SOCRATE, Simona (83 Grove Street, Winchester, Massachusetts MA, 01890, US)
STRINGFELLOW, Richard G (83 Grove Street, Winchester, Massachusetts MA, 01890, US)
SOCRATE, Simona (83 Grove Street, Winchester, Massachusetts MA, 01890, US)
1. An energy absorption system for protective headgear comprising an internal liner of at least two layers of a honeycomb structure comprising a thermoplastic elastomer material, wherein each layer of the honeycomb structure comprises a plurality of cells defined by walls comprising the thermoplastic elastomers.
2. An energy absorption system of Claim 1, wherein the honeycomb structure includes at least one facing sheet which substantially closes one end of a layer.
3. The energy absorption system of Claim 1 or Claim 2, wherein the thermoplastic elastomer
material is a thermoplastic urethane.
4. The energy absorption system of any preceding claim, wherein a first layer of honeycomb
material comprises a first thermoplastic elastomer and a second layer of the honeycomb material comprises a second, different thermoplastic elastomer.
4. The energy absorption system of any preceding claim, wherein one of the layers of the liner has a first honeycomb structure and a second layer of the liner has a second honeycomb structure, wherein the second honeycomb structure is different to the first.
5. The energy absorption system of any preceding claim, wherein at least some of the cell walls include at least one orifice.
6. The energy absorption system of any preceding claim, wherein honeycomb layers each have a density in the range 0.05g/cc to 0.2g/cc.
7. The energy absorption system of any preceding claim, wherein the internal liner comprises at least three layers of the honeycomb structure.
8. The energy absorption system of Claim 7, wherein the internal liner comprises a pair of opposed outermost facing sheets which are not perforated, such that gases within the liner are trapped therein.
9. The energy absorption system of Claim 7 or Claim 8, wherein at least some of the cells in at least one of the layers include orifices to allow gas flow through a portion of the liner.
10. A headgear protector including an energy absorption system of any of Claims 1 to 9.
This invention relates to an energy-absorbing system for use in protective headgear that protects the wearer by minimizing the acceleration levels that the head experiences due to blunt impact. The system employs an internal liner of thermoplastic elastomer (TPE) materials that have been formed into multiple layers of a honeycomb structure. The selection of the TPE materials and the construction of the honeycomb layers are designed to optimize the dynamic force-displacement characteristics of the energy absorption system so as to limit the acceleration of the head. Each layer of honeycomb may be comprised of different TPE materials and have a different honeycomb structure. Each honeycomb layer may have a facing sheet on one or more of its sides. Patterns of holes or other features may be introduced into the walls of the honeycomb and/or the facing sheets so as to modulate air-cushioning effects.
Protective headgear is typically comprised of a relatively hard outer shell and a relatively soft internal liner. In the event of blunt impact against the outside of the headgear, the hard outer shell acts to prevent penetration of the impacting object. The softer liner acts to limit the acceleration of the head by absorbing the kinetic energy of the impact through deformation. Energy is absorbed through transverse compression of the liner between the head and the hard outer shell.
In certain headgear designs (e.g., bicycle helmets), the energy-absorbing component of the liners are made from polystyrene, and a significant mechanism of energy absorption is fracture of the polystyrene upon impact. Such designs are not effective for use against repeated impacts, due to damage of the liner. Other, generally more expensive, headgear designs often use viscoelastic foams in the liner, and are designed to withstand repeated impacts.
Viscoelastic foam energy absorption systems provide some degree of protection against blunt impact; however, foam materials are effectively isotropic (i.e., they respond in a similar manner when loaded in any direction). The micro-structural distribution of the viscoelastic material in such foams is not optimal when one considers that there is typically one primary direction of loading during a blunt impact event, namely transverse compression between the head and the hard outer shell of the headgear. For this reason, systems constructed from viscoelastic foams do not exhibit optimal 'crush efficiency', where crush efficiency can be measured in terms of the extent of energy that can be absorbed by the system while maintaining the crush force— and therefore the acceleration of the head— below a certain value.
Material systems with tailored anisotropic behavior have been used extensively in energy absorbing systems, particularly when the predominant direction of loading is well known. Aluminum honeycombs are among the engineering solutions with the highest crush efficiency. However, while aluminum honeycombs can be tailored to have the desired force-displacement response under impact conditions, they crush in a non-recoverable manner, and therefore do not meet the requirements of certain energy absorption systems for protective headgear.
