Patent Publication Number: US-8528862-B2

Title: Systems and methods for reducing noise in aircraft fuselages and other structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 13/280,204, filed Oct. 24, 2011, entitled “SYSTEMS AND METHODS FOR REDUCING NOISE IN AIRCRAFT FUSELAGES AND OTHER STRUCTURES,” which is a divisional application of U.S. patent application Ser. No. 12/851,431, filed Aug. 5, 2010, entitled “SYSTEMS AND METHODS FOR REDUCING NOISE IN AIRCRAFT FUSELAGES AND OTHER STRUCTURES,” which is a divisional of U.S. patent application Ser. No. 11/084,779 filed Mar. 18, 2005, entitled “SYSTEMS AND METHODS FOR REDUCING NOISE IN AIRCRAFT FUSELAGES AND OTHER STRUCTURES,” all of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The following disclosure relates generally to acoustic damping systems and, more particularly, to acoustic damping systems for use in aircraft fuselages and other structures. 
     BACKGROUND 
     Wing-mounted engines on commercial aircraft can generate “shock-cell” noise that excites low frequency vibrational modes in the fuselage. The vibrational energy is transmitted through the fuselage by stiffeners and frames, and can cause substantial noise in the passenger cabin. This noise is often difficult to dampen, and is typically addressed by bonding relatively thick, metallic layers to portions of the stiffeners and/or frames to provide what is commonly referred to as constrained layer damping. 
       FIGS. 1A and 1B  are end views of two constrained layer damping systems  110   a  and  110   b , respectively, configured in accordance with the prior art. Referring first to  FIG. 1A , the damping system  110   a  is attached to a longitudinal stiffener  102   a  which in turn is attached to a fuselage skin  108   a . The damping system  110   a  includes a constraining layer  104   a  which is bonded to the stiffener  102   a  by an adhesive layer  106   a . The constraining layer  104   a  is typically aluminum, and the adhesive layer  106   a  is typically a viscoelastic adhesive, such as one of the Scotch Damp Viscoelastic Adhesives products provided by the 3M™ Company under the ISD-112, ISD-113, or ISD-830 part numbers. Referring next to  FIG. 1B , the prior art damping system  110   b  includes an angled constraining layer  104   b  attached to a stiffener  102   b  by means of an adhesive layer  106   b . With the exception of the angle, the constraining layer  104   b  and the adhesive layer  106   b  can be similar in structure and function to their counterparts in  FIG. 1A . 
     One downside of the prior art damping systems described above with reference to  FIGS. 1A and 1B  is that they can add significant weight to the base structure. For example, a typical installation of the configuration illustrated in  FIG. 1A  can weigh up to 0.9 pound per square foot of damping system. Another downside of these damping systems is that they can be difficult to manufacture and install. 
     SUMMARY 
     This summary is provided for the benefit of the reader only, and is not intended to limit the invention as set forth by the claims. The present invention is directed generally toward systems and methods for reducing noise in aircraft fuselages and other structures. A noise reduction system configured in accordance with one aspect of the invention includes an auxetic core supported by a structural member. The auxetic core can have a first surface facing at least approximately toward the structural member and a second surface facing at least approximately away from the structural member. The noise reduction system can further include a damping layer sandwiched between the second surface of the auxetic core and a constraining layer. In one embodiment, the auxetic core can include a material that expands in a first direction when stretched in a second direction perpendicular to the first direction. In another embodiment, the damping layer can include a viscoelastic adhesive. 
     A method of manufacturing a structural assembly in accordance with another aspect of the invention includes attaching a stiffener to a skin and attaching an auxetic core to a portion of the stiffener. The method can further include covering at least a portion of the auxetic core with a damping layer, and sandwiching the damping layer between the auxetic core and a constraining layer. In one embodiment of this method, attaching a stiffener to a skin can include bonding a fiber-reinforced resin stiffener to a fiber-reinforced resin skin during a co-curing process. 
