Rory P. Bigger

Dynamic response due to behind helmet blunt trauma measured with a human head surrogate

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.

Composite materials dynamic back face deflection characteristics during ballistic impact

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.

Measurement of damage velocities in impacts of transparent armor

A series of impact experiments were conducted to examine the response of transparent material to ballistic impact. The experiments consisted of impacting 15 mm of borosilicate glass bonded to 9.5 mm of Lexan. The projectile was a 0.30-cal hard steel bullet designed specifically for the experiments. High-speed imaging of the impact event and post-test analysis quantified damage propagation and the rate of propagation.

Dynamic Strain Measurement of Welded Tensile Specimens Using Digital Image Correlation

The high strain rate behavior of a welded interface was evaluated using a direct tension Kolsky (Hopkinson) bar setup. The weld and the Heat Affected Zone (HAZ) were included in the gage section of the tensile specimen to evaluate the effects of structural constraint (vs. weld metal only). The welds of interest are under-matched Metal Inert Gas (MIG) butt welds between identical aluminum alloys. Limitations on specimen geometry and maintaining the weld bead intact were imposed to provide a specimen that was most representative of the material and application. Strain determination for the welded specimen with non-uniform cross section was challenging using traditional techniques. A Digital Image Correlation (DIC) system was employed for dynamic strain measurement during high strain rate tensile testing. The results obtained with the DIC system are compared with strain gage data and post-test deformation measurements. Numerical simulations were also employed to aid with interpretation. These will be presented in further detail at the conference.

Experimental and computed results investigating time-dependent failure in a borosilicate glass

Symmetric plate-impact tests of borosilicate glass were performed from low (116 m/s) to higher (351 m/s) velocities. The tests were recorded with an ultra-high-speed camera to see the shock and failure propagation. The velocity of the back of the target was also recorded with a PDV (Photon Doppler Velocimeter). The images show failure nucleation sites that trail the shock wave. 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 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 decrease in intensity with time, probably due to the damage growth behind the shock wave.

A new technique for monitoring inhomogeneous deformation during flyer plate impact

A new and unique experimental configuration was developed and demonstrated to measure the inhomogeneous deformation of heterogeneous materials during flyer plate impact tests. Flyer plate experiments were performed on a granite material with a small scale structure; strain rates ranged from 105 to 107 s−1. A cross section of an impacted target was monitored and photographed during, and immediately following, passage of the shock wave through the material. Up to fourteen images were taken during passage of the shock wave. This was accomplished using an ultra-high speed Imacon camera with very short exposure times; for example, in one experiment the exposure time was 5 nanoseconds with a framing rate of 5 million frames per second. Continuous wave lasers were used as the illumination source. Edge and notch filters were used to lessen the intensity of the impact flash in the image. The photographic data was analyzed using a digital image correlation (DIC) system. These experiments examined inhomogeneous defo...

High-Speed DIC on Inside Perma-Gel During Ballistic Peneration

Perma-Gel® is a melt-castable, reusable synthetic ballistic gelatin used as a soft tissue simulant in research, forensics, and shooting sports. It can also be used to study the temporary wound cavity that forms during a penetrating injury from a ballistic projectile such as a small arms bullet or a blast fragment. This cavity is typically imaged with high-speed video cameras and its measured manually in each video frame. The dynamic cavity size alone, however, does not directly provide data on the strain that material nearby, but outside of, the wound cavity is subject to. In a recent project, SwRI® determined that it would be feasible to embed a dot pattern inside of the Perma-Gel block and use DIC to directly measure the strain field surrounding the wound cavity. Successfully and reliably embedding a dot pattern on a flat plane in the middle of the gel block proved to be challenging, in part because casting on top of a set layer with the pattern tended to melt the bottom layer and distort the pattern. After some modifications, a successful technique was developed. The method developed and example applications of it and the resulting DIC strain fields measured from a variety of projectiles and impact speeds are presented. The data demonstrate that DIC is an effective technique for measuring dynamic response of ballistic penetration in Perma-Gel, and can provide new insights that may be difficult or impossible to achieve with other measurement techniques.

High-Speed DIC on Flat Panels Subjected to Ballistic Impacts

Over the past several years, Southwest Research Institute® (SwRI®) has developed and applied DIC to measure back-face deflection and strain response on hundreds of flat panel test articles subjected to ballistic impacts from both small arms fire and bird-strike. The data collected has been used to develop new composite panel layups and validate material models to a level that no previous measurement technique was able to achieve. Getting consistent, high-quality DIC data during a ballistic impact test, however, presents some unique challenges not present in lower-rate testing methods. Proper pattern application techniques to avoid spalling during impact and protection of the expensive stereo high-speed camera pair must be considered and are addressed here. For small arms tests, the required image exposure time to avoid motion blur is on the order of 1 μs, requiring appropriate illumination methods. Bird strike presents a separate set of challenges including an outdoor environment, larger measurement area, and biohazard. The methods used to address these will also be presented. Materials tested include metals, transparent plastics, and composites including carbon, aramid, and polyethylene. Target sizes range from 6 × 6 to 36 × 36 in. and data rates from 10,000 to 50,000 Hz. Comparison of the DIC data with traditional measurement techniques such as strain gauges and calipers was also made. Agreement proved to be good, prompting the most recent series of tests to be run with DIC as the exclusive data collection method.

Comparison of breakout modes in analytic penetration modeling

When a projectile approaches the back surface of a target, complicated mechanics and different failure modes ensue that can lead to perforation. There have been a number of attempts to model these different failure modes, for example models due to Ravid and Bodner [1] and Walker [2]. Of particular interest are analytic models (as opposed to hydrocode models) where the model attempts to explicitly identify relevant failure modes and back surface breakout modes. This paper examines back surface breakout and target failure models with different failure modes to see how they are invoked in specific impact geometries and scenarios. Comparisons are made of thresholds and sensitivities for the different failure modes to gain a greater understanding of their usefulness and applicability. The model results are compared to data to ensure reasonableness of the computations. Good agreement is shown with a relatively simple back‐surface‐strain target failure mode for predicting ballistic limit velocity for large aspec...