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Figure 4. Prototype blocks (from left to right : RTV silicone, polyurethane, SEBS gel). Silicone materials exhibit excellent stability and nearly no dependence on ...
Ministère de l'intérieur, de l'outre-mer, des collectivités territoriales et de l'immigration Direction des ressources et compétences de la police nationale Sous-direction de l'équipement et de la logistique Centre de recherche, d'expertise et d'appui Logistique

A substitute of gelatin for the measurement of dynamic back face deformation O. Mauzac1, C. Paquier1, E. Debord2, F. Barbillon2, P. Mabire2 and J.F. Jacquet2 1

CREAL, Ministere de l'Intérieur, 168 rue de Versailles, 78150 Le Chesnay, France [email protected] 2 DGA Techniques Terrestres, rocade Est- Echangeur de Guerry,18021 Bourges cedex, France

Abstract. The measurement of dynamic back face deformation of ballistic protections by means of ultra-high speed cameras is a well established method. In this method the ballistic protection is classically placed against a translucent gel of 20% gelatin in which the deformation can be observed. In this paper it is shown that gels based on synthetic SEBS polymers can be used in place of gelatin, with a number of practical advantages and a similar quality of results. The matters of repeatability and reproducibility are discussed. The applications to the characterization of behind armour blunt trauma and of the effects of less lethal kinetic projectiles are presented.

1. DYNAMIC BACK FACE DEFORMATION (BFD) ANALYSIS OF GELATIN 1.1 Experimental method This method is already quite well known and is a NATO standard [1, 2], therefore only a short description will be given here. The bullet resistant vest or other ballistic protection to evaluate is placed on a block of gelatin, and a bullet is fired to the vest. Typically the blocks are 25x25x30 cm in size and are made of gelatin at 20% concentration. This method is normally used in non perforating ballistic conditions. At impact, a temporary elastic deformation of the block occurs as the bullet is decelerated and finally stopped in the vest, and ultimately the block of gelatin returns to its initial shape. As the gelatin is a translucent material, the dynamic back face deformation can be recorded by a high speed camera placed on one side, with a projector placed on the other side to illuminate the block. The frame rate is typically of 10.000 to 12.000 fps. With a proper geometry of the experimental set up, the subsequent analysis allows to derive the deformation profile as a function of time, as well as an estimate of the corresponding volume (under the assumption of axial symmetry) and the associated velocities of deformation. A pseudo "viscous criterion" can also be calculated for each time increment by multiplying the depth of the BFD by its derivative, and dividing by a typical value of thorax thickness [3]. The velocity can be derived by differentiating the moving average of the data points (DGA approach) or via the application of a mathematical model (CTSI, damped oscillator model).

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Figure 1. Block of 20% gelatin (left) and example of test result (right : graphs of back face deformation, velocity of deformation and pseudo-viscous criterion versus time) MAUZAC & al. Personal armour systems symposium 2010, Quebec, Canada

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1.2 Feedback on the method with gelatin This method has been used satisfactorily for about 15 years by DGA Techniques Terrestres for the evaluation of hard ballistic plates, and reasonable correlations have been established with data generated on biologic materials. CTSI have worked with DGA Techniques Terrestres on the adaptation of this method to the case of soft ballistic materials, and later to the direct impact of less lethal kinetic projectiles (LLKE) to an unprotected block. The principal difficulty with this method is that each face of a gelatin block can withstand only one shot, therefore a large number of blocks are necessary, yet the preparation of a block is time consuming and its gelation takes 24 to 48 hours at 10°C, and the shelf life is no more than 3 or 4 days before biodegradation occurs. Besides, the properties of gelatin are quite dependent on temperature (Figure 2), which implies strict precautions in order to keep the block at constant 10°C temperature in a laboratory, under the high level of lighting needed due to the limited transparency of the gel.

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Figure 2. Dynamic mechanical analysis (DMA) characterization of 20% gelatin (G' 33 kPa at +10°C). Melting starts at +30°C. In addition, some problems of reproducibility were encountered during the initial tests campaigns on less lethal kinetic projectiles. The overall response of the material, not even the values of the BFD, were sometimes not consistent although the experimental conditions were carefully controlled (Figure 3). 60 2007/10/29 shot # 01 50

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Figure 3 : test of repeatability and reproducibility of the BFD in gelatin at 20% concentration MAUZAC & al. Personal armour systems symposium 2010, Quebec, Canada

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These drawbacks and difficulties have lead to the investigation of alternative reference materials for the measurement of the dynamic BFD.

