Patent Application
20130312287
The present disclosure relates generally to the mitigation of impact forces and, more particularly, to a material and process for producing a material for mitigating impact forces with collision durations in nanoseconds to milliseconds range.
A dynamic impact is characterized by the force-time curve during the collision. Finding strategies (material or structural) to manage an impact depends upon the collision duration or the force amplitude of the force-time curve. High velocity impacts, such as those arising during combat missions, have durations in the nanoseconds to microseconds range (armor protection). Collisions with durations in the 1 ms to 100 ms range are common to virtually all day-to-day recreational and sports activities. Examples include, walking, running, jumping, aerobics, collisions between football helmets, a fast baseball striking a helmet, collisions during skiing, boxing, all racquet sports, debris/bird hitting an aircraft, etc.
Most impact resistance or absorbing materials are effective in managing impact in a very specific collision duration range. There has been no indication that such materials are effective in managing impacts having durations ranging from 1 to 100 milliseconds, which is the duration range for impacts occurring during sports and recreational activities. Most importantly, conventional materials are unlikely to be effective in reducing impact forces when the material strains are minimal as basic scientific principles require such materials to stretch significantly to absorb and dissipate impact energy. Because the force applied during sport and recreational related activities occurs over such a short time, the materials used in such activities experience a very small amount of stretching or deformation (i.e., less than 1′)/0 strain) due to the impact force. In general, the dynamic energy absorbing properties of most impact absorbing materials are only effective when the material is stretched 50 to 100 times more than what would actually occur as a result of impact forces applied during sport and recreation related activities and when the duration of the impact force is 100 to 1000 times longer than the duration of impact forces applied during sport and recreation related activities.
Material for mitigating impact forces with collision durations in nanoseconds to milliseconds range is disclosed. According to one embodiment, an object wearable on or against a human body comprises one of a layer of polyurea material and plug of polyurea material, and the layer of polyurea material or plug of polyurea material is positioned within the object or on the object between the human body and an outer structure of the object.
The systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. It is also intended that the invention is not limited to require the details of the example embodiments.
The accompanying drawings, which are included as part of the present specification, illustrate the presently preferred embodiment and, together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain and teach the principles of the present invention.
It should be noted that the figures are not necessarily drawn to scale and that elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. It also should be noted that the figures are only intended to facilitate the description of the various embodiments described herein. The figures do not necessarily describe every aspect of the teachings disclosed herein and do not limit the scope of the claims.
A material recipe and the process for producing a material including thin layers of polyurea are provided herein. Polyurea is a name given to a general class of viscoelastic elastomers prepared by mixing a soft (oligomeric diamine prepolymer), and a hard (modified diphenylmethane diisocyanate curative) phase. Both of these components are commercially available. For example the soft phase is marketed under the trade name Versalink® P1000 by Air Products Inc., while the hard phase is marketed under the trade name of Isonate 143L by Dow Chemical. A 1:1 ratio by weight results in an extremely hard polyurea while a ten to one ratio of the soft to hard phase by weight results in a gel-like material. Polyurea has been used widely as an adhesive and also as a water resistant and chemical resistant coating on truck beds and building facades because of its extreme chemical and water resistant properties.
The present polyurea material, made in accordance with the recipe and process described herein, mitigates impact forces with collision durations in the nanoseconds to milliseconds range. The polyurea material at the molecular level can be viewed as being composed of hard and soft phases. These phases deform synergistically to give the material its viscoelastic property. The material is able to engage the energy of impact pulses of widely varying durations (or frequencies) by deformation of its hard and soft phases in unison through resonance. The molecular structure is such that the material has a very high bulk modulus (which means it is very difficult to compress the sheet of this material in its thickness direction) and a very low shear modulus (meaning that it is easy to deform the material within its plane much like easy inter-card sliding in a deck of playing cards). Thus, the material can deflect the vertically-directed impact force and energy horizontally in its own plane. Because of this attribute, the material is ideal for applying to athletic surface or adding to shoes, socks and protective helmets to lower the impact force to an athlete or other user.
According to one embodiment, thin sheets of polyurea material are formed and then applied to a surface for protection. For example, thin sheets of a polyurea material prepared as described herein (the material referred to herein as “polyurea”) are glued to surfaces including: the inner protective foam or on the inner shell surface of protective helmets used in any sport (football, baseball, boarding, skiing, rafting, water sports, auto racing); inside shoes as inserts or insoles; in hip protector pads; in any protective pad including shin guards, chest and body guards for umpires or baseball catchers; and on the gym floors as mats for carrying out various aerobic exercises. The material can also be realized as bottom linings of socks, caps that can be worn over the skull before putting on helmets (bicycle, motorcycle, baseball, football, whitewater rafting, boarding, skiing, etc.), used by soldiers to cover their skulls and ears under their helmets to protect against traumatic brain injury, and used for body suits for entire body protection. Additionally the material can be used to protect against any impact-related forces arising in any sports or day-to-day activities. Surface application examples include playground surfaces in children's indoor and outdoor gyms, and machine floors of factories for vibration control.
