Gas gun

This paper presents the results from experiments and AUTODYN-3D s simulations on the ballistic impact of a KEVLAR s helmet. In the experiment, spherical projectile ($11.9 g), launched from a light gas gun, strikes the helmet with an impact velocity of 205 m/s. The interaction of the projectile with the KEVLAR s helmet is captured using high-speed photography. This helmet-projectile interaction is compared with that obtained from the AUTODYN-3D s simulation. Post-test damage photos from the experiments are also compared with those from the simulations. The response of the helmet from the simulations is consistent with those from the experiments. Also included in this paper are AUTODYN-3D s simulations on two ballistics test standards for KEVLAR s helmets. They are namely the NIJ-STD-0106.01 Type II and the V 50 requirement of the US military specification for Personal Armor System Ground Troops (PASGT) Helmet, MIL-H-44099A. For the simulation on MIL-H-44099A, a fragment-simulating projectile (FSP) strikes the helmet with an impact velocity of 610 m/s. The simulation revealed that an impact velocity above 610 m/s is required to perforate the KEVLAR s helmet. For the simulation on NIJ-STD-0106.01 Type II helmet, the projectile is a 9 mm full-jacketed bullet with a striking velocity of 358 m/s. Results from the simulation show that the KEVLAR s helmet is able to defeat a 9 mm full-jacketed bullet traveling at 358 m/s.

The quasi-static and dynamic compressive behavior of pyramidal truss cores made of 304 stainless steel were investigated using a combination of experimental techniques. Quasi-static tests were performed using a miniature loading stage while a Kolsky bar apparatus was used to investigate intermediate deformation rates. High deformation rates were examined using a light gas gun. Optical imaging of the sample deformation was performed in real time by means of high-speed photography. In this article, we provide a quantification of load-deformation response and associated failure modes across the sample as captured by high-speed photography. A finite element model is formulated and thorough simulations performed to understand the roles of material strain rate hardening and structural microinertia. Deformation modes were identified from acquired images, force-deformation histories and numerical modeling. Comparison between force-deformation histories under quasi-static and Kolsky bar loading reveals a moderate microinertia effect as manifested by a small increase in peak compressive stress. At high deformation rates, gas gun experiments, a totally different deformation mode is manifested with a major increase in peak compressive stress. In this case, the inertia associated to the bending and buckling of truss struts played a significant role. This effect appears to dominate the early truss core response because of two effects: (i) the propagation of a plastic wave along the truss members; (ii) buckling induced lateral motion. These findings are consistent with prior theoretical and computational work carried out by [Vaughn, D. Canning, M., Hutchinson, J.W., 2005. Coupled plastic wave propagation and column buckling. Journal of Applied Mechanics 72 (1), 1-8]. At larger deformations, the material strain rate hardening contribution to the total energy is as pronounced as the contribution arising from microinertia effect.

Herein, we study the technique of improving the gun performance by installing a diaphragm in the pump tube of the gun. A CFD study is carried out for the 0.28 in. gun in the Hyperveloeity Free Flight Radiation (HFF RAD) range at the NASA Ames Research Center. The normal, full-length pump tube is studied as well as two pump tubes of reduced length (,'-75% and ,,,33% of the normal length). Significant improvements in performance are calculated to be gained for the reduced length pump tubes upon the addition of the diaphragm. These improvements are identified as reductions in maximum pressures in the pump tube and at the projectile base of ,,,20%, while maintaining the projectile muzzle velocity or as increases in muzzle velocity of-,0.5 km/sec while not increasing the maximum pressures in the gun. Also, it is found that both guns with reduced pump tube length (with diaphragms) could maintain the performance of gun with the full length pump tube without diaphragms, whereas the guns with reduced pump tube lengths without diaphragms could not. A five-shot experimental investigation of the pump tube diaphragm technique is carried out for the gun with a pump tube length of 75% normal. The CFD predictions of increased muzzle velocity are borne out by the experimental data. Modest, but useful muzzle velocity increases (2.5-6%) are obtained upon the installation of a diaphragm, compared to a benchmark shot without a diaphragm. Particularly for larger guns, a way of reducing pump tube length while maintaining performance would be desirable. The technique studied here is to use multiple compressions, instead of a single compression, in the pump tube of the light gas gun. This would be done by installing one or more extra diaphragms in the pump tube. The diaphragms would be ruptured by the increasing pressure upon approach of the piston. In this paper, we present detailed CFD calculations that show that increases in 81

