Composite Helmet Liner to Mitigate Blast Shockwave Energy and Reduce Traumatic Brain Injuries

BACKGROUND

For thousands of years, head protection has been a key element in combat gear. The main role of a combat helmet today is to protect a soldier from an injury from blunt, or ballistic impacts. Until the 1980s, the main material for a combat helmet was steel. Despite their excellent puncture resistance, steel helmets have a number of significant drawbacks. First, the steel does not provide for any significant ballistic impact energy dissipation/absorption. The energy absorption of steel is approximately an order of magnitude lower than that of fiber-reinforced composites (FRP), and energy damping materials as rubbers or foams are even better absorbers than FRPs. Thus, a steel helmet does not provide adequate protection from a concussion. Second, steel helmets are very heavy and are a burden to a soldier if worn for a considerable period of time. There is a tradeoff between the thickness of a helmet, its degree of ballistic protection, and its weight. Thus, steel helmets offered only limited protection against the direct impact of a projectile, and almost no protection against the concussive or blast forces that cause brain injury.

The new era in combat helmet development began in 1980s, when all-steel helmets were replaced by the plastic composite Personnel Armor System for Ground Troops (PASGT) helmet which was made of phenolic resin reinforced with a lightweight, synthetic fabric. Yet, the PASGT helmet still didn't offer a significant protection from concussive or blast forces as it didn't have a mechanism for ballistic energy dissipation. Later developments in combat helmet design replaced phenolic resin reinforced by fabric with an Ultra High Molecular Weight Polyethylene (UHMWPE) resin reinforced by alternating carbon and UHMWPE fabrics. While the switch to all-plastic helmets provided more protection against ballistic impact without adding additional weight, the protection against concussive/blast forces remained limited.

Since early times in history, the head protection in a combat gear has been a key element of a warfighter gear design. In particular, the main role of a combat helmet has been to the protection of a soldier from three main sources of head injury: blast (bombs, artillery shells, IEDs), blunt (falls, vehicle crashes, blasts), or ballistic (bullets, artillery shells, flying debris) impacts. Traumatic Brain Injuries (TBIs) come both from ballistic projectile penetrations into a skull, and also from over pressurizing forces from explosion or impact shockwaves. Depending on the nature of head injuries, they have been classified as primary (direct hit of a body by a blast), secondary (a body is being hit by flying debris, which can cause both penetrating and direct impact trauma); tertiary (when a person is physically thrown by the blast, which usually causes direct impact trauma), and quaternary (burns, chemical exposure). The combat helmet needs to offer protection against all of these types of injuries. Recent studies showed that TBIs resulting from explosions represent a significant percentage of military personnel injuries. According to the Defense and Veterans Brain Injury Center (DVBIC), more than 150,000 US Military personnel have been medically diagnosed with TBIs since 2001. The severity of TBIs ranges from so-called mild injuries, such as concussions, all the way to head penetration injuries.

IED detonations involve chemical explosions, whereby a condensed explosive material (either solid or liquid) rapidly oxidizes, creating very hot high-pressure gases. Large amount of energy are released, leading to local high pressures. In chemical explosions, almost all this energy expands outwards in the form of a shockwave. As the blast wave moves outwards, the air pressure falls back to atmospheric pressure. Because of the momentum of the gas particles, the pressure briefly falls below atmospheric pressure, causing the characteristic “underpressure” of the blast. A typical pressure time trace of a point in the path of an “ideal blast wave” is shown in FIG. 1a, where P5 value represents the overpressure shockwave that causes the major TBIs. FIG. 1b also shows the actual time scale of a shockwave frequency and dissipation; thus, any protective device needs to be able to mitigate the shockwave effect within a fraction of a second.

There are not yet any systems to reduce the risk of a soldier's brain health during combat and training operations. These potentially injurious mechanical forces caused by a blast include, but are not limited to, overpressure, accelerative forces, and impact forces on the subject from the helmet dislodging. Blast overpressure is in the order of milliseconds for initial peak rise time, with a total event time-scale less than 10 milliseconds for an improvised explosive device (IED). This is a near instantaneous exposure; thus, any innovative technology must respond virtually instantaneously. Typically, the blast overpressure attenuation to 3 psi or less (so-called very low blast overpressure range), is considered to be a safe range that doesn't cause Traumatic Brain Injuries (TBI); however, it has been shown recently that even such low pressures could cause transient symptomatology that overlaps with sub-concussive like effects, and/or amylose β-peptide increase in a brain, which could lead to Alzheimer disease. Therefore, even better blast overpressure attenuation may be needed to fully protect a soldier from TBI.

Multiple solutions for the blast wave mitigation have been explored to date, including foam shock absorbent pads and full-scale liners. Still, none of these provide all of the following: 1) significant shockwave energy dissipation/absorption, 2) full coverage of the area which needs to be protected, and 3) wear comfort. No existing helmet design can protect a soldier against the full array of combat threats. While some combat helmets in the current state-of-the-art address ballistic projectile impact protection (primary and secondary injuries), the protection against blast or concussive forces (tertiary injuries) remains inadequate.

SUMMARY OF THE INVENTION

The present disclosure describes a protective liner that can be worn by a soldier underneath of a regular combat helmet's shell to protect against the head injuries caused by high pressure blast and concussive shockwaves. The use of shear thickening fluids (STFs) impregnated in rubber to improve the energy absorption of the material is a critical embodiment of the disclosure. In the multilayered design, each layer carriers out its own set of functions. The outer layer of the helmet liner is made of STF-impregnated rubber, and absorbs the majority of the shockwave energy. Preferably, the rubber is Ethylene/Propylene/Diene Monomer (EPDM) rubber, reinforced with STF (also known as liquid armor). The use of STFs herein is to provide an additional blast overpressure shockwave energy absorption through the motion of the nanoparticles. The STF and rubber act synergistically to dissipate and absorb the shockwave energy. The inner layer is made of a soft polyurethane foam, which completes the dissipation and absorption of the shockwave energy, providing a complete protection against trauma to the brain. An intermediate polyethylene plastic layer provides additional constrains between the two soft layers, protecting them from excessive vibrational deformations and also absorbing/dissipating additional energy.

The disclosure provides a layered composite material for mitigating blast shockwave energy, the layered composite material comprising: a first layer comprising a soft foam; a second layer comprising a hard foam; a third layer comprising a thermoset or thermoplastic compound; and, a fourth layer comprising a rubber compound, wherein the rubber compound does not comprise any fibers or yarns; wherein, the first layer is bound to the second layer, the second layer is bound to the third layer, and the third layer is bound to the fourth layer; wherein, the rubber compound is impregnated with a shear thickening fluid, wherein the shear thickening fluid comprises a silica aerogel and an organic glycol; wherein, the layered composite material has a height less than 0.85 inches; and, wherein, the layered composite material attenuates at least 62% of an incident blast shockwave impulse for an incident blast shockwave with an incident blast pressure of 50 psi.

Preferably, the rubber compound comprises 15-25 wt % silica aerogel. More preferably, the rubber compound is ethylene propylene diene monomer.