The polyurea material described herein can be implemented in the existing manufacturing processes for various athletic helmets. Presently available helmets are inadequate for preventing concussion injuries, and an addition of the polyurea material described herein can help prevent these injuries. The same is the case with soldiers and marines in combat missions where the current helmet designs are inadequate for protecting them from shockwaves-induced traumatic brain injuries.
When molded into thin flexible layers (0.5 mm and higher), the polyurea material recipe described herein significantly reduces impact forces with collision times ranging from few nanoseconds to tens of milliseconds. High velocity impacts, such as those arising during combat missions, have durations in the nanoseconds to microseconds range (armor protection). As described above, collisions with durations in the 1 ms to 100 ms range are common to virtually all day-to-day recreational and sports activities. Again, examples include walking, running, jumping, aerobics, collisions between football helmets, a fast baseball striking a helmet, collisions during skiing, boxing, all racquet sports, vehicle impact, and debris/bird hitting an aircraft.
The present polyurea material performs in a very large range. Additionally, within each collision duration range, the use of a mere 0.5 mm to 1 mm thick layer on top of commercially available products and structures can cut down impact forces dramatically (over 15-20%).
The present system exhibits a rare combination of mechanical properties, a very high bulk modulus and a very low shear modulus. The high bulk modulus allows retention of high through-thickness stiffness such as against the foot pressure applied by a runner while the low shear modulus results in long relaxation times which in turn brings down the peak dynamic impact force.
The addition of a polyurea layer is effective in mitigating the effects of impact in situations governed by force-time curves on the order of 1 ns to 100 ms. Test data has shown that an optimized thin layer of polyurea preferably 0.5 mm to 1 mm in thickness can significantly reduce the negative effects associated with impacts on this time scale. Data is included herein for a hip protection pad simulating a falling person, for a runner, and for a football helmet. Similar benefits are shown for baseball helmets, situations resulting in traumatic brain injury, and motorcycle helmets.
Many athletic activities involve repeated foot impacts with the ground surface. For example, during a 5 km run, body experiences approximately 3,000 impacts with the ground. Each impact produces a shockwave. The amplitude and duration of the shockwave depend upon several factors such as the running speed, the nature of the ground surface (soft vs. hard), the material and thickness of the sole of the running shoes, the anatomy of the foot, and above all, the running style of the runner, including whether running under shod or barefoot conditions.
Reducing the dynamic forces upon foot impact greatly benefits the soft tissue components of the knee and hip joints as it is their deterioration over time which eventually leads to arthritis. This beneficial effect can be quantified as follows.
The benefits of reducing the shockwave amplitude can be appreciated by relating it to the fatigue life of articular cartilage. How the force amplitude A affects the fatigue life of articular cartilage has been researched by Weightman who performed cyclic fatigue experiments in-vitro (tested outside the body) on cartilage specimens extracted from human cadavers of different ages. The extracted cartilages were loaded into a tension machine by straining the specimen from a zero stress (or Force) to a peak stress S (proportional to amplitude A, discussed here) and then the specimen was unloaded slowly to zero stress. This constituted one loading cycle. The machine was programmed to load the specimen with a large number of continuous cycles, each with an amplitude S. The number of loading cycles N that caused failure (fracture, damage, etc) of the cartilage was recorded. The data is represented by the following empirical equation: S=23-0.41 a −1.83 log10 N, where S is the failure stress measured in units of MN/m2, a is the age of individual in years, and N is the number of cycles to failure, each with a maximum amplitude S. In the context of discussion here, the amplitude A of the shock wave is directly proportional to the stress S and each heel strike with the ground constitutes one cycle of loading. An average person makes approximately 106 heel strikes per year on each foot. This is based on about 4.8 km (3 miles) of impact-running each day that could arise from simple running, stair climbing, or spot running during aerobic exercises in a gym. The value of S at the joints during normal walking has been estimated between 1.5 and 3 MN/m2, which will correspond to A values of about 1.0 g. Magnitudes of both S and A increase during running, with the specific value depending upon the speed of the run. Since S and A are proportional, the factor by which S will increase will be roughly the same amount as the increase in the G-force (or A) measured in our experiments during running. This factor was found to be about 3 to 5 times. The increase in the cartilage life when the impact force level is reduced by 20% can then be estimated.
