The effect of inclination and stand-off on the dynamic response of beams impacted by slugs of a granular material

The dynamic response of end-clamped sandwich and monolithic beams to impact by highvelocity tungsten carbide (WC) particle columns (slugs) has been measured with the aim of developing an understanding of the interaction of ejecta from a shallow-buried explosion with structures. The monolithic beams were made from stainless steel, while the sandwich beams of equal areal mass comprised stainless steel face sheets and an aluminium honeycomb core. High-speed imaging was used to measure the transient transverse deflection of the beams, to record the dynamic modes of deformation, and to observe the flow of the WC particles upon impact. The experiments show that sandwich beams deflect less than the monolithic beams both in normal and inclined impact situations. Moreover, the deflections of all beams in the inclined orientation were less than their respective deflections in the normal orientation at the same slug velocity. Intriguingly, the ratio of the deflection of the sandwich to monolithic beams remains approximately constant with increasing slug velocity for inclined impact but increases for normal impact; i.e. inclined sandwich beams retain their advantage over monolithic beams with increasing slug velocity. Dynamic force measurements reveal that (i) the momentum transferred from the impacting slug to both monolithic and sandwich beams is the same, and (ii) the interaction between the impacting particles and the dynamic deformation of the inclined monolithic and sandwich beams results in a momentum transfer into these beams that is equal to or greater than the momentum of the slug. These experimental findings demonstrate that contrary to intuition and widespread belief, the performance enhancement obtained from employing beam inclination is not due to a reduction in transferred momentum. Finally, we show that increasing the stand-off distance decreases beam deflections. This is because the slugs lengthen as they traverse towards their target and thus the duration of loading is extended with increasing stand-off. However, combining increased stand-off with sandwich construction does not yield the synergistic benefits of sandwich construction combined with beam inclination.The work was supported by the Office of Naval Research Grant N00014-09-1-0573 (Program manager, Dr. David Shifler) and the Defense Advanced Projects Agency under grant number W91CRB-11-1-0005 (Program manager, Dr. J. Goldwasser).This is the accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0020768314004466

Coupled discrete/continuum simulations of the impact of granular slugs with clamped beams: stand-off effects

Coupled discrete particle/continuum simulations of the normal (zero obliquity) impact of granular slugs against the centre of deformable, end-clamped beams are reported. The simulations analyse the experiments of Uth et al. (2015) enabling a detailed interpretation of their observations of temporal evolution of granular slug and a strong stand-off distance dependence of the structural response. The high velocity granular slugs were generated by the pushing action of a piston and develop a spatial velocity gradient due to elastic energy stored during the loading phase by the piston. The velocity gradient within the “stretching” slug is a strong function of the inter-particle contact stiffness and the time the piston takes to ramp up to its final velocity. Other inter-particle contact properties such as damping and friction are shown to have negligible effect on the evolution of the granular slug. The velocity gradients result in a slug density that decreases with increasing stand-off distance, and therefore the pressure imposed by the slug on the beams is reduced with increasing stand-off. This results in the stand-off dependence of the beam's deflection observed by Uth et al. (2015). The coupled simulations capture both the permanent deflections of the beams and their dynamic deformation modes with a high degree of fidelity. These simulations shed new light on the stand-off effect observed during the loading of structures by shallow-buried explosions

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

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 show the show momentum transmitted from the slugs into the targets. The targets were coated with Alumina, PTFE, show Aluminium or sand-paper layers in order to investigate the effect of varying surface hardness and show surface show roughness. In all the cases, the fraction of the slug momentum transferred into the target was show equal show for dry and water saturated sand slugs and also independent of the slug velocity over the range 73ms−1−137ms−1that 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 sand-paper 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 sand-paper: 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.The work was supported by the Office of Naval Research Grant N00014-09-1-0573 (Program manager, Dr. David Shifler) and the Defense Advanced Projects Agency under grant number W91CRB-11-1-0005 (Program manager, Dr. J. Goldwasser)

On the Implications of a Sex Difference in the Reaction Times of Sprinters at the Beijing Olympics

Elite sprinters offer insights into the fastest whole body auditory reaction times. When, however, is a reaction so fast that it represents a false start? Currently, a false start is awarded if an athlete increases the force on their starting block above a given threshold before 100 ms has elapsed after the starting gun. To test the hypothesis that the fastest valid reaction times of sprinters really is 100 ms and that no sex difference exists in that time, we analyzed the fastest reaction times achieved by each of the 425 male and female sprinters who competed at the 2008 Beijing Olympics. After power transformation of the skewed data, a fixed effects ANOVA was used to analyze the effects of sex, race, round and lane position. The lower bounds of the 95, 99 and 99.9% confidence intervals were then calculated and back transformed. The mean fastest reaction time recorded by men was significantly faster than women (p<0.001). At the 99.9% confidence level, neither men nor women can react in 100 ms, but they can react in as little as 109 ms and 121 ms, respectively. However, that sex difference in reaction time is likely an artifact caused by using the same force threshold in women as men, and it permits a woman to false start by up to 21 ms without penalty. We estimate that female sprinters would have similar reaction times to male sprinters if the force threshold used at Beijing was lowered by 22% in order to account for their lesser muscle strength

