Designing flocked energy-absorbing material layers into sport and military helmet pads

Abstract

A systematic study is reported on applying flocked energy-absorbing materials (FEAM) to designing sport and military helmet pad structures. An executed parametric study shows that the impact force absorbing (IFA) properties of FEAM elements are optimized when using (a) higher denier flock fiber (60 to 100 denier) and (b) longer flock fibers (3 to 4 mm length) at higher flock densities. Continuing work focuses on the importance of IFA/areal density ratios in helmet pad functional design. It is found that foam materials like vinyl-nitrile and ethylene vinyl acetate (EVA) inherently exhibit higher IFA/areal density (IFA/AD) ratios than FEAM material structures. With this finding, a new strategy for developing sport and military helmet pads was devised involving the combination of foam and FEAM layer elements. Here, the meritorious properties of foam materials (light weight and excellent IFA properties) and the excellent IFA and breathability (wearer comfort, sweat and heat management) properties of FEAM could be favorably encompassed. A plan was conceived and implemented whereby combination foam/FEAM test pads having high amounts of the high IFA/AD ratio VN-600 or EVA foam layer component were impact tested. By gradually introducing the more comfortable, breathable, body-heat managing FEAM layers into the helmet pad structure, some “trade-off” helmet pad configurations were designed and evaluated. Experiments showed that helmet pad designs having not more than 40% to 50% FEAM content should produce adequate IFA/AD ratio “trade-off” property helmet pad configurations.

Keywords

Impact force absorbing (IFA) properties composites < materials performance < materials structure-properties < materials flocked energy absorbing materials (FEAM)low velocity impact

Many types of polymeric viscoelastic materials such as foams are used in the field of impact energy-absorbing sport and military helmet padding structures.^1,2 In general, foam materials experience large deformations with relatively low stress levels in their mode of effectively decreasing the intensity of applied impact forces to the head. Such a process increases the overall contact time during the impact, accompanied by a more gradual decrease in impactor velocity.^3,4 Most of the studies have been carried out by subjecting the material to compressional impact loading.^4–8 Olivieri et al.⁵ studied the deformation mechanisms and energy absorption capability of expanded polystyrene (EPS) foams and polycarbonate shells for protective helmets using a combination of experimental methods and finite element models. Their study showed that the energy absorption capability of foam materials can be controlled at two different stages: (a) the macroscopic scale, by selecting the foam density which can minimize the transmitted load and the acceleration, and (b) the microscopic scale, by adjusting EPS’s internal structure in terms of hollow bead dimensions and wall thickness. Ouellet et al.⁶ examined three different types of foams (EPS, high-density polyethylene, and polyurethane) under a broad range of strain rates (0.008/s–2500/s). Here they employed a standard compression test device for low strain rates, a drop-weight tower device for medium strain rates, and a polymeric split Hopkinson pressure bar set up for high strain rates. Their prime result was that strain rate effects become more pronounced at rates above approximately 1000/s. Cui et al.⁷ fabricated model graded foam materials using numerical tools to investigate the effect of various gradient functions on the energy absorption under impact loads. They reported that the functionally graded foam showed superior impact force absorbing (IFA) compared with the uniform-gradient foam. Ramirez et al.⁹ investigated plateau stress and energy absorption of low-density (≤300 kg/m³) polyurea (PU) foams and EPS for a wide strain rate regime (0.04/s–5000/s). The plateau stress and energy absorption of low-density PU foams demonstrated a strong rate dependence. The strain-rate sensitivity of PU foams was found to have strong correlation with cell size for low strain rates and on cell wall aperture size for intermediate and high strain rates. However, the impact response for EPS foam was almost indifferent to strain rate. At low and intermediate strain rates, the plastic crushing in the EPS and the high plateau stress produced a much higher energy absorption than the viscoelastic dissipation in the PU foams. However, these EPS results were, of course, irreversible; structural damage occurs when these EPS foams are deformed beyond their elastic limit (i.e. “crushed”).

Although foam-based energy-absorbing materials are widely reported in the literature, very few studies are reported on fiber-based energy-absorbing materials. Fodor et al.¹⁰ investigated loss tangent values of flock layer-type fiber-based padding materials under compressional loading using an in-house designed dynamic mechanical analyzer. Loss tangent is a measure of a materials inherent ability to absorb energy when deformed. Here, a parametric study was conducted to study the effect of flock fiber diameter (denier), length, flock density, test frequency, and temperature on loss tangent values. Loss tangent of these materials showed improvement as the measuring frequency and flock fiber density (fibers per unit area) increases. Even though no significant changes were noticed as a function of temperature, the highest loss tangent was reported at room temperature. Furthermore, Correia et al.¹¹ performed a parametric study to investigate the effect of flock fiber-based IFA pad material properties such as fiber length, fiber diameter (denier), and fiber density on shear impact force absorption. These results were later compared with that of vinyl nitrile (VN) foam materials. Of importance, flock fiber-based padding materials showed 135% increase in shear strain energy density at high impact velocity loading conditions compared with VN foam-based pads. These results clearly showed the superiority of flocked fabric layer-based Impact Energy Absorbing (IEA) materials in absorbing shear-mode impact energy absorption. In this context, a study of flocked layer IEA materials for sport and military helmet pad applications is warranted. To this end, the goal of this paper is to present results of an experimental study that was carried out to establish some useful design parameters for creating sport and military helmet pads employing flocked energy-absorbing materials (FEAM) as component IFA material elements. For background, Figure 1 presents a rudimentary sketch and photograph of a single-side flocked surfaced fabric; this Figure represents the basic layer element of a FEAM IFA pad structure. It is here to be noted that a flocked surface by itself is not a FEAM IFA layer. To be referred to as a functioning FEAM entity, the flocked fibers must be sandwiched between two planar fabric or film layers. This arrangement allows for the distribution of force over the whole multitude of the flocked fiber ends when the assembly is compressionally deformed. Following on then, the mechanism of IFA by FEAM pad structures involves the compressional bending and buckling [see Figure 1(b)] of the upright flock fibers that occurs when the flocked surface is confronted by a force normal to its surface (compression). In addition to the spring action of these upright fibers, friction between these closely packed flocked fibers also occurs. It is hypothesized that this combination of upright fiber spring action and inter-fiber friction is what gives a FEAM material its IFA properties. Helmet pad designs involve the multi-layered assembly of these Figure 1 type flocked fabrics. As will be herein demonstrated, these FEAM layers can be combined with other IFA layer materials such as foams to create effectively designed helmet pad structures.

Figure 1.

Sketch and photograph of typical flocked surface element used to construct FEAM IFA pads.

