Patent Application
20250052542
The present disclosure generally relates to protective armor liners. In particular, the disclosure is directed to protective armor liners comprising a viscoelastic material and graphene nanoplatelets.
Protective armor has reduced injury and death by incorporating new technology and materials to defeat threats. An emerging threat, behind-armor blunt trauma (BABT), is a non-penetrative injury from the transfer of shock energy resulting in torn tissue and skeletal, thoracic, and abdominal injuries. Shock wave overpressure exposures can also result in blast-induced traumatic brain injury (bTBI).
Modern body armor has not been optimized to efficiently dissipate the energy responsible for BABT or bTBI from ballistic and blast threats. Trauma attenuating backings (TABs) have been investigated to dissipate shock energy but are typically made of single-use foam or trusses, limiting their practicality.
Therefore, a need exits for the development of improved protective armor that is capable of reducing or dissipating the energy responsible for BABT or bTBI. Still further, a need exists for protective armor liners which may be inserted in or placed on the body armor to dissipate this energy.
Other objects and features will be in part apparent and in part pointed out hereinafter.
Aspects of the present disclosure relate to viscoelastic trauma attenuating backings for use in body armor to reduce non-penetrating shockwave injuries.
Other aspects of the present disclosure are directed to protective armor liners comprising a viscoelastic material and graphene nanoplatelets. For example, protective armor liners wherein the viscoelastic material comprises a polyurethane viscoelastic material, silicon rubber-based viscoelastic material, or combinations thereof.
Additional aspects of the present disclosure are directed to a protective armor comprising a protective armor liner disposed therein or disposed thereon, wherein the protective armor liner comprises a viscoelastic material and graphene nanoplatelets.
Other objects and features will be in part apparent and in part pointed out hereinafter.
Corresponding reference characters indicate corresponding parts throughout the drawings.
The combat helmet is a vital piece of military equipment that has increased warfighter survival since its inception by creating a physical barrier between the wearer and kinetic threats. During the twentieth century, the combat helmet progressed from protection against artillery shell fragments to defeating direct impacts from 9 mm bullets.
Recently, research has progressed to the study of protecting the modern warfighter from the threat of blast-induced traumatic brain injury (bTBI). In 2021, a total of 18,773 TBIs were reported by the Department of Defense (DOD). Of particular note is blast-induced TBI (bTBI), which has been described as an occupational hazard as warfighters who may be exposed to repeated low-level blasts during training and deployment.
A limitation of modern helmets in preventing bTBI is the underwash effect. In this phenomenon, areas under the helmet experience higher overpressure due to interactions of the shock wave between the head and helmet as the wave moves over the head and gets trapped underneath the helmet. High, and even low, blast exposures can lead to bTBI depending on the severity and proximity.
Time-pressure waveforms are used to illustrate the shock wave exposure to a warfighter. Shock waveforms can be evaluated with the following characteristics: peak overpressure, rise time, waveform duration, and impulse. Peak pressure is the maximum pressure of a shock wave. Rise time shows how fast the peak pressure is reached after the pressure rises above ambient and can indicate the rate of loading during the interaction between the shock wave and the object. The duration is the length of time of the first positive phase, determined by the difference in time from the first increase above ambient pressure to the beginning of the negative phase of the waveform. Lastly, by taking an integral of the area under the curve of the shock waveform's first positive phase, impulse is found. This corresponds to the amount of force put on the head.
Importantly, these characteristics will change depending on the setting and source of the shock wave generated. The difference could be a breaching charge versus firing a mortar. Ideally, reduction of peak pressure, duration, and impulse results in less force applied to the warfighter and buildings around them, thus reducing harmful exposure for the warfighter.
As shock wave-helmet-head interactions are becoming better understood, it is essential to develop technologies that leverage that understanding to reduce under-helmet overpressures and enable helmets to provide better protection. Reducing aspects related to blast exposure such as overpressure/underwash effect could dramatically reduce the prevalence of bTBI. Therefore, it is one aspect of the present disclosure to provide for improved protective armor (e.g., helmets) that reduce the overpressure/underwash effect as compared to a standard helmet.
A potential solution is to adapt helmets to divert shock wave flow, thus preventing waves from getting under the helmet. This is typically done by incorporating thick foam pads inside the helmet, thereby creating a seal. Another tactic is to use a visor and mandibles to shield the front. Another solution considered is to add dampening materials directly to the helmet.
The inventors of the present disclosure have discovered that certain protective armor liners are especially beneficial in reducing the overpressure/underwash effect and providing a better reduction in the shock wave exposure to a wearer.
