CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     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. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     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. 
     
    
     TECHNICAL FIELD  
       [0003]     The present invention relates generally to methods and apparatus for pressure wave mitigation and, more specifically, to methods and apparatus for blast wave mitigation.  
       BACKGROUND OF THE INVENTION  
       [0004]     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.  
         [0005]     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.  
         [0006]     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%.  
         [0007]     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.  
         [0008]     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.  
         [0009]     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.  
         [0010]     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.  
         [0011]     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.  
       SUMMARY OF THE INVENTION  
       [0012]     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.  
         [0013]     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. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0014]     The present invention is described in detail below with reference to the attached drawing figures, wherein:  
         [0015]      FIG. 1  is a schematic of the blast wave mitigation apparatus of the present invention;  
         [0016]      FIG. 2  is a schematic of an exemplary embodiment of the blast wave mitigation apparatus for a testing of its effectiveness;  
         [0017]      FIG. 3  is a table listing the parameters of the blast wave mitigation apparatus for the embodiment of  FIG. 2 ;  
         [0018]      FIG. 4  is a diagram illustrating the relationship between pressure and the time of impact on the piston of the blast wave mitigation apparatus of the present invention;  
         [0019]      FIG. 5  is a diagram illustrating the difference of indentation between a control and the exemplary embodiment of the blast mitigation apparatus of the present invention; and  
         [0020]      FIG. 6  is a diagram illustrating the relationship between pressure and the time of impact on the base wall of the exemplary embodiment of the blast wave mitigation apparatus of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     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.  
         [0022]     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.  
         [0023]     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.  
         [0024]     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.  
         [0025]     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.  
         [0026]     The pressure on the base of the blast wave mitigation device is the key parameter determining the effectiveness of the device.  
       EXAMPLE  
       [0027]     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  FIG. 2 . A honeycomb structure, which can withstand a peak pressure of about 2 MPa, is placed on a steel test platform. The honeycomb structure was covered by either the blast wave mitigation device  10  of the present invention or a control device. The control device has the same dimensions and weight as the blast wave mitigation device. A rolled homogeneous armor steel plate is used to protect the honeycomb structure that is not covered by the blast wave mitigation and control device  10 . A hole was cut at the center of the steel plate to expose the blast wave mitigation device and control device. The diameter of the hole is the same as that of the blast wave mitigation device. The steel plate has a thickness of approximately 9 cm. The blast wave is generated by detonating 1.36 kg of Pentolite at a distance of 0.26 m. This setup is capable of testing the effectiveness of the blast wave mitigation device qualitatively.  
         [0028]     The design parameters of the blast wave mitigation device  10  used with the honeycomb structure are listed in the Table of  FIG. 3 . The parameters that significantly affect the effectiveness of the blast wave mitigation device are the thickness of the piston  6  and the length of the cylinder L. D is the diameter of the piston. The control device has the same dimensions and weight as the blast wave mitigation device. The blast wave mitigation device has a square base  16  of approximately 460×460 mm. One experiment was conducted with the control device and one with the blast wave mitigation device.  
         [0029]     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  FIG. 4 . When the Pentolite is detonated, the blast wave was transmitted through the control device with negligible attenuation. Since the blast wave pressure significantly exceeds the pressure rating of the honeycomb structure, the impact of the blast leaves an indentation of roughly 13 mm in the honeycomb structure, when the control device is used (see  FIG. 5 ).  
         [0030]     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.  
         [0031]      FIG. 7  shows that the peak pressure on the base of the blast wave mitigation device was predicted to be slightly higher than 2 MPa. The reduction of the peak blast wave pressure was predicted to be over 97%. As a result, the honeycomb structure should be adequately protected by the blast wave mitigation device. This was confirmed experimentally; the blast wave impact left a very shallow indentation along the edge of the blast wave mitigation device, which is where the stress concentration occurred. The indentations on the honeycomb structures with the blast wave mitigation device and the control device under the impact of the blast wave are shown in  FIG. 5 . The experimental results prove qualitatively that the blast wave mitigation device is effective in mitigating the blast wave impact.  
         [0032]     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.
A method and apparatus for mitigating blast compression waves is disclosed. The apparatus has a housing having an open end, and a piston slidably received in the open end of the housing in a substantially airtight engagement therewith. The piston and the housing define an interior wherein a compressible substance is confined. When a 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.