With the recent introduction of a new family of elastomeric materials that can be thermally formed— thermoplastic elastomers, or TPEs— manufacturing of elastomeric honeycombs has become economically feasible. They are currently used for a wide range of applications in medical, sports, automotive and aerospace industries.
Thermoplastic elastomers are a class of copolymers or a physical mix of polymers which comprise materials with both thermoplastic and elastomeric properties. A difference between thermoset elastomers and thermoplastic elastomers is the type of crosslinking bond in their structures or matrix. Typically, thermoset elastomers have covalent crosslinking bonds, whereas thermoplastic elastomers include weaker dipole or hydrogen bonded crosslink bonds.
According to a first aspect of the invention, there is provided an energy absorption system for protective headgear comprising an internal liner of at least two layers of a honeycomb structure comprising a thermoplastic elastomer material, wherein each layer of the honeycomb structure comprises a plurality of cells defined by walls comprising the thermoplastic elastomer(s).
The present invention employs multiple layers of honeycomb constructed from thermoelastic elastomers (TPEs) as the basis for the internal liner of an energy-absorption system for protective headgear. The use of honeycomb structures increases the crush efficiency of the system significantly, because the inherent structure of the honeycomb is anisotropic, and can be tailored for optimal energy absorption in one preferred direction. TPE materials suitable for use in energy-absorption systems of the present invention are commercially available with a wide range of properties (modulus, strength, extensibility, rate sensitivity, etc).
Moreover, the geometric characteristics of honeycomb structures (cell dimensions, wall thickness, facing sheet thickness, etc.) can be conveniently controlled in the manufacturing process. In addition, since honeycombs with bonded facing sheets are inherently closed-cell, characteristic times for air diffusion under impact conditions can be conveniently controlled through optimal distribution of orifices.
TPEs can be used to produce large volumes of honeycomb structures in a cost-effective manner using proven manufacturing methods such as thermal forming. They also have demonstrated an ability to quickly recover to their original shape following impact and, therefore, are particularly well suited for use in systems with repeated load requirements.
The skilled person will appreciate that internal liners of protective headgear may be formed as a continuous layer or may be formed as separate discrete sections or pads of the multi-layered honeycomb structure.
In an embodiment of the invention, the cells of the honeycomb structure include a facing sheet which substantially closes one end (i.e., one of the open faces) of the cells. Optionally, the cells include a facing sheet at both ends of the cell, i.e., a top facing sheet and a bottom facing sheet.
Previous experience with modeling the behavior of the crush of TPE honeycombs has indicated that, for honeycomb volume fractions typical for these applications, if the aspect ratio of the height of the honeycomb cell to the length (see the value Lw in Figure 4 appended hereto) of the cell wall is too great, the honeycomb will buckle in a cooperative manner involving several contiguous cells, severely degrading its crush performance. Maintaining cell height to cell wall length ratios in the range 0.5 to 5.0, such as 1.25 to 2.5, tends to eliminate such ineffective buckling modes. For system thicknesses (i.e., the height of the cells) that are on the order of 0.5 to 1.0 inches or greater, a single layer of honeycomb would tend to have very large cells to maintain such aspect ratios. If such large cells were used, the pressure transmitted to the head would be very non-uniform, causing the wearer to experience an unacceptable level of discomfort. Previous experience has indicated that separating the system into multiple layers can prevent this undesired mode of deformation while minimizing the non-uniformity of the pressure distribution. In addition, having multiple layers provides for a greater ability to control the crush characteristics of the system.
In one embodiment, which has been developed for use with the U.S. Army's Advanced Combat Helmet (ACH), the liner is comprised of a number of individual pads that are attached to the inside of the headgear. In a particular embodiment, the energy absorption system contained within each pad is comprised of three identical honeycomb layers fabricated using the thermal forming techniques outlined in U.S. Patent 5,039,567 [reference 1]. Each honeycomb layer is fabricated with a TPE, such as a thermoplastic urethane (TPU) or a thermoplastic vulcanizate (TPV). The TPE is suitably a TPU.