The layered composite material preferably attenuates at least 79% of an incident blast shockwave impulse for an incident blast shockwave with an incident blast pressure of 50 psi. More preferably, it attenuates at least 79% of an incident blast shockwave impulse and at least 61% of an incident blast shockwave pressure for an incident blast shockwave with an incident blast pressure of 50 psi. Preferably, the first layer, the second layer, the third layer, and the fourth layer are arranged such that the fourth layer contacts an incoming shockwave before the third layer, the third layer contacts an incoming shockwave before the second layer, and the second layer contacts an incoming shockwave before the first layer. Preferably, the fourth layer has a height of 0.0625-0.125 inches.

The disclosure also provides a helmet liner for mitigating blast shockwave energy, the helmet liner comprising: a first layer comprising a soft foam; a second layer comprising a hard foam; a third layer comprising a thermoset or thermoplastic compound; and, a fourth layer comprising a rubber compound, wherein the rubber compound does not comprise any fibers or yarns; wherein, the first layer is bound to the second layer, the second layer is bound to the third layer, and the third layer is bound to the fourth layer; wherein, the rubber compound is impregnated with a shear thickening fluid, wherein the shear thickening fluid comprises a silica aerogel and an organic glycol; wherein, the helmet liner has a height less than 0.85 inches; wherein, the layered composite material attenuates at least 62% of an incident blast shockwave impulse for an incident blast shockwave with an incident blast pressure of 50 psi; and, wherein, the helmet liner fits in a helmet such that the first layer contacts a user's head and the fourth layer contacts the helmet such that there are no gaps between the user's head and the helmet during use.

In an embodiment, the rubber compound comprises 15-25 wt % silica aerogel. Preferably, the rubber compound is ethylene propylene diene monomer.

The helmet liner preferably attenuates at least 79% of an incident blast shockwave impulse for an incident blast shockwave with an incident blast pressure of 50 psi. More preferably, it attenuates at least 79% of an incident blast shockwave impulse and at least 61% of an incident blast shockwave pressure, wherein the incident blast shockwave has an incident blast pressure of 50 psi. In an embodiment, the fourth layer has a height of 0.0625-0.125 inches.

In an embodiment, the helmet liner is a headband, wherein the headband fits in a helmet's inner edges such that there are no gaps between the user's head and the helmet during use. The helmet liner may further comprise at least one cut in the headband such that the headband lies flat when not in use, and wherein the headband can be folded into the helmet's inner edges such that there are no gaps between the user's head and the helmet during use. Preferably, the helmet liner can be reused more than 1 time.

Also, the disclosure provides a method for attenuating blast shockwave pressure and impulse, the method comprising the steps: a) providing a rubber compound; b) impregnating the rubber compound with a shear thickening fluid, wherein the shear thickening fluid comprises a silica aerogel and an organic glycol; c) forming a protective rubber layer with the rubber compound, wherein the protective rubber layer has a height of 0.0625-0.5 inches; and, using the protective rubber layer as a barrier for an incoming blast shockwave; and, wherein, the rubber compound and the protective rubber layer do not comprise any fibers or yarns. The method may further comprise the steps: e) providing a soft foam layer; f) providing a hard foam layer; g) providing a thermoset or thermoplastic layer; h) binding the protective rubber layer to the thermoset or thermoplastic layer, the thermoset or thermoplastic layer to the hard foam layer, and the hard foam layer to the soft foam layer and forming a layered composite liner; and i) orienting the layered composite liner such that the protective rubber layer receives an incoming blast shockwave before the thermoset or thermoplastic layer, the thermoset or thermoplastic layer receives an incoming blast shockwave before the hard foam layer, and the hard foam layer receives an incoming blast shockwave before the soft foam layer. Preferably, the method further comprises the step: j) attenuating at least 79% of an incident blast shockwave impulse and at least 61% of an incident blast shockwave pressure for an incident blast shockwave with an incident blast pressure of 50 psi. Preferably, the rubber compound is ethylene propylene diene monomer.

BRIEF DESCRIPTION OF FIGURES

FIG. 1: Blast shockwave profile a) time trace b) actual timescale of frequency and dissipation.

FIG. 2: Energy absorption data for the STF-impregnated EPDM samples vulcanized at 190° C. with DiCup® peroxide.

FIG. 3: Energy absorption data for the STF-impregnated EPDA samples vulcanized at 150° C. with Luperox® TBEC peroxide.

FIG. 4: Energy absorption data for the STF-impregnated EPDM/Luperox® TBEC/PPG/STF samples at 1/16″, ⅛″, and 3/16″ gages.

FIG. 5: Energy absorption data for STF-impregnated EPDA samples with flat vs. corrugated surfaces vulcanized at 150° C.

FIG. 6: Configurations of sandwich composite samples made for acoustic tests.

FIG. 7: Acoustic data a) raw and b) moving average.

FIG. 8: Normalized IL of the sandwich composite constituent materials and epoxy/Kevlar® helmet shell material.

FIG. 9: IL of the sandwich composite samples.

FIG. 10: Comparison of acoustic and blast attenuation data between samples with and without EPDM/STF rubber layer.

FIG. 11: Blast shockwave attenuation profiles for a composite coupon.

FIG. 12: Comparison of the sandwich samples to the foam-only control.

FIG. 13: Comparison of acoustic vs. blast tests results.

FIG. 14: Image of corrugated EPDM/DHBP/PPG STF compounded rubber.

FIG. 15: DMA trace of SBR and EPDM rubbers in −50 C to +50 C temperature range with varied gages.

FIG. 16: Headform with the helmet on a mount a) front view b) side view c) rear view.

FIG. 17: Headform with the helmet on a holding fixture inside the frame showing the positions of the exterior pressure sensors.

FIG. 18: Full liner/helmet assemblies of three principal designs: a) full hemisphere rubber/ABS/foam with some TW comfort foam pads b) full hemisphere rubber/ABS with a full set of TW foam pads c) edge band design with full hemisphere rubber/ABS/foam band and some TW comfort foam pads in the middle.

FIG. 19: Blast shockwave attenuation data.

FIG. 20: Pressure vs. Time blast profiles for the baseline vs. headband liner design.

FIG. 21: Liner parts and assembly process a) flat slabs of EPDM/STF rubber, ABS, and foam glued together b) die-cutting of flat composite liner strips c) die-cut head band strip d) hook-and-loop attachment to the strip e) folded headband liner f) sweat pads in a liner/helmet assembly.

FIG. 22: Example of a foldable headband liner folding into a net shape to fit the helmet shell.

FIG. 23: Cross-section of a composite liner.

FIG. 24: Full size (single cut) and 2-piece (double cut) headband liner designs.

FIG. 25: Head impulse of the liner samples vs. the baseline related to the mild TBI threshold.

FIG. 26: Shockwave attenuation for a helmet liner assembly in comparison with the incident wave profile and the helmet shell with no liner.

FIG. 27: Peak pressure attenuation comparison.

FIG. 28: Impulse attenuation comparison.

FIG. 29: Attenuation performance comparison in percentage.

FIG. 30: Shockwave attenuation for a helmet liner assembly.

FIG. 31: Impulse attenuation performance for 2-piece liners.

FIG. 32: Peak pressure attenuation performance for 2-piece liners.