     A structural assembly configured in accordance with yet another aspect of the invention can include a stiffener having a first stiffener portion configured to be attached to a skin panel, and a second stiffener portion configured to be offset from the skin panel. The second stiffener portion can include at least first and second plies of fiber-reinforced resin material, and the structural assembly can further include a layer of damping material sandwiched between the first and second plies of fiber-reinforced resin material. In one embodiment of this structural assembly, the layer of damping material can include a viscoelastic damping material. In another embodiment, both the stiffener and the skin panel can be composed of fiber-reinforced resin material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are end views of stiffeners having constrained layer damping systems configured in accordance with the prior art. 
         FIG. 2  is an end view of a stiffener with a constrained layer damping system configured in accordance with an embodiment of the invention. 
         FIG. 3  is an end view of another stiffener with a constrained layer damping system configured in accordance with an embodiment of the invention. 
         FIGS. 4A and 4B  are top views of a portion of auxetic core illustrating a particular strain characteristic associated with this type of material. 
         FIG. 5  is a graph illustrating the effect of Poisson&#39;s ratio on damping for a constrained layer damping system with an auxetic core. 
         FIGS. 6A-6G  are a series of cross-sectional end views illustrating various stages in a method for manufacturing structural assemblies with integrated viscoelastic damping layers in accordance with an embodiment of the invention. 
         FIG. 7  is a cross-sectional end view of a stiffener lay-up with an integrated viscoelastic damping layer in accordance with another embodiment of the invention. 
         FIG. 8  is a cross-sectional end view of a structural assembly with an integrated viscoelastic damping layer in accordance with a further embodiment of the invention. 
         FIG. 9  is a cross-sectional end view of a structural assembly with an integrated viscoelastic damping layer in accordance with yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure describes various systems and methods for reducing noise in aircraft fuselages and other structures. Certain details are set forth in the following description to provide a thorough understanding of various embodiments of the invention. Other details describing well-known structures and methods often associated with structural damping systems and composite manufacturing are not set forth below, however, to avoid unnecessarily obscuring the description of the various embodiments of the invention. 
     Many of the details, dimensions, angles and other features shown in the Figures are merely illustrative of particular embodiments of the invention. Accordingly, other embodiments can have other details, dimensions, angles and features without departing from the spirit or scope of the present invention. Furthermore, additional embodiments of the invention can be practiced without several of the details described below. 
     In the Figures, identical reference numbers identify identical or at least generally similar elements. To facilitate the discussion of any particular element, the most significant digit or digits of any referenced number refer to the Figure in which that element is first introduced. For example, element  210  is first introduced and discussed with reference to  FIG. 2 . 
       FIG. 2  is an end view of a structural assembly  200  that includes a constrained layer damping system  210  (“damping system  210 ”) configured in accordance with an embodiment of the invention. In the illustrated embodiment, the damping system  210  is attached to a cap portion  203  of a longitudinal “Z”-section stiffener  202  (“stiffener  202 ”), and the stiffener  202  is attached to a skin  208 . The damping system  210  can include a viscoelastic damping layer  214  sandwiched between an auxetic core  212  and a constraining layer  204 . The constraining layer  204  can include a number of different materials including both metallic materials (e.g., aluminum, steel, titanium, etc.) and non-metallic materials (e.g., fiber-reinforced resin materials or “composite materials” as they are commonly known). For stiffness and/or weight considerations, composite materials (e.g., graphite/epoxy materials) may be desirable. For example, in one embodiment, the constraining layer  204  can include multiple plies of graphite/epoxy material having a total thickness of about 0.006 inch to about 0.05 inch (e.g., about 0.01 inch). In another embodiment, the constraining layer  204  can include three plies of graphite/epoxy material oriented on a 0/90/0 bias. In other embodiments, the constraining layer  204  can include other materials having other thicknesses. For example, in one other embodiment, the constraining layer  204  can include aluminum sheet material. 
