The effect of inter-grain contact material on the dynamic fracture of short glass bead chains under impact

Impact induced dynamic fracture plays an important role in controlling the compaction behaviour of brittle granular media. Associated effects include dramatic changes to porosity and permeability, especially along earth-quake faults, and during mining and petroleum production processes. In this study, using split Hopkinson bar impact experiments on short chains of three glass beads whose contacts are separated by copper shims, we demonstrate that the front bead is much more sensitive to fracture than the rear bead, while the middle bead only damages slightly for the applied impact velocity range. The varying damage level of the rear bead indicates that irregular fracture mechanisms may exist even though the damage level of the front bead is almost the same. Corresponding numerical simulations with a simple brittle damage model for the glass successfully reproduce the crack initiation state of the failed glass beads. Comparisons between the glass bead chain systems, with and without contact materials, clearly indicate that the contact material changes the chain damage patterns, in particular for the middle bead that gets shielded by the shims. The contact material alters the initial contact state of the middle bead and consequently restrains the initiation of cracks, which is governed by the tensile stress near the contact surface. The chain system damage level is also inversely related to the amount of the contact material layers, while the insertion position generates additional effects. The thickness of the contact materials causes a slight fluctuation to the chain damage extent due to complex stress wave reflection and superposition. (C) 2018 Elsevier B.V. All rights reserved.