FIG. 33: Peak pressure and impulse attenuation improvement summary for different configurations of liner.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure teaches a composite helmet liner, offering increased protection from TBIs through a specific combination of STF-impregnated rubber/plastic/foam layers bound together to form a single liner. It is a teaching of this disclosure that the addition of an STF fluid to a rubber significantly improves the attenuation of the blast shockwave energy, thus effectively reducing the risks of TBIs caused by explosions. The STF fluid is a critical component to success, as non-impregnated rubber shows lower acoustic energy attenuation. Preferably, EPDM rubber compound is used due to its high compatibility with organic glycol solvent carrier. Other rubber compounds may include, but are not limited to, nitrile butadiene rubber, styrene butadiene rubber, natural rubber, silicone rubber, chloroprene rubber, butyl rubber, fluorosilicone rubber, fluoropolymer rubbers, polyurethane, hydrogenated nitrile, and the like. The best attenuation of shockwave energy occurs in a sandwich composite structure that comprises an STF-impregnated rubber (the layer facing the helmet shell inner side), intermediate plastic layer, and an outer foam layer (adjacent to a head).

The disclosure provides a method for mitigating incoming shockwave blast pressures using a rubber compound impregnated with STF. The blended rubber/STF composite does not comprise any yarns or fibers, which typically strengthen STF-impregnated composites. The rubber/STF layer alone provides effective blockage against incoming shockwave blast impulse and pressure. EPDM rubber is the preferred choice, but alternative rubbers or elastic materials may be used. The rubber/STF layer can be used in combination with a thermoplastic/thermoset later, a hard foam layer, and a soft foam layer, bound in that order, to have synergistic energy absorptive effects in the form of a composite liner.

The composite liner is intended to be used as a part of a combat helmet assembly to attenuate the blast shockwave energy and reduce TBIs to the minimum. Each layer in the protective helmet liner carries its unique function in the energy absorption/dissipation process. The rubber layer absorbs and dissipates the majority of the blast energy. The stiff plastic layer reflects most of the dissipated energy waves with reduced energy. Finally, the foam layer absorbs any remaining shockwave energy, and provides the wearer comfort. The plastic layer can be either a thermoplastic or a thermoset resin compound. All layers are bound together, by adhesive, plastic pins, or an alternative binding method.

The composite liner comprising STF-impregnated rubber/plastic/foam layers can be made as a flat strip that can be folded into a net shape headband configuration in a field by a soldier with no special tools or training. The folded head band can be inserted into the range of existing helmet shells used by military personnel. The folded headband can be unfolded back into a flat configuration and reused multiple times. The flat configuration of a liner allows for the large-scale conveyor type production at a low manufacturing cost and low cost of transportation. Additional cuts may be made in the band to form a 2-piece set with a more compact design, as shown in FIG. 24, but the single cut headband is preferred for protection against TBIs.

In the Summary of the Invention above and the Detailed Description of the Invention, and in the claims below, and in the accompanying drawings, reference is made to particular features of the invention. It is to be understood that the disclosure of the invention in this specification includes all possible combinations of such particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment of the invention, or a particular claim, that feature can also be used, to the extent possible, in combination with and/or in the context of other particular aspects and embodiments of the invention, and in the invention generally.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs.

“Comprises” and grammatical thereof are used herein to mean that other components, ingredients, steps, etc. are optionally present. For example, an article ‘comprising’ (or ‘which comprises’) component A, B, and C can consist of (i.e., contain only) components A, B, and C, or can contain not only components A, B, and C but also one or more other components.

The term “at least” followed by a number is used herein to denote the start of a range beginning with that number (which may be a range having an upper limit or no upper limit, depending on the variable being defined). For example, “at least 300” means 300 or more than 300. The term “at most” followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending on the variable being defined). For example, “at most 4” means 4 or less than 4, and “at most 50%” means 50% or less than 50%. When, in this specification, a range is given as “(a first number) to (a second number)” or “(a first number)-(“a second number)”, this means a range whose lower limit is the first number and whose upper limit is the second number. For example, “1-10” means a range whose lower limit is 1 and whose upper limit is 10, where both 1 and 10 are included in that range.

The term “soft foam” means a flexible, low compression modulus foam.

The term “hard foam” means an inflexible foam. The hard foam may have the same density as the soft foam, but is more resistant to compression than the soft foam.

The term “thermoset or thermoplastic compound” means a polymer compound that has crosslinking chemical bonds (thermoset) or does not have crosslinking chemical bonds.

The term “rubber compound” means an elastomeric polymer compound. Rubber compounds include, but are not limited to, EPDM rubber, nitrile butadiene rubber, styrene butadiene rubber, natural rubber, silicone rubber, chloroprene rubber, butyl rubber, fluorosilicone rubber, fluoropolymer rubbers, polyurethane, hydrogenated nitrile, and the like.

The term “any fibers or yarns” means any polymeric fibers, aramide fibers, polyethylene fibers, polypropylene fibers, polyvinyl alcohol fibers, fabrics, textiles, filaments, or combination thereof dispersed in the rubber compound or forming a network of fibers or yarns in the rubber compound or interacting with the STFs.

The term “shear thickening fluid” means a fluid comprising a colloidal filler suspended in an inert carrier liquid. The shear thickening fluid may be a colloidal dispersion of silica gel in organic solvent, such as an organic glycol (ethylene glycol, polyethylene glycol).

The term “silica aerogel” means fumed silica particles with a low density.

The term “incident blast shockwave impulse” means the impulse from the incoming blast shockwave.

The term “incident blast shockwave pressure” means the pressure from the incoming blast shockwave.

The term “bound” means tightly connected with a form of adhesive, glue, epoxy adhesive, tape, pins, or the like. Layers that are bound together will not come apart by user manipulation during use, or during regular wear. The layers must be bound together to achieve high energy absorptive properties.

The term “fits in a helmet such that the first layer contacts a user's head and the fourth layer contacts the helmet such that there are no gaps” means that the helmet liner is not too long to form overlaps or bent areas, and is long enough to fully surround the edges of a helmet without creating gaps of space that allow air to pass through between a user's head and the helmet. The helmet liner's measurements are precise to its use in the helmet.

The term “headband” means a single strip of material, around 1 inch in width, that surrounds a person's head or the inner edges of a hat/helmet when folded to a rounded shape.

The term “helmet's inner edges” means the inside circumference of a helmet that traces the end of the helmet material around the full helmet bottom, where the bottom is directionally referenced with a helmet worn normally atop a person's head.

The term “reused” means the helmet liner can be removed from one helmet and transferred to another helmet or used again in the same helmet with appropriate adhesives and achieve the same level of blast shockwave pressure and impulse attenuation. In the case of a flat, foldable helmet liner, the helmet liner will return to its original, flat shape between uses.

Aerosil® is an ultra-fine powdered Silica made by burning silicon tetrachloride in a hydrogen-oxygen furnace. It is a colloidal form of silica that is a very light and fluffy powder, and has spherical particle sizes of controlled and varying grades ranging from 4 to 25 micrometers.

DiCup® is dicumyl peroxide, used as a high temperature catalyst in the rubber and plastics industries. It is a pale yellow to white granular solid, which is soluble in a variety of organic compounds. It disperses readily in natural and synthetic rubber compounds, silicone gums, and polyester resins.