In previous work, a FEAM configuration employing a single, representative double-side flocked (20 denier, 2 mm long nylon) core element structure was studied in detail.^12-15 Here, the effects of the (a) number of flocked layers and (b) of combining FEAM elements with foam layers and/or spacer fabric layers were investigated. The effect of 6.4 mm (1/4”) diameter hole-perforated flocked and foam layers was also studied. This early work employed a ball-drop impact test as a screening tool to evaluate the IFA properties of the various fabricated FEAM configurations. Important conclusions about the IFA function of FEAM materials were derived from this early work: (a) multiple layer FEAM structures must be separated by a divider sheet (e.g. layer of thin plain-weave fabric) to prevent the contiguous flock fiber surfaces from inter-meshing during compressional impact deformation, (b) IFA properties of FEAM structures increase as the number of FEAM layers increases, and (c) there are little or no changes in the IFA properties of FEAM panels having perforated holes (below 30% open area) flocked layers. Advantageously, it was noted that with hole perforations, FEAM panels having higher IFA/areal density (IFA/AD) ratios are produced. In this work it was also observed that with some FEAM/foam layer combinations, a synergistic IFA-improving effect seemed to be operating. Having established these trends, new studies have been carried out for the purpose of developing some broader insights into designing FEAM employing IFA helmet pad structures. In this new study, the effects of flock fiber denier, length, and flock density on a FEAM panel’s IFA properties were determined. This was done to establish what FEAM design parameters are needed to establish an optimized FEAM-containing helmet pad structure.

Experimental

Background

In the present reported work, the ball-drop test (BDT) as well as another more “controlled” impact test procedure was used to evaluate the IFA properties of the experimental test pads. This new test involved dropping a rail-guided, 5 kg mass (projectile) onto the test sample. This test is referred to as the guided weight drop (GWD) test. This test measures the Force Loss per cent (FL%) and the projectile’s deceleration rate upon impact, denoted as “g”. Systematic studies have been carried out to determine the important design factors that are needed for optimizing FEAM-containing pad structures. Highlighting this present investigation was a study of relevant FEAM and foam layered pad structure combinations in an attempt to design the lightest weight (lowest areal density), highest IFA property functional pad structure. Here the IFA/AD ratio factor, interpreted as the FL%/AD ratio, was used to analyze the collected data. In this reported study, FEAM layers were systematically added to foam layers to provide wearer comfort to these designed pad structures. This approach was believed to be the most direct way of defining an optimized FEAM-containing IFA helmet pad configuration. While wearer comfort is a subjective term, there is no doubt that FEAM layers containing IFA pad structures will be more breathable, and will better manage sweat and body heat, and be more conformable and “textile-comfortable” than typical VN foam-containing helmet pads. VN and other foams used in helmet pad applications are closed-cell foams and therefore have no through-thickness pores; they are not “breathable,” their ability to transpire heat and especially sweat is practically non-existent. FEAM materials are characterized by having space between the upright flocked fibers; between-fiber, sideways, airflow readily exists. Furthermore, airflow in the z-direction, normal to the flocked surface, can easily be accommodated by the proper choice of flock adhesive or more directly by “non-barb” needle-punching holes in the plane of the flocked fabric material element as a textile (flocked fabric layer) post-treatment.

Materials

Flock fibers were obtained from Spectro Coating, Inc., Leominster, MA. The flocked-upon “base” fabrics were all obtained from a local JoAnn Fabric retail store. The flock adhesive used was FF 3822 from Key Polymer, Lawrence, MA. The foam materials were obtained from various vendors, namely, DerTex, Saco, ME and JoAnn Fabric Stores. The VelTex® loop fabric was obtained from Industrial Webbing Company, Boynton Beach, FL. VelTex® is a product manufactured by the Velcro® Corporation and is a “loop” fabric that readily accepts Velcro® hook fabric strips. Additionally, another type of studied Velcro® hook adaptable fabric, type WW1373, was obtained from Gehring-Tricot Corp., Hauppauge, NY.

Sample preparation

Flocking was done using University of Massachusetts Dartmouth (UMD)’s laboratory DC up-flocker (Maag-Flock HEK 200-80). After flocking, the samples were allowed to stand for at least 16 h at room temperature before they were post-cured in an oven at 120 degrees centigrade for 1 h. Most sewing machine-fabricated test samples were 10 cm × 10 cm (4” × 4”) in size. In multiple FEAM layer configurations, all the face-to-face internal flocked surfaces were separated from each other by a light-weight fabric, for example, a 100% polyester (PET) plain-weave “separator/divider” liner fabric obtained from JoAnn Fabrics. The assembled layers were finally encapsulated or wrapped in either a micro-suede (100% PET) or else a Velcro® hook adaptable fabric (VelTex®). All test measurements were an average of at least three (3) “strike” determinations performed on the same sample.

Impact testing methodology

The two IFA testing methodologies employed in this study were: (a) BDT and (b) GWD.

BDT

This impact test is fully described elsewhere.¹² The test involves gravity-dropping a ball projectile (Bocce Ball (BC), 102 mm dia., 0.543 kg or a field hockey (FH) ball 71 mm dia., 0.162 kg) from a pre-set height onto a 10 cm × 10 cm (4” × 4”) test sample mounted on a flat-solid force table where the impact force tracing of the impact is recorded. Here the Force Loss per cent (FL%) is calculated by comparing the impact force (peak) measured with and without the force-table mounted test sample. This BDT serves as a useful IFA property “screening tool” throughout this study.

GWD

Here a 5.0 kg steel weight projectile is gravity-dropped from a set height (continuously adjustable from a “set” 25 to 200 cm) onto a force table-mounted test panel (this is similar to the BDT device). The “strike face” shape of this projectile is of a 12.5 cm diameter hemi-spherical shape. The kinetic energy of this strike projectile is 49.0 Joules (at 100 cm drop height) and is measured by an accelerometer that is physically attached to the falling 5.0 kg mass. Strike velocities are thus measured from the accelerometer’s read-out. In this test, FL% and the impact-projectile’s deceleration value “g” (g-value) are measured and reported as a representation of the specimen’s overall IFA properties. A photograph of UMD’s GWD testing apparatus is presented in Figure 2.¹⁶

Figure 2.

Photograph of guided weight drop (GWD) material impact testing apparatus.

FEAM nomenclature scheme

To facilitate communication of FEAM technology to others, a FEAM nomenclature scheme was devised as a convenient way of describing the internal structure of FEAM elements. An illustration of this devised scheme follows:

Example FEAM Designation: FX601-3D//VN600P (7.8)//FX202-2D//FX452-3S:Micro-Suede

Here, FX refers to a generic flocked layer structure; 60 refers to the denier of the flock fiber used; 1 refers to PET flock fiber (2 here would refer to nylon flock fiber); 3 refers to the length of the flock fiber in millimeters; D refers to double-side flocked (S here would refer to single-side flocked layer). VN600P (7.8) refers to a perforated version of VN–type 600 foam; 7.8 mm thick [hole pattern was 6.4 mm (1/4”) diameter hole set apart in a 15.8 mm (5/8”) square pattern]. //refers to the placement of a “separator/divider” fabric (light weight plain-weave fabric) between the contiguous “bare” flocked surfaces. The FX452-3S: Micro-Suede refers to 45 denier, 3 mm long nylon single-side flocked onto the backside of micro-suede plain-weave fabric. This nomenclature scheme has been used throughout this document to define/describe the FEAM structures employed in the various pad configurations.

