Composite structure having reinforced core and method of making same

A polymer-based composite sandwich includes a reinforced core bonded between a pair of composite facesheets. The core includes a truss formed by groups of composite pins held in place by a layer of structural foam. The pins in the groups are radially arranged around nodes. The ends of the pins are splayed and sandwiched between the foam layer and the facesheets.

TECHNICAL FIELD

This disclosure generally relates to composite structures, and deals more particularly with a composite sandwich having a reinforced core, and a method of making the composite sandwich.

BACKGROUND

Composite sandwich constructions may be strengthened by placing structural reinforcement inside a core that is bonded between two facesheets. The core reinforcement may include structural elements that define load paths for transferring compressive, tensile and shear loads between the facesheets. The performance of the composite sandwich is dependent in part upon the type of core reinforcement and the quality of the bonds between the core and the facesheets. Common materials used in the core may include rigid plastic foam and honeycomb. While honeycomb cores exhibit good structural efficiency, they may subjected to higher core-to-facesheet loading in some applications, such as long duration space flights where a differential pressure may develop between the core and the surrounding environment.

Unreinforced closed cell rigid foam cores may exhibit reduced structural efficiency when subjected to moisture and to higher temperatures, or extreme low temperatures in space.

The problems associated with the sandwich constructions discussed above have been partially solved by the introduction of so-called X-COR structural cores which comprise a light-weight, closed cell polymethacrylimide (PMI) foam reinforced with small diameter, pultruded carbon fiber/epoxy pins arranged in a tetragonal truss network. The X-COR pins extend beyond the foam core and are embedded in the facesheets. A variation of X-COR is disclosed in U.S. Pat. No. 6,291,049 issued Sep. 18, 2001, in which the ends of the pins are bent so as to lie flat against facesheets to which the core is bonded.

The truss networks mentioned above that employ carbon fiber/epoxy pins may not provide adequate performance in some aerospace applications. Accordingly, there is a need for a composite structure having a reinforced core that is suitable for demanding aerospace, automotive and marine applications in which superior bond strength between the facesheets and core is required. Embodiments of the disclosure are intended to satisfy this need.

SUMMARY

Embodiments of the disclosure provide a composite sandwich construction in which improved facesheet-to-core bond strength is achieved while assuring that the structural integrity of the core is maintained. The construction and material selection used in the disclosed sandwich construction renders it suitable for high performance applications in the aerospace, automotive and marine industries. For example, and without limitation, the disclosed composite sandwich may be used in long duration spacecraft missions in which differential pressures may arise between the core and the surrounding environment. The improved bond strength provided by the disclosed embodiments may be maintained over a wide range of temperature and moisture conditions.

According to one disclosed embodiment, a composite sandwich comprises a reinforced core sandwiched between first and second composite facesheets. The reinforced core comprises a plurality of pins arranged in groups forming a truss, and a carrier surrounding the pins. Each of the pins includes medial portions extending between the first and second facesheets and distal portions respectively extending generally parallel with and bonded to the first and second facesheets. The pins in each of the groups may be spaced from each other and distributed around a node wherein the distal portions of the pins in each of the groups radiate outwardly from the node. The nodes of the pin groups may be spaced substantially equidistant from each other. The distal portions of each of the pins are splayed and sandwiched between the core and one of the facesheets. The pins may comprise carbon fiber reinforced resin and each of the facesheets may include laminated plies of carbon fiber reinforced resin. The carrier may comprise rigid foam such as a closed cell foam.

According to another disclosed embodiment, a reinforced composite sandwich core is provided that is formable into a curved shape. The core includes a plurality of pins arranged in groups forming a truss and a flexible carrier for supporting the pins. The pins in each of the groups are spaced apart from and displaceable relative to each other within the carrier upon forming of the carrier into the curve shape. The pins in each group are radially distributed around a node. Distal portions of each of the pins are splayed and lay substantially flush along a face of the foam layer.

