Formable heavy density honeycomb

Heavy density honeycomb structures which include alternating layers of primary corrugated sheets and bisector sheets which are bonded together. The primary corrugated sheets are offset so that the bisector sheets are bonded to the corrugated sheet nodes so that the upper and lower bonding locations on each bisector sheet are displaced from each other. This displacement provides flexibility regions in the bisector sheets which enhance the formability of the heavy density honeycomb. The displaced node configuration is useful for enhancing thermal formability of both metallic and non-metallic honeycomb structures. The offset configuration is used with both substantially flat bisector sheets and corrugated bisector sheets.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to heavy density non-metallic
 honeycomb structures. More particularly, the present invention involves
 increasing the formability of such honeycomb structures so they can be
 made into a wide variety of non-planar shapes.
 2. Description of Related Art
 Honeycomb structures which include bisector sheets are generally referred
 to as "high density honeycomb". These types of reinforced honeycombs are
 usually composed of a stack of alternating corrugated and bisector sheets
 which are glued or otherwise bonded together. A portion of a typical high
 density honeycomb is shown at 10 in FIG. 5. The honeycomb 10 includes
 bisector sheets 12 and corrugated sheets 14 which are bonded together at
 node junctures 16. As can be seen from FIG. 5, the bisector sheets 12
 split the hexagonal honeycomb cells down the center. This configuration
 adds density, strength and bonding surface to the core. The high density
 honeycombs are well-suited for use in situations where high structural
 strength is required. However, the inherent stiffness of high density
 honeycomb and the presence of the bisector sheets makes it difficult to
 form such structures into non-planar shapes without damaging the
 honeycomb.
 As shown in FIG. 5, honeycombs are three dimensional structures which are
 characterized as having a thickness (T direction) which is measured
 parallel to the honeycomb cell and provides a measure of the honeycomb
 depth. The width (W direction) of the honeycomb is measured perpendicular
 to the T direction and provides a measure of the height of the stacked
 honeycomb cells. The length (L direction) of the honeycomb is measured
 perpendicular to both the T and W directions and provides a measure of the
 length of the corrugated and bisector sheets present in the honeycomb (see
 FIG. 5).
 When forming non-planar high density honeycomb structures, planar
 honeycombs of the type shown in FIG. 5 are formed in the L and/or W
 directions by applying heat and/or pressure to the honeycomb. When forming
 in the L direction, the outside radius of the core must expand in the L
 direction and inside radius of the core must contract in the L direction.
 The bisector sheet passing through the cell will not allow the cell to
 expand on the top of the radius. As a result, the inside of the cell must
 condense more. This causes the inside cell to deform or crush to such a
 degree that the resulting core may have reduced strength and/or the
 corrugated and bisector sheets may separate at the node junctures.
 When forming non-planar high density honeycombs in the W direction, the
 outside radius of the cell must expand in the W direction and the inside
 radius of the core must contract in the W direction. The bisector sheets
 limit the movement of the cell walls so that the usual result is that the
 relatively stiff node junctures are torn apart.
 Various approaches have been taken to try and increase the formability of
 high density honeycombs. For example, attempts have been made to increase
 node strength by using higher strength adhesives. Various thermosetting
 resins have been used in the resin matrix of composite honeycomb walls to
 enhance heat formability and various thermosetting dip resins have been
 used to coat honeycomb walls. The use of hybrid weaves for composite
 honeycomb walls has also been proposed. Although all of these approaches
 have achieved some improvement in formability of high density honeycomb,
 there still is a continuing need to increase and enhance the formability
 of such honeycomb structures.
 SUMMARY OF THE INVENTION
 In accordance with the present invention, it was discovered that the
 formability of high density honeycomb can be enhanced and increased by
 orienting the primary corrugated sheets and bisector sheets in a specific
 fashion which increases honeycomb flexibility without unduly affecting the
 overall strength of the high density honeycomb. This increase in
 flexibility is achieved by offsetting the honeycomb nodes so that the
 stiff node structures are separated and redistributed throughout the
 honeycomb to provide for increased formability. The offsetting of the
 honeycomb nodes allows for more deformation of the inside cells without
 failure when forming in the L direction. In the W direction, the offset
 node configuration allows the outside of the cell to expand and the inside
 to condense more without undue crushing or failure of the structure.