Experimental results

Effect of flocked-upon base media

Before starting the planned parametric study, a study was carried out to determine the effect the flocked-upon base fabric has on the IFA performance of assembled FEAM structures. To this end, FEAM panels were prepared using various flocked-upon base fabric materials. Descriptions of these base media materials are summarized in Table 1; these materials are listed in order of their areal density. Similar construction flocked panels were prepared employing these base fabrics by fabricating them into double-side flocked FEAM configurations of the same overall structure: FX451-3D//FX451-3D. All these test panels were over-wrapped with polyester micro-suede cover fabric and perimeter sewn. These FEAM panels were then ball-drop tested. The FL% data obtained are summarized in Table 1. Here, we can conclude that the flocked-upon base material is not critically important in affecting the IFA properties of prepared FEAM test panels. Base material samples of a very wide areal density, from 31 g/m² to 318 g/m² and thickness of 0.1 mm to 1.9 mm (see Table 1) were all found to have FL% values in a very similar range. However, since the Pell-810 Tru-Grid® nonwoven fabric was found to exhibit the highest FL% properties, it was selected for use in the planned parametric (flock fiber dimension) experimental study.

Table 1.

Effect of various flocked-upon base-ply media upon measured IFA properties^a,b,c

Lab ID	Flocked Upon, Base Ply (Material Description)	Thickness (mm)	Areal Density (g/m²)^d	Overall Average FL%	IFA Rank
82-A	Pellon 810 Tru-Grid® PET nonwoven	0.1	31	27 ± 3	1
81-C	Pellon 90 Heavyweight polyester nonwoven	0.5	90	14 ± 3	5
90-A	Twill weave, 100% polyester fabric (black)	0.3	174	17 ± 3	4
95-A	Plain weave- 87% nylon, 13% Spandex®	0.4	190	21 ± 2	2
81-D	Pellon PELTEX 318, thick, nonwoven fabric	1.9	318	19 ± 3	3

^aThe IFA FL% data are an “overall” average of two measurements; Bocce Ball drop from 50 cm and a Field Hockey ball drop from 100 cm. Each measurement was an average of five replicate tests.

^bAll ball-drop test panel configurations were of the FX451-3D//FX451-3D construction. All flocked FEAM element were prepared with the designated central support as the base flocked-upon layer.

^cAll FEAM panels were micro-suede wrapped and perimeter sewn.

^dAreal density of unflocked base fabric.

Parametric study of FEAM pad structures

A parametric study was planned and executed to evaluate the IFA behavior of FEAM configurations of structures having various flock fiber denier, length, and flock density (at two levels: L = Low- about 100 fibers/mm² or H = High -about 200 fibers/mm²). Pell-810 “Tru-Grid®” nonwoven fabric was used as the flocked-upon base fabric. Here, two-flocked layer FEAM element test panels were fabricated and IFA tested using the BDT. Table 2 summarizes the test data obtained for these panels tested with BC and FH ball projectiles. This parametric study has led to the following conclusions:

Table 2.

Summary GWD FL% and “g” data for various baseline foams^a,b

Foam Material^c	Areal Density^d (g/m²)	FL%/“g” at 5 mm Thickness	FL%/“g” at 10 mm Thickness	FL%/“g” at 20 mm Thickness	FL%/“g” at 30 mm Thickness	FL%/”g” at 40 mm Thickness^e
LDPE	33	5/163	7/161	23/142	50/112	(70)/(80)
EVA	76	2/170	10/160	43/118	67/82	75/63
VN-600	100	20/160	32/130	60/84	71/76	75/63
VN-740	130	20/140	50/110	58/94	60/92	60/82
Polyurethane (memory foam)	246	8/155	20/122	52/98	59/80	(62)/(67)

^aGWD Projectile Drop height was 100 cm for all tests.

^bAll FL% and “G” values are extrapolated from drawn IFA property vs. Thickness plots.

^cAbbreviations: VN = vinyl nitrile, EVA = ethylene vinyl acetate, LDPE = low-density polyethylene.

^dAreal densities are normalized to 1 mm thick panels.

^eNumbers in parentheses were extrapolated beyond the actual tested thickness range.

Higher flock density flocked layers produce the highest FL%

Relative to Flock Length and FL%: 3mm > 2mm > 1mm

Relative to Flock Denier and FL%: 45 D < 60 D = 80 D.

The issue of the effect of flock density on FL% properties is a topic of interest but its study is not straightforward. Overall, one would expect that higher flock densities would lead to higher FL% because with high flock densities, a larger number of upright flock fibers (per area) exist for IFA bending during compressional impact. In the practice of flock processing, flock density is affected by the length and denier of the fiber being flocked; lower denier (smaller diameter) fibers lead to higher flock density because of a geometric areal cross-section effect. Also, longer flock fibers will lead to lower flock density, because of the “fiber interference” during electrostatic deposition. Therefore, it seems that there should be some combination of flock fiber denier and length that will produce optimum FL% properties. This optimum flock fiber denier/length parameter ratio will be sought in later studies.

Developing optimized FEAM-Based IFA helmet pad materials

All sport and military head-protecting helmets employ interior-mounted IFA pads. These integrally functional pads provide the helmet wearer with (a) head cushioning and protection from blunt impacts, (b) head conforming/fitting comfort, and (c) sweat and heat management. Many types of IFA materials are used for helmet pads including felts, spacer fabrics and, most commonly, foam materials.^17,18 Among the many types of foam materials, the most prevalently used helmet pad foams are VN, polyurethane (memory) and, to some extent, ethylene vinyl acetate (EVA) foam. Helmet pads fit into the interior of a sport or military helmet in large, semi-continuous patterned sections or as a grouping of several suitably positioned pad units. Most helmet pad types are fabricated in a thickness range of between 12.5 mm (1/2”) and 25.4 mm (1”). Many professional sports (football) and US military helmet pad systems tend to be in the higher thickness range of about 25.4 mm (1”).^16,19 This thickness has been found to be satisfactory. Therefore, our study has settled on the “standard” of designing experimental FEAM-containing helmet pads in the 25.4 mm (1”) thickness range. In addition to thickness, an optimum helmet pad structure should be light-weight (low areal density) and have high IFA performance properties. Therefore, this study seeks to develop IFA material combinations and geometries that will lead to comfortable-wearing helmet pads with high IFA to areal density ratios. This should be an overall design goal for our present helmet pad studies. However, prior to directing our experiments to designing IFA helmet pads, two preliminary studies were first carried out: (a) an investigation of the known IFA-enhancing effect of resin-coated flocked surfaces in developed FEAM structures, and (b) a comparative IFA evaluation of representative commercial foam materials.

