Patent Application
20070144337
The present invention relates generally to methods and apparatus for pressure wave mitigation and, more specifically, to methods and apparatus for blast wave mitigation.
Conflicts throughout the World have already caused thousands of deaths of military personnel. A large percentage of these deaths are due to land mines and improvised explosive devices (IEDs). Explosion of land mines and IEDs send out blast waves of extremely high pressures, which destroy vehicles and incapacitate personnel. With the rise of low intensity conflicts involving terrorists and guerillas, Humvees, armored personnel carriers and tanks are forced into urban warfare for which they were not designed. In some instances, soldiers have taken it upon themselves to armor their vehicles with extra layers of scrap materials. While hardening may be effective against projectiles, it is not effective against blast wave impacts. The blast wave easily propagates through the hardened armor with little mitigation, killing occupants and damaging equipment.
Extensive research has been conducted to reduce the damage potential of blast waves. It is known that the damage potential of an explosive blast depends on three main factors: the force exerted on the target; the duration of the applied force; and the ability of the target to withstand the effects of the blast wave.
Two main approaches are used to mitigate the damage potential of the blast wave: blast absorbing materials and heterogeneous systems. For example, water has been used as a blast-absorbing material to reduce the damage caused by blast waves. One known water-based attenuation method uses a liquid layer confined within an elastic envelop to mitigate the blast wave. For this device, it was shown that the blast wave pressure attenuation coefficient depends on the distance from the blast epicenter to the point of measurement as well as the thickness of the water layer. The water medium delays the shock front and reduces the magnitude of initial peak shock pressure by approximately 40%-50%.
Other blast-absorbing materials used to mitigate the blast wave include both aqueous and metal foams. For aqueous foams, the vaporization of the liquid component has been shown to be detrimental to blast wave mitigation. Specifically, the many reflections off the foam/air interface produce a complicated waveform in the aqueous foam. Further, the blast mitigation behavior of cellular materials have been investigated. It is known that the transmitted pressure can be attenuated by the foam layer if the input blast load is below a critical value. Thus, this material can be used only for the lower pressure blast wave. For the high blast wave pressure, the cellular material will be destroyed and the pressure on the target will increase with adverse impact.
The list of blast absorbing materials typically includes granular, particulate matter, porous material, and foam. The momentum and energy of a blast wave can be absorbed by these “soft” condensed matters. The density, porosity and relative geometrical size of the so-called “soft” condensed matter are the main parameters determining the effectiveness of blast wave mitigation. For example, a tapered chain of elastic beads has been used for blast mitigation. The elastic beads act as an absorber of kinetic energy and can reduce it by about 30%. Results show that the energy absorption is affected by the restitution coefficient, the size of the particles and the tapering ratio. For particulate matter, the mitigation of an explosion is enabled largely by the consolidation of low density particulate matter into compacts of greater density. Mitigation effects decrease with average particulate size for particulate material with low areal densities.
In spite of their successful application to date, current methods and systems for using aqueous foams in pressure attenuating roles are inefficient and unnecessarily bulky.
Heterogeneous systems have also been designed to attenuate blast waves. For example, geometrical parameters of a blast wall have been studied to protect a target structure. A relationship has been demonstrated between blast mitigation and geometrical configuration of the wall. This relationship may be used to optimize the parameters of the blast wall. Results showed that the overpressures behind the wall are 30% to 60% of those without a wall. Solid barriers for shock wave containment or protection suffer from several shortcomings. Blast walls are typically massive and are thus inherently immobile and expensive. They cannot, therefore, be used in the majority of mobile applications.
In view of the shortcomings for existing apparatus and assemblies to mitigate blast shock waves as noted above, there has been found to remain a need for an assembly for more effectively mitigating blast waves.
The present invention generally relates to an assembly for mitigating a blast compression wave. The assembly comprises a housing having a base wall, an outwardly extending wall, and an open end. A piston having an impact face and an interior face is slidably received in the housing in a substantially airtight engagement therewith. The piston, the base wall and the outwardly extending wall define an interior. A compressible substance is confined within the interior, whereby when the blast wave impacts the impact face of the piston and drives the piston toward the base of the housing, a shock wave is induced in the compressible substance. The shock wave is reflected by the base of the enclosure and the interior surface of the piston to mitigate the impact of the blast wave.
In accordance with another embodiment of the present invention, a method for mitigating blast compression waves is disclosed. The method includes the step of providing a housing having a base wall and an outwardly extending wall, and a piston slidably received in the housing and in substantially sealing engagement therewith. The housing and piston defining an interior. The method further includes the step of inducing a shock wave in the interior of the housing through the piston receding into the interior upon the impact of the blast wave. The shock wave is reflected within the interior of the housing.