 The honeycomb structures of the present invention include a plurality of
 primary corrugated sheets with each primary corrugated sheet having a
 plurality of alternating upper nodes and lower nodes. Each upper node
 includes a top surface and a bottom surface, and each lower node also
 includes a top surface and a bottom surface. A plurality of bisector
 sheets which each includes a top surface and a bottom surface are combined
 with the corrugated sheets to form the high density honeycomb structure
 which includes alternating layers of primary corrugated sheets and
 bisector sheets. The top surfaces of the upper nodes are bonded to the
 bottom surface of the bisector sheets at upper node bond locations on the
 bisector sheets. The bottom surfaces of the lower nodes are bonded to the
 top surfaces of the bisector sheets at lower node bond locations on the
 bisector sheets. As a feature of the present invention, the upper node
 bond locations and lower node bond locations on each bisector sheet are
 displaced from each other. This provides an offset node configuration
 which, as mentioned above, separates the node junctures and redistributes
 the density of the cells more evenly to allow for increased formability of
 the overall honeycomb structure.
 The offset node design provided by the present invention is well suited for
 use in a wide variety of metallic and non-metallic, high density honeycomb
 structures where it is desired to form non-planar structures from the
 initially prepared planar honeycomb. The invention does not depend upon
 the use of specialized high strength adhesives or specialized thermal set
 resins or specialized weave patterns. Instead, the invention involves a
 basic reorientation of the honeycomb layers to provide increased and
 enhanced flexibility regardless of the specific materials being used for
 the primary corrugated sheets and bisector sheets.
 The above described and many other features and attendant advantages of the
 present invention will become better understood by reference to the
 following detailed description when taken in conjunction with the
 accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 A first preferred exemplary heat formable heavy density honeycomb in
 accordance with the present invention is shown generally at 20 in FIG. 1.
 The honeycomb 20 is made up of a plurality of primary corrugated sheets 22
 and bisector sheets 24. The structure 20 shows only a portion of a
 honeycomb structure which includes four primary corrugated sheets and four
 bisector sheets. As is well known in the art, honeycomb structures may
 include hundreds of primary corrugated sheets and bisector sheets. For
 exemplary purposes, only a small portion of an actual honeycomb structure
 is shown.
 A more detailed view of a portion of a single primary corrugated sheet 22
 and bisector sheet 24 is shown in FIG. 3. Two sheets are shown prior to
 bonding together. Referring to FIGS. 1 and 3, each of the primary
 corrugated sheets includes a plurality of alternating upper nodes 26 and
 lower nodes 28. Each upper node 26 includes a top surface 30 and bottom
 surface 32 (see FIG. 3). Likewise, each lower node 28 includes a top
 surface 34 and a bottom surface 36. The bisector sheets 24 each have a top
 surface 38 and a bottom surface 40.
 The primary corrugated sheets 22 and bisector sheets 24 are stacked to form
 the honeycomb structure 20 which includes alternating layers of the two
 sheets. As represented in FIG. 3, by arrow 42, the nodes 26 and 28 of the
 corrugated sheets are bonded to the bisector sheets so that the top
 surfaces 30 of upper node 26 are bonded to the bottom surface 40 of the
 bisector sheets 24. This produces a series of upper node bond locations on
 the bisector sheets as shown at 44 in FIG. 1. The bottom surfaces 36 of
 lower nodes 34 are bonded to the top surface 38 of the bisector sheets 24
 at lower node bond locations 46 on the bisector sheets. In accordance with
 the present invention, the upper node bond locations 44 and lower node
 bond locations 46 on each individual bisector sheet are displaced from
 each other as shown in FIG. 1. This is a substantial departure from prior
 high density honeycomb structures as shown in FIG. 5 where the upper node
 bond locations and lower node bond locations coincide for each bisector
 sheet. This displacement of the upper node and lower node bond locations
 on each bisector sheet allows the sheet to flex in a way which is not
 possible when the upper node bond locations and lower node bond locations
 coincide. This displacement of the upper node and lower node bond
 locations on the bisector sheets allows the planar honeycomb structures 20
 shown in FIG. 1 to be formed into a wide variety of non-planar shapes.