     The damping layer  214  can include can include a viscoelastic pressure sensitive adhesive (PSA) having a thickness of about 0.005 to about 0.02 inch (e.g., about 0.01 inch). As those of ordinary skill in the art are aware, the term “viscoelastic” is often used to describe a material that demonstrates both viscous and elastic properties. Suitable PSAs can include acrylic, urethane, silicon, or epoxy-based adhesives. In this regard, the 3M™ Company provides a number of suitable viscoelastic adhesives having part numbers ISD-112, ISD-113, and ISD-830. In other embodiments, other viscoelastic materials can be used to attach the constraining layer  204  to the auxetic core  212 . Such viscoelastic materials can include, for example, viscoelastic foam materials (e.g., viscoelastic closed- and opened-cell foam materials) having suitable adhesive properties. 
     As described in greater detail below, in one embodiment of the invention the auxetic core  212  has a negative Poisson&#39;s ratio. That is, in contrast to most elastic materials, the auxetic core  212  expands in a first direction when stretched in a second direction perpendicular to the first direction. Suitable auxetic core materials can include Nomex® auxetic honeycomb core provided by the M.C. Gill Corporation of 4056 Easy Street, El Monte, Calif. 91731. Other suitable core materials can include other commercially available honeycomb materials and foam materials. In the illustrated embodiment, the auxetic core  212  can have a thickness ranging from about 0.03 inch to about 0.15 inch (e.g., about 0.125 inch). In other embodiments, the auxetic core  212  can have other thicknesses depending on a number of different factors including, for example, core density, Poisson&#39;s ratio, the level of damping desired, etc. 
     The auxetic core  212  is attached to the stiffener  202  by an adhesive layer  206 . The adhesive layer  206  can include a PSA that is at least generally similar in structure and function to the adhesive used for the damping layer  214 . As mentioned above, suitable PSAs can include acrylic, urethane, silicon, and epoxy-based adhesives. In the illustrated embodiment, the adhesive layer  206  can have a thickness ranging from about 0.003 to about 0.01 inch. In other embodiments, the adhesive layer  206  can have other thicknesses and can include other substances for bonding the auxetic core  212  to the stiffener  202  as shown in  FIG. 2 . 
     The skin  208  and the stiffener  202  can be manufactured from a plurality of different materials including both metallic materials (e.g., aluminum, steel, titanium, etc.) and/or composite materials. Conventional composite materials typically include graphite, glass, or polyarimide fibers in a matrix of epoxy or other resin. Although described herein in the context of an aircraft fuselage, the damping system  210  is not limited to this particular use. Accordingly, in other embodiments, the damping system  210  can be used to reduce noise in other structures including, for example, other air, land, and marine vehicles. 
       FIG. 3  is a partially exploded end view of a structural assembly  300  having a damping system  310  that is at least generally similar in structure and function to the damping system  210  described above with reference to  FIG. 2 . In this embodiment, the damping system  310  is bonded to a cap portion  303  of a hat-section stiffener  302 , and the hat-section stiffener  302  is attached to a skin  308 . Both the hat-section stiffener  302  and the skin  308  can be manufactured from multiple plies of composite materials (e.g., graphite/epoxy materials). For example, as described in greater detail below, in one embodiment the hat-section stiffener  302  can be co-cured with the skin  308  to bond the stiffener  302  to the skin  308 . In this embodiment, the damping system  310  can then be bonded to the cap portion  303  during a subsequent assembly step. 
       FIGS. 4A and 4B  are two different top views of a portion of auxetic honeycomb core  412 . In  FIG. 4A , the auxetic core  412  is in a relaxed, unexpanded state. In  FIG. 4B , the auxetic core has been stretched in a first direction  421 . As  FIG. 4B  illustrates, stretching the auxetic core  412  in the first direction  421  causes it to expand in a second direction  422  perpendicular to the first direction  421 . This atypical behavior is commonly characterized by a negative Poisson&#39;s ratio, because conventional materials with positive Poisson&#39;s ratios contract in the transverse direction when stretched in the longitudinal direction. 