Resin-coated flock fiber FEAM configurations

During this FEAM optimization study it was learned that when the flocked surfaces of a FEAM’s core element are coated with a resin (e.g., Minwax® solvent-based polyurethane semi-gloss varnish), an enhancement in the IFA properties results. Here, the flocked fiber coated fabric surfaces were directly spray-coated upon with the clear varnish lacquer coating. Fine-mist spray coating was used to assure that all the individual upright flock fibers were evenly coated with the resin. Different resin amounts were achieved by giving the flocked fiber surfaces multiple coats. All coated flocked surfaces were cured by drying them at room temperature for at least 24 h before testing. This technology is fully described elsewhere.²⁰ Experimentally, the GWD test, at 50 cm and 100 cm drop heights, was used to determine the IFA properties of a series of FEAM test panels having various amounts (0% to 13%) of applied (flocked surface) resin. The results of these experiments are presented in Figure 3, where it shows that the “g” and FL% properties of so-called “resin-assisted” (“RA”) FEAM panels are clearly enhanced by coating the flock fibers with resin. Here, the measured “g” values are lowered and the FL% values increase as the amount of resin coated onto the flocked surface increases. It should be noted that all the FEAM panels tested for Figure 3 data had the same FEAM configuration: [FX452-3S × 3/FX452-3D]. These data indicate that FEAM’s IFA properties could be further improved by resin-coating the flocked fibers to amounts higher than 13%. However, it must be noted that this enhancement in IFA properties is accompanied by a steady increase in areal density. Henceforth, by employing this resin-assist feature, one is confronted with the reality that the IFA properties of these FEAM structures increase at the expense of increasing the panel’s areal density. Therefore, employing this “RA effect” is only relevant in potential FEAM applications where areal density is not of primary concern.

Figure 3.

Effect of the amount of resin coating on FEAM panel’s flock fibers on its “g” and FL% IFA properties.

Impact force absorption of various foam materials

The meritorious effects of combining FEAM layers with IFA foam material layers have been previously investigated and reviewed.¹² Continuing this work, studies on the IFA properties of FEAM/foam layer combinations were first carried out to establish a better understanding of a foam’s IFA behavior. GWD FL% and “g” data at 100 cm drop height were obtained on a series of foam-only materials at various thicknesses. This was done to compare the IFA of some of the foam materials most commonly used in sport helmet pad applications. Table 2 summarizes the results of these GWD tests. Here it was determined that VN (VN-600) and EVA foam materials show the highest IFA properties among the tested foam materials in the 30–40 mm thickness range. VN-600 foam has a respectfully high FL% and lower “g” value accompanied by a low areal density throughout the overall 5–40 mm tested thickness range. Also, EVA foam was found to have IFA properties quite similar to VN-600 foam but has a slightly lower areal density. While the tested polyurethane memory foam had good IFA properties, it also has a higher areal density. From these observations, it was decided that VN-600 and EVA foam would be the foams of choice in any of our further studies of sport and military helmet pad material structures.

Comparing IFA properties of foam and FEAM materials

For this analysis, available FEAM panel and foam-only data of approximate thicknesses were compared and are summarized in Table 3

Table 3.

Comparison of selected FEAM and foam IFA test data^a

Material ID^b	Description	Thickness (mm)^b	“g” (mm/s²)	FL%
022-E	[FX602-2D]^c	6.3	96	16
VN-600	Foam	(6.3)	152	23
EVA	Foam	(6.3)	167	4
Polyurethane	Foam	(6.3)	146	10
005-A	FX452-3D^c	10.6	86	28
VN-600	Foam	10.0	130	32
EVA	Foam	10.0	160	10
Polyurethane	Foam	10.0	122	20
022-D	FX601-3D^c	9.2	90	23
VN-600	Foam	10.0	130	32
EVA	Foam	10.0	160	10
Polyurethane	Foam	10.0	122	20
016-B	FX1002-3D^c	12.2	77	41
VN-600	Foam	(12.2)	119	38
EVA	Foam	(12.2)	150	18
Polyurethane	Foam	(12.2)	116	27
010-C	[FX452-3S][FX452-3D]^d 8.4 % “RA”	18.6	69	50
VN-600	Foam	20.0	84	60
EVA	Foam	20.0	118	43
Polyurethane	Foam	20.0	98	52
010-D	[FX452-3S][FX452-3D]^d13.0% “RA”	20.8	49	68
VN-600	Foam	20.0	84	60
EVA	Foam	20.0	118	43
Polyurethane	Foam	20.0	98	52
055-A (see Table 5)	/FX1002-3D//FX1002-3D//FX1002-3D/^d	23.3	56	66
VN-600	Foam	(23.3)	81	64
EVA	Foam	(23.3)	86	51
Polyurethane	Foam	(23.3)	92	55
055-C (see Table 5)	/FX202-2D-six layers/^d	27.3	53	70
VN-600	Foam	(27.3)	78	68
EVA	Foam	(27.3)	92	61
Polyurethane	Foam	(27.3)	85	57

^aAll GWD data here are for 100 cm drop heights.

^bAll foam data taken or interpolated from the Table 4 data.

^cFlocked layer “core” enveloped in micro-suede fabric and over-wrapped in VelTex® fabric.

^dThese test samples were micro-suede fabric wrapped and perimeter sewn.

. This comparative analysis has been limited to only comparing FEAM configurations with some of the better IFA-performing foam types, namely, VN-600, EVA, and polyurethane. In Table 4, FEAM samples 016B (FX1002-3D) and 010-D (Resin Assist at 13%) were the only 100% FEAM sample configurations measured that had a higher FL% value and a lower “g” value than any of the foam-only samples. Furthermore, all the measured FEAM FL% values are somewhat higher than those for EVA foam (only) and are somewhat comparable to the polyurethane foam’s FL% values. This study’s most striking revelation is the observation that the “g” values for all the 100% FEAM panels were all comparably lower than the “g” values for the like-thickness foam materials. This is a new behavioral observation, as high FL% and low “g” usually follow together as being indicative of typical IFA material performance. If FEAM structures are somehow prone to exhibit an enhanced “g” property, compared with other IFA materials, this presents another distinguishing feature of FEAM-type materials.

Table 4.