Luperox® is a brand representing a wide range of organic peroxides. Crosslinking of polyolefins and elastomers increases their mechanical strength and resistance to chemicals or weather.

Kevlar® is a type of organic fiber in the aromatic polyamide family, also known as aramid fibers. It is known for its strength, durability, and versatility. It has a molecular structure that includes many inter-chain bonds, and its rigid molecules form sheet-like structures similar to silk protein.

Synbone® is a type of anatomical model manufactured with a specially formulated polyurethane foam comprising of a cancellous inner core and a hard outer shell simulating a bone. Synbone® fracture during ballistic testings in a manner very similar to natural bones providing an excellent reproducible simulation.

Exfil® is a type of protective headgear designed by Team Wendy® for military and civilian use. It offers a customizable, stable fit and has foam pads and liner to protect against impact and allow for comfort.

Team Wendy® is a Cleland-based company that supplies helmet systems for military, law enforcement, search and rescue, and adventure sports.

One critical embodiment to the present disclosure is the optimized formulation of STF fluid. STFs are very efficient within the broad range of frequencies up to 22 kHz, which covers the typical frequencies of a blast overpressure wave. Adding STFs allows for a reduced base material, and thus reduces the overall weight while increasing the strength of the helmet liner. In the present disclosure, STF additives are blended with viscoelastic rubbers to synergistically absorb/dissipate energy frequencies from blast overpressure waves and protect a wearer from TBI injuries.

The amount of STF added to the rubber material is limited by its miscibility with the rubber, which disintegrates at high STF concentrations. The optimized composite mixture comprises 15-25 wt % STF, more preferably 18-22 wt %. The precise amount depends on the choice of rubber, silica nanoparticles, and organic solvent. There are no yarns or fibers present in the rubber material, STF, or organic solvent. A preferred blend maximizes the STF nanoparticle concentration, while minimizing disintegration of the rubber or other side reactions.

The solvent medium for STF blending should be chosen based on compatibility with the silica nanoparticles. The STF nanoparticles should be chosen based on compatibility with the rubber. For example, Aerosil® R816 nanoparticles have surface modifying groups compatible with EPDM rubber. For the solvent medium, the choices are 2-ethyl-1,3-hexandiol (EHD) and polypropylene glycol (PPG); both are compatible with Aerosil® particles and have some miscibility with EPDM rubber. A preferred combination comprises 18-22 wt % PPG/Aerosil® compounds blended into EPDM rubber.

Preferably, the STFs are added into Ethylene/Propylene/Diene Monomer (EPDM) rubber. EPDM/STF rubber gauge thickness is optimized between ⅛″ and 1/16″, where increasing thickness further increases the liner's weight and doesn't provide significant benefits in terms of blast overpressure energy attenuation. The maximum impact for the energy absorption at ⅛″ thickness for the STF-modified samples averaged at 13%, followed by 6% improvement for the next gage at 3/16″; although the data have some overlap within the standard deviations of the measurements. Results are shown in FIG. 4. Given that the increased thickness of the rubber layer will add to the weight of a helmet/liner assembly and will become a burden for a soldier to wear, any further increases of the rubber thickness layer in a liner will only yield to diminishing returns. Therefore, the rubber layer thickness is capped between 1/16″ and ⅛″ limits, and the overall multilayer liner has a maximum overall ½″ liner thickness limit.

The rubber surface of the EPDM/STF blend may be corrugated or flat. To create a corrugated surface, 0.02″ thick aluminum wire mesh may be used. The wire mesh was placed on top of the unvulcanized rubber swatch, shimmed to ⅛″ thickness, and cured in a platen press at 150° C. for 45 minutes; the results are displayed in FIG. 5. Alternative corrugating methods may be used. There is a slight increase of energy absorption for a corrugated surface, with tests showing 5.35 kJ/m²compared to 5.05 kJ/m²for a flat surface analog demonstrating a ˜6% improvement.

There are no yarns or fibers present in the rubber compound impregnated with STF. Typically, STF modifies the coefficient of friction between fibers or yarns, rigidizing the material during an impact event. The fibers are impregnated with particles, rather than the rubber itself. Blending the STF into a rubber with no fibers or yarns is an important embodiment of the present disclosure.

It is a teaching of this disclosure that each layer in the composite helmet liner serves a specific purpose in shockwave energy dissipation/absorption. The layers combine to synergistically protect a wearer against TBIs. The synergistic effects occur in the order (from the outmost, first to contact a blast shockwave) STF-impregnated rubber/plastic/hard foam/soft foam. The layers must be bound together by an adhesive, pins, tape, or alternative binding method to achieve synergistic energy dissipation/absorption.

Each constituent component was screened at ½″ thickness in FIG. 8. The hard HDPE plastic material has the best acoustic damping properties, attributed to its relatively high density compared to a foam, and amorphous structure where the intertwined polymer chains dissipate the energy transforming it into molecular motions. However, the HDPE alone is not a suitable material for a helmet liner for several reasons. First, the HDPE plastic is hard and not breathable; hence it doesn't provide the wearer comfort and breathability. Second, the HDPE plastic doesn't offer a good protection against the other head injury modes, such as blunt impact trauma and flying high energy projectile. As such, a combination of HDPE plastic with an inner soft breathable foam layer and the rubber layer capable of preventing blunt impact and kinetic projectile caused injuries is needed to build a successful liner that offers a full protection against all types of head injuries.

FIG. 8 shows that the STF-impregnated EPDM rubber is the second-best material for the acoustic wave energy absorption. The ˜20% STF content of the blend shows approximately 8% better energy absorption increase than a neat rubber. Out of all foam materials tested, the soft foam showed the best damping properties among the foam materials. The epoxy/Kevlar® composite, the core material of a helmet shell, also showed a significant attenuation and/or reflection of an acoustic wave. The 3D fabric, the very lightweight 3D fabric material, showed almost no energy loss.

The sandwich composites samples containing STF-impregnated rubber, a variety of commercial foams, and an optional HDPE plastic layer were explored. FIG. 6 shows the various combinations of sandwich composites tested. The results are summarized in FIG. 9. The relative ranking of the sandwich composite samples show that the top two best performing samples were DK-1403-32 with consecutive hard foam/EPDM rubber/HDPE/soft foam, followed by DK-1403-32: EPDM rubber/HDPE plastic/soft foam. Samples DK-1403-33 and DK-1403-42 were slightly above the average; however, the latter had a much higher thickness (0.62″) compared to the regular range 0.45″-0.55″ for the other samples. Importantly, all the samples with EPDM/STF layer outperformed the control foam-only sample by at least 27% in acoustic tests.

It is a teaching of this disclosure that the STF/rubber layer is critical to shockwave energy reduction. To evaluate the impact of EPDM/STF in a sandwich composite, two samples similar to the best top performers, DK29 and DK32, were made with no rubber layer. The results are illustrated in FIG. 10. Removing the rubber layer reduces the acoustic energy attenuation by 40-50%, and samples with no EPDM/STF rubber perform no better than the control sample with foam only. Thus, the presence of EPDM/STF layer is indeed critical to the acoustic energy absorption performance.