Thermoplastic urethanes typically comprise linear polymeric chains in block structures. Such chains suitably contain low polarity segments which are relatively long (sometimes referred to as "soft" segments), alternating with shorter, high polarity segments (sometimes referred to as "hard" segments), wherein both types of segments are joined by covalent bonds such that they effectively form block copolymers. The relatively high polarity of the hard segments creates a strong attraction between the chains, which causes a high degree of aggregation and order in this area of the polymer matrix, resulting in crystalline or pseudo crystalline regions which are located within the relatively flexible regions of the matrix formed by the soft segments. The crystalline or pseudo crystalline regions act as physical crosslinks within the polymer matrix, which provides the elastomeric properties. However, these physical cross-links reduce or disappear upon the application of heat to the polymer matrix, which results in the thermoplastic properties of the material and is what allows the polymer to be extruded, moulded and/or calendered.
Thermoplastic urethanes may be prepared by reacting diisocyanates with (1) short chain diols; and (2) long chain bifunctional or multifunctional alcohols (polyols) to form the hard and soft segments as desired.
In view of the above, the TPE may be a thermoplastic urethane (TPU).
TPEs suitable for use in the present invention have a durometer value that is high enough to have the strength for use in the protective headgear and low enough to have the required recoverability. The TPE material suitably has a durometer of 80A to 100A, suitably 85A to 95A. A particularly suitable TPE for use in the honeycomb energy absorption systems of the present invention is an 87A durometer TPU. Such materials are commercially available. In one embodiment, 0.015-inch thick sheets of an 87A TPU are used to fabricate the honeycomb layer by thermal forming. The TPU sheet material has a density of 1.12 g/cc. The TPU material was tested and found to have an excellent combination of strength and recovery over a wide temperature range.
Other TPE materials with a durometer of 80A to 100A that could be used in the present invention include:
• 83A durometer TPU;
• 85A durometer TPU;
• 87A durometer TPU;
• 90A durometer TPU;
• 95A durometer TPU.
These TPU materials are commercially available.
These materials have a desired range of tensile properties. However, as the stress vs. strain curves are nonlinear, it is believed that the range of tensile properties should be cast in terms of a "secant modulus", a measure that is defined as the slope of the curve from the origin to a point on the curve at a particular strain level. It is further believed that 40% strain is a reasonable value given the strains that arise during the crushing of the honeycomb. Under quasi-static loading conditions, i.e., for strain rates between 0.001/s and 0.1/s, the materials of the invention suitably have a calculated secant tensile modulus at 40% strain of 10 to 40MPa, more suitably 15 to 35MPa, more suitably 17 to 32MPa.
When the TPE used in systems of the invention is an 87A TPU material, a suitable honeycomb layer is approximately 0.25 inches (6.35mm) thick, with a honeycomb cell wall width (the width of one side of a hexagonal cell) approximately equal to 0.2 inches (5.08mm). A cell wall width-to-thickness ratio of approximately 0.075 corresponds to a volume fraction of solid material of about 11.5%, resulting in a honeycomb density of about 0.13 g/cc. Both of these values are comparable to those of viscoelastic foams commonly used in headgear energy-absorption systems for the ACH. Each layer may include facing sheets on both sides. Where present, the facing sheets may also be made from 0.015-inch (0.38mm) thick sheets of the TPU material. A thin layer of viscoelastic foam may be added to the head- side of the system to improve comfort. The entire energy absorber may be encapsulated in an enclosure of fabric or other suitable material.
In an embodiment of the invention, the honeycomb structure is hexagonal; however, other cell profiles, such as rectangular, triangular, etc., may also be effective in certain cases.
In a further embodiment of the invention, one of the layers of the liner has a first honeycomb structure and a second layer of the liner has a second honeycomb structure, wherein the second honeycomb structure is different to the first. In this respect, the layers may have different sizes of cells and/or different shapes of cells.
In an embodiment of the invention, each of the layers has the same thickness. Our experience indicates that constructions with layers of different thicknesses may provide more optimal performance in some circumstances. Thus, in an embodiment of the invention, the layers of the liner have different thicknesses.
In further embodiments of the invention, a first layer of honeycomb material comprises a first thermoplastic elastomer and a second layer of the honeycomb material comprises a second, different thermoplastic elastomer. Thus, where the liner comprises two honeycomb layers, it may have the structure A-B, where A is a first thermoplastic elastomers and B is a second, different thermoplastic elastomers. Furthermore, when the liner comprises three honeycomb layers, it may have the structure: A-A-B, A-B-B, A-B-A or B-A-B. Alternatively, it may have the structure A-B-C, B-A-C or A-C-B, where C is a third thermoplastic elastomer.