One aspect of the present disclosure is directed to a protective armor liner comprising a viscoelastic material and graphene nanoplatelets. Other aspects are related to protective armor comprising a protective armor liner disposed therein or disposed thereon, wherein the protective armor liner comprises a viscoelastic material and graphene nanoplatelets.
While discussion herein is directed to a protective armor liner, it will be understood that the discussion is applicable to the protective armor liner itself, or its incorporation into a protective armor system. For example, discussion of a protective armor liner is equally applicable to a helmet (i.e. protective armor) having the protective armor liner disposed therein or disposed thereon. For further clarification, the materials of the protective armor liner are applicable to the protective armor liner itself, or a protective armor liner incorporated into a protective armor.
Although discussion is primarily directed to the protective armor of a helmet, it will be understood that the disclosure is not limited to this embodiment. The discussion herein is also applicable to use of an armor liner in any other suitable protective armor. For example, leg armor, chest armor, arm armor, elbow pads, knee pads, boots/shoes, etc.
In certain embodiments, the armor liner comprises a viscoelastic material.
Viscoelastic materials were found to absorb, dampen, and redirect kinetic energy (blunt-ballistic load and blast waves) by converting mechanical energy into heat. This process is characterized by hysteresis and frequency-dependent damping, and results in a reduction in the magnitude of mechanical disturbances. The response of a particular viscoelastic material to such kinetic energy depends on the frequencies and amplitude of stress waves traveling through the material. That is, the process is dependent on the viscoelastic materials' complex modulus, which varies with the frequency it is subjected to, thus determining the extent of damping provided. For this reason, viscoelastic materials were further investigated for vibration suppression (i.e. dampening of kinetic energy from blast waves, blunt-ballistic loads, etc.).
The viscoelastic material of the present disclosure may be selected from any viscoelastic material that is suitable for shock wave or pressure reduction. In some embodiments, the viscoelastic material is selected from the group consisting of polyurethanes, rubbers, or combinations thereof. In other embodiments, the viscoelastic material comprises a polyurethane viscoelastic material, silicon rubber-based viscoelastic material, or combinations thereof. In one embodiment, the viscoelastic material comprises silicon rubber, urethane rubber, or combinations thereof. In additional embodiments, the viscoelastic material may comprise a platinum-catalyzed silicone rubber, urethane rubber, or combinations thereof. For example, Ecoflex 00-30 silicone rubber or PMC 121/30 (both commercially available from Smooth-On, Inc.). In still further embodiments, suitable viscoelastic material may be Ecogel, Ecoflex 00-30 silicone rubber, Vytaflex, or PMC 121/30 (each commercially available from Smooth-On, Inc.).
Ecoflex 00-30 is a platinum-catalyzed silicone rubber. The cured rubber is soft, strong, and “strechy” (stretching many times its original size without tearing and rebounding to its original form without distortion).
PMC 121/30 is a urethane rubber that is strong and abrasion resistant.
Ecoflex is a platinum silicone rubber gel. Ecoflex is soft and has a Shore hardness of 000-35.
Vytaflex is a urethane rubber especially suited for casting as a mold rubber. Vytaflex is available in 10 A, 20 A, 30 A, 40 A, 45 A, 50 A or 60 A Shore hardness'. Vytaflex has a Die C tear strength of from 38 pli (lbs per lincar inch) to 136 pli (i.e. 6.66 kN/m to 23.8 kN/m).
In other embodiments, the armor liner comprises a viscoelastic material and graphene nanoplatelets.
Graphene nanoplatelets are multilayer particles of graphene sheets. In certain embodiments, the graphene nanoplatelets have a thickness of 5-10 nm. In other embodiments, the graphene nanoplatelets have a thickness of up to 50 microns. In some embodiments, graphene nanoplatelets are characterized as those graphene sheet particles having at least one dimension less than 100 nm.
In some embodiments of the present disclosure, the armor liner comprises a viscoelastic material and graphene nanoplatelets, wherein the graphene nanoplatelets are mixed into the viscoelastic material before hardening (i.e. the liner comprises a viscoelastic material with graphene nanoplatelets therein). In other embodiments, the armor liner comprises viscoelastic material with graphene nanoplatelets disposed thereon. In still further embodiments, the armor liner comprises a viscoelastic material having nanoplatelets both therein and thereon (i.e. inside and externally attached to the viscoelastic material).
