In the aerospace industry, honeycomb core is commonly employed in the fabrication of various panels and assemblies to provide strength and low weight. A honeycomb core panel comprises honeycomb core bonded between face sheets that close the transverse cells defining the core.
Engine nacelle panels made of honeycomb core can be modified to serve as Helmholtz resonators for attenuating engine noise. One prior art method of making such resonators involves perforating one or both of the face sheets such that the cells of the honeycomb core communicate with the atmosphere via small holes. The cell and hole sizes as well as the thickness of the face sheet are selected such that the panel is tuned to resonate at a pre-selected engine frequency. The cells of the honeycomb core act as multiple resonant chambers wherein the sound waves are broken down into waves of different, and usually higher, frequencies and wherein some of the acoustic energy is transformed into heat, which dissipates into the atmosphere. U.S. Pat. Nos. 3,948,346 and 3,910,374 illustrate examples of such acoustic liners.
Another design for a honeycomb resonator involves a structure comprising a double layer of honeycomb core with a septum bonded between the two layers. Facing material is then bonded with an adhesive on both sides of the double layer to form a panel. This type of construction requires four adhesive bond lines, which degrades the strength of the panel. Also, it can be very difficult to align the individual cells of the adjacent layers. Further, heat transfer is less efficient and varies due to the septum adhesive bond lines and the misalignment of the individual cells.
A typical embodiment of such prior art panels with a honeycomb core septum structure comprises an aluminum or composite outer skin, a first honeycomb core layer adhesively bonded to the skin, a perforated aluminum or composite septum adhesively bonded to the first core layer, a second layer of honeycomb core adhesively bonded to the septum, and a perforated face sheet adhesively bonded to the second layer of core. In such structures, alignment of the cells of the two core layers is very difficult. Also, the presence of the adhesive creates certain difficulties. Such acoustic panels can be used only at relatively low temperatures, the upper temperature limit being determined at least in part by the temperature resistance of the adhesives used in the structure. If the honeycomb core also is constructed using adhesively bonded metal strips, that adhesive also will have an upper temperature limit. Further, the adhesives will become brittle after repeated thermal cycling, and will crack and deteriorate due to the mechanical vibrations experienced by the nacelle panels during normal engine operation.
In the fabrication of a core structure by prior art methods where a septum is provided having various depths within the individual cells, complex machining and assembly techniques are required. This is expensive and time-consuming.
A prior art acoustical core has been reported wherein a single layer of honeycomb core is provided with an intermittent zone in the core which is crushed to form a sound barrier therein. Also, there has been reported a single layer honeycomb core structure wherein the individual cells are filled with foam plugs to a single cell depth. This structure does not provide for varying depth cells. Yet another sound absorption honeycomb core structure provides a cellular array cut obliquely so that the effective lengths of the cells vary along the plane of the cut.
U.S. Pat. No. 4,257,998 and U.S. Pat. No. 4,265,955 disclose a “lost wax” method of making a honeycomb core with a septum. A wax mold is provided with a desired top surface configuration, and a curable material such as a resin is spread over the top surface of the mold. The honeycomb core is then pressed through the resin and wax mold to a desired depth. The entire assembly is heated to cure the resin and set it in place. The assembly is then heated further to allow the wax to melt away. The resulting structure is a core with a resin septum. The resin septum can be perforated or non-perforated. If perforations are desired, the resin septum must be perforated after insertion into the core. Often, each piece of resin septum in each cell must be touched up around its edges to adhere to the cell walls. Also, the resin septum provides no strength to the overall structure and can carry no mechanical load.
U.S. Pat. No. 4,475,624 and U.S. Pat. No. 4,594,120 disclose a method of forming an acoustical core with a resin septum wherein the septum is first sandwiched between two layers of support material having a melting temperature in between the fusing and curing temperatures of the septum material. The honeycomb core is pressed into the supporting materials and septum to position the septum at the desired depth in the honeycomb cells. The resultant assembly is heated sufficiently to allow the septum to fuse and adhere to the honeycomb core. The resultant assembly is then heated sufficiently to melt the support material and allow it to be drained away, leaving the segmented septum in place within the core. Like the lost wax system, this system has the disadvantage that, after the support material has melted away, the pieces of resin septum in each honeycomb cell must be individually touched up at their edges where the cured adhesive or resin has pulled away from the cell walls. Also, this resin septum provides no strength to the overall structure and can carry no mechanical load.
U.S. Pat. No. 4,336,292 discloses a multi-layer honeycomb thermo-barrier material comprising at least four metal honeycomb cellular cores, at least three metal septum sheets disposed between the honeycomb cores, and first and second face sheets disposed on either side of the thermo-barrier material. The thermo-barrier material is made entirely of titanium alloys, TD nickel, niobium, or other super alloys; strips of such materials can be resistance welded together to form a honeycomb core structure. The resistance welded cores are bonded to the septum by brazing or by a technique described in that patent as liquid interface diffusion bonding. The resulting structure is rigid.
It is one object of the invention to provide a honeycomb acoustic barrier unit having a deformable septum metallurgically secured to the honeycomb.
It is another object of the invention to provide a honeycomb acoustic barrier unit that can be constructed using no adhesives.
It is still another object of the invention to provide a honeycomb acoustic barrier unit that is simple to manufacture and assemble.
It is yet another object of the invention to provide a honeycomb acoustic barrier unit having a septum that is capable of carrying a mechanical load.