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                            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

                                                                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
                                                                   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


                                                                    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
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