Patent Publication Number: US-2019176992-A1

Title: Asymmetric core sandwich structure for heated floor panels

Description:
BACKGROUND 
     An aircraft may include heated floor panels to mitigate the effects of cold underfloor temperatures and to help maintain a comfortable cabin temperature. The floor panels are typically supported by an aircraft structure arranged, for example, in a grid-like pattern. The floor panels have structural integrity sufficient to support the weight of people and objects resting on the panels. A metal cover sheet typically forms the top surface of the panel to protect the underlying layers from punctures from high heels, chips from dropped objects, scratches from dragged luggage and/or other floor-traffic related hazards. Some type of floor covering (e.g., carpeting, tiling) is typically placed over the panels for comfort and/or appearance. A heated floor panel can include a weight-supporting layer and a heating layer. The floor panel can also include an insulating layer to prevent heat from exiting the aircraft compartment. 
     The heating layer of the heated floor panels can be placed just under the metal sheet or near the top surface of the floor. This makes the heating elements of the floor panels susceptible to damage (mechanical or due to fluid intrusion) during installation, maintenance or general use. To address this issue, additional protective layers can be placed over the heating layer. However, such layers can reduce thermal conductivity to from the heating layer to the panel surface, thus requiring a greater power input to achieve the desired panel temperature. 
     SUMMARY 
     A composite sandwich structure for heating an environment includes a first core layer made from a first cellular material and having a first thickness, and a second core layer made from a second cellular material and having a second thickness less than the first thickness. The structure further includes a heating layer having a thermoelectric heating element and disposed between the first and second core layers. The first core layer is positioned between the heating layer and the environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded cross-section showing the various layers of a sandwich panel. 
         FIG. 2  is a cross-section of an asymmetric core structure shown separately from the sandwich panel of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a composite sandwich panel, and more specifically, to a heated floor panel having an asymmetric core structure. The core structure includes two core layers surrounding a heating layer. The upper core layer is thinner than the lower core layer, which facilitates heat transfer to the panel surface while providing impact protection to the heating layer. 
       FIG. 1  is a simplified cross-section of floor panel  10 . Panel  10  includes heating layer  12 , core layers  14  and  16 , structural layers  18  and  20 , and cover sheets  22  and  24 . Surface  26  represents the upper (external) surface of panel  10 . Panel  10  is positioned over substrate S, and provides heat to environment E, which is located on a side of panel  10  opposite substrate S. 
     Heating layer  12  can include a thermoelectric heating element (not shown). The heating element can be a resistive heating element formed, for example, from a metallic material, Positive Temperature Control (PTC) ceramic, PTC polymer, or carbon allotrope material. The heating element can be arranged as an etched foil, wire, or printed-ink element. Other suitable heating elements are contemplated herein. Heating layer  12  can be used to control the temperature of surface  26  of panel  10 , which can be installed, for example, in an aircraft cabin or cockpit. In certain embodiments, the heating element can extend across the entire area of heating layer  12 . In other embodiments, the heating element can be inset some distance from the edges of heating layer  12  in order to protect the element from fluid spills along or damage at the edges of panel  10 . 
       FIG. 2  is a simplified cross-section of lower core layer  14  and upper core layer  16 , shown for simplicity removed from panel  10 . Together, core layers  14  and  16  form asymmetric core structure  28 . Core layers  14  and  16  provide impact resistance to panel  10 , and carry shear loads to stiffen floor panel  10 . Core layers  14  and  16  can also have insulating and/or flame retardant properties. Core layers  14  and  16  can be formed from an expanded honeycomb, or an open cell or closed cell foam material. For example, core layers  14  and  16  can be formed from an expanded Kevlar® or Nomex® honeycomb, which provide sufficient support to panel  10 . Suitable foam materials include polyamide and polyimide foams. In some embodiments, core layers  14  and  16  can be formed from the same material. In other embodiments, core layers  14  and  16  can be formed from different materials. For example, upper core layer  16 , which can be subject to greater impact forces from environment E, can be formed from a honeycomb material, while lower core layer  14  can be formed from a foam material. Material choices and combinations can further be based on the specific structural and heating requirements of panel  10 . For example, honeycomb materials are lighter than certain foam materials, and may be ideal for applications in which panel weight is a critical factor. 
     As can be seen in  FIG. 2 , lower core layer  14  has a first thickness t 1 , and upper core layer has a second thickness t 2 , which is less than thickness t 1 . The sum of t 1  and t 2  is core thickness T, which in various embodiments, remains constant in order to maintain the overall thickness of panel  10 . That is, increasing or decreasing t 2  to suit a particular application requires a corresponding adjustment in t 1  to maintain thickness T. Depending on the application, t 2  can range from about 0.05 in (1.27 mm) to about 0.25 in (6.35 mm). Upper core layer  16  is formed from a material (honeycomb or foam) having relatively low thermal conductivity, which can impair the heating of surface  26  of panel  10  compared to heated floor panels with no upper core layer. Therefore, reducing thickness t 2  can increase the thermal transfer properties of upper core layer  16  while still providing the desired structural support and impact resistance for panel  10 . Upper core layer  16  also offers increased protection to the heating element of heating layer  12 . Upper core layer  16  protects heating layer  12  from mechanical damage during installation, maintenance and use and from fluid intrusion during operation (spills, etc.) and maintenance (e.g., cleaning). Moving heating layer  12  away from surface  26  also allows for localized repair of upper core layer  16  (or other upper layers) without damage to heating layer  12 . 
