Abstract:
A method of making a composite panel having subsonic transverse wave speed characteristics which has first and second sheets sandwiching a core with at least one of the sheets being attached to the core at first regions thereof and unattached to the core at second regions thereof.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 11/129,755, filed May 13, 2005 now abandoned, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to composite panels. More specifically, the invention is a composite panel that has subsonic transverse wave speed characteristics in regions thereof for reduced sound radiation efficiency and increased sound power transmission loss. 
     2. Description of the Related Art 
     Composite materials are used in many construction applications (e.g., structures, aircraft, trains, vehicles, industrial machines, etc.) because of their light weight and strength. The materials are frequently formed into what are known as composite panels where two sheets of one type of material are sandwiched about another type of core material. For example, one type of composite panel has two sheets of a material such as graphite-epoxy, epoxy, fiberglass or aluminum sandwiched about a honeycomb core made from materials such as NOMEX, aluminum or paper. The resulting composite panel is light and stiffer than any of its component parts. However, as can be the case with most lightweight and stiff materials, sound can be radiated very efficiently because the transverse wave speed through the panel can be greater than the speed of sound in air. In other words, the composite panel has a supersonic transverse wave speed. If the composite panel is to be used to define a human-occupied interior space, noise radiated by the composite panel into the interior space may be unacceptable. Current methods of addressing this noise problem have involved the addition of noise control material to the composite panel such that the noise-controlled composite panel is characterized by a subsonic transverse wave speed. Suggested additions include a limp mass (e.g., lead vinyl) applied to one or both of the composite panel&#39;s face sheets and/or the inclusion of foam within the composite panel&#39;s core in the case of a honeycomb core. However, the extra noise-control material adds cost and weight to the composite panel. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a composite panel having subsonic transverse wave speed characteristics. 
     Another object of the present invention is to provide a composite panel that does not require the addition of noise control material to achieve subsonic transverse wave speed characteristics. 
     In accordance with the present invention, a composite panel has first and second sheets sandwiching a core. At least one of the first and second sheets is attached to the core at first regions thereof and unattached to the core at second regions thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
         FIG. 1  is an exploded perspective view of a composite panel having a core with recessed regions in accordance with an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the composite panel of  FIG. 1  in its assembled form; 
         FIG. 3  is a cross-sectional view of a composite panel in accordance with another embodiment of the present invention where the recesses are different sizes; 
         FIG. 4  is a cross-sectional view of a composite panel in accordance with another embodiment of the present invention where the recesses are formed on either side of the core in a mirror-image fashion; 
         FIG. 5  is a cross-sectional view of a composite panel in accordance with another embodiment of the present invention where the recesses are formed on either side of the core in a non-mirror-image fashion; 
         FIG. 6  is a cross-sectional view of a composite panel in accordance with another embodiment of the present invention where areas of non-attachment are provided between the core and face sheets; 
         FIG. 7  is a cross-sectional view of the composite panel of  FIG. 2  further having acoustically absorbent material in the panel&#39;s recesses; and 
         FIG. 8  is a cross-sectional view of a composite panel in accordance with another embodiment of the present invention where recesses are formed in one of the face sheets. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIGS. 1 and 2 , a composite panel in accordance with an embodiment of the present invention is shown and is referenced generally by numeral  10 . For illustration, composite panel  10  is a flat panel. However it is to be understood that composite panels constructed in accordance with the present invention can also be shaped to define contoured panels as needed. 
     Composite panel  10  has face sheets  12  and  14  sandwiched about a core  16 . Face sheets  12  and  14  can be the same or different materials. Suitable materials for face sheets  12  and  14  include, but are not limited to, graphite epoxy, aluminum and fiberglass. Core  16  is a lightweight material that is bonded, attached or adhered (in ways well understood in the art) to face sheets  12  and  14  to foam composite panel  10  such that the stiffness of composite panel  10  is greater than the stiffness of its component parts. As a result, while the transverse wave speed for typical materials and thicknesses of face sheets  12  and  14  is subsonic, the transverse wave speed is very often supersonic for a composite panel using these face sheets. Suitable constructions for core  16  include, but are not limited to, a honeycomb structure, a truss structure, or a foam structure. Suitable materials for core  16  include, but are not limited to, NOMEX, paper and aluminum in the case of honeycomb cores, and polymers and carbon in the case of foam cores. The core can be of varying thicknesses depending, for example, on a particular application, without departing from the scope of the present invention. 
