Static structures (e.g. – buildings) – Machine or implement
Reexamination Certificate
1998-04-21
2001-05-29
Kent, Christopher T. (Department: 3635)
Static structures (e.g., buildings)
Machine or implement
C052S167600, C052S167800, C052S749100, C052S749100, C052S742130, C052S745190
Reexamination Certificate
active
06237302
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the use of granular materials of low bulk sound speeds for acoustic damping.
BACKGROUND OF THE INVENTION
Vibration in built-up structures causes problems ranging from radiation of acoustic noise to damage to sensitive instrumentation. Built-up structures are any composite of beams, plates, joints and other structural components connected to form a nominal single unit. Examples of built-up structures are bridges, electronics cabinets, sports equipment, and automobiles. All of these structures suffer from vibrations to one degree or another. Structural vibration is caused most commonly by external forcing from impact, attached equipment, or fluid flow. In many structural applications it is desirable to reduce the level of such vibrations.
The use of granular material to effect structural damping is well known. Sand and lead shot have been used for some time to provide such damping. For example, sand is used to fill the hollow spaces of tubular beams. Sand and lead shot, though effective in providing damping primarily through a mass loading effect, are very heavy, so that their use extracts a high weight penalty. For example, sand has a bulk specific gravity of not less than 1.5 and lead shot has a bulk specific gravity of approximately 7.
The present inventor has previously established that some lighter weight materials can provide significant vibration damping. U.S. patent application Ser. No. 08/662,167 (allowed) discloses and claims the use of granular materials with a bulk specific gravity of less than 1.5, such as glass micro-spheres or polyethylene beads, for vibration damping. The teachings of this allowed application are incorporated herein by reference. The materials described therein do not rely upon the mass-loading effect of high-density materials, nor on the high intrinsic attenuation of viscoelastic materials. Rather, the dominant damping mechanisms are disclosed to be the small intrinsic attenuation within the solid pieces of granular material, the friction between pieces of granular material, and the non-linear hysteresis in the pieces of granular material arising from deformation during the wave propagation process. The effectiveness of these materials for vibration damping is attributed to a low bulk wave speed characteristic of granular materials.
Thus the prior art acknowledges several desirable characteristics for a granular vibration damping material. The material may have significant mass relative to the host structure such as sand or lead shot, high intrinsic damping characteristics such as viscoelastic spheres, or a low bulk wave speed which allows several other damping mechanisms to come into play. While studies have tried to increase each of material density and viscoelasticity to enhance damping, to date, little, if any, attention has been paid to the reduction of bulk wave speed of materials to improve their vibration damping characteristics.
SUMMARY OF THE INVENTION
In one aspect, the invention comprises a material for damping vibration in a structure. In this aspect, the material is composed of a mixture of at least two granular materials. In a related aspect, the material is a granular material selected to have a bulk sound speed of less than 90 meters/second. An example of the first aspect is a 50/50 mixture of two materials selected from the group consisting of lead shot, sand, rice, low-density polyethylene beads, high-density polyethylene beads, perlite, vermiculite, glass microspheres, and expanded polystyrene beads. An example of the second aspect is a granular composition of perlite, which has been found to exhibit a bulk sound speed of about 62 meters/second. Particularly when the material is a mixture, it may be desirable for the individual grains of the material to adhere at least partially to one another. Such adherence may also be desirable for homogeneous materials such as perlite, for example to reduce rattling.
In further aspects, the invention comprises a method of damping vibration in a structure, whereby at least one of the materials of the previous paragraph is placed in intimate contact with the structure, in an arrangement whereby particles of the material are in intimate contact with each other.
In other related aspects, the invention comprises a damped structural member. The member is damped by intimate contact with one of the granular materials described above, where the granular material is disposed so that its particles are in intimate contact with each other.
In other aspects, the invention comprises methods for tailoring specific mixtures which have a desired sound speed and/or level of damping. The methods include determining damping levels and sound speeds for mixtures of two materials and for mixtures of three or more materials.
As it is used in this application, “Lodengraf™ materials”
1
are granular materials intended for use for vibration damping. “Lodengraf™ damping” refers to methods of vibration damping comprising placing Lodengraf™ materials in intimate contact with at least a portion of a structure subject to vibration.
1
“Lodengraf™” is a trademark of Edge Innovations & Technology, LLC, the assignee of this application.
In this application, “homogeneous” granular materials are composed of a mixture of particles, where substantially all particles are composed of the same material. “Inhomogeneous” or “heterogeneous” granular materials are composed of a mixture of particles of at least two materials.
As it is used in this application, the “sound speed” of a granular material refers to the bulk sound speed of the material in granular form. Similarly, “density” refers to bulk density of a granular material, not the density of the solid, nonporous material.
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Choate Hall & Stewart
Edge Innovations & Technology, LLC
Kent Christopher T.
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