     With reference to  FIG. 2 , the negative Poisson&#39;s ratio of the auxetic core  212  has the effect of amplifying the shear strain in the damping layer  214  during bending motion of the stiffener  202 . The amplified shear strain in the damping layer  214  increases the ability of the damping system  210  to dissipate the noise energy transmitted through the stiffener  202 . In this manner, the damping system  210  can effectively dissipate vibrational energy in the structural assembly  200  through exaggerated shearing motion in the damping layer  214 . Indeed, in some embodiments, the auxetic core  212  has been shown to amplify the strain energy dissipation characteristics of damping systems by as much as fifty percent. 
       FIG. 5  is a graph  530  illustrating the effect of Poisson&#39;s ratio on damping for damping systems that use auxetic core in accordance with embodiments of the invention. Damping percentage is measured along a vertical axis  531 , and Poisson&#39;s ratio is measured along a horizontal ratio  532 . As a plot  534  demonstrates, in this embodiment vibrational dampening can increase by about fifty percent when the Poisson&#39;s ratio of the auxetic core drops from about 0.2 to about −1. In a broader sense, the plot  534  reflects the fact that damping increases as Poisson&#39;s ratio becomes more negative. 
       FIGS. 6A-6F  are a series of end views describing various aspects of a method for manufacturing a stiffened structure with an integrated viscoelastic damping layer, in accordance with an embodiment of the invention. Referring first to  FIG. 6A , a plurality of uncured fiber-reinforced resin plies  640  (identified individually as plies  640   a - i ) are positioned on a stiffener tool  642  to form the basis of a stiffener lay-up  641 . The fiber-reinforced resin plies can include a number of different fiber/resin materials, including graphite/epoxy materials. In addition, the plies can be arranged in various orientations known in the art for providing the desired structural characteristics. The stiffener tool  642  includes a plurality of tool surfaces  644  (identified individually as tool surfaces  644   a - e ) which give the stiffener lay-up  641  a hat-section cross-sectional shape. The hat-section cross-sectional shape includes a cap portion  645  offset from opposing base portions  647   a  and  647   b . In other embodiments, stiffeners having other cross-sectional shapes (e.g., “I,” “Z,” “C,” “T,” “L,” etc. cross-sectional shapes) can be manufactured in accordance with the methods described herein. 
     Referring next to  FIG. 6B , a damping layer  646  is laid down against the cap portion  645  of the stringer lay-up  641 , and one or more constraining layer plies  648  (identified individually as a first constraining layer  648   a  and a second constraining layer  648   b ) are laid over the damping layer  646 . The damping layer  646  can include a thin, viscoelastic polymer material having a thickness from about 0.005 inch to about 0.02 inch (e.g., about 0.01 inch). Suitable damping materials can include, for example, acrylic, urethane, silicon, rubber, etc. 
     In some embodiments, the damping layer  646  may need to withstand elevated temperatures and pressures during subsequent curing processes. For example, in one embodiment, the damping layer  646  may need to withstand temperatures up to about 350° Fahrenheit and pressures of about 90 pounds per square inch (psi) for extended periods of time (e.g., up to six hours). At these temperatures and pressures, viscoelastic materials can become soft and squeeze out of the laminate. This can have a negative effect on the porosity, strength, and/or stiffness of the resulting part or assembly. In addition, some viscoelastic materials may chemically interact with the resin portion of fiber-reinforced resin materials. This interaction can alter the basic characteristics of the viscoelastic material and/or the fiber-reinforced resin materials. For example, interaction resulting in co-polymerization can reduce the damping properties of some viscoelastic materials. One method for reducing the likelihood of co-polymerization and/or viscoelastic squeeze is to add a barrier ply around the viscoelastic layer. Various types of materials are suitable for a barrier ply. One type includes a scrim, which is a thin, mesh-like material that helps to maintain the shape of the viscoelastic material during curing. Another approach involves laminating the viscoelastic layer between two plies of thin (e.g., 0.0005 inch thick), perforated material, such as polyvinyl fluoride sold under the brand name of Tedlar®. 