According to a further embodiment, a method is provided of fabricating a reinforced composite sandwich. The method includes fabricating a core, forming the core into a curved shape, applying a pair of facesheets respectively to opposite faces of the core and curing the core and the facesheets. Fabricating the core may include arranging structural pins into groups forming a truss, and supporting the pins in the groups by surrounding the pins with a layer of uncured structural foam. Arranging the pins may include inserting the pins into the layer of foam at differing angles and fabrication of the core may include bending and flaying the ends of the pins onto the faces of the core. Fabricating the core may further include splaying the ends of the pins and sandwiching the splayed ends of the pins between the faces of the foam layer and the facesheets.

According to still another embodiment, a method is provided of fabricating a reinforced composite sandwich core. The method comprises producing a layer of structural foam and inserting reinforcement pins into the foam layer, including arranging the pins into groups forming a truss within the foam layer.

Other features, benefits and advantages of the disclosed embodiments will become apparent from the following description of embodiments, when viewed in accordance with the attached drawings and appended claims.

DETAILED DESCRIPTION

Referring first toFIGS. 1-9, a composite sandwich construction generally indicated by the numeral30broadly comprises a reinforced core32sandwiched between and bonded to a pair of outer facesheets34,36. Each of the facesheets34,36may comprise multiple plies38of fiber reinforced polymer resin, such as graphite fibers in cloth or other form, held in an epoxy binder. The embodiment of the composite sandwich construction30shown inFIG. 1is substantially planar or flat, however, as will be discussed below, in other embodiments, the sandwich construction30may have one or more curvatures and may be formed into a variety of non-planar shapes.

The core32may broadly comprise a reinforcing truss33held in a matrix or carrier which may comprise a light weight, low density layer of foam46. The foam layer46may comprise, without limitation, a polymethacrylimide (PMI) rigid closed cell foam known by the trade name ROHACELL®. ROHACELL® is commercially available in differing densities and thicknesses, and has a relatively low coefficient of linear thermal expansion. The foam layer46functions to hold the truss33in place during fabrication of the core32and also may add some degree of structural strength to the core32. In some embodiments, the foam layer46may comprise a fugitive foam that is removed as by subjecting the finished structure30to elevated temperatures in an oven in order to incinerate the foam, leaving the truss33intact.

The reinforcing truss33may comprise an array of structural pins40which are arranged in groups42that may be regularly spaced from each other, as best seen inFIG. 3, using pre-selected pitches “x” and “y”. In one embodiment, the “x” and “y” pitches are equal, resulting in a square pitch that aligns the groups42along diagonal axes44.

In one embodiment illustrated inFIGS. 5-8, the pins40are symmetrically arranged or distributed around a central axis50in each group42, and are substantially circumferentially spaced equally from each other. In other embodiments however, depending on the application, the arrangement of the pins40around the central axis50may not be symmetrical, and/or the circumferential spacing of the pins40may not be equal. Each of the pins40includes medial portions40athat are inclined relative to the planes of the facesheets34,36, and distal portions40b,40cwhich extend substantially parallel to the facesheets34,36. The medial portions40aof the pins40are inclined from vertical at an angle φ (FIG. 4) which, in one embodiment may be approximately 30 degrees; other angles are possible.

As best seen inFIG. 6, when viewed in plan, the pins40in each group42are arranged in a crossing pattern such that they overlap each other and radiate outwardly from a node52that is aligned with the central axis50. While four pins40per group42are illustrated in the embodiment shown inFIGS. 5-8, more or less than four pins40may be employed. Although the pins40shown inFIGS. 5-8overlap each other when viewed in plan, they do not touch each other but rather are spaced from each other as indicated by the gap or space53between the pins40at the node52shown inFIG. 6. In some embodiments of the sandwich construction30that are substantially flat, such as that shown inFIG. 1, the pins40in each group42may touch each other, however, in other embodiments where the sandwich construction30is intended to have one or more curvatures therein, the provision of the gap53between the pins40allows the pins40in each group42to move relative to each other, without interfering with each other, during fabrication process, as will be discussed in more detail below.