IFA and impact duration time properties of various FEAM and foam materials

Lab ID	Description^a	Thickness (mm)	Areal Density (g/m²)	“g”^b (mm/s²)	IDT^c,d (seconds)	FL%^b
FOAM MATERIALS
054-A	VN-600 – (six, 4 mm thick layers)	23.6	3160	66.6 ± 0.8	0.013	63.5 ± 0.4
054-B	VN-600P – (three, 7.7 mm thick layers	23.1	2225	50.3 ± 0.5	0.016	73.3 ± 0.2
054-C	VN-740 – (six 3.6 mm thick layers)	21.9	3365	78.2 ± 0.3	0.013	53.5 ± 0.3
054-D	EVA – (eight, 2.9 mm layers)	23.9	2941	44.9 ± 0.3	0.020	75.7 ± 2.0
FEAM STRUCTURES^e
055-C	FX202-2D – (six layers)	27.3	5379	53.2 ± 1.3	0.023	69.2 ± 0.6
055-D	FX202-2DP-(six layers)	24.7	4261	62.5 ± 0.7	0.018	62.6 ± 0.7
055-B	FX602-3D–(four layers)	24.2	4374	59.2 ± 3.0	0.020	65.2 ± 2.2
055-A	FX1002-3D-(three layers)	23.3	4159	56.3 ± 4.7	0.018	66.5 ± 3.3

^aAll test samples wrapped with micro-suede fabric and perimeter sewn.

^bAll testing at 100 cm drop height.

^cIDT refers to impact duration time (projectile’s approximate “strike-event” time).

^dOverall average IDT for all foam materials = 0.016 s; FEAM = 0.020 s.

^eMultiple stacked FEAM layers are separated by divider fabric.

To gain further insight into this surprising observation, the basis for the “g” and FL% measured values were closely reviewed. This was done by examining the force table’s output tracings during the impact “strike.” In these tracings, the target panel material’s FL% and “g” values can be calculated. Some typical force/time plots/tracings are shown in Figure 4. Experimentally, FL% values are based on measuring the peak impact force from these time-force tracings; this force is read as the peak of these “bell-shaped” tracings. Note that in the Figure 4 examples, the recorded force peak for the foam material reads about 3000 N while the force peak for the FEAM sample is only 2000 N. This is a clear indication that the FEAM-type material is much better in absorbing the impact force compared with the foam material; less force is transmitted through the FEAM-type pad. In practice, FL% is calculated by comparing the peak force measured when the test panel is in place (on the force table) to when no test sample is in place. Alternatively, “g” values are the measured deceleration (velocity change/time) of the projectile when it strikes the target.

Figure 4.

Comparing typical force-time curve for foam and FEAM IFA helmet pad. Material specifications are found in Table 4.

Deceleration here is measured by dividing the projectile’s velocity at the instant of impact, V_i (as measured from the falling projectile’s mounted accelerometer signal), by the total time duration of the impact event. From a dimensional analysis viewpoint, it is clear that the dimensional units of Force (normalized here as FL%) and “g” are different. Force has units of mass × acceleration (or mass × distance/time squared) while “g” has the basic units of distance/time squared. In UMD’s GWD test, an accelerometer fixed onto the falling mass provides the signal information used to calculate the impact velocity of the striking projectile. From this, the deceleration value “g” is then determined by dividing the projectile’s initial impact velocity change by the impact duration time (IDT). Consequently, the longer the measured IDT, the lower will be the projectile’s deceleration rate “g” for a given impact velocity. It is therefore concluded that FEAM material structures should have slightly longer IDT compared with comparable thickness foam materials. Table 4 presents some IDT values for some representative foam and FEAM pad structures. These Table 4 data show that overall, the IDTs for all the FEAM structures are found to be slightly longer than the IDT values for all the foam materials. This provides some support for the supposition that the IDT of FEAM-type IFA panels should be slightly longer than those for comparable thickness foam materials. These observations provide some insight into why the “g” values of FEAM structures are proportionally lower than comparable foam materials.

Upon further review of the Figure 4, we see that the shape of the force vs. time tracings for the foam and the FEAM are somewhat different. Comparing Figures 4(a) and 4(b) we see a different real-time deformation response between the foam and the FEAM when compressionally impacted. The FEAM materials show a slightly more gradual increase in force per time compared with the foam materials. Of additional interest, these observations support the earlier discovery that FEAM IFA materials have the propensity of eliminating the so-called “hump effect” that seems to characterize all IFA foam materials.¹² This “hump effect” relates to a foam’s more rapid increase in force at low compressional strain levels during impact deformation; there exists a ubiquitous “initial force-bump” in a foam’s stress–strain curve. Overall, then, the inherent ability of FEAM layer IFA materials to respond more gradually to a high-velocity mechanical impact should be a critically important issue to sport and military helmet wearers. Overall, these subtle, but important, inherent FEAM properties should be of considerable interest when designing FEAM layers into sport and military helmet pad structures.

Summarizing, in pursuing the goal of designing an optimum sport/military helmet pad structure, several features must be considered. One sought-after property is that these pads should have a high IFA to areal density ratio; i.e. these pads should have high impact-absorbing properties at the lightest possible weight. Additionally, the pads must be wearer-comfortable by being soft and conformable against the head and able to manage sweat and body heat. In practice, most interior helmet pads are geometrically fitted into the helmet in patterned locations. As any geometric pad structures will only fit to specific helmet designs, this pad geometry topic will not be covered in this paper. This present study will thus focus on only the pad’s generic material composition and IFA properties. For this continued work, a “model” helmet pad test sample structure was chosen having the form of a square, 10 cm × 10 cm (4” × 4”), pad that is about 25.4 mm (1”) thick.

Designing sport and military helmet pad structures

IFA properties in terms of IFA/areal density ratios

Equipment and garment weight and wearer comfort have always been important factors in the field of sport and military paraphernalia. Burdening the wearer with needless weight will definitely affect endurance and performance. This weight factor is therefore considered important in the design of in-helmet IFA pads. Consequently, a relevant goal for helmet pad materials development is to create a pad that has the highest IFA properties to areal density (weight) ratio. This IFA/AD ratio factor has therefore been the focus of UMD’s helmet pad materials development. To this end, measurements of the FL%/AD ratios of various FEAM and foam combination test materials were carried out on a series of three-component IFA pad structures. Data collected in these measurements are summarized in Tables 5 and 6.

Table 5.

FL%/AD ratios for various baseline foams^a,b

Foam Material^c	Areal Density (g/m²) per 1 mm thickness	FL%/AD^d 10 mm Test Sample Thickness	FL%/AD^d 15 mm Test Sample Thickness	FL%/AD^d20 mm Test Sample Thickness
LDPE	33	0.21	0.45	0.70
EVA	76	0.13	0.34	0.57
VN-600	100	0.32	0.46	0.60
VN-740	130	0.38	0.53	0.45
Polyurethane (memory foam)	246	0.08	0.15	0.21

^aGWD Projectile Drop height was 100 cm for all tests.