The layers must be bound together. Samples DK 29 and DK32 were made with no adhesive and tested acoustically exactly the same was as their bound analogs. The no-adhesive analog of DK29 showed 23% drop of Insertion Loss (IL), and even greater decline of 36% was observed between original DK32 and no-adhesive analog of DK32. These results indicate that the synergistic effect of the multiple layers in sandwich samples requires tight bonding between the layers; e.g., the adhesive layer is necessary.

The impact of using a thinner layer of EPDM/STF rubber in a sandwich composite found that the layer size is optimized between 1/16″ and ⅛″, preferably between 3/16″ and ⅛″. For comparison, an analog of sample DK29 using 1/16″ of the rubber vs. ⅛″ of the same rubber in original DK29 coupon was made and tested. The observed reduction of the sample with 1/16″ gage was approximately 8%, similar to the decline between ⅛″ and 1/16″ rubber gages of previous tests. This illuminates that the Izod impact energy absorption test (ASTM D256) can also be used as effective screening tool in optimizing of the EPDM/STF rubber formulation.

Representative blast shockwave attenuation profile is shown in FIG. 11. The comparison of the 3 samples: the Control, DK29, and DK32, are shown in FIG. 12. The chart clearly shows the significant reduction of the blast shockwave attenuation, the improvements over the control sample were 17% for DK29 and 23% for DK32 (FIG. 12). Importantly, the trend remained consistent between acoustic tests observations and the blast shockwave attenuation. The samples DK32 and DK29 were again the best and the second-best performers in regard to the energy attenuation. A comparison of acoustic vs. blast shockwave energy absorption trends are shown in FIG. 13. Sample DK32 was the best performer in both tests, while the DK29 stayed at the second place in both methods.

The helmet liner may form a net shape that fits inside a combat helmet, or fold into a net shape by a user. The edge of the helmet must be completely sealed by the edge of the composite liner, in other words the circumference of the net-shaped liner or the length of the foldable liner must be equal to the inner circumference of the helmet. An edge band design is preferred to a full hemisphere design because the edge band design provides a much tighter seal around the edge of a helmet shell; thus, preventing a blast shockwave to come underneath of a helmet and penetrate into a skull. The plastic or hard-core material, such as a composite helmet shell, will reflect the significant amount of the incoming blast shockwave. Thus, the helmet shell adequately protects the majority of a skull, and the edge band design effectively seals the gap between the edge of a helmet and a skull to dissipate and absorb any shockwaves that would have penetrated into a head through this gap. It is critical that the length of the band matches the circumference of the inner edge of a helmet so there is no overlapping or gaping material that would break the seal.

Preferably, the composite helmet liner is a flat and bendable band that can be inserted into a helmet. A soldier can fold the flat strip into a shape, seal it with a hook and loop or similar attachment method, and insert into a helmet shell quickly in a field without any special knowledge, training, or tools. Having a liner in a flat configuration before the use will greatly reduce the time and cost of the manufacturing process, as the 3-layered EPDM/STF rubber∥ABS plastic∥foams can be manufactured as a flat triple layer slab in large volumes, and then the flat piece will be folded into a shape right before its insertion into a helmet shell. Optional sweat pads can be inserted between the liner and the standard comfort pads as shown in FIG. 21f.

Another addition to the second iteration edge band design are ABS reverts that can be quickly inserted in a large-scale fabrication step by a robotic arm. As was found previously, the adhesive itself may not be strong enough to hold EPDM/STF rubber together with the rest of the slab during the die cutting procedure. Thus, the combination of ABS reverts and adhesive will provide enough interlaminar strength to hold the layers together during the manufacturing step when a slab is being die cut into the band strips. FIG. 22 shows the representative example of a flat head band strip being folded into a net shape shown in FIG. 21e and inserted into a helmet shell. FIG. 23 shows a cross-section of the composite liner indicating the constituent layers of: STF-impregnated rubber (the outer layer), ABS plastic layer, hard foam, and soft foam (the inner layer).

Optionally, the liner may be cut in the middle to form a 2-piece set. FIG. 24 shows an example of the 2-piece design, where the liner was cut in the middle at 450 angle. The 2-piece design is more compact for a user to carry than a single, long headband. However, it is preferred that there is only a single cut in the edge band (1 cut forms the single headband) to maintain the most complete seal of a user's head.

Examples
STF/Rubber Blending

In a typical compounding procedure, the raw rubber stock was milled on the rubber mil at ˜65° C. on the rollers at 3 RPM speed, whereupon the rubber was first blended with 1% wt. of a vulcanizer followed by the consecutive additions of STF fluid and dry Aerosil® silica. In the case of EHD STF (˜22% weight Aerosil® concentration), the overall EHD/Aerosil® blended concentration reached approximately 12% wt. increase (determined by the weight difference) in two consecutive additions before exceeding the limit of EHD/STF miscibility with EPDM rubber at which point the rubber sample disintegrates. According to the Thermogravimetric Analysis (TGA), the breakdown of the 12% weight increase was approximately 5% to EHD solvent and 7% of Aerosil® particles. However, EHD showed significant off-gassing. PPG/STF demonstrated no off-gassing issues in TGA analysis due to the polymeric nature of PPG and much higher molecular weight. Furthermore, when PPG/STF blend was compounded with an EPDM rubber, the impact and acoustic energy absorption studies showed 8% and 33% energy absorption increase, respectively, over the unimpregnated baseline for an optimized EPDM/STF compound.

DiCup® peroxide was then investigated as a vulcanizer for the system. DiCup® was blended with EPDM in a neat solid form at approximately 1 wt. % loading and vulcanized at 190° C. (slightly higher than the onset of DiCup® thermal reaction). The results are presented in FIG. 2. Alternative peroxides or vulcanizers may be used or added in the system as a vulcanizing agent. Because of the pungent smell produced during the DiCup® peroxide cure, the lower curing temperature Luperox® TBEC peroxide was also investigated as a vulcanizing agent. The results are presented in FIG. 3 and Table 1, which indicate ˜33% improvement in the impact energy absorption over the baseline with no STF. However, that the overall energy absorption values were significantly higher for the high temperature 190° C. vulcanization rather than for the lower temperature 150° C. process. Thus, the vulcanizing component of the blend can be further optimized by using a combination of DiCup® and Luperox®.

TABLE 1

Energy absorption Izod impact test data for EPDM samples with

DiCup ® and Luperox ® vulcanizers.

Energy Absorption,

Sample
kJ/m²

EPDM/DiCup baseline
7.52

EPDM/DiCup/PPG STF 1st
9.98

attempt, 15% STF

EPDM/DiCup/PPG STF 2nd
8.93

attempt, 13% STF

EPDM/DiCup/EHD STF 12%
10.35

EPDM/Luperox/PPG STF 19%
5.05

EPDM/Luperox/PPG STF ⅛″
5.71

EPDM/Luperox/PPG STF 3/16″
5.36

Next, the EPDM/STF blending trials using PPG/STF mixture in order to maximize Aerosil® content and achieve the best energy absorption performance. The concentration was raised to 18.5% wt., at which point the mixture could not be no longer shear mixed and processed on the rubber mill. The Aerosil® concentration was scaled to 18 wt. %. At this concentration the STF fluid is fully processable. Even though no more Aerosil® could be added directly to the STF fluid, silica aerogel was successfully added during the blending trials, which further improved the overall STF reinforcement performance. In the process, 5 consecutive blending steps were used, in each of which the PPG/STF was added first to a rubber compound followed by addition of dry Aerosil® particles until all liquid phase of PPG was consumed and STF fluid was fully absorbed. 6 consecutive blending steps were attempted; however, the rubber began to disintegrate after the sixth addition. Hence, for the PPG/STF mixture, the number of blending steps was optimized at 5. According to the weight gain measurement, approximately 22% of the PPG/Aerosil® compounds were blended into the EPDM rubber.