FIG. 6Aillustrates, on a larger scale, the distal ends40cof the pins40having been flayed and flattened against the top surface46aof the foam layer46following a processing step that will be described below.FIG. 6Aalso better illustrates the spacing53between the pins40and their circumferential arrangement around the node52.

As best seen inFIG. 4, in one embodiment, the distal portions40b,40cextend parallel and are bonded to the inside face of the facesheets34,36respectively. As shown inFIG. 10, the length “L” of the distal portion40b,40cwill depend upon the particular application, however in one embodiment the length “L” may be approximately 4 to 6 times the diameter of the pin40. As will be discussed later in more detail, the length “L” may be determined by the process used to fabricate the core32

In one embodiment, the pins40may be formed of pultruded graphite held in an epoxy binder. When the facesheets34,36are bonded to the core32, the resin binder in the distal ends40b,40cof the pins40fuses (i.e. co-mingles) with resin binder48that migrates from an adjacent facesheet ply34a(FIG. 10), so that the flayed distal ends40b,40cof the pins40become bonded to and form a part of the facesheets34,36. Desirably, the resin binders used in the pins40and the facesheet ply34aare the same, or are at least compatible, so that when cured, the co-mingled resin binders form a solidified matrix that exhibits maximum strength. Alternatively, the distal ends40b,40c(see for example,40cinFIG. 11) may be bonded between adjacent plies34a,34bof the facesheets34,36, thereby locking the ends of the pins40within the facesheets34,36. A dry film adhesive is placed between core32and facesheets34and36to improve bonding of distal ends40b,40cwith the facesheets34,36. The epoxy binders in pins40, the dry film adhesive and the facesheets34,36should be chosen for their compatibility so that they fuse during the cure process at the same cure temperature. The amount and type of the dry film may significantly affect the strength of the finished structure.

As will be discussed below, the selection of the values for certain parameters characterizing the core32including the truss33, provide a particularly durable and reliable sandwich construction30that may be readily scaled to meet the requirements of various applications. The parameters of particular interest in constructing the sandwich structure30include: the type of carrier foam46, the diameter of the pins40, the orientation angle φ of the pins40(from vertical), the spacing of the pins from each other, the reveal height (“L”) of the pins40, the number of pins in each pin group42, and the particular type of material used to fabricate the pins40.

FIG. 12illustrates the superior structural properties of two embodiments relative to a sandwich construction employing a un-reinforced core. Curves60and64represent the shear strength as a function of temperature for a sandwich construction30employing a reinforced core according to the disclosed embodiments using foam densities of 12 and 6.9 pounds per cubic foot, respectively. In contrast, the curves represented by62and66show the shear strength for a ½ inch core using un-reinforced ROHACELL® foam of 12 and 6.9 pounds per cubic foot, respectively. As is apparent from the test results shown inFIG. 12, embodiments of the disclosure employing the reinforced core32exhibit superior shear strength compared to unreinforced cores of the same density.

Referring toFIGS. 13 and 14, a series of tests were performed that were used to identify the parameters of the sandwich structure30that could be used to provide substantially improved structural properties for the sandwich structure30while assuring adequate bond strength and avoiding core cracking or other deterioration of the core32. A key for interpreting the test result curves inFIG. 13is shown inFIG. 14. For example, a sandwich construction was fabricated using values for various parameters that provided test results represented by curve “A” inFIG. 13. The particular embodiment represented by curve “A” included a core32having a density of 12.08 pounds per cubic foot, ½″ thick, pins40having a diameter of 0.020 inches inclined at 35 degrees relative to vertical, a reveal height (“L”) of 0.080 inches and a pin density of 8.8. Using the test results shown inFIG. 13, values for a group of parameters have been developed for various applications, as shown inFIG. 15. These parameters include core density68, core thickness70, pin diameter72, pin angle from vertical74, pin spacing (pitch), pin reveal length78, number of pins per node and the type of foam carrier82. The desired foam density ranges between 6.9 and 12 pounds per cubic foot. The core thickness ranges from to 1 inch, while pin diameter is between 0.02 and 0.028 inches. The preferred pin angle is approximately 30 degrees and the square pitch spacing between nodes52ranges from 0.168 to 0.191 inches. The reveal height (“L”) is approximately 0.055 inches. Four pins per node were employed and the carrier foam is a PMI such as a type 51WF ROHACELL®.

