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INNOVATIVE MATERIALS AND DESIGN FOR THE IMPROVEMENT OF WARFIGHTER HEAD PROTECTION Lionel R. Vargas-Gonzalez* and Shawn M. Walsh U.S. Army Research Laboratory Aberdeen Proving Ground, MD 21005-5069 ABSTRACT (thermoplastic/thermoset composites with metals, ceramics, or other composite systems), composite A great deal of effort has been focused in the Armed architecture and through innovative system design. forces community on development of new warfighter head protection that is lighter and more resistant to Specifically the focus of the research in head ballistic, blast, and blunt impact. Research has been protection includes: directed toward utilizing new materials and innovative design strategies to achieve these goals. Ultra-high a) Determining the impact properties of hybridized molecular weight polyethylene (UHMWPE) was thermoplastic composites and panels of varying hybridized with other polymer and carbon-based architecture materials, and with itself in various architecturally b) Evaluating the performance of hybridized complex hybrids, and evaluated for impact response. An magnesium alloy/UHMWPE composites exceptional hybrid system was discovered that exhibited c) Exploring methods for improved adhesion low levels of backface deformation and high penetration between magnesium alloy and UHMWPE velocity. In addition to this work, innovative design surfaces concepts have been proposed that could lead to higher d) Designing innovative head protection chassis impact resistance. A chassis-type concept has been tested structures for improvement of blunt impact and and has exhibited high levels of impact resistance. These blast performance. developments will optimistically lead to higher performance and greater safety in the next generation of Specific experimentation and results in these areas will be warfighter head protection systems. expanded and discussed in the following sections. 1. INTRODUCTION 2. INNOVATION IN MATERIALS Efforts within the Armed Forces community have 2.1. Hybridized and architecturally complex been increasingly focused on developing new lightweight thermoplastic composites materials and novel designs for head protection for the warfighter, in hopes of minimizing head and brain tissue Backface deformation performance has been of injuries in the event of ballistic impact, shock, and blunt concern in the thermoplastic-based helmet as of recent. trauma. Advanced materials, such as thermoplastic An attempt to improve the performance of the helmet in composites made from ultra-high molecular weight this aspect, thermoplastic composites (in this specific case polyethylene (UHMWPE), have had impact in both Dyneema HB25 and Spectra Shield II 3130) were reducing the overall system weight (UHMWPE has a hybridized with other fibers (aramids, carbon) and made specific gravity of 0.97 versus Aramid’s 1.44) and into flat composite panels. A flat plate configuration was increasing the ballistic penetration resistance (up to 35%). employed for this testing as it eliminated any processing Previously, a great deal of work had been focused on the variability inherent in the various methods of helmet manufacturability of these lightweight, high-strength manufacturing and performance variance due to helmet thermoplastic materials in ballistic helmets1‒5, as methods curvature. All of the panels were made with the same areal density (10.74 kg/m2 or 2.2 lbs/ft2); layers of HB25 for manufacturing were not previously in place. The were removed to accommodate the weight of the benefits of thermoplastics are not completely without stiffening materials. All laminates were consolidated and cost, as improvement in weight and ballistic properties cured using a uniaxial press (Wabash 800 Ton Press, have come at the expense of other important attributes, Wabash MPI, Wabash, IN) at 338 tons (20.8 MPa over specifically backface deformation, resistance to part) and 125°C for one hour. Hybrid laminates flammability, and impact resistance. However, it is incorporating Tensylon® materials were processed at a believed that these trade-offs could be mitigated and marginally lower temperature (115.5°C) and 13.8 MPa overall system properties could be improved through pressure, as per manufacturer specifications. For the hybridization of various materials architecture evaluation, HB25 plies were laid down in a quasi-isotropic fashion, with every two plies rotated While reconciled to the fact that the higher stiffness clockwise by 22.5°. The panels oriented in this manner in the quasi-isotropic hybrids would potentially lead to were not necessarily symmetric; however, there were no lower penetration resistance, it was not clear exactly how issues with out of plane warpage. Several panels were much lower the performance would be. There was no made with a mixture of both [0/90] and quasi-isotropic readily available guidance in previous literature, or from layers, where for instance 75% of the panel would be any of the composite materials manufacturers. Therefore, [0/90], and 25% would be