Single-yarn impact results have been reported by multiple authors in the past, providing insight on the fundamental physics involved in fabric impact. This insight allowed developing full fabric models that were able to reproduce properly wave propagation, deflection, and ballistic limits. This paper proposes a similar experimental methodology but for a specific composite material made of ultra-high molecular weight polyethylene. The presence of the polyurethane matrix in the composite is expected to slow down wave propagation. But the high-speed photographic tests reported in this paper indicate that wave propagation in strips and single-layer material is similar to that expected for dry fiber. An explanation is proposed for this unexpected result. This paper also reports the critical velocities (i.e., impact velocities that fail the fibers immediately) measured for the composite material and compares them to the velocities expected from the theory. The velocity is accurately predicted when taking into account wave interactions in front of the projectile. Finally, tests on multilayer composites are presented. In particular, a flash produced under the projectile during the first few microseconds was recorded with high-speed video cameras. A simplified study of the temperature increment upon impact indicates that the material may be reaching the autoignition point. This mechanism is speculated to be the origin of the flash systematically observed.
A Human Head Surrogate has been developed for use in behind helmet blunt trauma experiments. This human head surrogate fills the void between Post-Mortem Human Subject testing (with biofidelity but handling restrictions) and commercial ballistic head forms (with no biofidelity but ease of use). This unique human head surrogate is based on refreshed human craniums and surrogate materials representing human head soft tissues such as the skin, dura, and brain. A methodology for refreshing the craniums is developed and verified through material testing. A test methodology utilizing these unique human head surrogates is also developed and then demonstrated in a series of experiments in which non-perforating ballistic impact of combat helmets is performed with and without supplemental ceramic appliques for protecting against larger caliber threats. Sensors embedded in the human head surrogates allow for direct measurement of intracranial pressure, cranial strain, and head and helmet acceleration. Over seventy (70) fully instrumented experiments have been executed using this unique surrogate. Examples of the data collected are presented. Based on these series of tests, the Southwest Research Institute (SwRI) Human Head Surrogate has demonstrated great potential for providing insights in to injury mechanics resulting from non-perforating ballistic impact on combat helmets, and directly supports behind helmet blunt trauma studies.
The current rationale for development of composite combat helmets is to either maintain performance at reduced weight or maintain weight with a significantly higher level of ballistic performance. Typically, weight reduction with maintained performance is the design approach used. In order to reduce weight with the same materials requires a reduction of material thickness. Thinner structural materials then introduce the complicating and often limiting factor of greater back face deflection. To further understand the tradeoffs of ballistic performance and efficiency, weight and back face deflection, a research project was undertaken. In this research project, a set of 17 composite materials were investigated. The digital image correlation method was used to directly measure the characteristics of the dynamic back face deflection of targets engaged by a set of threats. The analysis of this data, which includes dynamic deflection time histories, back face velocity time histories, strain time histories and spatial distributions of these quantities, allowed for assessment of candidate material performance and characterization of back face deflection. The details of this experimental program and key data results are presented in this paper.
Symmetric plate-impact tests of borosilicate glass projectiles into borosilicate glass targets were performed at stress levels from 0.7 to 2 GPa (impact velocities from 116 to 351 m/s). As far as the authors know this range of velocities has not been explored in the literature previously. The tests used an ultra-high-speed camera to record shock and failure propagation. The velocity of the back of the target was also recorded with a photon Doppler velocimeter (PDV). The images clearly show the shock wave and its propagation at the speed expected. The reflected tensile wave is also apparent. What the authors interpret as the failure wave/front is clearly observed and appears at stress levels of 0.8 GPa (velocities as low as 130 m/s). The images apparently show failure nucleation sites that trail the shock wave. These seem to be closer to the shock wave for higher speeds. A possibility is that the defects open faster at higher impact velocity while at lower velocities they are still present but remain undetected. Interestingly, even though the failure wave is clearly seen, the PDV never detected the expected recompression wave. The reason might be that at these low impact velocities the recompression wave is too small to be seen and is lost in the noise. This, in the past, may have confused researchers by thinking that the failure wave was not present at these low impact velocities. This work also presents a new way to interpret the signals from the PDV. By letting part of the signal travel through the target and reflect on the impact side, it is possible to see the PDV signal decrease in intensity with time. This would be consistent with having damage in the interior of the specimen, something not straightforward to confirm through just high-speed photography.
The deformation response of welded aluminum plate was evaluated at high and low strain rates. Mechanical and ballistic experiments were conducted on 2.5 cm thick samples obtained from full penetration welds for welded aluminum 5083-H131 plate. Similar experiments were also conducted for the aluminum 5083 alloy as a baseline for comparison. Experiments were designed to compare the deformation response and ballistic performance differences for fusion welds versus friction stir welds. The fusion welds were processed using gas metal arc welding. The low strain rate deformation response was evaluated with three-point bend tests at an approximate strain rate of 1 s-1. The high strain rate response of the three materials was assessed using ballistic impact experiments at a range of velocities. Digital image correlation analysis was applied to gain insight into the deformation response through quantification of the strain and deflection profiles. The deformation response differences are compared for the welds versus the baseline aluminum alloy.
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