In an embodiment of the invention, the honeycomb has a density of approximately 0.13g/cc Our experience suggests that honeycomb layers with a density in the range 0.05g/cc to 0.2g/cc, optionally 0.09g/cc to 0.17g/cc, will be most effective, while accounting for weight constraints related to use in military helmet liners.
In the aforementioned embodiment, the materials used to construct each of the honeycomb layers are TPEs. For systems in which recoverability is not required, our experience indicates that the use of aluminium honeycomb is likely to be an effective way to increase the energy absorption capacity of the system with little or no increase in weight. Thus, the liner of the invention may include an aluminium honeycomb layer.
In an embodiment of the invention, the liner includes a pair of opposed facing sheets. Suitably, these are made from the same material and, optionally, have the same thickness as the honeycomb. Each of the honeycomb layers may be sandwiched/bonded between a pair of opposed facing sheets, so that two facing sheets are in contact with each other when two honeycomb layers are stacked. However, our experience has shown that bonding contiguous honeycomb layers to a single intermediate thinner facing sheet and/or a facing sheet made from the same or a higher durometer TPE may provide a decrease in the thickness of the system with little or no decrease in performance.
In a further embodiment of the invention, the honeycomb structure does not have any features cut into either the walls of the cells or in the facing sheets. However, such features can easily be added during the manufacturing process. When present, such features would modulate the flow of air through the honeycomb during transverse impact loading. Modulating the air flow has been shown to have a significant effect on the crush characteristics of the system. Adding holes to certain layers may also provide a comfort benefit, as it would enable air circulation. Note that holes or other features may be selectively added in only some of the honeycomb layers. Also, only certain portions of a complete energy-absorption system might be fitted with holes or other features. Accordingly, some or all of the cells of the honeycomb may include an orifice or aperture such that the cells which include the orifices are in fluid communication with the adjacent cell which shares the cell wall (i.e., the cell wall which defines the orifice/aperture).
In an embodiment of the invention, the energy absorption system comprises three layers of honeycomb. Only the middle layer of honeycomb has holes (i.e., orifices) cut into the walls of the cell. Facing sheets with no holes are bonded to the outside of the top and bottom layers. Facing sheets with holes are bonded between the middle and top layer and between the middle and bottom layer. With this construction, air flow out of the system is limited to the middle layer, increasing the overall resistance to flow.
It should be appreciated that the terms "embodiment" and "an embodiment of the invention" should be understood to refer to any embodiment or aspect of the invention as defined or described herein. Therefore, it should be understood that the features of specific embodiments can be combined with one or more other specific features described herein or be combined with any aspect or embodiment of the invention described herein. All such combinations of features are considered to be within the scope of the invention defined in the claims.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Figure 1 shows stress/strain test results for sample materials;
Figure 2 shows stress/strain results for further sample materials;
Figure 3 shows stress/strain results for further sample materials;
Figure 4 shows an example honeycomb cell;
Figure 5 shows a mesh for a preliminary FEA model;
Figure 6 shows a predicted force-displacement graph for different materials;
Figure 7 shows example honeycomb structures;
Figure 8a shows a mesh for a three-cell honeycomb model;
Figure 8b shows predicted force-crush curves for different models;
Figure 9 shows models of mono, bi and tri-layer structures;
Figure 10 shows force-displacement curves for the layers of Figure 9;
Figure 11 shows honeycomb deformations;
Figure 12 shows force-displacement graphs for sealed and vented cells;
Figure 13 shows comparative first impact force-displacement graphs;
Figure 14 shows comparative second impact force-displacement graphs;
Figure 15 shows force-displacement graphs for a test material at different temperatures; and
Figure 16 shows force-displacement graphs for a different test material.
The following examples describe how materials and structures of particular embodiments of the present energy absorption system were made and tested. The examples are not to be understood to limit the scope of the invention. Example 1 - Mechanical Testing
For tension tests, samples were cut from sheets to a width of approximately 5 mm and a length of about 20 mm. Specimens were positioned in the tensile grips so that they had a gauge length of approximately 10 mm.