quasi-isotropic. The processed a test of the quasi hybrid panels was completed to panels, measuring 0.38 m × 0.38 m ( 0.145 m2), were determine the V50 performance as an indicator of ballistic ballistically tested with a 9mm 124-grain FMJ round shot resistance. All testing was performed at Aberdeen Proving at a velocity of 473.13 ± 3.60 m/s. Backface deflection Ground. The quasi hybrid panels (0.45 m × 0.45 m) were measurement was taken with high-speed imaging and shot with 17 grain .22 caliber FSP, each shot into a digital image correlation (DIC). The deflection extent previously un-delaminated area. The maximum number was measured in the high-speed camera acquisition of partials and completes were collected as possible to software with the optical length scale calibrated using a determine the penetration velocity parameter. Panels standard calibration scale, and sourced from both an were tested using both sides of the hybrid structure as the overhead and a side perspective to correct any aberration strike face. in optical methods of measurement. Figure 1 shows a graph illustrating the relationship The entire range of ballistic testing included a variety between the normalized penetration velocity values using of stiffening fibers: carbon (IM7), aramid (with both the 17 grain round to the 9 mm deformation data collected thermoplastic and thermoset matrices), polyethylene previously. The data clearly shows a relationship (Tensylon, Spectra), and the quasi-isotropic hybrid layers. between the deformation and penetration response. The While the entire test matrix would be too consuming for fully quasi-isotropic panel had the lowest penetration the scope of this report (over 100 tested panel value compared to all the rest of the panels, which was combinations), several key results are listed below in expected, however the monolithic [0/90] HB25 panel did Table 1. In the table, all of the hybrids made with not exhibit the highest penetration resistance, as was stiffening agents (LF1, K745, Tensylon) were tested with thought to be expected. Many of the hybridized quasi- the stiff layer on the strike face. The quasi-oriented isotropic panels performed well as compared to the [0/90] hybrid panels were tested with the quasi layers facing standard, while still providing added deformation inward. response. The critical “sweet spot” (i.e., good V50 and low back face deformation) however encompasses two Table 1. Measured Panel Deformation with 9mm panel architectures, the 60/40 and 75/25 hybrids that were ballistically excited on the [0/90] side of the composite. O Deflection S Deflection This result was counter-intuitive to the surmised outcome, Panel Type (mm) (mm) and demonstrates the fact that architecture in a composite Quasi-isotropic (QI, HB25) 5.563 7.5946 can change the behavior of a panel in drastic and 50/50 Hybrid (HB25 + QI) 7.036 6.6294 unexpected ways. 60/40 Hybrid (HB25 + QI)) 8.052 6.6294 75/25 Hybrid (HB25 + QI)) 9.042 7.7216 Figure 2 shows the image of two separate 60/40 90/10 Hybrid (HB25 + QI)) 13.081 12.1412 panels, each shot on opposing sides of the hybrid. Both 60/40 HB25/LF1 15.646 16.256 panel types exhibit nearly the same penetration response, HB25 + 2 Layer K745 16.891 16.2052 Dyneema HB25 17.12 17.12 however, the 60/40 hybrid with the [0/90] strike face has 60/40 HB25/Tensylon IV 17.78 17.78 the dominant deformation resistance. The panel strikes Spectra SSII 3130 26.264 26.264 see more involvement, inferred from the extent of delamination of each shot, which can be one of the factors Given the slight variance in deflection between the influencing higher performance. overhead (O) and side (S) camera, the trend is similar and several observations are strikingly apparent. Firstly, the quasi-oriented hybrids performed substantially better than the rest of the hybridized panels. The deflection decreases with increasing quasi-isotropic content in the hybrid panel. Secondly, most of the stiffening agents did not bestow higher deformation resistance to the HB25 composite; in fact, most panels performed relatively equally to the monolithic HB25 panel. their low density (1.6-2.0 g/cm3), strength and damping properties. The aim of the magnesium shell is to blunt the incoming penetrator’s tip, mushrooming the tip to enable more interaction between the penetrator and the polymer composite, which could lead to lower backface deformation and higher penetration resistance. Magnesium alloy WE43 has been developed and studied through work between ARL and Magnesium Elektron North America. Initial trials attempting to bond the WE43 alloy with UHMWPE were not successful however. Therefore, an attempt was made to modify the adhesive behavior of the composite to improve the bond to the adhesive system. Previous literature7-12 exists on the modification of polymer surfaces (specifically polyethylene-based) and coatings through atmospheric plasma treatment methods. In plasma treatment, a glow discharge is generated in a controlled atmosphere of gases Figure 1. Penetration response (V50, such as helium, oxygen, nitrogen, or argon (or a