Advanced composites are increasingly used in aerospace, naval, and automotive vehicles due to their high speci®c strength and stiness. However, the mechanical properties of composite materials may degrade severely in the presence of damage. Damage due to impact in composite plates is often dicult to detect using any single technique. In this paper, the use of multiple sensing techniques to characterize high-velocity impact damage in advanced composites is reported. Broadband wave-based acoustic emission (AE) sensors are used to capture wave signals due to impact while shearography and ultrasonic (UT) immersion techniques are used to assess location and extent of damage after the impact. Five 48-ply [0= 45=90= À 45]6s laminated AS4/PEEK composite plates were used as test specimens. Shearography images of all ®ve test specimens were taken before impact testing to detect any preexisting internal damage from fabrication. Three broadband AE sensors were mounted on the surface of the composite plates to capture the AE signals due to impact. A 3/8-inch diameter stainless steel ball ®red from a gas gun facility was used as a projectile to in¯ict damage to the composite plates. The AE signals were instantaneously acquired during the impact tests and stored in a computer. The AE signals show existence of both the extensional and¯exural modes, with extensional modes typically showing ®rst. AE energy also increases to a threshold as the kinetic energy of impact increases. Shearography fringe patterns show existence of damage and this is con®rmed and quanti®ed with C-scan images from the UT immersion test. There is good correlation between AE parameters such as AE energy, AE amplitude, and AE count with impact energy and with damage on the composite plates. Due to the low contrast of the shearograms, UT C-scans are used to show extent of damage. This research demonstrates how multiple sensing techniques can be used to characterize high-velocity impact damage in advanced composites. Ó

This paper presents a novel experimental methodology for the study of dynamic deformation of structures under underwater impulsive loading. The experimental setup simulates fluid-structure interactions (FSI) encountered in various applications of interest. To generate impulsive loading similar to blast, a specially designed flyer plate impact experiment was designed and implemented. The design is based on scaling analysis to achieve a laboratory scale apparatus that can capture essential features in the deformation and failure of large scale naval structures. In the FSI setup, a water chamber made of a steel tube is incorporated into a gas gun apparatus. A scaled structure is fixed at one end of the steel tube and a water piston seals the other end. A flyer plate impacts the water piston and produces an exponentially decaying pressure history in lieu of explosive detonation. The pressure induced by the flyer plate propagates and imposes an impulse to the structure (panel specimen), which response elicits bubble formation and water cavitations. Calibration experiments and numerical simulations proved the experimental setup to be functional. A 304 stainless steel monolithic plate was tested and analyzed to assess its dynamic deformation behavior under impulsive loading. The experimental diagnostic included measurements of flyer impact velocity, pressure wave history in the water, and full deformation fields by means of shadow moiré and high speed photography.

There is a need for more faithful simulation of space debris impacts on various space vehicles. Space debris impact velocities can range up to 14 km/sec and conventional two-stage light gas guns with moderately heavy saboted projectiles are limited to launch velocities of 7-8 km/sec. Any increases obtained in the launch velocities will result in more faithful simulations of debris impacts. It would also be valuable to reduce the maximum gun and projectile base pressures and the gun barrel erosion rate. In this paper, the results of a computational fluid dynamics (CFD) study designed to optimize the performance of the NASA Ames 0.5" gun by systematically varying seven gun operating parameters are reported. Particularly beneficial effects were predicted to occur if (1) the piston mass was decreased together with the powder mass and the hydrogen fill pressure and (2) the pump tube length was decreased. The optimum set of changes in gun operating conditions were predicted to produce an increase in muzzle velocity of 0.7-1.0 km/sec, simultaneously with a substantial decrease in gun erosion. Preliminary experimental data have validated the code predictions. Velocities of up to 8.2 km/sec with a 0.475 cm diameter saboted aluminum sphere have been obtained, along with large reductions in gun erosion rates. The present CFD optimization was performed for the NASA Ames 0.5" gun. The effects of varying the gun parameters will be discussed in detail in the following sections. Support for DWB by NASA (Contract NAS-2-14031) to Eloret is gratefully acknowledged. lI. CFD Code Description The CFD code and its validation are described in detail in reference 5; hence, only a brief description of the code will be given here. The code is quasi-one-dimensional (quasi-l-D), uses the Godunov method and is third-order accurate in space and second-order accurate in time. The Riemann solver used is nearly exact. Friction and heat transfer to the tube wall for gases and dense media are modelled and a simple nonequilibrium turbulence model is used for gas flows. Realistic equations-of-state are used for all media. The code also models the gunpowder burn in the first stage breech. The code was validated against a number of analytical solutions and by comparing its predictions against experimental data from the Ames 1.5" and 0.28" light gas guns, taken over a considerable range of gun operating conditions. The agreement between the CFD predictions and the experimental data was judged to be very good. Further details on the description and validation of the code can be found in reference 5. Form Approved REPORT DOCUMENTATION PAGE OMBNoo7o4-o188 Public reporting burden for this collection of informationis estimated to average 1 hour per response, including the time for reviewinginstructions,searching existingdata sources, gathering and maintaining the data needed, and completingand reviewingthe collectionof information. Bend comments regardingthis burden estimate or any other aspect of this collectionof information, includingsuggestions for reducing thisburden, to Washington HeadquartersServices, Directoratefor informationOperations and Reports,