     The constraining layers  648  can include uncured fiber-reinforced resin materials (e.g., graphite/epoxy materials) that are at least generally similar in structure and function to the fiber-reinforced resin plies  640  described above with reference to FIG.  6 A. In the embodiment illustrated in  FIG. 6B , the edges of the constraining layers  648  extend beyond the damping layer  646  so that the constraining layers  648  can be bonded to the fiber-reinforced resin plies  640  during a subsequent curing process. 
       FIG. 6C  illustrates another embodiment in which a stiffener lay-up  651  includes a damping layer  656  having a thickness from about 0.02 inch to about 0.06 inch (e.g., about 0.03 inch). When damping layers of this thickness are used, it may be advantageous to taper the edges of the damping layer  656  as shown in  FIG. 6C  to avoid creating a step and/or gaps in the lay-up.  FIG. 6D  illustrates a further embodiment in which a constraining layer  668  “floats” on a corresponding damping layer  666 . Here, the term “float” is used to describe the condition in which the edges of the constraining layer  668  do not extend over the damping layer  666  for attachment to the fiber-reinforced resin plies  640 . 
     After assembling a stiffener lay-up as shown in  FIG. 6B ,  6 C, or  6 D, the stiffener lay-up (e.g., the stiffener lay-up  651 ) is positioned in a shell tool  650 , a portion of which is illustrated in  FIG. 6E . The shell tool  650  includes a plurality of stiffener tool surfaces  654  (identified individually as stiffener tool surfaces  654   a - e ) and a plurality of adjacent skin tool surfaces  652  (identified individually as skin tool surfaces  652   a  and  652   b ). The stiffener tool surfaces  654  are dimensionally similar to the tool surfaces  644  described above with reference to  FIG. 6A , and provide support for the stiffener lay-up  651  in the desired hat-section shape. 
     As described in greater detail below, the skin tool surfaces  652   b  are configured to support one or more plies of fiber-reinforced resin material (not shown in  FIG. 6E ) which overlay the stiffener lay-up  651  to form a composite hat-stiffened shell structure. The over-laying fiber-reinforced resin material can include fabric plies, tape, and/or filament wound toes. In this regard, in one embodiment, the shell lay-up tool  650  can include a stationary or rotating lay-up mandrel or similar tool for forming a one-piece composite aircraft fuselage. Various methods and systems for forming one-piece composite fuselages are described in detail in co-pending U.S. patent application Ser. Nos. 10/853,075; 10/851,381; 10/949,848; and 10/996,922; each of which is incorporated herein in its entirety by reference. 
     Referring next to  FIG. 6F , a flexible bladder  662  is positioned inside the stiffener lay-up  651 . The bladder  662  can include an elongate tubular membrane or similar material for sealing the stiffener lay-up  651  during the subsequent vacuum-bagging and curing processes. After the bladder  662  is in place, a first adhesive strip  664   a  is positioned along the inboard edge of the first base portion  647   a , and a second adhesive strip  664   b  is positioned along an adjacent inboard edge of the second base portion  664   b . An adhesive layer  669  is then positioned over the stiffener lay-up  651  from the extents of the first base portion  647   a  and the second base portion  647   b.    
     As shown in  FIG. 6G , one or more fiber-reinforced resin plies  670  (identified individually as plies  670   a - d ) are laid-up on the shell tool  650  to form a skin lay-up  671 . The skin plies  670  can include various types of known fiber-reinforced resin materials including, for example, preimpregnated bidirectional and/or unidirectional fabrics, tapes, and/or filaments, in various orientations or biases. For instance, in the illustrated embodiment, the first ply  670   a  and the fourth ply  670   d  can include a fabric (e.g., a bidirectional graphite/epoxy fabric), while the intermediate plies  670   b  and  670   c  can include tape (e.g., a unidirectional graphite/epoxy tape). Once the skin plies  670  have been applied to the tool  650 , the combined skin/stiffener lay-up can be vacuum-bagged and positioned in a suitable autoclave or oven for curing at an elevated temperature and/or pressure. Alternatively, if raised temperatures and/or pressures are not required for satisfactory curing, then the lay-up can be cured at room temperature. 