Using the values for the parameters shown inFIG. 15, a series of tests on sandwich samples were performed; the results of these are shown inFIGS. 16-23.FIG. 16shows the results of tests performed on various sandwich constructions30having a ½″ core32using a three point bend shear strength test in accordance with ASTM C-393. ASTM C-393 is a standardized test method used to determine the core shear properties of flat sandwich constructions subjected to flexure in a manner such that the applied moments produce curvature of the sandwich facing planes. Graphs86represent the test results for three embodiments of the truss reinforced core32having a density of 6.9 pounds per cubic foot, while graph84represents the test results using an unreinforced core comprising ROHACELL® foam. The test results are provided in terms of the average shear strength in pounds per square inch as a function of temperature.

The samples represented by the test results shown inFIG. 16were also subjected to flat-wise compression strength testing in accordance with ASTM C365, resulting in the test results shown inFIG. 17. The test results inFIG. 17are provided in terms of compression strength in pounds per square inch as a function of temperature.FIGS. 18 and 19show test results similar toFIGS. 16 and 17, but for test samples employing densities of 12 pounds per cubic foot.

FIGS. 20 and 21provide comparative test results for samples having ¾″ thick cores32and densities of 6.9 pounds per cubic feet. Similarly,FIGS. 22 and 23provide test results for samples having ¾″ thick cores and densities of 12 pounds per cubic feet.

As is evident from the test results represented by the graphs shown inFIGS. 16-23, test samples employing values of the parameters within the ranges listed inFIG. 15exhibit substantially superior shear and compressive strengths compared to sandwich constructions with un-reinforced cores.

Referring now concurrently toFIGS. 24-26, a method of fabricating a composite sandwich30begins at step88with laying up facesheets34,36using prepreg which may comprise graphite fabric or other forms of graphite fiber impregnated with a polymer resin such as epoxy. In other embodiments, the facesheets34,36may be fabricated by infusing resin into a preform of dry fabric or tacked fabric. Next, at step90, the facesheets34,36are debaulked. Then, at step92, a dry film adhesive is applied to the facesheets34,36and the lay-up is again debaulked.

Separately, the core32is prepared, by following steps96-112. Beginning at step96, the pin material is developed by pultruding graphite/epoxy, which comprises pulling fine carbon fibers through a die and resin bath. The pin material is partially cured and taken up on a spool at step98. At step100, the graphite/epoxy pins40are inserted into a layer of PMI foam46in a three dimensional lattice pattern. The pin insertion process may be performed using commercial equipment (not shown) that includes, without limitation, an automated tool head operated by a programmed computer. The insertion head inserts the pin material from any desired angle from vertical, and following the insertion, a fixed length is automatically cut and the insertion depth is adjusted so that a desired reveal height “L” is exposed at the top and bottom surfaces of the foam layer46. The pins40are inserted along trajectories that are indexed around the central axis50.FIG. 24shows one of the pins40having just been inserted into the foam layer46, with the distal portion40cextending above the upper surface of the foam layer46corresponding to a reveal height “L”.

Next, at step102, the distal portions40b,40care flayed and bent in a process shown inFIG. 25, wherein a hot press platen47moves downwardly into contact with the distal portions40c, bending the fibers and partially melting the epoxy binder, so as to cause the fibers to splay open and separate into whiskers, generally parallel to the outer surfaces of the foam layer46. Since the pins40comprise multiple fine, whisker-like fibers and pultruded resin, when pressure is applied to the distal portions of the pins40by the hot platen press, the fibers in the distal portions open like a fan instead of bending as a unit. Step108represents completion of the formation of the truss33within the foam layer46.