At running speed of 6.3 mph, S value is estimated to be about 7 MN/m2. Based on equation (1), at this level of S, the cartilage life is about 38 years, 17.4 years, 3 years, and 0.9 year for population with 30 years, 40 years, 50 years and 60 years of age, respectively, assuming an individual makes 106 foot-impacts each year. For a 30-year-old person, a 20% reduction in A (and hence in S) will result in an astonishing 5 times (500%) increase in the number of cycles to failure for a 30 yr old population. This will roughly correspond to 60 years of increased cartilage life. Similar calculations yield, on average, an enhancement of 17.4 years for 40 yr old, and 4.9 years for 50 yr old population. Similar calculations can be made at higher running speeds and different impact force levels.
In summary, the benefits of reducing impact forces, even in the small 10-20% range, can have a significant effect on fatigue life of articular cartilage. One way to do this is to use compliant insoles or midsole shoes, such as those provided in the modern footwear industry (Dr. Scholl's insoles). Unfortunately there is a limit as to how compliant the material can be made as the foot will sink more the more compliant the material is made (like running in sand). So what is needed is a material that supports the weight of the person but at the same time it reduces the impact by increasing the time T2 in
Moving on to head protection, the NFL has come under increasing public pressure to reduce the number of concussion-related injuries.
Fall related injuries in the growing geriatric population results in the fracture of the femur (hip bone) at the hip joint. Hip protector pads made from polyurethane foams placed inside a pouch are sold today which can be placed on the side of the undergarments. This reduces the dynamic force to below the fracture level of the femur. Since the collision time for this impact is also in the milliseconds range, use of polyurea was explored as a way to further make these commercially available hip protector pads more efficient.
Other protective pads suitable for the present system involving a layer of the present polyurea material include shin guards, knee pads, elbow pads, chest pads, leg pads, and any protective pad for use in a sport environment or otherwise environment where impact may occur.
According to one embodiment, the layer of the mixture has a thickness in the range of 0.1 mm and 10 cm. The vacuum includes a pressure range of 2-3 millitorr.
It will be appreciated that, while the 4:1 ratio is described herein as a preferred ratio, the ratio can be changed from 2:1 to 8:1 (soft: hard phase) to give properties that are still superior to existing materials.
It is understood that the exemplary shoe designs of
In an alternate embodiment, a layer of the present polyurea material can be included in armor for vehicles, aircraft, and other structures exposed to impacts in the microseconds to milliseconds durations. A layer of the present polyurea material can be included in a wall of a building, a wall or door of an aircraft, on the exterior of an aircraft, on a bumper of a vehicle, on a tank, on a hum-vee, or anywhere on a ship.
Four specific applications were explored using the same material polyurea recipe, discussed above. Within each application, the performance of the new polyurea impact manager is compared with the currently available solutions. Details of these tests are given below. A very thin layer of polyurea is effective in managing these impacts because it exhibits a rare combination of mechanical properties, a very high bulk modulus and a very low shear modulus (as previously mentioned). The high bulk modulus allows retention of high through-thickness stiffness such as against the foot pressure applied by a runner while the low shear modulus results in long relaxation times which in turn brings down the peak dynamic impact force. For most materials available prior to the present polyurea material, the bulk modulus and shear modulus are positively correlated. That is, the higher the bulk modulus, the higher is the shear modulus. This is the technological reason why polyurea is quite effective in attenuating dynamic forces comparable to existing materials, but with significantly less thickness.
An unmodified sneaker was used as a control. The modified sneaker included a layer of 0.5 mm thick polyurea insole glued to the inside of the shoe after removing the thin insole that came with the shoe. The original insole was then replaced on top of the polyurea insole (refer to the configuration illustrated in
Since impact shock is known to vary systematically with running speed and surface gradient, changing the speed and gradient of a motorized treadmill provided a convenient means for manipulating the levels of impact shock in the laboratory environment. The use of a treadmill allowed maintaining a proper control over walking and running speeds across different runners. Axial acceleration of the lower right leg was recorded by means of a piezo-resistive accelerometer attached to the skin overlying the tibia with the sensitive axis of the accelerometer aligned with the long axis of the bone. This site was selected because the soft tissue overlying the bone is relatively thin at this point. The output was collected and recorded by a digital oscilloscope. Speeds used in testing were 2.9 mph, 6.3 mph, 7.4 mph, and 9.2 mph. Both the unmodified sneaker (control) and barefoot samples were taken for reference. A 75 kg male heel-strike runner was used as the test subject.
Additional tests with thicker polyurea (1 mm) have shown the same performance as a conventional running insole albeit still at one-fifth the thickness.