2. THEORETICAL RESEARCH OF ALTERNATIVE REFERENCE MATERIALS 2.1 Specifications The following criteria were defined for the research of candidate reference materials. Mechanical and thermal properties : the material should exhibit a linear elastic behaviour with a low dependence on temperature ; its elastic modulus at room temperature should be near that of 20% gelatin at 10°C (about 33 kPa) and preferably be adjustable via formulation. It is highly desirable that it does not keep memory of previous impacts (either by mechanical damages or by rheological effects) so that it can be used for series of subsequent impacts, in order to increase the productivity and to allow for analyses of repeatability. Optical properties : high level of transmission and low level of diffusion of visible light are mandatory, a low refractive index is preferred in order to minimize distortion Logistic properties : excellent stability of properties and long shelf life are required (unlike gelatin that exhibits fast biodegradation). The material should be usable at ambient temperature. It should be easy to process, reusable and recyclable, and most preferably not include toxic ingredients. It should be based on industrial ingredients with good traceability warranting the reliability of procurements and the batch-tobatch consistency. The possibility to measure characteristic properties for the quality control is a must. Finally, overall cost should remain moderate. 2.2 Comparison of candidate materials The analysis of the specifications above and a review of the literature led to the identification of three material chemistries : silicones [4,5], polyurethanes, and physically associated gels of triblock thermoplastic elastomers [6]. For each of them, a prototype block of 25 cm size was prepared in an initial study of feasibility.

Figure 4. Prototype blocks (from left to right : RTV silicone, polyurethane, SEBS gel) Silicone materials exhibit excellent stability and nearly no dependence on temperature or ambient conditions in general and on rate of deformation. However, they are quite expensive and very few convenient translucent grades are commercially available. It was not possible to evaluate any physical gel of silicone in this study. The RTV (chemically crosslinked) grade that was evaluated exhibited a correct translucence in low thickness, but was far too cloudy when molded in the size of a block, despite of a careful processing including high level of degassing (Figure 4). Polyurethanes are easier to process, although again thorough degassing is mandatory. A few convenient grades are commercially available but their compositions are proprietary and will not be disclosed. Quite large possibilities exist for developing formulations with different levels of elasticity and of viscoelasticity (e.g. polyester- vs. polyether-urethanes), however this requires significant chemical

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processing equipment. Among the drawbacks of polyurethane are the toxicity of one constituent (isocyanate), the cost, and the fact that the softer grades tend to be very tacky and take up dust. SEBS are commercial thermoplastic elastomers consisting of triblock (styrene - ethylene / butylene – styrene) copolymers. When mixed with mineral oil, these can form non-crosslinked, thermally reversible gels that exhibit very good transparency (Figure 4). Gel candles are a familiar application of such gels. Due to their structure, these gels are self-healing and readily recyclable via re-melting. The manufacturing of the gels is relatively straightforward and does not require complex equipment. The material cost is similar to gelatin. The initial analysis indicated that this candidate was by far the most compliant to the specifications, therefore only this one was used for the subsequent steps of this research project.

3. PARAMETRIC STUDY OF SEBS GEL FORMULATIONS Nine formulations were defined, based on five different grades of SEBS and two ratios of polymer to oil in the gel (15:85 and 30:70 by weight). The SEBS were all from the same manufacturer and differed by their molecular weights (low / medium / high) and the styrene ratios in the block copolymer (13% to 31%). After preparing small blocks of about 10 cm size, three formulations had to be discarded because either it was not possible to form the gel, or the gel was too tacky or exhibited creep. The remaining formulations were submitted to mechanical testing and the results are given in Table 1. Table 1. Main characteristics and mechanical properties of the 6 formulations within the processability window Formulation # Color tackiness Shore 00 hardness Elongation at break (%) Modulus at 100% elongation (kPa)

1.1 none No 35 115 70

1.2 none No 61 380 200

2.1 none No 35 400 60

2.2 yellow Yes 62 800 180

3.1 yellow Yes 30 1200 50

4.2 None No 52 400 80

Formulation # 2.1 was characterized in the DMA (dynamic mechanical analyzer). It was found that the behaviour at 23°C is very linear up to large levels of deformation (Figure 5 left), with some dependence on the frequency (40% increase of elastic modulus from 5 to 1000 Hz, see Figure 5 right). As can be seen in Figure 6, the elastic modulus is independent of temperature over the range from -10°C up to at least 60°C. 1E+05

