  We study experimentally the fracture mechanisms of a model cohesive granular
medium consisting of glass beads held together by solidified polymer bridges.
The elastic response of this material can be controlled by changing the
cross-linking of the polymer phase, for example. Here we show that its fracture
toughness can be tuned over an order of magnitude by adjusting the stiffness
and size of the polymer bridges. We extract a well-defined fracture energy from
fracture testing under a range of material preparations. This energy is found
to scale linearly with the cross-sectional area of the bridges. Finally, X-ray
microcomputed tomography shows that crack propagation is driven by adhesive
failure of about one polymer bridge per bead located at the interface, along
with microcracks in the vicinity of the failure plane. Our findings provide
insight to the fracture mechanisms of this model material, and the mechanical
properties of disordered cohesive granular media in general.