Rubber Types

The STF fluid was comprised of PPG with a molecular weight of 1,200 Da and ˜18-18.5% wt. of Aerosil® R816 silica aerogel particles. During the process, a rubber stock was compounded with 1% DBPH peroxide vulcanizer, followed by five consecutive additions of the STF fluid and dry Aerosil® R816 silica aerogel particles. The final STF concentration in EPDM rubber was 19.1 weight % and 18.5 weight % for the SBR rubber. It took a significantly longer time (by approximately 30 min) to obtain a homogeneous blend of SBR rubber than with the EPDM rubber; moreover, the SBR rubber showed a somewhat lower blending limit of STF than an EPDM rubber. EPDM rubber has a similarly structured polypropylene unit as the PPG medium, while SBR rubber's structure is entirely dissimilar to PPG. In the first trial, the rubber stocks were shimmed at approximately 3/32″ gage for both SBR and EPDM rubbers. In a typical compounding procedure, the raw rubber stock was milled on the rubber mil at ˜65° C. on the rollers at 3 RPM speed. However, the SBR rubber stock came at the significantly thicker gage at 0.094″ vs. similarly processed EPDM stock, whose thickness was recorded at 0.078″. Rubber swatches with corrugated surfaces produced the best energy dissipation results. As such, both rubber stocks were covered with 0.02″ aluminum wire mesh, shimmed to the actual gages of the rubber stocks, and cured in a platen press at 150° C. for 45 minutes at 3 tons constant pressure as found in the previous series of experiments. FIG. 14 shows a representative image of a vulcanized rubber swatch.

The energy-damping performance of the SBR and EPDM swatches was evaluated by Dynamic Mechanical Analysis (DMA) temperature sweep from −50° C. to +50° C., which covers the full operational temperature range for a helmet liner. Loss Modulus of a material that shows the energy-damping properties was plotted and integrated over the temperature range. FIG. 15 shows the DMA traces. The results show that although the integrated Loss Modulus curve area was 6% greater for the SBR rubber sample, it was also thicker by 13% compared to the EPDM sample: both were taken at the same diameter. As such, the SBR sample was thicker than the corresponding EPDM sample and contained approximately 11% more material mass. Therefore, the EPDM sample would have a higher energy-damping than SBR rubber samples of identical thickness and mass. The acoustic data also confirm these findings. Thus, the EPDM rubber has a higher blast overpressure attenuation capability than its SBR rubber analog.

Gage Thickness

The energy-damping efficacy of the EPDM/STF rubber was taken at three different thicknesses: 0.11″, 0.078″, and 0.069″. The rubber swatches were impregnated with STF and DBPH vulcanizer and cured at 145° C. for 45 min at 3 tons, using the aluminum wire mesh to create a corrugated surface. The Loss Modulus traces for all three compounds are shown in FIG. 15. The 0.069″ ( 1/16″) gage showed the lowest energy attenuation. Between the other two gages, the thickness of 0.078″ offered the best attenuation to 0.11″ while having lesser weight.

Layers

A series of flat sandwich samples were screened for acoustic energy loss from acoustic waves, which are a good mimic of blast overpressure shockwaves. The sandwich samples use the following materials: soft foam of different gages supplied by commercial suppliers; ¼″ of hard foam; ½″ Novel foam; 1/16″ soft foam; 1/32″ HDPE; and ⅛″ STF-impregnated EPDM rubber with a corrugated surface. Alternatives to the foam layers may include a special lightweight 3D fabric. The configurations of the test samples are shown in FIG. 6. The thickness of the entire samples was kept in the range between 0.45-0.55″, except for the sample DK42 which was ˜0.62″ thick. Using commercial epoxy adhesive, the 4″×4″ plates of each constituent material were glued together.

So-called “pink noise” in the range of 300-1,800 Hz, where each octave (begins when a frequency is doubled) has the same amount of energy was recorded and measured. First, the background acoustic noise with no pink noise frequency sweep and no sample insert was collected and further subtracted from each sample. The second test was a baseline test, where the data were collected for a 300-1,800 Hz frequency range but with no sample insert. Finally, the composite samples were inserted in the window one at a time and screened in the frequency range of 300-1,500 Hz After the subtraction of the background noise, the integrated Insertion Loss (IL) was calculated according to the formula:

$IL = 10 \times \log_{10} {(P_{sample} / P_{baseline})}^{2}$

where P_sampleis the root means square of an amplitude at a certain frequency for a sample; and P_baselineis the root means square of an amplitude at a certain frequency for a sample. The raw data for each sample is shown in FIG. 7a. The moving average line is shown in FIG. 7b. Once the background noise was subtracted, the IL was calculated according to the equation above for each individual frequency, and then integrated through the sweep range 300-1,800 Hz. Individual constituent materials (EPDM rubber, EPDM/STF rubber, HDPE, helmet shell material) were screened similarly, and the results are shown in FIG. 8, normalized to ½″ thickness.

Prototype Tests

A 50 psi pressure sensor was placed inside of a headform and sealed with the silicone caulk to prevent any blast shockwave penetration directly inside the headform. The hole in the bottom of the neck was sealed with a steel plug and two foam plugs to insulate the interior of the headform with a built-in sensor from direct penetration of the blast shockwave inside of the headform. A helmet with a liner was then placed on the helmet as shown in FIG. 16.

The headform with the 50 psi inside pressure sensor was mounted on the aluminum 80/20 frame structure, and two exterior 100 PCB psi pressure sensors were mounted on the Lexane holding fixtures as shown in FIG. 17. One pressure sensor was aligned with the back plane of a helmet, whereas the second pressure sensor was 3.25″ behind the first one. The holding fixture was positioned on the 80/20 structure so that the back plain of the helmet was 7″ away from the shock tube exhaust. The entire holding assembly with the headform and the 80/20 frame structure was bolted firmly to one end of the shock tube, and the entire assembly was then placed inside of the blast chamber. The shock tube was 10′ long. Approximately 4 grams of RDX explosive was charged in the shock tube to create 3-45 psi blast pressure.