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4. PRESENTATION OF THE SEBS BLOCKS USED IN THIS STUDY The test series presented in this paper amount to 180 shots. They were performed on three blocks of SEBS gel prepared according to formulation #2.1. The blocks were 25x25x25 cm in size. Each of them has been used in two to five campaigns and has been regenerated between one and four times by re-melting, over a period of 22 months (mainly in order to evaluate the effect of this thermal treatment). The blocks were characterized in the DMA at each campaign and after each melting, and were found to have all three very similar mechanical properties that remained unchanged throughout the total period. Only some degree of tackiness and yellowing did develop with ageing and with the repeated thermal treatments, but these did not seem to affect the results, as will be seen further in this paper. Table 2. History of the three blocks of SEBS gel. Block # Initial preparation Regenerations by re-melting at 150°C

# 165 February 2008 July 2009

# 166 March 2009 June 2009 July 2009 December 2009

# 167 March 2009 June 2009 (3 times) July 2009

5. DIRECT IMPACTS OF LESS-LETHAL KINETIC PROJECTILES ON SEBS GEL 5.1 Experimental details Two kinds of projectiles were used in this campaign : a commercial less-lethal kinetic projectile (called DEF in this study) of 40 mm diameter with a round nose made of deformable elastomeric foam and a mass of 60 g, and a non-deformable projectile (called INDEF) made of rigid thermoplastic, with a diameter of 40 mm, a round nose and a mass of 29.9 g. The projectiles were shot at velocities between 50 m/s and 100 m/s using a gas launcher at a distance of ca. 20 cm from the block, which allowed a very good control of the location of impact and the obliquity of the projectile. The projectile velocity was measured by a laser gate at the exit of the launcher. Several shots were performed on each face of each block. Three SEBS blocks were used. Block #165 had been prepared 18 months ago and was never remelted since although it had been used for several tests series (about 40 shots). Blocks #166 and #167 had been prepared 3 months ago and used for one campaign, and had been re-melted respectively one time and three times before the tests. All three blocks were characterized in DMA and exhibited very similar properties.

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5.2 Shots of LLKE projectiles onto unprotected strike face In this phase the strike face of the block was totally unprotected. Substantial elastic deformations of 100 to 150 mm were formed at impact, with a rise time of about 5 ms, after what the projectiles would rebound and the block returned to its initial shape. Some SEBS material sticked to the projectile as it went away and finally came back to the block. This resulted in damages to the face of the block, in the shape of circular crack patterns (Figure 7). However, such damages did not modify the results with the subsequent impacts, as can be seen in Figure 8 where the results of three series of impacts in similar conditions on the three different blocks have been plotted. The overall scatter was less than 5 mm (for a BFD of about 140 mm), both for the variations between blocks and between impacts on a same block.

Figure 7 Impact of LLKE projectile on SEBS block : penetration (left, the projectile can be seen in the cavity), rebound of the projectile with gel sticking to its base (centre), circular mark left on the strike face of the block (right). The rest time between successive shots was varied between several hours down to 7 minutes, and was not found to affect the results, which rules out the possibility of memory effects of rheological nature for this time frame.

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Figure 8. Assessment of repeatability and reproducibility of the BFD on three unprotected blocks of SEBS gel (DEF projectile at 68 m/s) The graph in Figure 9 (below) shows that the amplitude of the BFD is effectively related to the kinetic energy of the projectile, and in Figure 10 it can be seen that the initial velocity of deformation of the block is proportional to the velocity of the projectile, even actually very close to it. The difference is not due to the deformation of the projectile itself, since it is also observed with the non-deformable projectile. The uncertainties on the derivation of both block deformation and projectile velocities may account for much of the difference, though. Nevertheless, these observations confirm that this gel of SEBS provides a linear elastic characterization of the impacts on its surface.

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5.3 Shots of LLKE projectiles with protection on the strike face Although they did not seem to alter the results nor impair the reusability of the block, the features of SEBS gel sticking to the projectile as it bounced away, and of cracks appearing on the strike face, lead to the research of a protective layer to be placed on the reference material. The rubbery artificial skin of the crash-test dummies like Hybrid III was an option that could not be evaluated. Instead, a 6 mm thick neoprene foam was chosen, as it is a readily available material in the laboratories performing stab resistance tests according to the HOSDB procedure N°39/07/C (2007) [7], and it is a standard product with constant characteristics. The deformations in this configuration were about three times lower than without protection. It was found necessary to replace the neoprene sheet at least at every 5 th shot because of a marked phenomenon of fatigue of this product. Despite of this precaution the reproducibility was sometimes poor, and it was preferable to repeat several series of shots in similar conditions in order to get clear trends. The general relationship between kinetic energy and depth of deformation is given in Figure 11.