In certain embodiments, the armor liner comprises about 5 wt % or less, about 4 wt % or less, about 2 wt % or less, about 1.9 wt % or less, about 1.8 wt % or less, about 1.7 wt % or less, about 1.6 wt % or less, about 1.5 wt % or less, about 1.4 wt % or less, about 1.3 wt % or less, about 1.2 wt % or less, about 1.1 wt % or less, about 1.0 wt % or less, about 0.9 wt % or less, about 0.8 wt % or less, about 0.7 wt % or less, about 0.6 wt % or less, about 0.5 wt % or less, about 0.4 wt % or less, about 0.3 wt % or less, about 0.2 wt % or less, or about 0.1 wt % or less of the graphene nanoplatelets.
In other embodiments, the armor liner comprises from about 0.25 wt % to about 5 wt %, from about 0.25 wt % to about 4 wt %, from about 0.25 wt % to about 3 wt %, from about 0.25 wt % to about 2 wt %, from about 0.5 wt % to about 2 wt %, from about 1.0 wt % to about 2 wt %, or from about 1.5 wt % to about 2 wt % of the graphene nanoplatelets.
It will be understood that the benefits of any protective armor or armor liner is balanced against the negative effects of added weight. That is, at a certain point the ability to protect against shock waves or impacts are negated by the burdensome weight or size of the protective armor.
Therefore, in certain embodiments, the armor liner has a thickness of about 10 mm or less, about 9 mm or less, about 8 mm or less, about 7 mm or less, about 6 mm or less, about 5 mm or less, about 4 mm or less, about 3 mm or less, about 2 mm or less, or about 1 mm or less. For example, in some embodiments, the armor liner has a thickness of from about 0.5 mm to about 10 mm, from about 0.5 mm to about 9 mm, from about 0.5 mm to about 8 mm, from about 0.5 mm to about 7 mm, from about 1 mm to about 7 mm, from about 1 mm to about 6 mm, from about 1 mm to about 5 mm, from about 1 mm to about 4 mm, or from about 1 mm to about 3 mm.
In some embodiments, the protective armor liner comprises multiple thin layers of the viscoelastic materials and graphene nanoplatelets. That is, a protective armor liner may comprise a discrete first layer of viscoelastic material and graphene nanoplatelets stacked on a discrete second layer of viscoelastic material and graphene nanoplatelets. The multiple layers in the liner may be any integer of layers. For example, 2, 3, 4, 5, 10, 15, 20, 25, 50, 75, or 100 layers. In such embodiments, the viscoelastic materials or graphene nanoplatelets of each layer may be the same or different.
In embodiments wherein the armor liner comprises multiple thin layers of the viscoelastic material and graphene nanoplatelets, each layer of viscoelastic material and graphene nanoplatelets may have a thickness of about 10 mm or less, about 9 mm or less, about 8 mm or less, about 7 mm or less, about 6 mm or less, about 5 mm or less, about 4 mm or less, about 3 mm or less, about 2 mm or less, about 1 mm or less, about 0.5 mm or less, about 0.1 mm or less, about 0.05 mm or less, about 0.01 mm or less, about 0.005 mm or less, about 0.001 mm or less, about 0.0005 mm or less, or about 0.0001 mm or less.
In other embodiments, the protective armor layer comprising multiple layers has a total thickness of about 10 mm or less, about 9 mm or less, about 8 mm or less, about 7 mm or less, about 6 mm or less, about 5 mm or less, about 4 mm or less, about 3 mm or less, about 2 mm or less, or about 1 mm or less. For example, about 0.5 mm to about 10 mm, from about 0.5 mm to about 9 mm, from about 0.5 mm to about 8 mm, from about 0.5 mm to about 7 mm, from about 1 mm to about 7 mm, from about 1 mm to about 6 mm, from about 1 mm to about 5 mm, from about 1 mm to about 4 mm, or from about 1 mm to about 3 mm.
Further embodiments of the present disclosure are directed to protective armor comprising the protective armor liner described herein. That is, a protective armor comprising a protective armor liner disposed therein or disposed thereon, wherein the protective armor liner comprises a viscoelastic material and graphene nanoplatelets.
The viscoelastic material and graphene nanoplatelets may be as described above.
The assembled protective armor (i.e. comprising the protective armor liner disposed therein or disposed thereon) may exhibit a certain dissipation factor when tested at a frequency of from about 20 Hz to 1,000,000 Hz. For example, a dissipation factor of about 1 or less, about 0.75 or less, about 0.5 or less, about 0.25 or less, or about 0.1 or less. In other embodiments, the dissipation factor may be about 0.015 or less, about 0.01 or less, about 0.005 or less, or about 0.001 or less.