In the series of tests, the helmet liner baseline that had a set of standard foam liners from a commercial supplier attached to the helmet shell with hook-and-loop fasteners. Three blasts shots were tested for each helmet liner design, and the data was averaged between the three data points of blast pressure and impulse reduction. The blast shockwave attenuation obtained for the helmet baseline liner was then compared to the blast pressure and impulse attenuation for our three experimental designs. First design was a full hemisphere of bonded rubber/plastic foldable pattern followed by the full hemisphere foldable pattern of a foam bonded to the rubber/plastic slab with hook-and-loop attachments. Such design provides the full coverage of the area inside of the helmet and is considered the most protective. Second design was also a full hemisphere of bonded rubber/plastic foldable pattern; however, in this case the standard configuration of the commercial pads were skived to meet the overall liner's thickness requirement. The third design used an edge-band design of the rubber/plastic liner that acts as an edge seal for a helmet. For the third design the standard configuration of the commercial pads were skived to meet the overall liner's thickness requirement. The helmet/liner assemblies of all 3 designs are shown in FIG. 18. All 3 samples of the liners and a baseline helmet assembly with the standard set of commercial pads were screened in the external blast pressure range of 30-78 psi. The dimensions for the constituent composite liner layers were as follows:

- Full Hemisphere 1: 0.092″ EPDM/STF∥10.062″ ABS∥0.345″ soft foam all glued together (FIG. 18a).
- Full Hemisphere 2: 0.092″ EPDM/STF∥0.062″ ABS glued together with standard commercial pads (FIG. 18b).
- Edge-band: 1″ wide, 0.092″ EPDM/STF∥ 0.062″ ABS∥0.242″ soft foam∥ 0.345″ hard foam all glued together (FIG. 18c).

3 blast tests were carried out for each design and the average pressures for the incident waves from the pressure sensor that was aligned with the back plane of a helmet were compared to the pressure reading of internal pressure sensor mounted inside of the headform. The average pressures for the baseline helmet, full hemisphere EPDM/ABS/foam, full hemisphere EPDM/ABS with standard set of commercial pads, and the edge band were at 64 psi, 31 psi, 39 psi, and 78 psi, respectively. Thus, the edge band and the baseline assemblies were tested at ×2 higher pressures than the full hemisphere designs. To account for such pressure discrepancies, the data was normalized to 64 psi pressure that was recorded for a baseline helmet assembly.

The comparative data analysis is presented in FIG. 19 below. The blast pressure and impulse attenuation were both normalized to 100% and compared to the 3 our designs. Both full hemisphere EPDM/ABS designs did not perform to the expectation and exhibited the reduced pressure and impulse attenuations. However, the edge band design showed approximately 50% improvements in both pressure and impulse attenuations as compared to the baseline helmet with the standard set of commercial pads. The representative pressure vs. time curves for the baseline and the edge band liner design is shown in FIG. 20. The overall % pressure and impulse attenuations for an edge band liner were 94% and 87%, respectively, measured at 78 psi and normalized to a 64 psi pressure (the pressure recorded for the baseline assembly. For the baseline assembly, the absolute attenuations were 86% and 81% for the pressure and impulse, respectively.

2-Piece Set

In a round of tests, the impact of the STF fluid on the blast shockwave energy absorption/dissipation was assessed. Thus, a liner that had an STF-impregnated EPDM rubber layer was compared to identical EPDM rubber with no STF. Two different designs of a headband liner were also compared: full size and 2-piece set, where the liner was cut in the middle at 45° angle (FIG. 24). Finally, the foam-only liner was used to assess the impact of both EPDM/STF and ABS plastic layers on the blast shockwave energy absorption and dissipation. The blast test setup was similar to the one used in the prior study with exception of using a soft plastic Synbone® headform as opposed to the hard magnesium Humanetics headform used in the previous study.

The specific configurations of the liner samples that were tested in this study of the blast tests were as follows:

Baseline with the standard TW pads.

- Full-size liner with 3/16″ STF/EPDM rubber∥ 1/32″ ABS plastic∥⅜″ soft foam∥¼″ hard foam.
- 45° cut half-size liner with 3/16″ STF/EPDM rubber∥ 1/32″ ABS plastic∥ 13/8″ soft foam∥ 11/4″ hard foam.
- 45° cut half-size liner with 3/16″ STF/EPDM rubber∥ 11/32″ ABS plastic∥⅜″ soft foam∥¼″ hard foam with additional perforations in a foam layer.
- Full-size liner with 3/16″ EPDM rubber (no STF)∥ 1/32″ ABS plastic∥⅜″ soft foam∥¼″ hard foam.
- 45° cut half-size liner ⅜″ soft foam∥½″ hard foam (another control sample).

The results of the pressure and impulse attenuation of the liner samples vs. the baseline standard pads are shown in Table 3.

TABLE 2

Pressure and impulse attenuation results of the liner samples.

Head pressure,
Head pressure,
Pressure
Impulse
Pressure
Impulse

% of the
% of the
attenuation,
attenuation,
attenuation
attenuation
Pressure,
Impulse,

incident
incident
%
%
improvement, %
improvement, %
psi
psi*ms

Baseline helmet
4.25
17.14
96
83
0
0
2.94
1.4

Full size STF/EPDM
1.28
2.57
99
97
3
18
0.86
0.21

Full size EPDM no STF
2.10
4.90
98
95
2
15
1.41
0.4

Half size STF/EPDM
2.59
7.35
97
93
2
12
1.74
0.6

Half size STF/EPDM
2.01
7.35
98
93
2
12
1.35
0.6

perforated

Half size foam only
3.19
19.59
97
80
1
−3
2.14
1.6

The data in Table 2 shows that addition of a cut decreases the efficacy of a liner to reduce the penetrated shockwave impulse. However, the net shaped liner is cost prohibitive for a mass production and logistics due to looser packaging. Therefore, the flat shape liner (full-size or half-size) is preferred.

The results also show the significance of the STF fluid. The same liner design where the STF/EPDM rubber layer was replaced by EPDM rubber with no STF didn't show as good impulse attenuation as the comparable STF/EPDM compound. These tests confirmed that the 3-layered liner structure is also critical to the successful pressure/impulse attenuation. When the half-size foam-only liner was used instead of STF/EPDM∥ABS∥foam triple layer liner, the impulse attenuation dropped from 12% improvement to −3% below the baseline. Finally, the half-size perforated liner designed to improve the perspiration mitigation didn't show any difference with the non-perforated liner of the similar design.

Literature values indicate that the impulses higher than 1.36 psi*ms can cause mild TBIs. The comparison of our liner samples to the mild TBI threshold is shown in FIG. 25. The chart clearly shows that both the standard pad baseline and 450 cut half-size liner are above the mild TBI threshold and, therefore, are unacceptable for the TBIs mitigation. The other liner designs can mitigate the TBIs effectively. Whereas the full-size liners are the best in terms of the TBI mitigation efficacy, the half-size liners still offer an adequate protection against TBIs. The efficacy of the liners may be increased where an improved fit to a user's head and helmet is used.

Liner Assembly

In the assembly process, 3 flat slabs: EPDM/STF rubber, ABS plastic, and a foam (FIG. 21a) are glued together with a specialty Neoprene High Performance Contact Adhesive from 3M. The dimensions of the head band liner were as follows: 1″ wide 0.092″ EPDM/STF 1∥0.062″ ABS∥0.242″ soft foam∥0.345″ hard foam. The flat full-size strip is either die-cut or laser-cut from a flat slab of required configuration (FIG. 21b,c). After enclosing the assembly into a RF-sealed fabric, hook-and-loop buckles are attached at the ends of the strip. The strip can be folded in a net shape by a user before inserting a liner into a helmet shell assembly (FIG. 21d,e).