mixture normalized) with respect to backface deformation for of these gases). The glow discharge ions attack the the quasi-isotropic hybrid panels. A relatively polymer surface, breaking apart the carbon bonds in the promising compromise is shown in the 60/40 and 75/25 polymer chain, which enables the grafting of the ions in [0/90] strike face hybrid panels. the plasma onto the surface. This process is often used to increase the wettability of the polymer film or fiber to polar compounds. A large experimental test grid (Tables 2 and 3) was created to explore varying plasma treatment variables (such as plasma power, treatment time, gap spacing and atmospheric makeup) and determine how the respective variables affect the UHMWPE surface. Strips of HB25 laminate (0.54 cm × 15.24 cm × 1.06 cm, processed at 20.6 MPa, 125°C as previous experiments) were plasma treated using an atmospheric plasma system (Sigma Technologies, Tucson, AZ) available at ARL. The atmosphere in the reactor was helium dominant, with oxygen ranging from 1‒10% within the mixture. Two voltage settings (2 and 10 kV) were used in conjunction with three plasma treatment times, varying from 7‒200 seconds. After the treatments, samples were characterized through contact angle measurements using liquids of varying polarity (water, Formamide, Methylene iodide, Figure 2. Images of the penetration velocity tested glycerol) to determine extent of wettability and to 60/40 hybrids with opposite strike faces. More calculate data points to generate a solid surface energy for delamination is evident in the second panel; however, each sample. Scanning electron microscopy (SEM) and the deformation response is half. X-ray photoelectron spectroscopy (XPS) were also used to characterize the surface treatments. Afterward, the 2.2. Magnesium/UHMWPE Composites treated samples were bonded to strips of WE43 using a commercially available grade of polyurethane-based A second research thrust involves exploring the use adhesive (Sikaflex®-234, Sika Corporation, Lyndhurst, of magnesium alloy as a potential strike face for a head NJ) and tested for lap shear strength according to a testing protection system. Magnesium alloys are a promising procedure similar to the ASTM D5868 standard13. The material to hybridize onto ultra-high molecular weight bonded samples were evaluated in a tensile load frame polyethylene (UHMWPE) ballistic helmet shells, due to (5500R, Instron, Norwood, MA) equipped with a 5000 lb (22.2 kN) load cell. Eight to ten samples were sheared at a crosshead rate of 0.02 mm/second until the peak bond strength was reached. Figure 3 shows the change in the contact angle between the various plasma treatment specimens; this reveals a generally decreasing trend with increasing plasma treatment time. This is a good indicator that the polymer composite surface is being modified chemically by the plasma species. XPS data, shown in Figure 4, confirms the assertion and gives the quantified change in each species listed with respect to the original content of each element in the untreated UHMWPE surface. Oxygen content, which is added to the atmosphere of the reactor, increased in amount with increasing plasma treatment time and in samples treated at a higher electrode gap spacing. Conversely, silicon concentrations were reduced extensively with increasing treatment time and voltage, but the silicon depletion is lower with the larger electrode gap spacing than it is with the smaller one. Figure 3. Contact angle measurement (water) as a function of plasma treatment. Table 2. Experimental Test Grid for Plasma Surface Treatments of UHMWPE Sample O2 % in He Plasma Voltage Gap Number atmosphere Treat Spacing Time (mm) 1.T1.P1.2 1 T1 P1 2 1.T2.P1.2 1 T2 P1 2 1.T3.P1.2 1 T3 P1 2 1.T1.P1.4 1 T1 P1 4 1.T2.P1.4 1 T2 P1 4 1.T3.P1.4 1 T3 P1 4 1.T1.P2.2 1 T1 P2 2 1.T2.P2.2 1 T2 P2 2 1.T3.P2.2 1 T3 P2 2 5.T1.P1.2 5 T1 P1 2 5.T2.P1.2 5 T2 P1 2 5.T3.P1.2 5 T3 P1 2 10.T1.P1.2 10 T1 P1 2 10.T2.P1.2 10 T2 P1 2 10.T3.P1.2 10 T3 P1 2 5.T1.P2.2 5 T1 P2 2 5.T2.P2.2 5 T2 P2 2 5.T3.P2.2 5 T3 P2 2 Figure 4. Change in elemental concentration as a 10.T1.P2.2 10 T1 P2 2 function of plasma treatment. Oxygen content 10.T2.P2.2 10 T2 P2 2 increases substantially in most cases, and silicon 10.T3.P2.2 10 T3 P2 2 content is reduced more readily in samples treated at 5.T3.P2.4 5 T3 P2 4 the lower electrode gap spacing. 