 The exemplary embodiment shown in FIG. 1 depicts the upper node bond
 locations 44 and lower node bond locations 46 being displaced apart from
 each other on each bisector sheet 24 in a uniform manner. The present
 invention also contemplates displacing the upper node and lower node bond
 locations 44 and 46 in a non-uniform manner when specialized formability
 properties are desired. For example, the upper and lower node bond
 locations are shown in FIG. 1 as being centered over each other with the
 corrugated sheets being uniformly displaced. It is possible in accordance
 with the present invention to shift one or more of the corrugated sheets
 so that the upper and lower node bond locations do not follow the uniform
 pattern shown in FIG. 1. The only requirement is that the upper node bond
 locations 44 and lower node bond locations 46 for a given bisector sheet
 do not coincide. Instead, they are displaced from each other a sufficient
 amount so that the bisector sheet may flex in the areas located between
 the node bonds. A few of these bisector sheet flex regions are shown at 48
 in FIG. 1. The bisector flex regions 48 shown in FIG. 1 are all the same
 size. As mentioned above, the corrugated sheets 22 may be shifted during
 the bonding process to achieve a wide variety of different flex region
 sizes within a given honeycomb structure. However, it is preferred that
 the flex regions 48 be uniform in size throughout the honeycomb structure
 so that the various node bond locations line up and remain co-planar
 within the honeycomb structure.
 A second preferred exemplary heavy density honeycomb structure in
 accordance with the present invention is shown generally at 50 in FIG. 2.
 As was the case with the first embodiment, only a portion of an overall
 honeycomb structure is shown for exemplary purposes. The honeycomb
 structure 50 is basically the same as the first exemplary honeycomb
 structure 20, except that the bisector sheets 52 are not substantially
 flat as are bisector sheets 24 in the first embodiment. Instead, bisector
 sheets 52 are corrugated. As shown in FIGS. 2 and 4, each bisector sheet
 52 includes alternating upper bisector nodes 54 and lower bisector nodes
 56. Each upper bisector node 54 includes a top surface 58 and bottom
 surface 60. The lower bisector nodes 56 include top surfaces 62 and bottom
 surfaces 64. As shown in FIGS. 2 and 4, the bottom surface 60 of each
 upper bisector node 54 is bonded to the top surface 130 of upper node 126
 of the primary corrugated sheet 122. The top surface 62 of each lower
 bisector node 56 is bonded to the bottom surface 136 of each lower node
 128 of the primary corrugated sheet 122. This particular honeycomb 50
 differs from honeycomb 20 in that the flexible regions of the bisector
 sheets 148 shown in FIG. 2 are at an angle relative to the honeycomb
 nodes. This particular configuration is well suited for situations where
 high degrees of formability are required.
 A third exemplary honeycomb structure in accordance with the present
 invention is shown generally at 300 in FIG. 6. The honeycomb structure 300
 is similar to the honeycomb structure 50 in that it includes alternating
 primary corrugated sheets 310 and corrugated bisector sheets 320. The
 corrugated sheets 320 have angled corrugated portions 322 which are
 oriented at a steeper angle than the corrugations 312 of the corresponding
 primary corrugated sheet 310. Specifically, the corrugated angle portions
 322 are at an angle of about 70.degree. relative to the flat portion of
 the bisector sheet, whereas the corrugated portions of the primary sheet
 are at an angle of about 77.degree. relative to the flat portion of the
 primary corrugated sheet. This is to be contrasted with the honeycomb
 structure in FIG. 2 wherein the corrugated portion 148 of the bisector
 sheet is at an angle relative to the flat portion of the bisector sheet
 which is greater than the angle of the corrugated portion of the primary
 sheet.
 Another difference between the honeycomb structure 300 and honeycomb
 structure 50 is that the bisector nodes 330 are wider than the underlying
 upper surface of the primary sheets 310. As a result, the primary sheets
 310 are bonded to the corrugated sheet nodes 30 only at the center portion
 of the nodes 330 as shown at 332. This leaves further flex portions 324 on
 either side of the node bond. In the honeycomb structure 50, the size of
 the bisector sheet nodes and primary corrugated sheets are selected so
 that they bond together across the entire bisector sheet node. Honeycomb
 structures of the type shown in FIG. 6 are preferred where additional
 flexibility and formability is desired. The size of the bisector sheet
 node can be increased or alternatively the size of the corrugated sheet
 bonding surface decreased in order to provide a wide range of adhesive
 node fingerprints. The fingerprints can range from complete bonding of the
 entire surface area of the bisector node to the primary sheets as shown in
 FIG. 2. Alternatively, the fingerprint can be a partial bonding of the
 bisector sheet to the primary sheet as shown in FIG. 6. The principal
 limitation on the node bond fingerprint is that a sufficient area of the
 bisector sheet and primary corrugated sheet must be bonded to achieve
 desired honeycomb strength.
