Auxetic Granular Packings Absorb All Your Energy

Granular materials are often used because of their energy absorbing features; think of coastal protection by tetrapods, “packaging peanuts'' in shipping boxes or sand bags for flood control. The energy absorbing features from granular materials originate from the many dissipative interparticle contacts that are activated during compression and shear of a granular packing. To maximize dissipation energy during deformation, one should aim for maximizing the strain amplitude over which the dissipative forces can act, whilst simultaneously maximizing the dissipative forces between the grains. However, increasing strain and force at the same time is typically not possible: large deformation forces come from stiff particles, but stiff particle packings deform only very little. Here, we solve these conflicting aims by making granular packings of mixtures of 3-dimensional auxetic and normal grains. The auxetic grains become smaller when stressed, which entirely changes packing dynamics. We find that the dissipation that such packings experience in a compression-decompression cycle is close to five times higher than the dissipation experienced in a similar test with a packing of only non-auxetic or positive-Poisson's ratio grains. Furthermore, we show that the dissipation in a an auxetic granular packing can be tuned by changing its compression history and the ratio of regular and auxetic grains. Mixing grains with different Poisson's ratios is thus a very promising means to tune the compressive and dissipative properties to a specific need.