In one embodiment, the viscoelastic material comprises polyurethane and the dissipation factor for a frequency of from about 20 Hz to 1,000,000 Hz is about 1 or less, about 0.75 or less, about 0.5 or less, about 0.25 or less, or about 0.1 or less.
In another embodiment, the viscoelastic material comprises a silicon rubber-based viscoelastic material and the dissipation factor for a frequency of from about 20 Hz to 1,000,000 Hz is about 0.015 or less, about 0.01 or less, about 0.005 or less, or about 0.001 or less.
In further embodiments, the assembled protective armor (i.e. comprising the protective armor liner disposed therein or disposed thereon) may achieve a shock wave velocity reduction as compared to the identical protective armor that does not have the protective armor liner. For example, a shock wave velocity reduction of about 1% or greater, about 2.5% or greater, about 5% or greater, about 6% or greater, about 7% or greater, about 8% or greater, about 9% or greater, about 10% or greater, about 11% or greater, about 12% or greater, about 13% or greater, about 14% or greater, about 15% or greater, about 16% or greater, about 17% or greater, about 18% or greater, about 19% or greater, or about 20% or greater.
Certain specific embodiments of the present disclosure are directed to a protective armor liner comprising one or more layers of viscoelastic material such as polyurethane and silicone rubber with beneficial mechanical properties-resulting in high glass-transition phases-nanostructured with up to 2% by weight graphene nanoplatelets to enhance these properties.
Various polyurethane and silicon based viscoelastic materials with and without incorporation of nanosized graphene particles were prepared and tested by dielectric spectroscopy and mechanical testing methods (as discussed in further detail in the Examples).
Evaluation of these materials occurred in series using dielectric spectroscopy and open-air blasts of composition C-4—an explosive mixture of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), plasticizers, and motor oil. Data was collected during blast testing using a pressure sensor directly behind material samples and filmed with a high-speed camera. Dissipation factor of various materials measured as a function of frequency showed increased dielectric loss within the frequency range of blast waves (−10-300 Hz), indicating their effectiveness at absorbing blast waves. It was also confirmed that these materials were effective for energy release and redirection determined by mechanical testing methods using sensors.
Reductions in peak pressure, impulse, and velocity change of the shockwave before and after impact were analyzed using a data acquisition system (DAS) and Phantom camera control software (PCC). The electrical properties of the materials—recorded during dielectric testing—were also compared to assess energy absorption capacities. Through this, the liner thicknesses and graphene nanoplatelet additives were evaluated against design constraints such as weight. By applying these types of materials and through active research in shockwave reduction, it was possible to design protective armor liners and protective armor comprising such protective armor liners that protect against BABT.
Still further, experiments were conducted for Composition C-4 (C-4) free field blast overpressures from 27.5 kPa (4 psi) to 165 kPa (24 psi) in 27.5 kPa (4 psi) increments to compare not wearing a helmet, wearing a helmet, and wearing a helmet modified with a viscoelastic helmet liner. Piezoelectric pressure sensors in the skull of a surrogate model of a human head and torso were used to gather overpressure data on the surface of the head. Peak pressure and impulse were higher with the addition of a helmet but lowered with the addition of a helmet liner for all peak pressures and five of the six pressure regime impulses. The higher duration due to the presence of the liner was the reason for this impulse increase. At lower pressure exposures, the liner kept impulse on the head at or slightly lower than the impulse and peak pressure felt when not wearing a helmet. Based on this experimental investigation, the dampening effects of the liner are most apparent below 82.7 kPa (12 psi). Although some dampening effects occurred past those pressures, the effect of the longer duration caused by the use of the liner was noticed. Thus, the use of viscoelastic materials and armor liners comprising a viscoelastic material and graphene nanoplatelets added to helmets are shown to dampen overpressure under the helmet.
In certain of these tests it was determined that the protective armor liner described herein provides significant improvement in limiting the impact on the wearer. For the lowest overpressure tested, 27.5 kPa, the helmet liner decreased the overpressure on the top of the head by 37.6%, with reduction reaching 26% at the highest overpressure exposure of 165 kPa. Additionally, the inclusion of the viscoelastic material extended the shock waveforms' duration, resulting in decreased total strain rate over which the shock is applied to the head.
Having described the above in detail, it will be apparent that modifications and variations are possible without departing from the scope of the disclosure defined in the appended claims.