10.T3.P2.4 10 T3 P2 4 From this data, it was deduced that the smaller gap Table 3. Testing Parameters for Plasma Treatments spacing caused more physical modification to the surface T1 T2 T3 V1 V2 than the larger spacing. The larger spacing created a 1 Pass 15 Passes 30 Passes 2 kV 10 kV larger mean free path for the ions in the plasma, so the 6.72 100.8 201.6 collisions with the plasma and the composite were seconds seconds seconds reduced in frequency. It is assumed that this led to higher collisions of monatomic oxygen with the surface, which then gave the surface a higher chemical functionalization of oxygen. SEM confirmed this assumption, as it was seen that the silicon removal was tied to the matrix being preferentially etched away from the surface, leaving exposed UHMWPE fibers. With the lower gap spacing, more fiber was exposed after plasma treatment. Figure 5 shows the lap shear strength data obtained in the tensile pull tests of the treated UHMWPE/WE43 composite. The untreated sample exhibits the lowest bond strength of all the tested samples. The general trend is increased bond strength with increased treatment time, increased oxygen content in the atmosphere, increased voltage, and higher electrode gap spacing. The highest bond strength exhibited (in the 10.T3.P2.2 sample) is 113.72% greater than the untreated sample. From the analysis of all the variables, it is shown that the dominant factor governing the behavior of the adhesive bond improvement is the physical modification (the matrix depletion) of the surface. It is evident from the pulled surfaces (Figure 6) that the exposed UHMWPE fibers are in direct contact with the adhesive and are creating a Figure 6. Bond lines of A) untreated and B-D) stronger mechanical bond between the composite surface plasma-treated UHMWPE surfaces, illustrating the and the adhesive. In many cases, the bond fails in the effect of plasma treatment on the bond line adhesion. interlaminate layers of the composite or at the WE43/adhesive interface, and not at the adhesive/composite interface. This is an indicator that the 3. INNOVATION IN DESIGN bond line is optimized beyond the capacity of the actual system it is bonding. Improving the overall impact protection in soldier- borne helmets will benefit from a more comprehensive assessment of possible material and mechanism-based management of adverse impulses. To date, the traditional approach has been to rely heavily on the materials (e.g., visco-elastic foam pads) and modifications thereof to provide the improved impact resistance. While the introduction of new or disruptive materials concepts is certainly warranted and encouraged, it should be augmented wherever possible with mechanistic approaches. For example, the use of a secondary “chassis” type concept was advocated by Walsh2 for both mechanical performance and manufacturing. A carbon composite rim-stiffened, UHMWPE prototype of this concept is shown in Figure 7. Such a concept offers other potential benefits that can be exploited to reduce adverse impact events, such as partial, or near total, decoupling of the ballistic shell from direct contact with the head. A variation of the concept shown in Figure 7 was manufactured and evaluated for impact resistance. Although the weight of the overall system increased Figure 5. Lap shear bond strength of slightly (due to the parasitic weight of the chassis), the WE43/UHMWPE composites after treatment and concept exhibited a significant improvement in impact bonding. High oxygen atmosphere, high power, and resistance, as shown in Figure 8. The novel chassis small gap spacing lead to higher bond strength. concept was able to sustain 14 ft./s (vs. the current ACH baseline of 10 ft./s) without exceeding 150 g in any of the crucial areas of the helmet (e.g., front, back, sides, and crown). The research conducted in this work has shown that architecture of composite panels can be tailored to provide a high level of ballistic deformation resistance and provide high levels of penetration resistance. Adhesive bonding improvements have been made between magnesium alloy and UHMWPE materials through use of atmospheric plasma treatment methods. In addition, improvements in the impact behavior of thermoplastic helmets have been realized through innovative design in the structure of the helmet system. All of these advances are laying the support for further improvement in future head protection. Future work will be focused on further evaluating the effect of various adhesives and plasma treatment on the ballistic and impact behavior of WE43/UHMWPE composites. Innovative processing methods for composites, such as hydroclaving, will be studied for their merit in creating low cost, lighter weight and higher performance ballistic solutions for both head and body Figure 7. Chassis concept with integrated carbon rim protection. stiffener and UHMWPE ballistic shell ACKNOWLEDGEMENTS The authors would like to acknowledge and thank James Wolbert and the composites laboratory for their efforts in processing and consolidation of the composites used in this work, Daphne Pappas, Benjamin Stein, and Victor Rodriguez Santiago for their assistance with the plasma reactor and surface characterization, Paul Moy for his assistance with the mechanical testing, and Peter Dehmer and Jian Yu for their assistance with the high speed imaging and digital image correlation; and SLAD PEEP Site personnel. Finally, the authors acknowledge Chassis Concept Ceradyne-Diaphorm, Magnesium Elektron., DSM, Baseline UHMWPE ACH Honeywell, and DuPont. Figure 8. Baseline ACH Geometry (Using UHMWPE) REFERENCES Compared with Novel Chassis Concept 1. Walsh, S. M. Scott, B.R., Spagnuolo, D.M., Wolbert, J., “Examination of Thermoplastic 4. 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