Several loading and unloading cycles were performed for each specimen. Total strains ranging from 20% to 80% were applied at strain rates ranging from 0.001/sec to 0.1/sec.
The following sheet materials were acquired and tested in tension:
• Test Sample A: a 55A durometer TPV, having an 0.031" thickness;
• Test Sample B: an 80A durometer TPV, having an 0.031" thickness;
• Test Sample C: a 65A durometer TPV, having an 0.020" thickness;
• Test Sample D: a 90A durometer TPV, having an 0.012" thickness;
• Test Sample E: an 87A durometer TPU, having an 0.015" thickness;
• Test Sample F: an 85A durometer TPU, having an 0.012" thickness;
• Test Sample G: a 72A durometer TPU, having an 0.020" thickness.
The TPV materials were tested first. Test results for these materials indicate that TPV exhibited considerable residual (irrecoverable) deformation after the first cycle and a subsequent loss of elastic modulus on subsequent loadings (conditioning). Figure 1 shows results for Test Sample A and Test Sample B. The harder 80A material had a much larger initial slope but also exhibited much higher irrecoverable extension.
The decrease in modulus of TPVs for subsequent loading cycles is undesirable for certain applications, which require consistent performance under repeated impacts. Attention was therefore focused on TPUs. We performed tension tests on the three TPUs. The 87A durometer Test Sample E exhibited considerable strength and much less conditioning than the TPVs that were tested. The 85A durometer Test Sample F behaved similarly. The softer 72A durometer TPU (Test Sample G) was not nearly as strong, with a room temperature elastic modulus of 10.4 MPa (1510 psi), as compared to 26 M Pa (3770 psi) for Test Sample E. Representative tensile data for Test Sample E are shown in Figure 2a. This TPE exhibits a fairly linear stress-strain response over the first 20% or so of strain, with a subsequent slight decrease in slope. Conditioning effects are not as severe as those observed for TPVs, and the material stiffness is maintained in subsequent loadings. As evident in Figure 2(b), the strain-rate dependence of the material is significant— there is an increase of 30% or so in strength with an order of magnitude increase in strain rate.
Further tests were conducted using the following materials:
Test Sample H: a 90A durometer TPU
Test Sample J: an 85A durometer TPU
Test Sample K: a 90A durometer TPU
Figure 3 shows tensile data (true stress vs. true strain) for the three different TPE materials that were tested. Data for Test Sample E are included for comparison.
As the curves of Figure 3 show, each of these other materials is stiffer than Sample E. For this reason, each may be more optimal for a liner system. The four together establish a desirable range of tensile properties for the invention. Because the curves are nonlinear, it is believed that the range of tensile properties should be cast in terms of a "secant modulus", a measure that is defined as the slope of the curve from the origin to a point on the curve at a particular strain level. 40% strain was selected as the appropriate reference point, given the strains that arise during the crushing of the honeycomb. The calculated secant moduli at 40% strain are as follows for these four materials:
Test Sample E -- 17.7 MPa (2570 psi)
Test Sample H -- 30.4 M Pa (4410 psi)
Test Sample J -- 22.7 M Pa (3290 psi)
Test Sample K -- 28.0 M Pa (4060 psi)
Example 2 - Determination of Geometry of Honeycomb Structure
Individual Cell Structure
In order to refine an appropriate range of properties for the TPE materials that would be used to construct the prototype pad, we performed preliminary finite element analyses (FEAs) of crushing honeycomb structures using material models based on the stress-strain information for each TPE material. To determine a preliminary geometry/configuration for the honeycomb FEA model, we weighed helmet pads from Team Wendy, MSA and Oregon Aero, and determined that the density of the pads ranged from about 0.09 to 0.15 g/cc. We then compared these values to estimated weights for the honeycomb materials. Figure 4 shows a schematic of a honeycomb cell. For a cell structure that is formed by thermally bonding sheets together, every third cell wall (i.e., the horizontal walls in Figure 4) is a double-wall. It can easily be calculated that the volume fraction of such a honeycomb is
approximately 1.54xTw/Lw, where Tw is the thickness of the sheet material and Lw is the length of a cell wall. For these preliminary calculations, typical properties of TPVs were used to characterize the sheet material comprising the honeycomb walls. The density of TPV is about 0.92 g/cc, so the density of a TPV honeycomb is about 1.42xTw/Lw g/cc. Based upon this expression and the weight range for the existing pads, we chose an initial Tw/Lw ratio of 1 to 10, corresponding to a honeycomb density of about 0.14 g/cc.