The following non-limiting examples are provided to further illustrate the present disclosure.
A first experiment was conducted to select viscoelastic materials for use as a liner in a protective armor.
A study was conducted to examine the reduction in reflected shock wave velocity from the following tested materials: Ecogel, Ecoflex 00-30 silicone rubber, Vytaflex, and PMC 121/30, and a Kevlar plate. Ecoflex 00-30 exhibited one of the highest reductions in reflected shock wave velocities. The other viscoelastic materials could be used in further testing or as the viscoelastic material for use as a liner in a protective armor. However, Ecoflex 00-30 silicone rubber and PMC 121/30 were selected for further experimentation discussed herein.
A series of air blasts was designed to expose an instrumented manikin to predicted overpressures. The predicted pressures ranged from 27.5 kPa (similar to that created in breaching, to 165 kPa (comparable to an improvised explosive device (IED) event), with testing conducted at increments of 27.5 kPa. This method allowed for the shock wave interactions on different parts of the head to be evaluated using shock parameters of overpressure, rise time, duration, and impulse.
A surrogate model was constructed from the torso and head of a manikin model MM-BC8S with the following modifications: holes were drilled into the head, metal threads were epoxied on the inside, and PCB Piezotronics Model 102B18 and 102B15 high-frequency Integrated Circuit-Piezoelectric (ICP) sensors were mounted flush with the exterior of the head. The sensor wires were fed out the back of the torso for connection to a Synergy Hi Techniques data acquisition system Synergy P (DAS), with a sampling rate of 1 MHz bandwidth.
The sensor locations are shown in
The manikin was filled with Clear Ballistics ballistic gel to give the interior a flesh-like density. The torso was mounted onto a metal stand capable of bolting to the ground.
A helmet (with or without the protective armor liner as discussed in further detail below) was affixed to the head. The helmet straps were securely tightened onto the manikin's chin. Care was taken to ensure the helmet pads were touching the manikin head.
The helmet pad orientation for both helmet regimes is shown in
The clear viscoelastic material added 200 grams (g) to the helmet, resulting in a total weight of 1446 g, a 16% weight increase. While casting the protective armor liner into the helmet, variations in thickness occurred. The majority of the helmet liner had a thickness ranging from 1 mm to 2.5 mm, while some small sections reached a thickness of 6 mm.
The helmets set forth in Example 1 were then tested with suspended air blasts.
During a suspended air blast, ground reflected waves complicate waveform profiles. Interaction between the incident and ground reflected waves creates a triple point with the Mach stem below. Lowering the charge height relative to the target increases the height of the triple point, enabling just a Mach Stem interaction with the target, thus simplifying the resultant waveforms. To make this simplification, the charge height and mass were adjusted for the desired side on pressure. Distance and charge size were chosen to predict overpressures at the top mid-line of the head (sensor 4). This information is reported below in Table 1.
The range of tested overpressures was chosen to encompass the safety standard threshold for the U.S Army of 27.5 kPa (4 psi) and an IED event of 165 kPa (24 psi) in increments of 27.5 kPa (4 psi). Table 1 lists the predicted static pressures, Composition C-4 (C-4) charge size, Trinitrotoluene (TNT) equivalence based on overpressure, distances from manikin, and charge height. Three tests were conducted at each overpressure: without the helmet, with a helmet, and with a modified helmet with a protective armor liner comprising a viscoelastic material. The modified helmet comprised the viscoelastic material Ecoflex.
The sensors unprotected by the helmet and facing the blast head were expected to receive a total reflected overpressure, whereas those on the top and side of the head would be expected to receive a side-on overpressure. The sensors protected by the helmet would be expected to report a combination of head-on and side-on overpressures as well as oblique angle reflected waves caused by helmet-head reflections.
Overpressure data was analyzed for peak pressure, duration, rise time, and impulse. Statistical tests were not performed due to the limited number of trials but the root mean square formula was used for overall trends. Normalization of the data was performed to remove any environmental variances.
In addition to the collection of overpressure data, a monochrome Phantom V2012 high-speed camera recording at 100,000 frames per second documented the blast.
As noted above, eighteen free-field blasts were conducted (three tests at each overpressure) and analyzed using time-pressure waveform data to evaluate shock wave behavior at different parts of the head using no helmet, helmet, and modified helmet liner regimes. Slight variations in pressure data were observed during the test period, attributed to changing environmental conditions such as humidity, wind, and air temperature, as well as charge configuration factors like density, geometry, and mass which are typical in free-field blasts. To account for these effects, an average peak pressure from sensor 1, the eye, in all three scenarios, was taken and used as a multiplier correction factor to adjust values for all waveforms in each overpressure exposure.