^bAll FL% values are extrapolated from drawn IFA property vs. thickness plots.

^cAbbreviations: VN = vinyl nitrile, EVA = ethylene vinyl acetate, LDPE = low-density polyethylene.

^dFL%/AD is abbreviation for FL%/(AD per mm thickness) empirical ratio; AD has units of grams/square meter.

Table 6.

FL%/AD ratios for various FEAM configurations

Lab ID	Description	Thickness(mm)	Areal Density (g/m²) per 1 mm	FL%/ (AD per mm thickness)
005-A^a	VT//FX452-3D//VT	10.6	177	0.16
022-D^a	VT//FX601-3D//VT	9.2	225	0.10
016-B^a	VT//FX1002-3D//VT	12.2	210	0.20
010-A^b	[FX452-3S × 3] [FX452-3D] - - 0% “RA”	16.2	191	0.17
010-B^b	[FX452-3S × 3] [FX452-3D] - - 2.9 ±0.6 “RA”	15.4	212	0.21
010-C^c	[FX452-3S × 3] [FX452-3D] - - 8.4 ±1.5 “RA”	18.6	180	0.28
010-D^c	[FX452-3S × 3] [FX452-3D] - - 13.0 ±2.2 “RA”	20.8	172	0.40

^aCompare FL%/AD data with 10 mm thick foam data of Table 5.

^bCompare FL%/AD data with 15 mm thick foam data of Table 5.

^cCompare FL%/AD data with 20 mm thick foam data of Table 5.

For the data review, all data were “standardized” by our selecting to analyze only the 100 cm drop height GWD data. Furthermore, the areal density values have all been “normalized” to represent the grams/square meter area for 1 mm thick layers; this number physically represents the expected areal density of a 1 mm thick layer of the GWD test sample material. Using these conventions, an empirical evaluation parameter, FL%/(AD per mm thickness), has been established. Such FL%/AD data have been summarized for foam materials in Table 5 and for selected FEAM configurations in Table 6. Reviewing the foam data of Table 5 shows that all the FL%/AD ratios of the tested foam materials increased as the thickness of the tested sample foam material increases. Comparing these Table 5 foam data with the FEAM data of Table 6, we find that with the exception of polyurethane memory foam, all of the tested foam materials are found to have much higher FL%/AD ratios than the selected three-layer FEAM configurations. This comparison suggests that an impact pad material’s FL%/AD (per thickness) ratio appears to be a meaningful material property index for rating helmet pad material functional performance. It is noted that for 20 mm thick panels, the foam-only materials have FL%/(AD per thickness) ratios in the 0.45 to 0.70 range. Comparable thickness range FEAM materials have IFA/AD ratios of 0.28 to 0.40.

It is obvious that helmet pad configurations employing FEAM-only IFA material elements will not satisfy the needs for creating optimally high FL%/AD ratio helmet pads. From this finding, it would be advantageous to find a way to combine the good IFA and light-weight material properties of foam with FEAM’s desirable breathable, sweat, and heat-managing properties. This approach should lead to helmet pad structures whose properties are a “trade-off” between foam’s light-weight and FEAM’s meritorious comfort and wearability properties. Now, using a combination of FEAM and foam layers, an integrated balance of these two properties should lead to developing an acceptable helmet pad design. Employing this approach, experiments were planned and executed to evaluate the efficacy of foam/FEAM combinations in helmet pad structures. From this it is concluded that pads made using a combination of FEAM and foam would be the most logical approach to integrating FEAM components in helmet pad design.

Evaluating various FEAM helmet pad component configurations

Studies involving the IFA testing of FEAM/foam-layer combination panels for high IFA, low weight, and wearer comfort were started. Function-wise, all these studies involved fabricating experimental GWD test pads having three-functional components: (a) a FEAM/foam layered IFA “core,” (b) an against-the-helmet outer layer of a Velcro® hook adaptable fabric (like VelTex®), and (c) an against-the-head smooth-to-the-touch, comfortable, fabric layer. A diagram of this generic three-component helmet pad structure is presented in Figure 5. This sketch presents a specific helmet pad design composed of a single-fabric Velcro®-adaptable outer first layer, a FEAM/foam/FEAM central core, and a smooth-to-the-touch single-fabric second outer layer. Recognize here that the central core component can have a varied assembly of FEAM and foam layers such as a - - foam/FEAM/foam central core or any multiple thereof. Furthermore, the use of backside flocked Velcro® first side outer fabric layer and/or a backside flocked smooth-to-the-touch second outer fabric are also feasible configurations for this generic three-component helmet pad design. The need for a Velcro® hook adaptable fabric as against-the-helmet layer is because most helmet pad units are customarily fitted into the helmet by attaching themselves to Velcro® hook-strips that are fixed/adhesively bonded/mounted on the inside of the helmet. Furthermore, a most promising choice for the “soft” smooth, comfortable against-the-head fabric is a “velvety-soft” flocked fiber fabric; like commercial garment quality “simulation velvet” fabric. Alternatively, lighter-weight polyester-based micro-suede fabrics could also be used here. With this variety of component features in mind, panels were fabricated and GWD tested where the effect of using outer-wrap fabrics of plain-weave, micro-suede polyester versus the VelTex®, Velcro® hook adaptable, and other outer fabric types were evaluated. In other tests, the variation of employing central cores of multiple layers of thin VN-600 foam as opposed to fewer thicker layers of VN-600 foam was evaluated. Additionally, GWD test pads prepared employing much lighter-weight, low-density polyethylene (LDPE) were prepared and tested. These LDPE studies were carried out to settle the question of: Can the much lighter LDPE foam replace VN-600 foam in helmet pad applications? The results of these experiments are listed below:

Figure 5.

Sketch of a generic three-component FEAM/foam IFA helmet pad structure.

VelTex®-covered helmet pads are found to result in pads having IFA properties (“g” and FL%) that are 20% better than fully micro-suede fabric-wrapped pads. As VelTex® outer-wrap fabrics (or the like) result in this IFA improvement, it should be the material of choice for any newly designed helmet pad configurations.

Multiple layers of thinner VN-600 foam gave virtually identical IFA results to test panels containing fewer layers of thicker foam. There seems to be good mechanical coupling between laid-together foam layers.

Employing LDPE foam in place of VN-600 foam in a fabricated FEAM panel resulted in a significant drop in sample weight (areal density). However, the LDPE-containing pads were found to exhibit an over 30% poorer “g” and FL% performance. From this observation, LDPE foam has been eliminated from our continuing helmet pad study.

