Effect of surface properties on momentum transfer to targets impacted by high-velocity sand slugs

Abstract

The response of dry and water saturated sand slugs impacting normally oriented and inclined rigid-stationary targets with four different surface coatings is measured with an emphasis on the quantification of the momentum transmitted from the slugs into the targets. The targets were coated with Alumina, PTFE, Aluminium or sandpaper layers in order to investigate the effect of varying surface hardness and surface roughness. In all the cases, the fraction of the slug momentum transferred into the target was equal for dry and water saturated sand slugs and also independent of the slug velocity over the range that is investigated here. For normal impacts, the surface coatings had no measurable influence on the momentum transfer into the targets and this was attributed to the symmetry of the impact event. However, the break of symmetry in the inclined impact cases resulted in two non-zero components of the net transmitted momentum into the targets and a strong influence of the surface coatings. This is attributed to friction between the sand particles and the target surface with the resultant transmitted momentum increasing in the order Alumina to PTFE to Aluminium to sandpaper surface coatings. In all cases, the transmitted momentum was less than the corresponding value under normal impact. Coupled discrete particle/ Lagrangian simulations of these experiments with the sand particles modelled as spheres captured the normal impact measurements with a high degree of fidelity. However, the simulations underestimated the transmitted momentum for the inclined impacts especially for the rough surface coatings such as the sandpaper: increasing the friction coefficient between the particles and the target in the simulations did not improve the predictions. We demonstrate that this discrepancy is due to the spherical particle assumption: in the experiments the sand particles are sub-spherical and this reduces the tendency of particles to roll on the target surface and thereby increases frictional interactions. Increasing the radius of gyration of particles decreased the discrepancy between the measurements and the predictions but yet could not accurately predict all components of the transmitted momentum. Most numerical calculations tend to use spherical particles to represent the impacting granular media. However, this study demonstrates the need to appropriately parameterise particle shape in such discrete particle calculations to accurately capture the granular media/ structure interactions.

Impact of granular slugs on rigid targets: Effect of grain shape and fracture

“…The sub-particles were assumed to be made from a solid of density ; = 2700 kg m oC (approximately equal to that of the quartz phase of silica) and the sub-particle contact model was defined by four parameters M , , O and ; . Liu et al 2013and Goel et al (2017) demonstrated that these parameters have a negligible effect on the interaction of the granular slug with the targets and thus following these studies we chose them to be M = 0.69 MN m op , O M ⁄ = 2 7 ⁄ (Bathurst and Rothenburg, 1988;Silbert et al, 2001), = 0.7 and ; = 0.6. The friction coefficient f between the particles and the target surface depends on the target surface and was varied over the range 0.0 ≤ f ≤ 2.0 in the calculations presented here.…”

“…Since the objective is to investigate only the effect of grain shape, all the simulations were restricted to grains comprising either a single spherical particle or grains constructed by joining together sub-particles via rigid connections. A number of studies (Goel et al, 2017;Liu et al, 2013;Pingle et al, 2012) have shown that for slugs comprising spherical grains, the transmitted momentum normalised by the initial slug momentum 7 , is independent of the slug velocity 7 . Here, we confirmed via spot calculations that this result extrapolates to non-spherical rigid grains that do not fracture.…”

“…These measurements indicated that while the momentum transmitted into the inclined targets was less than that for the normally oriented counterparts, it was significantly higher than that anticipated from analyses which neglect frictional interactions between the granular particles and the target; see for example Cheng et al (2007) as well as Johnson and Gray (2011). Goel et al (2017) subsequently showed that frictional interactions between the particles of granular medium and the target surface significantly increase the momentum transfer to the target. Moreover, Goel et al (2017) demonstrated that discrete particle numerical simulations using spherical particles were unable to capture these frictional effects.…”

“…Goel et al (2017) subsequently showed that frictional interactions between the particles of granular medium and the target surface significantly increase the momentum transfer to the target. Moreover, Goel et al (2017) demonstrated that discrete particle numerical simulations using spherical particles were unable to capture these frictional effects. They argued that modelling the angular particle shapes is critical in order to accurately predict the interaction of high velocity granular media with a target.…”

“…1. The experiments conducted by Goel et al (2017) indicated that friction between the grains and targets strongly influence the momentum transferred during oblique impacts and that coupled discrete particle/Lagrangian finite element (FE) simulations with spherical grains were unable to capture the measurements with sufficient fidelity. Artificially increasing the rotational inertia of the grains, and thereby reducing the rolling of the grains improved the predictions and suggested that grain shape might have a strong influence on momentum transfer.…”