Our preliminary FEA model for the honeycomb consisted of one-half cell (actually one-sixth of three adjacent cells), as depicted in Figure 5. We selected dimensions of 0.02 inches (0.5 mm) for Tw, and 0.2 inches (5 mm) for the cell wall length, with a single honeycomb cell through the 0.75 inches (19mm) allowed for the pad thickness. Two rigid surfaces are used to quasi-statically crush the honeycomb. Constraints are imposed on the deformation of the outer edges of the three cell walls to model the effects of adjacent cells. (Note that this preliminary FEA model was later modified to include multiple cells, as is described in more detail hereinbelow.)
The three force-displacement responses of the honeycomb structures, shown in Figure 6, correspond to material properties fitted to the response of the three grades of TPVs. Note that predicted forces have been scaled to represent force per unit honeycomb cross-sectional area (crush pressure). As is evident, the very different strengths associated with the three TPVs result in very different force levels.
To estimate the desired force per unit area for our helmet pad design, we considered the energy that must be absorbed, the total area of pad that absorbs the energy, and the distance that the pad crushes. Table 1 lists the average force required to absorb 67 J of kinetic energy (5kg mass traveling at 17 feet/sec) for pad areas ranging from a single trapezoidal front/back pad to a single crown pad, and for usable crush distances of 0.5, 0.625 and 0.75 inches prior to consolidation of the pad material. As the table indicates, the average crush pressure (strength) is estimated to be somewhere in the range of about 40 psi to 100 psi.
Based upon this range, we determined that, for the assumed honeycomb structure, the TPV with durometer 40D is relatively hard, and the TPV with durometer 55A is relatively soft. The 87A durometer TPV appears to be an acceptable compromise. It is worth noting that, for a honeycomb with a bigger cell size-to-cell wall thickness ratio, the harder 40D durometer material could be made to have the correct average crush pressure (and would be even lighter); however, our experience tells us that this material may not provide repeatable behavior for multiple impacts. Its composition features a high ratio of thermoplastic component to elastomer component. Similar behavior is found in other classes of TPEs of high hardness (e.g., high-hardness TPUs, where a larger fraction of hard segments results in substantial irrecoverable deformation following the first loading cycle).
Based on these preliminary findings, we confined our more extensive material characterization efforts (detailed below) to TPEs in the 65A to 95A durometer range.
Table 1. Estimated average crush pressure (strength), in psi, required to absorb 67 J of energy. i ad area (square inch es)
limit crush 19.6 13.6 10.3
distance (in) (center pad) (two side pads) (front/back pad)
0.5 60 87 1 15
0.625 48 70 92
0.75 40 58 77
We developed several different FEA models and conducted an extensive number of analyses, both to get a sense of how different parameters affect the behavior of the honeycomb pad system and determine how detailed the models should be to capture this behavior with sufficient accuracy. In addition to the half-cell model shown in Figure 5, we developed a multi-cell cell model, with 33 cells, using over 150,000 elements, as depicted in Figure 7. This model, while still employing constraints along its outer boundary conditions to capture the effect of neighboring cells, simulates a large enough number of cells that the effects of edge boundary conditions should have a minimal affect on its response. Because it is so large, the solution time is quite large for this model, so it is not practical for running most cases. However, it is very useful as a validation of the behavior of other models.
A three-cell model that we developed to improve computational efficiency is shown in Figure 8(a). The behavior of the multi-cell model is compared with that of the three-cell model in Figure 8(b). As is evident, the comparison is favorable, and demonstrated to us that the more computationally-efficient three-cell model was sufficient for purposes of designing the pad.
Having settled on a three-cell model, we added facing sheets, as we determined these to have a significant effect on crush behavior, and we constructed models with one, two, and three layers of honeycomb cells, as shown in Figure 9.
A comparative evaluation of the effect of layering is shown in Figure 10. As is evident, the multi-layer pad exhibits a more efficient, level force-crush curve than does the single-layer and bi-layer pads. The intermediate facing sheets tend to stabilize the response.