Blast Chamber Tests

Two headforms, two types of liners, and two types of a helmet shell were tested in a closed blast chamber using the pressure membranes opposite to the prior tests carried out in an open chamber with explosive. Specifically, the headform/liner/helmet shell combinations tested during the reported period were as follows:

- Baseline Exfil® helmet with a standard set of comfort pads with Synbone® headform.
- Exfil® helmet with a full-size liner with Synbone® headform.
- Exfil® helmet with a 2-piece liner with Synbone® headform.
- 3D printed helmet made from 30% glass-filled Nylon 6 with a 2-piece liner with Synbone® headform.
- Baseline Exfil® helmet with a standard set of comfort pads with Humanetics headform.
- Exfil® helmet with a 2-piece liner with Humanetics headform.
- 3D printed helmet from TW made from 30% glass-filled Nylon 6 with a 2-piece liner with Humanetics headform.

The headform was placed 2.94 m away from a membrane section of the blast chamber. The outer pressure sensor was positioned at approximately 2.64 m away from a membrane section of the blast chamber. The headform was placed on a base plate ˜20 cm to the side of the outer pressure sensor and ˜25 cm upfront of the outer pressure sensor. The chamber also had a ˜2 m distance in front of the headform to extinguish the blast shockwave. In a typical experiment, a plastic membrane was inserted and clamped between the membrane-held pressurized section of the blast chamber and the blast tunnel section of the chamber. The membrane was then pressurized and burst to create a shockwave in the range 40-50 psi. Each headform was equipped with either 100 psi or 50 psi PCB pressure sensor which recorded the pressure under a helmet. Each pressure sensor readings: both internal and external were recorded and analyzed.

The representative blast shockwave profile for the incident wave vs. the attenuated pressure wave under the helmet/liner assembly and the comparison to the attenuated pressure wave under the helmet without a liner are shown in FIG. 26. For each helmet/liner/headform combination a set of 3 consecutive tests was performed and the average attenuation of the peak pressure and impulse were calculated. The summary results are presented in FIG. 27 and FIG. 28, and the percent attenuation comparison is shown in FIG. 29. The results show significant attenuation of both impulse and pressure for the full liner and 2-piece liner in comparison to the helmet shell without a liner. As before, the impulse attenuation for a full-size liner was almost 2-fold greater than for a 2-piece liner due to having fewer cut, as each cut introduces additional pressure wave penetration path. However, no significant peak pressure reduction was observed for a full-size vs. 2-piece liners.

Overall, based on the tests the composite EPDM/STF/ABS/foam liners showed between 49%-79% improvement for pressure and impulse attenuation over the baseline (Exfil® helmet with a standard set of foam comfort pads) depending on a liner (full vs. 2-piece), headform (Synbone® vs. Humanetics), and the helmet shell (molded vs. 3D printed). These comparisons are shown in FIG. 29. The full-size liner showed the best impulse 79% impulse attenuation as compared to the baseline helmet, whereas the 2-piece design was at ˜62% impulse attenuation level for both types of the headform.

Blast Chamber Tests for 2-Piece Design

In the final series of tests, four different designs of a 2-piece liner were studied for the blast shockwave attenuation. The first design was a hard/soft foam-only design with the thicknesses of ¼″ and 9/16″, respectively. The second design was the best-to-date design that had EPDM/STF runner, followed by ABS plastic, hard foam, and soft foam layers, which was tested previously a few times. The thicknesses of each layer were ⅛″, 1/16″, ½″, and ⅜″, respectively. In design 3, the ABS plastic layer was removed and the thickness of the inner soft foam layer was increased to 7/16″ to compensate for the thickness loss. Finally, design 4 was a different layout of design 3, where the EPDM/STF rubber was glued directly to a helmet shell, whereas the hard/soft foam liner piece was enclosed into a fabric and inserted separately on the top of the rubber strip through hook-and-loop attachments. Otherwise, design 4 had the same thicknesses of each layer as design 3. All liners were enclosed into RF sealable fabric. In all four designs, the overall thickness of a liner was maintained at 0.8125″; the same thickness as used before in previous tests. In these series of tests, the headform was placed 271 cm away from a membrane section of the blast chamber. The outer pressure sensor was positioned at approximately 2.64 m away from a membrane section of the blast chamber. The headform was placed on a base plate ˜20 cm to the side of the outer pressure sensor and ˜10 cm upfront of the outer pressure sensor. The chamber also had a ˜244 cm distance in front of the headform to extinguish the blast shockwave. In a typical experiment, a plastic membrane was inserted and clamped between the membrane-held pressurized section of the blast chamber and the blast tunnel section of the chamber. The membrane was then pressurized and burst to create a shockwave in the range 40-50 psi. The Synbone® headform was equipped with 100 psi PCB pressure sensor which recorded the pressure under a helmet. Each pressure sensor readings: both internal and external were recorded and analyzed. The average frontal blast pressure was approximately 54 psi (FIG. 30). The representative blast shockwave and attenuated wave profiles are shown in, and the impulse and pressure attenuations are shown in FIG. 31 and FIG. 32.

The test results once again reconfirmed that the design consisting of bonded together EPDM/STF rubber, ABS plastic, hard foam, and soft foam (configuration 2 in the current round) was still the best for the blast shockwave pressure attenuation. Thus, it performed 66% better for the impulse attenuation and 50% better for the peak pressure attenuation compared to the baseline helmet with the standard set of comport pads. Compared to the foam-only liner (configuration 1), configuration 2 liner decreased the impulse and peak pressure by 6% and 14%, respectively. The tests of configuration 4, where the rubber layer was disattached from the rest of a composite liner and glued directly to the shell, showed no improvement for the impulse attenuation compared to the foam-only configuration 1. Such result indicates emphasizes the earlier observation that all constituent layers of a composite liner need to be bonded together to act synergistically for the best shockwave attenuation performance. Finally, the test of configuration 3 validated the need for using a hard intermediate plastic layer as an additional reflective surface. Both impulse and pressure attenuation for configuration 3 were worse than those for the foam-only configuration 1 liner.

FIG. 33 shows analysis of the peak pressure and impulse attenuation improvement of the full-size and 2-piece liners compared to the Exfil® helmet shell equipped with the standard set of comfort pads. The full-size liner with EPDM/STF/ABD/hard/soft foam liner demonstrated the best blast shockwave attenuation performance reducing the impulse to the mild TBI threshold approximately at 1.35 psi*ms. The full-size liner showed the 79% impulse reduction improvement and 61% peak pressure reduction improvement over the baseline. The 2-piece liners were all above the mild TBI threshold level. Among the different variations of the 2-piece design, the fully bonded EPDM/STF/ABD/hard/soft foam liner had the best performance in attenuating the blast shockwave pressure. These tests also confirmed that that when either constituent layer is removed or not bonded to the rest of composite, the overall attenuation performance declines. For example, the removal of ABS layer led to significant decrease in both impulse and peak pressure attenuation. In another example, when the EPDM/STF rubber was disattached from the rest of the composite liner (configuration 4), the contribution of the rubber layer was negligible, and the attenuation was comparable to the foam-only liner (configuration 1) with bonded hard and soft foams.

Composite Helmet Liner to Mitigate Blast Shockwave Energy and Reduce Traumatic Brain Injuries

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