The present invention is described in detail below with reference to the attached drawing figures, wherein:
The present invention provides an apparatus for blast wave mitigation, generally indicated at 10. The apparatus 10 comprises a piston 12 slidably received in and in substantially sealing engagement with a housing 14, such as in a piston-cylinder assembly. The housing 14 may be a cylindrical housing having a base 16 and a cylindrical wall 18 extending outwardly from the base wall. Alternatively, the housing may be generally rectangular in shape having opposing side walls and opposing upper and lower walls extending outwardly from the base wall 16. The housing 14 is preferably made of steel, such as rolled homogeneous armor steel. It is to be understood by those skilled in the art that other materials may be selected without departing from the scope of the present invention.
The outwardly extending cylindrical wall 18, or alternatively opposing side walls and upper and lower walls, may extend outwardly for different lengths from the base depending on the particular requirements of the blast wave mitigation device 10. In an exemplary embodiment, the length the cylindrical wall extends from the base is between approximately 1 to 20 cm. More specifically, the length is more preferably between approximately 1.5 to 10 cm. It is to be understood that the particular length dimension may differ and still be within the scope of the present invention.
The piston 12 is operably configured to be slidably received within the cylindrical housing in a substantially airtight engagement. The piston 12, wall 18, and base 16 together define an interior 20 of the piston-cylinder assembly 10. The piston includes a blast-impact face 22 and an interior face 24. Depending on the materials selected and the particular usage of the blast mitigation apparatus 10, the piston may have a thickness of between about 1 to 10 cm. and preferably a thickness of about 5 cm. Preferably, the piston is likewise made of rolled homogeneous armor steel. It is to be understood by those skilled in the art that other materials may be selected and that the piston may present numerous face configurations and thicknesses without departing from the scope of the present invention.
The interior 20 is filled with air or other inert gases having desirable characteristics. Blast waves, when traveling through air or other gases, produce increases in pressure (referred to as “overpressure”), temperature; and also accelerate gas molecules in the direction of wave travel. For all blast waves, the wave speed, overpressure, and temperature increase they induce in the local medium are mathematically linked.
Having described an exemplary embodiment of the present invention, an exemplary operating environment for the present invention is described. A continuous grid of the blast wave mitigation apparatus 10 of the present invention may be used to cover the surface of military structures. The base 16 of the housing 14 is mounted to the structure through known means. As a blast wave impacts the blast impact face 22 of the piston 12, the piston is forced to recede into the interior 20 of the assembly 10. This piston motion induces a weaker shock wave that propagates toward the base 16 of the device 10 at supersonic speed. When the shock wave impacts the base 16, it is reflected back and travels toward the interior face 24 of the piston 12. When the reflected shock wave hits the interior face 24 of the piston, it is reflected again. This process repeats until the piston 12 comes to a complete stop. Each time the shock wave is reflected, the pressure of the gas in the interior of the assembly 10 increases. The pressure reaches its maximum when the piston 12 comes to rest. The repeated reflection of the shock wave within the blast mitigation device 10 significantly increases the duration of the force on the base 16 of the housing 14 as compared to the duration of the blast wave alone. Because the impulse of the blast wave is almost conserved, this results in a significant decrease of the force on the base of the cylinder. The duration of the force on the base of the device is increased to several orders of magnitude of the duration of the blast wave, and, thus decreases the maximum pressure on the base and the surface to which it is mounted by several orders of magnitude.
The pressure on the base of the blast wave mitigation device is the key parameter determining the effectiveness of the device.
In an effort to determine the effectiveness of the blast mitigation device 10 of the present invention, the blast mitigation device was setup as shown in
The design parameters of the blast wave mitigation device 10 used with the honeycomb structure are listed in the Table of
Based on compiled experimental data, the peak blast wave pressure generated by 1.36 kg Pentolite is approximately 140 MPa and the duration of the blast wave is roughly 0.2 ms. Experimental data of the blast wave pressure as a function of time is shown in
When the blast wave mitigation device 10 in accordance with the present invention is used, the impact of the blast wave caused the piston 12 to recede. The piston movement induced a weak shock wave inside the blast wave mitigation device 10. The shock wave propagated inside the blast wave mitigation device and was reflected repeatedly. Each time, the shock wave was reflected, the pressure, temperature and density increased. The increased pressure slowed down the piston, which eventually came to a complete stop. At this moment, the pressure on the base of the device reached its maximum. The shock wave propagation process inside the device lengthened the duration of the force on the base of the device to several orders of magnitude of the duration of the blast wave, while it decreased the maximum pressure by several orders of magnitude.
It is to be understood that the specific embodiments of the present invention that are described herein is merely illustrative of certain applications of the principles of the present invention. It will be appreciated that, although an exemplary embodiment of the present invention has been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Therefore, the invention is not to be limited except as by the appended claims.
This application claims priority to commonly owned U.S. provisional application Ser. No. 60/721,798, filed Sep. 29, 2005, which is incorporated by reference in its entirety.
This invention was made with government support under grants from the U.S. Army Research Office, Contract number W911NF-04-2-0011. The government does not have any rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60721798 | Sep 2005 | US |