Optimizing helmet pad configurations

The development of an optimized helmet pad structure has been approached with foam serving as the base starting material. The design process will proceed by fabricating test panels that start out with close to 100% foam content, and subsequently prepare successive test samples by replacing the foam layers with FEAM element layers. The strategy here was to determine the “trade-off” level between the foam/FEAM component ratio; the foam/FEAM ratio where the IFA/wearer comfort (breathability, heat and sweat management) properties are optimized. This “trade-off” property helmet pad structure will then be used as the model for creating an optimum design sport or military helmet pad. In this final experimental series, VN and EVA foams were selected as the starting (base) foam materials. The experimental plan called for first fabricating GWD test samples having close to 100% foam, followed by a series of samples where the laminate’s foam layer/thickness cross-section are replaced by select FEAM layers.

Experimental results on test panels where foam layers were replaced with FEAM layers are presented in Table 7. This Table includes a data column specifying the foam content (in terms of the approximate percent cross-sectional foam thickness) of each panel. For consistency, all of the 100% test foam (actually 96% foam as all samples were fabric wrapped) samples were micro-suede fabric wrapped and perimeter sewn. Note too that all the fabricated test panels were fabricated to be as close as reasonable to the 25.4 mm (1”) thickness range. Additionally, the Table 7 test samples also include panels containing hole-perforated foam, and in some cases hole-perforated flocked FEAM layers. It should be also noted that the selected FEAM/foam configurations were prepared using laboratory-available foam and FEAM configurations; at this time, the fabricated test panel selection was selected to establish general trends. As shown, the sample configurations in Table 7 have been grouped in VN-600 and EVA foam-containing configurations. The data have also been tabulated in terms of decreasing foam content. Overall, as expected, it is found that helmet pad test samples containing hole-perforated foam layers (96% foam content) and no FEAM layers showed the highest overall FL%/AD performance.

Table 7.

Some experimental-model foam/FEAM helmet pad designs

Lab ID	Description^a,b	Foam Content (%)^d	Thickness (mm)	Areal Density (g/m²)	“g” (100 cm)	FL% (100 cm)	FL%/ (AD per mm thickness)
Vinyl Nitrile Foam VN-600 Containing
036-H	/VN-600 Foam/	96	25.1	3254	53.6 ±1	53.6 ±1	0.41
030-A	/VN-600P/-1/4” dia. hole perforated^c	96	24.9	2462	54.2 ±1	68.4 ±1	0.69
030-B	WW-1733/VN-600(2.1)/VN-600P (8.1)/VN-P (8.4)/ VN-600(1.8)/FX452-3S: plain weave micro-suede fabric base	76	27.1	3125	49.3 ±1	70.0 ±1	0.62
044-A	WW-1733:FX1002-3S//FX1362-2D//FX1362-2D//FX601-3D //FX602-3S:plain weave micro-suede fabric base	0	23.4	5353	61.0 1	62.5 ±1	0.27
Ethylene Vinyl Acetate (EVA) Foam Containing
032-A	/EVA Foam, twelve 2 mm thick layers/	96	25.4	2846	42.9 ±1	74.6 ±2	0.67
034-A	/EVAP Foam, one 2 mm/ten 2 mm thick, ¼” dia. hole-perforated layers/one 2 mm thick layer/	96	24.7	2461	42.1 ±1	74.6 ±2	0.75
038-A	WW1733;FX1002-3S/EVA 2mm /EVAP three 2mm thick layers /EVA 2mm/FX601-3DP//FX601-3D//FX601-3S: plain micro-suede weave fabric	40	25.3	3327	44.7 ±2	72.4 ±1	0.55
038-C	WW1733/EVA-2mm/FX1002-4D//FX1002-3D//FX602-2D//FX801-3S-plain weave fabric	8	26.1	4144	53.8 ±1	63.3 ±1	0.42

^aVinyl Nitrile VN-600 foam, DerTex, Saco, ME. EVA foam obtained from JoAnn Fabric as “little Makers” Art Foam and confirmed by JoAnn Fabric that it is EVA foam.

^bAll foam test samples were wrapped with micro-suede fabric and perimeter sewn before testing.

^cThis panel was a VN-600 foam lay-up of VN (3.4), VN-P (7.8), VN-P (1.6), VN-P (7.8), VN (2.3). Overall micro-suede fabric wrapped and perimeter sewn.

^dFoam % values is based on the approximate thickness percentage of foam in the fabricated test sample.

Sample 030-B with only one FEAM layer had a FL%/AD ratio of 0.62 with VN-600 as the foam component. When using the FL%/AD ratio as the IFA performance parameter, several conclusions have been made relative to the Table 7 data:

The best IFA/AD performing helmet pad structure was the perforated EVA foam material (034-A).

The second best IFA/AD performing helmet pad structure was the perforated VN-600 foam material (030-A).

The best performing FEAM-containing helmet pad configuration was sample (030-B); having only one FEAM component. This component had a micro-suede outer fabric whose backside was flocked with 45 denier, 3 mm long nylon. This micro-suede fabric was positioned to be the against-the-head fabric layer of the helmet pad structure; this panel was composed of only 24% FEAM. Furthermore, layers of solid and perforated VN-600 foam were also positioned in this single FEAM layer structure.

The next best performing FEAM-containing helmet pad configuration was (038-A); having a 40% FEAM composition; two layers of 60 denier, 3 mm long polyester fiber flock (one of the FEAM layers was hole perforated) and a final against-the-head layer of micro-suede fabric whose backside was flocked with 60 denier, 3 mm long polyester fiber flock.

All experimental IFA helmet pads in this study having more than one FEAM layer exhibited lower FL%/AD ratio values. As expected, helmet pad configurations having no or very low foam layer content were the poorest performing IFA configurations.

It appears that an optimized FEAM/foam combination material sport or military helmet pads should have a FEAM layer content in the range of not more than 40% to 50%.

Overall, the Table 7 data set the foundation for further studies on optimizing foam/FEAM helmet pad material configurations. It appears that the demonstrated approach is viable. Additional systematic studies must be carried out to arrive at a final, optimized, sport and/or military helmet pad configuration. Furthermore, an important topic not covered in this present paper is in-helmet helmet pad wearer comfort. This is, of course, a more “subjective” issue but certainly is a topic that all sport and military helmet pad studies must eventually consider. Wearer comfort information can only be obtained by carrying out real-time field trials of these newly designed in-helmet helmet pads with athletes and warfighters. These field studies should be scheduled when a finalized helmet pad structure configuration is achieved. Nevertheless, these Table 7 data solidly set some technical material-choice guidelines for all future work on using FEAM IFA materials in helmet pad applications.

