ensemble of isotropic compression experiments, all starting from the same initial particle configuration but with varying fraction of bonded particles, we use photoelastic force measurements to identify the causes of this phenomenon at the particle scale and mesoscale. By introducing a variable percentage of bonded dimers, we observe the expected decrease in the critical packing fraction at which jamming occurs. Above jamming, the local pressure increases predominantly for the bonded particles, measured relative to the unbonded case. The use of bonded dimers allows us to directly measure the relative contributions of tensile and compressive forces at the bond, which we observe occur with approximately equal probability. Histograms of the magnitude of the interparticle forces become broader for systems with more bonded particles. We measure both pressure and coordination number as a function of distance from a bonded dimer, both of which are locally enhanced for nearest neighbors, indicating that dimers appear to act as areas of concentrated force and connectivity that improve rigidity.