We also examined the effects of air cushioning using a model in which an internal air cavity was defined (see Figure 11). The results of this model suggest that air cushioning can have a significant effect on impact response, especially as the cell crushes to a small percentage of its original volume, as shown in Figure 12. It is not clear that this effect is always beneficial. The crush load increases dramatically at crush strokes above 50% of the initial pad thickness, and the rapid rise in load can hurt the overall crush efficiency of the pad.
To summarize the results of the preliminary modeling, we learned, among other things, the following:
• A three-cell model of the honeycomb structure is sufficient to capture uniaxial compression
• A three-layer model provides a reasonably flat force-displacement curve;
• If the cell is too tall, lateral buckling of the cell occurs; otherwise, the cell is insensitive to cell
height/cell area ratio; • When lateral buckling is not present, the key design parameter becomes the ratio of cell wall length to cell wall thickness— absolute cell size has a weak effect, as does the angle between cell walls (i.e., if the cell is not hexagonal, but stretched in one direction).
Example 3 - Construction of Sample Pad
Sample pads were fabricated, based on the results of the materials characterization and pad design activities described in Examples 1 and 2, above, following a fabrication process outlined in [reference 1]. Test Sample E with a sheet thickness of 0.015" (0.38 mm) was used as the material for both the honeycomb and the facing sheets. The cells were formed with wall length of 0.18 inches. Because the density of Test Sample E is about 1.12 g/cc, and the volume fraction of solid material in the honeycomb core is approximately 12.8%, the resulting density of the honeycomb core is about 0.14 g/cc. Laminates of three layers of honeycomb material were formed.
Example 4 - Comparative Performance Tests
In a comparative set of dynamic impact tests, the sample pads of Test Sample E produced as described in Example 3, above were compared to existing helmet pads. Figures 13 and 14 compare the measured crush force versus pad displacement for a trapezoidal front/back pad at room temperature for an impact energy of approximately 46 J (consistent with a helmet drop test standard mass of 5 kg impacting at 14 feet per second). Figure 13 shows results for the initial impact test. The measured force stays below the limit specified in the standard for ACH pads (150 G, or 7.36 kN for a 5 kg impact weight). Figure 14 shows results for a repeated impact test, conducted about one minute later. The comparative performance of the honeycomb-based pad is particularly favorable for the second impact.
In accordance with our models, we expected the pads to absorb over 50 J of energy at room
temperature while maintaining acceleration levels below 150G. The test results above are consistent with this expectation and further suggest that this pad could absorb approximately 65 J of energy at 0°C while maintaining acceleration levels below 150G. It is our expectation that, with further optimization of the design, these systems can be made to absorb 50 to 70 J of energy over the temperature range - 10°C to 55°C (the temperature range specified in ACH test standards) while maintaining acceleration levels below 150G (as also required by these test standards). Further dynamic impact tests were conducted using three-layer stacks of honeycomb panels.
Figure 15 shows the results of a dynamic impact test result for a three-layer stack of honeycomb panels formed from Test Sample K having a 0.012-inch sheet thickness and a cell wall length of approximately 0.13 inches. The material is perforated, so air is not trapped when it is crushed. These panels have no facing sheets.
We measured the density of these panels and determined them to be approximately 0.17 g/cc.
The test results shown in Figure 15 are for a three-layer stack of five-inch round panels cut from this honeycomb, with a total thickness of approximately 0.9 inches. These results show that panels without facing sheets perform very well in the dynamic impact test. The high-temperature results show that there is only about a 20% loss of strength from room temperature to 55C, which is quite favorable. The repeatability of the results for the 2nd impacts at room temperature show that the permanent set for this material is limited, which is also a favorable result.
As a result of this test, it appears that honeycombs without facing sheets perform well. This is an important result because the facing sheets add a lot of weight to the system. Also, FEA-based simulations of helmet impact tests indicate that facing sheets are not as easy to conform to the curved inside surface of a helmet.
The results of a further dynamic impact test result are shown in Figure 16. This test used test Sample J material with facing sheets and no perforations (and thus traps air). This result indicates that trapping of air leads to an increase in force that limits energy efficiency of crush.
. Landi, C.L, Wilson, S.L, "Resilient Panel Having Anisotropic Flexing Characteristics and Method of Making Same," U.S. Patent #5,039,567, Aug. 13, 1991.