     One feature of the manufacturing method described above with reference to  FIGS. 6A-6G , is that the stiffener lay-up  641  bonds to the skin lay-up  671  during the co-curing process. One advantage of this feature is that it provides a continuous, high-strength bond that alleviates the need to fasten the stiffener to the skin with additional mechanical fasteners. Another feature of the method described above is that the damping layer (e.g., the damping layer  646  of  FIG. 6B ) is integrally formed with the stiffener during the manufacturing process. An advantage of this feature is that it streamlines the manufacturing process by eliminating the need to attach a separate damping system to the stiffener after the corresponding shell structure has been produced. Another advantage of laminating the damping layer into the stiffener during the manufacturing process is that it provides a relatively efficient damping mechanism without the additional weight typically associated with conventional constrained layered damping systems that utilize a metallic constrained layer. 
       FIG. 7  is an end view of a stiffener lay-up  741  that is at least generally similar in structure and function to the stiffener lay-up  641  described above with reference to  FIGS. 6A and 6B . For example, the stiffener lay-up  741  includes a cap portion  745   a  to which a first damping layer  746   a  is laminated. In this particular embodiment, however, a second damping layer  746   b  is wrapped around a bladder  762  positioned inside the stiffener lay-up  741 . During a subsequent curing process, the second damping layer  746   b  bonds to the inner walls of the stiffener lay-up  741  and the adjacent skin lay-up (not shown). One advantage of this embodiment is that it can provide additional noise reduction with a relatively minor increase in weight. 
       FIG. 8  is an end view of a stiffener lay-up  841  configured in accordance with another embodiment of the invention. The stiffener lay-up  841  is at least generally similar in structure and function to the stiffener lay-up  641  described above with reference  FIGS. 6A and 6B . In this particular embodiment, however, a damping layer  846  extends beyond a cap portion  845  of the stiffener lay-up  841 , and onto adjacent sidewall portions  849   a  and  849   b.    
       FIG. 9  is an end view of a stiffener lay-up  941  configured in accordance with a further embodiment of the invention. The stiffener lay-up  941  is at least generally similar in structure and function to the stiffener lay-up  641  described above with reference to  FIGS. 6A and 6B . In this particular embodiment, however, a damping layer  946  is attached toward an outer surface of a cap portion  945  of the stiffener lay-up  941 . In all the embodiments described above including the embodiment illustrated in  FIG. 9 , the damping layer (e.g., the damping layer  946 ) can be segmented so that it does not extend over the full length of the corresponding stiffener. In this manner, the damping layer  946  could be omitted in areas where it is desirable to reduce stiffener height, for example, at those locations where the stiffener extends under or through a frame or other structure. 
     Although the discussion above referring to  FIGS. 6A-9  has focused on hat-section stiffeners, the various manufacturing methods and damping systems described herein can be used with virtually any type of stiffener and/or other structural member, including flat panels, curved skins, and various other structural members having a wide array of different cross-sectional shapes. For example, in one embodiment, the manufacturing methods and/or damping systems described above can be used with an inverted “T” stiffener (or “blade” stiffener). In this embodiment, the stiffener can include one or more viscoelastic damping layers bonded or otherwise adhered to the upstanding leg portion of the “T” stiffener. Similar implementations are possible with “L,” “C,” “Z,” and “S”-shaped stiffener configurations, among others. In yet another embodiment, a torque-tube damping element having an outer constrained layer and/or an internal damping medium can be positioned inside a hat-section stiffener to provide noise reduction. 
     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. For example, aspects of the invention described in the context of particular embodiments may be combined or eliminated in other embodiments. Further, while advantages associated with certain embodiments of the invention have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the invention. Accordingly, the invention is not limited, except as by the appended claims.