An example of this adjustment is shown in
Exposure Heat maps were created in Python to present a comparison of peak overpressures and impulses from separate sensor locations (i.e the sensor locations shown in
Overpressures and impulses were evaluated for a 27.5 kPa (4 psi) blast exposure with no helmet, helmet, and helmet comprising a protective armor liner comprising a viscoelastic material are shown in
Overpressures and impulses were evaluated for 165 kPa (24 psi) blast exposure with no helmet, helmet, and helmet comprising a protective armor liner comprising a viscoelastic material are shown in
In each of
For the 27.5 kPa exposure (
For the highest tested exposure of 165 kPa (
For each of the 27.5 kPa exposure (
Peak pressure and impulse were notably higher on the back of the head when a helmet or protective armor liner helmet were present compared to tests without them, likely due to shock waves under the helmet colliding after they have moved over and around the head. An additional collision also occurs as the shock wave reflects off the shoulder upwards and into the helmet. These collisions are known to cause a higher overpressure upon interacting with each other than the sum of the separate overpressures, so that P1+P2<P3.
Examining the shockwave overpressures with the unmodified helmet during the 165 kPa exposure showed an overpressure of 155 kPa on the side of the head (sensor 9), whereas overpressure reached 269 kPa at the back of the head (sensor 7) due to shock waves colliding. Conversely, when no helmet is worn, the back of the head experienced a much lower overpressure of 147 kPa. When incorporating the protective armor liner, peak pressure reached 205 kPa at the back of the head, a decrease of 27% relative to the helmet. Interestingly, the side of the head (sensor 9) experienced a slightly higher overpressure with the protective armor liner of 173 kPa compared to the unmodified helmet (155 kPa).
The increased overpressure at the back of the head and top of the head with helmets on compared to no helmet being worn indicate that shock wave collisions occurred. However, the protective armor liner helmet overpressure decrease indicates that dampening from the viscoelastic material did occur. Because viscoelastic materials are made up of polymer chains, force interaction at one part of the polymer will affect the rest of the linked chains, leading to a possibility that the entire mass of the viscoelastic liner is causing this dampening effect.
To provide more insight into the data collected in the above experiment, the root mean squared (RMS) method (shown below in Equation 1) was employed. This analysis method was used to search for trends in the front (sensor 2), top (sensor 5), and side (sensor 9) compared to the back (sensor 7), top and side sensor waveform data.
Calculations were performed where ‘n’ represented the number of measurements (three in the current calculations-corresponding to the front, top, and side of the head, or the back, top, and side of the head), and ‘x’ denoted the waveform parameter value from the head's respective area.
The results are presented below in Table 2, 3, and 4 for peak pressure, impulse, and positive phase duration, respectively. Rise time was not evaluated using this method but was included in later analysis when examining the shock waveforms.
These RMS evaluations for no helmet, helmet, and protective armor liner helmet focused on the peak pressures and impulse, using the durations to provide context for the impulse values. Subsequently, heat maps were generated using these values and are presented in Tables 2, 3, and 4, the lowest (white), intermediate (grey), and high (black) values are highlighted. This combined analysis offers insight into the force directed to the head from the shock wave in the tested scenarios.
For the peak pressure reported in Table 2, the RMS equation was applied to the peak pressure values collected for different test regimes and parts of the head. The shading indicates differences in the magnitude of overpressure. Values of the higher magnitude comparing the front and back of the head are bolded and italicized to show the difference.
Table 2 shows clear trends in peak overpressure, with nearly all of the greatest peak overpressures occurring using the helmet regime. Trends indicate the protective armor liner reduced peak overpressure compared the unmodified helmet and no helmet regimes. In the protective armor liner helmet, eight occurrences were noted where peak overpressures were lower than both the unmodified helmet and no helmet regimes. These trends were not seen in the 110 kPa blast exposure, potentially attributed to testing variance. For this reason, pressure regime 110 kPa was chosen to be examined further later in the analysis.