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6. BALLISTIC IMPACT ON BODY ARMOUR USING SEBS GEL AS REFERENCE MATERIAL 6.1 Experimental details The study used four different threats (9x19 mm, 0.357 magnum, 0.44 magnum and 7.62x51 NATO) and five different ballistic protections, soft and hard, based on different types of materials. The same three blocks of SEBS gel were used as in the study on LLKE projectiles described in section 5. A neoprene protection was not used on the strike face. Again, the shots were repeated in series and on different blocks in order to evaluate the repeatability and the reproducibility of the results. 6.2 Results and comments Figure 12 shows a typical back face deformation. The graph in Figure 13 gives a general picture of the results with the different ballistic protections versus the different threats. It is obvious that the different levels of performance in term of behind armour blunt trauma can be characterized with this method. The graph also shows that the method is applicable throughout the range of ballistic threats.

Figure 12. Back face deformation of a ballistic protection on SEBS gel

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7. COMPARISON OF SEBS AND GELATIN Several experiments were repeated using both 20% gelatin and SEBS gel #2.1, in order to provide direct comparisons of the results. Figures 14 and 15 illustrate the typical difference of behaviour between these two gels at impact. The maximum back face deformation in SEBS gel #2.1 is substantially higher and the kinetic is slower. The repeatability of the results is often better with SEBS than with gelatin, like in the example of Figure 14. 120 100

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8. CONCLUSIONS The results presented in this paper correspond to 180 shots (half ballistic and half LLKE), and were generated over 10 days and with just 3 blocks of SEBS gel that are still usable. With gelatin, this would have needed 90 blocks (thus 250 work hours to make these) and at least 5 more days for the shots due to the limited production capacity of 6 blocks per day. During the 22 months of this study, the blocks were always available without the need to refrigerate them or to condition them at a specific temperature. The good transparency of the SEBS gel made it possible to use exposure times of the high speed camera about 12 times lower than for gelatin, which opens possibilities for recording at higher frame rates or for using less powerful, cheaper light sources. The repeatability and the reproducibility of the results were found to be correct. Similar results were obtained with different blocks even when they were not from the same batch or when they had very different thermal histories. The overall scatter of the results was just like with gelatin : generally within +/- 5 mm, and in some very unfavourable cases within +/- 20 mm. The specific SEBS gel formulation that was evaluated was softer than 20% gelatin. Although this may improve the sensitivity of the method, it could be a drawback as the link with the existing database generated with gelatin is not straightforward. It should however be possible to develop a formulation with the same response as gelatin, as well as stiffer formulations to approach the response of different body areas. This will be our next area of research, together with the improvement of the manufacturing process in the laboratory and the investigation of a better protection system for the impact face in LLKE experiments.

Acknowledgments This paper describes research that has been conducted in cooperation by the French MOD ( DGA Techniques Terrestres) and the French national police (CTSI). The authors wish to express their thanks to the teams who participated, and to the managers who allowed them to devote some time to this project.

References [1] Volff, "Caracterisation d'un blindage souple pare-eclat", Note N°17 CTME/MBE, DGA/DRET/ETCA internal report, 1991 [2] Stanag 2920, NATO standardization agreement [3] Bir C.A., "Female body armor assessment : current methods and future techniques", in Proceedings of the Personal Armour Systems Symposium 2004, The Hague, Netherlands [4] Smorenburg K., van Brug H., Cheng L., van Bree J., Djikstra M., Braat J., “optical measurements on shock waves in human tissue stimulants”, in Proceedings of the Personal Armour Systems Symposium 2002, The Hague, Netherlands [5] van Bree J.L.M.J., Volker A., van der Heiden N., “tissue stimulant response at projectile impact on flexible fabric armour systems”, in Proceedings of the Personal Armour Systems Symposium 2006, Leeds, UK [6] Juliano T.F., Moy P., Forster A.M., Weerasooriya T., VanLandingham M.R., “Multiscale mechanical characterization of biomimetic gels for army applications”, Journal of Materials Research vol.21 No.8, 2084-2092, 2006 [7] Croft J., Longhurst D., HOSDB body armour standards for UK police (2007), part 3 : knife and spike resistance, Publication No. 39/07/C

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