Summarizing discussion

UMD’s interest in IFA helmet pad material research was initiated by two events. In 2014, UMD was awarded a sub-contract through the National Football League (NFL) Head Health Challenge IV program to evaluate the efficacy of using FEAM for football helmet pad applications.²¹ This event was followed in 2015, by UMD being awarded an 18-month Department of Defense (DoD) research contract to evaluate FEAM for military helmet pad applications.¹⁶ While the NFL sponsored research was most preliminary, it provided good background information on the FEAM concept that was initially conceived in 2012. The results of this background work have been documented in UMD’s publications and patents.^12-16,20 Now, this present paper highlights the results of this DoD contract as well as helmet pad research that has been carried out by UMD as an extension of this military helmet pad work. The main conclusions of this early DoD contract work were that while FEAM-containing helmet pad structures were found to have IFA properties that were equal to or better than the existing “Team Wendy” military helmet pad system, FEAM-utilizing helmet pads were all much heavier (higher areal density) by almost double the weight of standard issue “Team Wendy” helmet pads. These conclusions were the starting point for UMD’s continued helmet pad research. From this, it was deemed relevant to examine the field of helmet pad research in terms of studying helmet pad materials in the context of maximizing their IFA to areal density (IFA/AD) ratio properties. More specifically, the material property of most interest in this present paper was the helmet pad structure’s IFA/AD ratio. In practice, this ratio has been experimentally defined as the material’s FL%/AD (per thickness) ratio, where FL% is the experimentally measured FL IFA parameter.

As new research progressed, the FL/AD ratios of various foam materials were determined and tabulated. It was learned that the two most appropriate helmet pad foams to use in our study were VN VN-600 and EVA foams. The FL%/AD (per thickness) ratio for VN-600 foam was 0.41 and for EVA foam it was 0.67. Hole-perforated VN-600 had a ratio of 0.69, while the perforated EVA foam the ratio was 0.75. These data reflected the known fact that the FL% of hole-perforated IFA materials is very similar to the FL% of non-perforated material, so long as the hole per cent is lower than 30% open area. These data have set a FL%/AD ratio target range for our helmet pad materials research. For comparison, FL%/AD ratios of several variants of FEAM-only IFA pad structures were measured. Here it was learned that most all-FEAM-containing pad materials had FL%/AD ratios that were lower than the foams; ratio from 0.10 to 0.40; much lower than the ratio for the foams. From this comparison, it was surmised that an optimum sport or military helmet pad might best be developed by not employing FEAM material alone. It was decided that employing a strategic combination of FEAM and foam layer materials would be best. In this context, it is conceivable that a final pad composition/design could be developed that would be a “trade-off” material design featuring the meritorious high FL%/AD properties of foam and the meritorious wearer comfort properties of FEAM. In other words, foam could provide the pad’s light-weight properties and the FEAM would give breathability and superior wearer comfort qualities. This final “trade-off” choice would then have to be settled by subjective in-field/in-practice field trial judgments. As it was concluded that foam/FEAM IFA material combinations could lead to an optimum performing sport or military helmet pad, research dedicated to refining this idea followed. A simple approach was taken whereby a series of GWD test panels was made and tested, whereby foam-only test panels were first prepared followed by test panels where the foam layer material was sequentially replaced by FEAM layer elements. Two sets of foam/FEAM panels were prepared in an effort to evaluate VN-600 foam and EVA foam as the foam component of the proposed foam/FEAM “trade-off” property helmet pad design. While these present foam/FEAM panel studies were of limited scope, it was learned that either VN-600 or EVA foam could be used in these foam/FEAM pad structures. Furthermore, foam/FEAM pads having a FEAM content of 40% to 50% or lower should produce optimum foam/FEAM combination helmet pads.

This reported work should be considered as background for potential future work. In new work, additional FEAM material property variations must be focused on. Topics should be addressed such as (a) hole-perforated FEAM and/or foam layers; (b) strategic placement of foam or FEAM layers of different compressional stiffness; and (c) an investigation of the previously observed “synergistic” effect that has been cited in an earlier publication.¹² This synergistic IFA enhancement effect of foam/FEAM material combinations has indicated the ability of foam/FEAM layer combinations to IFA perform better than each of the individual components of the pad materials by themselves. The use of this “synergistic effect” indicates that it might be possible to create an optimum performing foam/FEAM layer design that is NOT a “trade-off” between high (light weight) IFA performance and wearer comfort. There could possibly be an optimum combination of foam type/FEAM geometry what will give us higher FL%/AD ratios than would be inherently better than any light-weight 100% foam pad structures. This suggests that higher than expected IFA performing foam/FEAM layer configurations are indeed possible. The key to such a development will be an investigation into the cause of this observed synergistic effect.

Conclusions

This paper summarizes experimental results carried out in an attempt to evaluate the efficacy of using FEAM-type IFA materials for sport and military helmet pad applications. Conclusions from this study are listed below:

A parametric study showed that flock fiber of (a) longer length (3 to 4 mm) (b) higher denier (45 to 100 denier range) and (c) higher flock density (fibers/area) are the flock fiber properties that can optimize the IFA properties of FEAM materials.

Applying a thin resin (lacquer) coating onto flock fiber surfaces can increase the IFA (FL% and “g”) properties of FEAM layered panels by at least 20%. This improvement is accompanied by an increase in the panel’s weight (areal density, AD).

The FL%/AD (per thickness) ratio has been established as an important experimental parameter in research and developing effective, light-weight sport and military helmet pad structures. In practice, the optimum range of FL%/AD ratio range for successful helmet pad structures has been found to be between 0.60 and 0.75.

Employing hole (¼” diameter) perforated foam and FEAM layers in layered helmet pad panel structures can significantly increase (25% to 50%) the FL%/AD ratio of fabricated foam/FEAM assembled helmet pad test pads.

It was decided that an optimum sport or military helmet pad design can be created by strategically combining assembled layers of (a) high IFA property, light-weight foam and (b) high IFA performing, wearer-comfortable, breathable FEAM layers. This can be referred to as the property “trade-off” helmet pad design.

Configurations of this “trade-off”, foam/FEAM helmet pad design have been fabricated and IFA tested indicating that layered configurations having a FEAM content of less than about 40% to 50% should be the key to developing an optimized helmet pad structure.

Studies should continue on optimizing foam/FEAM configurations for helmet pad applications. Investigations of hole perforation shape and geometry should be carried out. Additionally, the previously reported on phenomenon of the “synergistic effect”¹² improvement in IFA properties of foam/FEAM should be further investigated.

Footnotes

Acknowledgments

The authors wish to thank the CCDC Soldier Center, Natick, MA for partial support of this reported work.¹⁶ The authors would also like to thank the American Flock Association (AFA) for their support of UMass Dartmouth’s overall flock research effort; the latter parts of this herein reported study were carried out using this AFA support. Finally, the authors would like to acknowledge the invaluable help provided by the UMass Dartmouth laboratory students, especially Ramzi Bechara and Austin Taylor who carried out most of the material testing reported herein.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Yong K Kim

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