Overall, the underwash effect was observed as pressure increase due to the presence of a helmet, notably in the back of the head. When a comparison of the front versus the back of the head was conducted it showed that the front had higher overpressures than the back. This trend was evident for all the regimes, except in one instance of the 55.1 kPa exposure, by a narrow margin. This difference in overpressure was likely because the front of the head was closer to the blast than the back. The front sensors, when a helmet was not worn, received a head-on impact of the shock wave, while the back sensors received an oblique reflection impact as the shock wave engulfed the head. When an unmodified helmet and protective armor liner helmet were worn this trend persisted. However, the front sensors did not receive a head on impact. Rather, they received a combination of multiple oblique reflection impacts, making proximity a likely cause of overpressure differences. To understand the damage these overpressures caused during these exposures the impulse was examined in Table 3.
In the testing of Table 3, the RMS equation was applied to the impulse values collected for different test regimes and parts of the head. The shading indicates differences in magnitude of impulse exposure. Values of the higher magnitude comparing the front and back of the head are bolded and italicized to show the difference.
Analysis of Table 3 shows the lowest impulses occurred without a helmet worn except in two cases. These two cases were the 82.7 kPa and 110 kPa exposures where the protective armor liner helmet only had a slightly lower impulse. The highest impulses occurred throughout the helmet regime except in one case. Generally, this follows the trend of the helmet having higher overpressures under the helmet compared to without a helmet. Comparing specifically the two helmet regimes, incorporating the viscoelastic linear (i.e. protective armor liner) reduced the impulse experienced on the head.
An examination of the impulse in the front versus the back of the head showed the impulse was lower in the back for the no helmet regime, but impulse in the front was lower in the helmet regime. However, the protective armor liner helmet had a mixture of these results comparing front to back. The reason the impulse was lower in the back of the head for the no helmet regime was that the shock wave was able to reflect off the head and the overpressure was not sustained. However, when the helmet was added the shock wave reflecting in the back of the head likely had difficulty escaping from under the helmet, leading to a larger impulse. If the case was that the shock wave had a hard time escaping from under the helmet, then this would be shown by examining the positive phase duration of the shock waves as depicted in Table 4.
The most notable trend observed from the duration data was that the protective armor liner increased the duration of the shock waveforms compared to the unmodified helmet. For example, in the 55.1 kPa exposure, the protective armor liner's RMS values of 4.4 and 4.6 were higher compared to the unmodified helmet's 3.7 and 4.2. For the unmodified helmet at 27.5 kPa and 55.1 kPa pressure exposures, the lowest durations were observed compared no helmet and the protective armor liner helmet. The protective armor liner helmet's higher duration may explain why its impulse values were not the lowest despite having some of the lowest overpressure values observed, such as in the 27.5 exposure, with 29.7 and 28.3 RMS values. The highest durations were found to be without the helmet, but to understand why this occurred, shock waveforms were examined as discussed further below.
The front of the head experienced a lower duration than the back based on these RMS values. This does not completely support the notion that higher duration values show that the shock wave stuck under the helmet as this trend was observed in the no helmet regime. Considering all RMS results, the shock wave behavior in the helmet liner regime could be explained by the following: a) the shock wave's travel slowing with diminishing pressure and increasing duration, b) the shock wave being unable to escape from under the helmet, and c) the shockwave unable to move quickly out under the helmet. These behaviors resulted in a generally higher impulse than no helmet regime.
A longer duration would indicate more time during blast loading, and possibly more time for the material to manage the load being impressed. However, if this longer duration is coupled with a slower rise time and overpressure decay, it could be better than a short duration fast rise time exposure because the rate of injury would lower. Thus, an argument could be made that longer durations of the no helmet and protective armor liner helmet could have caused less damage. As a metric, duration has been shown to be useful for understanding the two common direct metrics of injury, impulse and peak pressure, but is less conclusive as a direct metric from a blast injury standpoint. These metrics can be fully understood by examining shock waveforms of an exposure.
Phantom camera control (PCC) software was calibrated and used to measure the movements of the helmet in the footage, and shock waveforms were visually examined.
The shock waveforms from the 27.5, 110, and 138 kPa blast exposures were examined to analyze the general trends described in Table 2 and Table 3. The waveforms collected exhibited a negative phase before returning to ambient pressure, but that will not be the focus of this analysis.
As shown in
From each of the results of this examples, it is clear that it is possible to reduce the loading on the head by adding a protective armor liner comprising a viscoelastic material to a helmet. The waveforms in
A further experiment was conducted to evaluate the shifting of the helmet during a blast event. Phantom camera control (PCC) software was calibrated and used to measure the movements of the helmet in the footage, and shock waveforms were visually examined.