     In an exemplary embodiment, t 2  can be 50% to 60% of t 1 . In such an embodiment, heating layer  12  can sufficiently heat upper cover sheet  24  and environment E, while requiring less power than if t 2  and t 1  were equal. The layers above heating layer  12  (on the side of environment E), meanwhile, experience similar impact stresses to equal-thickness models, while the stresses on heating layer  12  are slightly greater. In other embodiments, t 2  can be anywhere from 20% to 80% of t 1 . The variation in t 2  can be based on, for example, heating requirements and power of the heating element, or the impact/puncture risk from environment E. 
     Lower structural layer  18  and upper structural layer  20  surround heating layer  16  to secure it at the center of panel  10 . Structural layers  18  and  20  can be formed from a material that is not electrically conductive. For example, layers  18  and  20  can be a pre-impregnated material, such as a carbon fiber or fiberglass with a resin system such as epoxy, polyurethane, phenolic, cyanate ester, bismaleimide, or other appropriate resins. The resin system in structural layers  18  and  20  can additionally contain short or chopped fibers. Each of structural layers  18  and  20  can include a single ply, or a plurality of plies, depending on, for example, the material chosen to form the structural layers, or the robustness of heating layer  16 . 
     Cover sheets  22  and  24  are the outermost layers of panel  10 . Lower cover sheet  22  provides structural support to panel  10  on a side of substrate S. Upper cover sheet  24  provides structural support to panel  10  on the side of panel  10  exposed to environment E, which can be, for example, an aircraft cabin, cockpit, or other compartment. Cover sheets  22  and  24  are relatively stiffer than core layers  14  and  16  (having a higher value of elastic modulus E). Cover sheets  22  and  24  can be formed from a composite material, such as a pre-impregnated carbon fiber, fiberglass, Kevlar®, or combinations thereof. Other suitable reinforced polymer matrix materials are contemplated herein. In some embodiments, cover sheets  22  and  24  can be formed from the same material, while in other embodiments, cover sheets  22  and  24  can be formed from different materials. 
     Under normal loading, cover sheets  22  and  24  function similar to the flanges of an I-beam, whereby lower cover sheet  22  is in tension and upper cover sheet  24  is in compression. Core layers  14  and  16  function similar to the web of an I-beam by evenly spacing cover sheets  22  and  24 . That is, the arrangement of core layers  14  and  16  between cover sheets  22  and  24  gives panel  10  a bending stiffness. Bending stiffness can be increased, for example, by spacing cover sheets  22  and  24  father apart. Therefore, the bending stiffness of panel  10  is generally proportional to core thickness T. 
     The disclosed sandwich panel having an asymmetric core structure maximizes both the heating of the floor panel surface, the structural integrity of the panel, and the protection of the embedded heating element. The sandwich panel can optionally include adhesives between any of the disclosed layers, or within the layers themselves, in order to solidify the panel structure. Other means of attachment are contemplated herein. In addition to aerospace applications, the disclosed sandwich panel can be used in maritime, railroad, and automotive applications, as well as the construction industry. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A composite sandwich structure for heating an environment includes a first core layer made from a first cellular material and having a first thickness, and a second core layer made from a second cellular material and having a second thickness less than the first thickness. The structure further includes a heating layer having a thermoelectric heating element and disposed between the first and second core layers. The first core layer is positioned between the heating layer and the environment. 
     The structure of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The above structure can further include a first structural layer disposed between the heating layer and the first core layer, and a second structural layer disposed between the heating layer and the second core layer. 
     In any of the above structures, the first and second structural layers can be formed from a composite fabric pre-impregnated with a resin. 
     In any of the above structures, the resin can include an epoxy, a polyurethane, a phenolic resin, a cyanate ester, a bismaleimide, or combinations thereof. 
     Any of the above structures can further include a first cover sheet abutting the first core layer on a side opposite the first structural layer, and a second cover sheet abutting the second core layer on a side opposite the second structural layer. 
     In any of the above structures, a bending stiffness of the structure can be proportional to a sum of the first thickness and the second thickness. 
     In any of the above structures, the first and second cover sheets can be formed from a reinforced polymer material. 
     In any of the above structures, the first cover sheet can abut the environment. 
     In any of the above structures, the first and second cover sheets can have higher elastic moduli than the first and second core structures. 
     In any of the above structures, the first and second core layers can form an asymmetric core structure having a core thickness. 
     In any of the above structures, the second thickness can range from 20% to 80% of the first thickness. 
     In any of the above structures, the second thickness can range from 50% to 60% of the first thickness. 
     In any of the above structures, the second thickness can range from 0.05 in (1.27 mm) to 0.25 in (6.35 mm). 
     In any of the above structures, the first or second cellular material can be a honeycomb material. 
     In any of the above structures, the first or second cellular material can be a foam material. 
     In any of the above structures, the first and second cellular material can be the same. 
     In any of the above structures, the heating element can include a metallic material, PTC ceramic, PTC polymer, or carbon allotrope material. 
     In any of the above structures, the structure can be an aircraft floor panel. 
     In any of the above structures, the environment can be an aircraft compartment. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.