 The honeycomb structures 20 and 50 shown in FIGS. 1 and 2 are made in
 accordance with conventional processes for making high density honeycombs
 of the type shown in FIG. 5. In general, an adhesive is applied to the
 primary corrugated sheet along the top of the upper nodes and along the
 bottom of the lower nodes. Bisector sheets are then placed on the top and
 bottom of the corrugated sheet. Adhesive is then applied to another
 corrugated sheet in the same manner as the first sheet and this additional
 corrugated sheet is then placed on top of the previously placed top
 bisector sheet. This process is repeated until the honeycomb stack has
 reached the desired height.
 The materials which can be used for the primary corrugated sheets, bisector
 sheets (both flat and corrugated) and adhesives may be any of those which
 are used to form high density honeycomb structures of the type shown in
 FIG. 5. Although the present invention may be used in connection with
 metallic honeycomb structures, its preferred use is in connection with
 non-metallic structures which are intended for heat forming into
 non-planar structures. Exemplary materials which may be used as the
 primary corrugated sheets include plastics and composite materials which
 include a wide variety of fiber configurations which are combined with a
 resin matrix. Exemplary fibers which may be used in the composite
 materials include glass, carbon, boron and ceramic fibers. Preferred
 resins for use as the resin matrix include those which are amenable to
 heat forming. Such resins include high temperature polyimides, phenolics
 and epoxies. Any of the glass reinforced honeycomb materials, aramid-fiber
 reinforced honeycomb materials and resin-dipped paper honeycomb materials
 may be used in accordance with the present invention.
 A wide variety of adhesive materials may also be used. Exemplary adhesives
 include nitrile phenolic adhesives, epoxy adhesives, polyamid adhesives,
 urethane adhesives, polyimide adhesives and other high temperature
 adhesives.
 The honeycomb structures shown in FIG. 1 and FIG. 2 are planar in shape. In
 accordance with the present invention, these structures may be formed into
 a variety of non-planar structures. The preferred forming procedure
 involves application of heat to the honeycomb structure to raise the
 temperature of the honeycomb to a sufficient level to allow thermal
 forming. For such thermal forming processes, the particular resin used in
 the fiber reinforced composite is selected to be sufficiently
 thermoplastic to allow thermal forming. For example, high temperature
 polyimides, phenolics and epoxies are suitable resins which may be thermal
 formed in accordance with the present invention when used in combination
 with various substrates, such as glass or carbon fibers. In general, the
 high density honeycomb is heated to temperatures on the order of between
 about 200.degree. C. to 350.degree. C. and then formed to the desired
 final structural shape by using molds or other conventional thermal
 forming equipment. This procedure is well suited for forming honeycomb
 structures which require both strength and small radiuses in complex
 shapes.
 Exemplary honeycomb material combinations are set forth in the following
 Table.

Resin Node
 Honeycomb Type Fiber Matrix Adhesive
 High Temp (&gt;350.degree. C.) Carbon Polyimide Polyimide
 High Modulus
 High Temp (&gt;350.degree. C.) Glass Polyimide Polyimide
 Low Modulus
 Low Temp (&lt;350.degree. C.) Carbon Polyimide or Polyimide or
 High Modulus Phenolic Phenolic
 Low Temp (&lt;350.degree. C.) Glass Phenolic Polyimide or
 Low Modulus Phenolic
 Having thus described exemplary embodiments of the present invention, it
 should be noted by those skilled in the art that the within disclosures
 are exemplary only and that various other alternatives, adaptations, and
 modifications may be made within the scope of the present invention.
 Accordingly, the present invention is not limited to the specific
 embodiments as illustrated herein, but is only limited by the following
 claims.