The polyurea material was applied to the best performing helmet from a recent NFL-sponsored study. Based on this study, a drop weight standard was developed, which involved dropping an inverted helmet draped over a head form onto an anvil with an equivalent energy of 74 Joules. A comparable test was performed herein where the helmet was kept stationary and the load was dropped on top of it with an energy equal to 74 J. The best performing helmet from a study that was recently conducted by NFL was used for a control. The force that was transmitted through the helmet was measured using a piezoelectric load cell (Kistler Model 9201A). A cylindrical impact head was used to mimic the curvature of a helmet that would impact the tested helmet and cause injury in a sport. The load cell was preloaded as per the specifications of the manufacturer prior to each impact. Polyurea layers of different thickness were tested. The location of the polyurea layer on the helmet was also examined.
A 0.5 mm thick layer of polyurea 806 adhered to the inside of the helmet (
Since the collision duration for riders impacting the pavement from a bicycle or a motorcycle in an accident are very similar to those in the helmet-to-helmet impact on a football field, the exact same strategy works for protecting the riders by designing polyurea-based helmets. The energy during impact is however much higher.
Another helmet application for polyurea layers described herein is for protecting soldiers from the traumatic brain injury (TBI). The latter is a long-term effect caused by the trauma to the brain tissue, which in turn is caused by the shockwaves generated by the explosions.
The effectiveness of the present polyurea material was also investigated in managing hypervelocity impacts, which are generated by weapons in modern warfare, such as shaped charges and explosively formed projectiles which can attain speeds between 9,000 ft/s to 30,000 ft/s. Threats from these deadly hypervelocity penetrators pose significant challenge to the armor design community as no data is available with respect to conventional armor materials at these extreme high loading rates. In contrast to the above applications where collision times ranged from 1 millisecond to tens of milliseconds, hypervelocity impacts are characterized by collision times in 1 to tens of nanoseconds (million times faster than those discussed above), even microseconds. Layered armors were fabricated herein where polyurea layers are placed sandwiched between steel, aluminum, glass, acrylic, and polycarbonate plates to see if these combinations of materials result in defeating such impacts.
As a result, advanced helmets and advanced multilayer armors involving the present polyurea material and metal layers can be made for protecting ground vehicles and aircrafts (e.g. helicopters, drones, spaceships).
To test protective pad embodiments, samples were prepared by placing thin polyurea layers on top of commercially available hip protector pads.
Soft tissue covering the hip was simulated using Plastazote (Atlas International Model PL-34W, Rancho Cordova, Calif.), a polyethylene foam. A vertical loading drop weight machine (Instron 8250 DynaTup, Canton, Mass.) was used to impact the mechanical hip. The total energy at impact was 80 joules. A stainless steel impact platen was attached to the crosshead tup. The impact surface area was 0.0045 m2. The entire drop weight mass was guided by two vertical rails, which insured the impact area was the same for each repeated impact.
Impact force data was measured with the aforementioned piezoelectric load cell (Kistler Instruments Model 9021A, Amherst, N.Y.) affixed to the pelvic mass, recording the impact load proximal to the femoral head. After calibration of the test setup was complete, the configuration was not altered during the course of experimentation. All values for force refer to force transmitted through the bone and recorded by this load cell. The impact test was also repeated without the surrogate hip model.
Unlike conventional impact absorbing or resistant materials in which dynamic energy absorbing properties are only effective when the materials are stretched 50 to 100 times more than what would actually occur in a typical sports or recreational related impact applications, the present polyurea material provided significant reduction of such impact forces. When thin layers of the polyurea material were tested using impact tests that mimicked collision durations typical in contact sports, the impact forces were significantly reduced even with small deformations. The reason for this is that the material relaxes in shear (in the plane of the sheet) because of its low shear modulus and this allows the impulse time to increase long enough to reduce the peak dynamic forces via the Newton's second law. This unexpected behavior has not previously been discussed by others.
In the description above, for purposes of explanation only, specific nomenclature is set forth to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that these specific details are not required to practice the teachings of the present disclosure.
The various features of the representative examples and the dependent claims may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. It is also expressly noted that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter.
Material for mitigating impact forces with collision durations in nanoseconds to milliseconds range has been disclosed. It is understood that the embodiments described herein are for the purpose of elucidation and should not be considered limiting the subject matter of the disclosure. Various modifications, uses, substitutions, combinations, improvements, methods of productions without departing from the scope or spirit of the present invention would be evident to a person skilled in the art.
The present application claims the benefit of and priority to U.S. Provisional Application No. 61/393,735 titled “Material For Mitigating Impact Forces With Collision Durations In Nanoseconds To Milliseconds Range,” filed on Oct. 15, 2010, which is hereby incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US11/56458 | 10/14/2011 | WO | 00 | 8/7/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61393735 | Oct 2010 | US |