High-speed footage was used to examine if the helmet shifted the during a blast event, causing an increased pressure on the sensors. Measuring the distance between the shoulder and the bottom of the helmet with the PCC software was used to determine if the center of gravity changed due to increased weight from the protective armor liner, possibly affecting overpressure exposure as the shock wave interacts with the head. This was done with the 165 kPa blast exposure footage because, as the maximum amount of force, it would show the most helmet shifting.
Frames from this PCC software with the helmet are shown in
Materials such as Ecoflex 00-30 silicone rubber exhibit significant stress recovery, but has long recovery times. This could potentially make the helmet more comfortable for the wearer under non-blast conditions. However, the added weight of viscoelastic materials may be a concern for wearer comfort and safety. Therefore, designs should balance shock wave dampening, ballistic protection, and lightness to avoid straining warfighters' necks.
Having discovered that a protective armor liner comprising a viscoelastic material provides a number of surprising benefits when added to armor (e.g., a helmet), the addition of further materials into the protective armor liner was tested.
In one experiment, protective armor liners comprising a viscoelastic material of Ecoflex 00-30 silicone rubber or PMC 121/30 were tested. Ecoflex 00-30 silicone rubber containing liners are referred to herein as “EcoFlex.” PMC 121/30 containing liners are referred to herein as “PMC.”
In blast testing, a PCB pressure sensor Model 102B18 was placed behind samples, with data collected using a High Techniques Synergy Data Acquisition System. A high-speed Phantom Camera was used to record shockwave velocities.
First, PMC and EcoFlex liners were tested in various thicknesses.
Having established a baseline for the protective armor liner comprising a viscoelastic material, the protective armor liners were further modified to include graphene nanoplatlets (GNP).
Varying concentrations of GNP were added to the viscoelastic material of PMC or EcoFlex. The results for varying concentration of GNP, but consistent thickness, are set forth in
Finally, the shockwave velocity reduction of a GNP-viscoelastic protective armor liner and a viscoelastic protective armor liner was evaluated.
The results of these experiments demonstrated favorable long rise times and lower peak pressures for polyurethane. There was also favorable positive phase duration for silicone rubber. Polyurethane was more effective up to 9 mm, with differences more apparent in thinner layers.
The same thicknesses of PMC and EcoFlex tested above, were subsequently tested to determine the dissipation factor at 100 and 200 kHz. Dielectric spectroscopy was performed using a QuadTech RLC Impedance Meter. The results are reported in
The dissipation factors for the GNP-viscoelastic protective armor liners tested above were tested across a frequency range of 20 Hz to 1 MHz. Dielectric spectroscopy was performed using a QuadTech RLC Impedance Meter. The results are reported in
Polyurethane generally showed higher dissipation at all frequencies, indicating a better energy dissipation for these samples. Increasing frequency resulted in exponential overpressure decay of energy dissipation for polyurethane in all samples.
Higher dissipation levels were observed for polyurethane with increased GNP additions below 2 kHz. Dissipation factors generally increased for silicone rubber at most frequencies with GNPs.
Overall, it was determined that the tested materials have efficient energy dissipation capabilities. Liners less than 3 mm were particularly effective at dissipating energy. GNPs were found to increase dissipation factors of silicone rubber and at low frequencies for polyurethane. Silicone rubber was found to dissipate energy by increasing the positive phase duration.
Still further experiments were conducted to evaluate the impulses, peak pressures, positive phase durations, and rise time for protective armor liners comprising varying viscoelastic materials (i.e. EcoFlex and PMC) and varying amounts of GNP in the liner.
As explained in Example 2, variations in data were observed during the test period attributed to changing environmental conditions such as humidity, wind, and air temperature, as well as charge configuration factors like density, geometry, and mass which are typical in free-field blasts. To account for these effects, an average peak pressure was taken and used as a multiplier correction factor to adjust values for all waveforms in each overpressure exposure. The results of such correction of the data is presented in
Still further, the average overpressure decay time of the varying viscoelastic materials (i.e. EcoFlex and PMC) and varying amounts of GNP in the liner are reported in
When introducing elements of the present disclosure or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
In view of the above, it will be seen that the several objects of the disclosure are achieved and other advantageous results attained.
As various changes could be made in the above methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
This application claims priority to U.S. Provisional Application No. 63/532,257 filed Aug. 11, 2023, the entire disclosure of which is incorporated herein by reference.
This disclosure was made with Government support under grant 0059271 awarded by the Leonard Wood Institute. The Government has certain rights in the disclosure.
| Number | Date | Country | |
|---|---|---|---|
| 63532257 | Aug 2023 | US |