     One embodiment of the present invention addresses this problem by forming recesses in core  16  adjacent face sheet  12 . More specifically, an array of recesses  18  are formed in core  16  so that face sheet  12  is only bonded/attached/adhered to core at regions  16 A while the entire side of face sheet  14  is bonded/attached/adhered to the other side of core  16  as indicated by  14 A. The number, size, depth and shape of recesses  18  and resulting size/shape of regions  16 A can vary without departing from the scope of the present invention. In general, a balance must be struck between stiffness requirements and noise requirements of composite panel  10 . With respect to noise reduction, the greater the area of the recesses, the greater the reduction in sound radiation efficiency and increase in sound power transmission loss. This is because each region  12 A of face sheet  12  adjacent to a recess  18  is uncoupled from core  16  so that transverse wave speed at this local region of composite panel  10  is reduced to the subsonic transverse wave speed of face sheet  12 . With respect to stiffness, composite panel  10  must have sufficient attachment regions  16 A (between face sheet  12  and core  16 ) to achieve the necessary stiffness requirements. Accordingly, any given application of the present invention will require these two criteria to be balanced. 
     In the illustrated embodiment discussed thus far, identically-sized recesses  18  are formed just on one side of core  16 . However, the present invention is not so limited. For example, composite panel  30  in  FIG. 3  has recesses  38  formed in core  16  that are of different sizes. Note that the shapes of recesses  38  could vary too. In  FIG. 4 , composite panel  40  has recesses  48  formed on either side of core  16  in a mirror-image fashion so that the regions of face sheets  12  and  14  contacting and attached to core  16  are similarly mirror images of one another. Composite panel  50  in  FIG. 5  utilizes recesses  58  on opposing sides of core  16 , but in a non-mirror-image fashion. 
     Another embodiment of the present invention is illustrated by a composite panel  60  in  FIG. 6  where, rather than forming recesses in core  16 , regions of non-attachment  16 B are formed between face sheets  12 / 14  and core  16 . That is, face sheets  12  and  14  are coupled to core  16  only at attachment regions  16 A while remaining uncoupled or unattached to core  16  at non-attached regions  16 B. As sound radiates through composite panel  60 , friction losses will be generated between the non-attached regions  16 B of core  16  and face sheets  12  and  14 . In many applications, this will be sufficient to produce a satisfactory low frequency response. However, higher-frequency buzzing may occur thereby making this embodiment most suitable for applications where high-frequency buzzing is not problematic. 
     Still another embodiment of the present invention involves adding an acoustically absorbent material (a wide variety of which are well known in the art) to some or all of the recesses formed in the composite panel&#39;s core. For example,  FIG. 7  illustrates the  FIG. 2  embodiment with recesses  18  further having an acoustically absorbent material  20  partially or completely filling recesses  18 . 
     The present invention is not limited to the formation of recesses in the core of a composite panel. For example, a composite panel  70  illustrated in  FIG. 8  has recesses  78  formed in face sheet  12 . Although not illustrated, recesses could also be formed in face sheet  14  in a mirror-image or non-mirror-image fashion with respect to recesses  78 . Still further, recesses could be formed in one or both of face sheets  12 / 14  and in core  16  without departing from the scope of the present invention. 
     The advantages of the present invention are numerous. Composite panels having local regions characterized by subsonic transverse wave speeds can be constructed without requiring the addition of noise control material. Rather, the present invention addresses the transverse wave speed problem of composite panels by actually eliminating material thereby decreasing the weight of the panel. Cores and/or face sheets with recesses formed therein can be easily achieved using automated manufacturing processes. The present invention can be used wherever composite materials are used in weight sensitive, noise environments such as aerospace and ground vehicles, trains and industrial machines. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function and step-plus-function clauses are intended to cover the structures or acts described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.