The football is medicine plaform-scientific evidence, large-scale implementation of evidence-based concepts and future perspectives

The idea that football can be used as therapy and as a high-intensity and literally breath-taking training regime goes back centuries. To take one prominent example, the French philosopher Voltaire describes in the Book of Fate (1747), how a patient is cured by playing with a sacred football: “… full-blown and carefully covered with the softest Leather. You must kick this Bladder, Sir, once a Day about your Hall for a whole Hour together, with all the Vigour and Activity you possibly can”, “Ogul, upon making the first Experiment, was ready to expire for want of Breath”, “In short, our Doctor in about 8 days Time, performed an absolute Cure. His Patient was as brisk, active and gay, as One in the Bloom of his Youth.”1 Today, Voltaire and his main character, philosopher Zadig, have been proved right: Football is indeed a breath-taking activity and it can be used as therapy. Albeit today's recommendations suggest a lower training frequency, longer training periods and encourage group-based training, and say that any football can be applied

Unsteady penetration of a target by a liquid jet.

It is widely acknowledged that ceramic armor experiences an unsteady penetration response: an impacting projectile may erode on the surface of a ceramic target without substantial penetration for a significant amount of time and then suddenly start to penetrate the target. Although known for more than four decades, this phenomenon, commonly referred to as dwell, remains largely unexplained. Here, we use scaled analog experiments with a low-speed water jet and a soft, translucent target material to investigate dwell. The transient target response, in terms of depth of penetration and impact force, is captured using a high-speed camera in combination with a piezoelectric force sensor. We observe the phenomenon of dwell using a soft (noncracking) target material. The results show that the penetration rate increases when the flow of the impacting water jet is reversed due to the deformation of the jet-target interface--this reversal is also associated with an increase in the force exerted by the jet on the target. Creep penetration experiments with a constant indentation force did not show an increase in the penetration rate, confirming that flow reversal is the cause of the unsteady penetration rate. Our results suggest that dwell can occur in a ductile noncracking target due to flow reversal. This phenomenon of flow reversal is rather widespread and present in a wide range of impact situations, including water-jet cutting, needleless injection, and deposit removal via a fluid jet

Response of clamped sandwich beams subjected to high-velocity impact by sand slugs

The dynamic response of end-clamped sandwich and monolithic beams of equal areal mass subjected to loading via high-velocity slugs of dry and water-saturated sand is measured using a novel laboratory-based method. The sandwich beams comprise aluminium face sheets and an aluminium honeycomb core: the effect of sandwich core strength and beam thickness on the dynamic beam deflection is investigated by varying the orientation and height of the anisotropic aluminium honeycomb core material. High-speed imaging is used to measure the transient transverse deflection of the beams and to record the dynamic modes of deformation. The measurements show that sandwich beams with thick, strong cores are optimal and that these beams significantly outperform monolithic beams of equal mass. The water-saturated sand slugs cause significantly higher deflections compared to the dry sand slugs having the same mean slug velocity and we demonstrate that this enhanced deflection is due to the larger mass of the water-saturated slugs. Finally, we show that the impact of sand slugs is equivalent to the impact of a crushable foam projectile. The experiments using foam projectiles are significantly simpler to perform and thus represent a more convenient laboratory technique. © 2014 Elsevier Ltd. All rights reserved

The effect of inclination and stand-off on the dynamic response of beams impacted by slugs of a granular material

The dynamic response of end-clamped sandwich and monolithic beams to impact by high-velocity tungsten carbide (WC) particle columns (slugs) has been measured with the aim of developing an understanding of the interaction of ejecta from a shallow-buried explosion with structures. The monolithic beams were made from stainless steel, while the sandwich beams of equal areal mass comprised stainless steel face sheets and an aluminium honeycomb core. High-speed imaging was used to measure the transient transverse deflection of the beams, to record the dynamic modes of deformation, and to observe the flow of the WC particles upon impact. The experiments show that sandwich beams deflect less than the monolithic beams both in normal and inclined impact situations. Moreover, the deflections of all beams in the inclined orientation were less than their respective deflections in the normal orientation at the same slug velocity. Intriguingly, the ratio of the deflection of the sandwich to monolithic beams remains approximately constant with increasing slug velocity for inclined impact but increases for normal impact; i.e. inclined sandwich beams retain their advantage over monolithic beams with increasing slug velocity. Dynamic force measurements reveal that (i) the momentum transferred from the impacting slug to both monolithic and sandwich beams is the same, and (ii) the interaction between the impacting particles and the dynamic deformation of the inclined monolithic and sandwich beams results in a momentum transfer into these beams that is equal to or greater than the momentum of the slug. These experimental findings demonstrate that contrary to intuition and widespread belief, the performance enhancement obtained from employing beam inclination is not due to a reduction in transferred momentum. Finally, we show that increasing the stand-off distance decreases beam deflections. This is because the slugs lengthen as they traverse towards their target and thus the duration of loading is extended with increasing stand-off. However, combining increased stand-off with sandwich construction does not yield the synergistic benefits of sandwich construction combined with beam inclination