The foam layer46may be either procured as shown at step104as a purchased component or fabricated, following which the foam layer46is heat treated at step106. Heat treatment of the foam layer46may be optionally required in some cases where the foam may have a tendency to absorb atmospheric moisture. Heat treating of the foam layer46both removes the moisture and may improve the mechanical strength of the foam layer46so that the foam layer46better supports the pins40and provides some degree of structural strength for the core32.

With the truss33having been formed in the foam layer46at step108, the core32is then heat treated at step110in order to cure the truss33. The heat treatment at step110results in a full cure of the partially cured pins40. The preformed core32is then dried at step112. The drying at step112may include a primary drying step followed by a final dry and pre-layup drying cycle. The purpose of this two step drying cycle is to remove any remaining moisture in the preform core32, as well as to assure that the truss33is completely cured. The primary drying step may comprise successively increasing the temperature according to a predefined schedule over time, however the exact schedule will depend upon the application. The final drying step may involve subjecting the core32to a constant temperature for a period of time, for example, 250° F. for a period of 8 to 24 hours, in one embodiment.

At step94, the fully formed and cured core32is deposited on facesheet34, and then layers of dry film adhesive are applied to the remaining, exposed face of the core32. The dry film adhesive may comprise, for example, a 350 degree F. cure epoxy film adhesive commercially known as FM300 film adhesive available from Cytec. Following debaulking at step114, the second facesheet36is applied to the exposed, remaining face of the core32, as shown in step116. Finally, the sandwich structure30is compacted and cured at step118.

Attention is now directed toFIGS. 27-30which illustrate the fabrication of a reinforced core32having one or more curvatures. In the illustrated example, the core32is curved in a single dimension, however depending upon the application, the core32may be formed into a variety of shapes having simple or compound curves and/or undulations, as well as a combination of flat and curved sections. The core32may be fabricated using the methodology previously described (seeFIGS. 24,25and26) in which composite pins40are inserted into a substantially flat layer of structural foam46, wherein the pins are arranged into groups42centered around nodes so as to form a truss-like reinforcement within the layer of foam46. Also, as previously described, the distal portions40b,40c(FIG. 30) are splayed and then folded or bent onto the faces125,127of the foam layer46. The flat core32may then be placed on a forming tool120(FIG. 27) having one or more curved tool surfaces122. The flat core indicated at32ain broken lines is uncured at this point, and is therefore possess a degree of flexibility.

The flat core32ais then bent or formed down over the curved tool surface122, thereby imparting a curvature into the core32, resulting in curved inner and outer core surfaces125,127. The bending or forming of the flat core32amay be carried out using vacuum bagging techniques, a press having an additional tool (not shown) that mates with tool120, or even by hand labor or any other suitable means. Once formed onto the tool120, the resulting core32maintains its curvature.

Prior to bending of the flat core32a, the relative positions of the pins40in each group thereof are substantially those in which the pins were initially inserted, similar to the positions shown inFIGS. 5-8. However, as the flat core32ais being formed over the tool120, some of the pins40may shift or be displaced relative to each other within the foam layer46in order to accommodate the change in curvature of the foam layer46.

Due to the initial placement of the pins40in each group thereof and the fact that there is a slight spacing or gap52(FIG. 6) between the pins40, the pins40are free to move slightly during the curvature forming process, without interfering with each other. For example,FIG. 7illustrates the position of a group of pins40that have been inserted into a flat layer of foam46. Following bending, however, as shown inFIG. 30two of the pins40′ can be seen to have been displaced relative to each other to accommodate bending of the foam layer46. It may thus be appreciated that the core32may be formed into a variety of curved shapes without materially diminishing the reinforcement strength provided by the truss33(FIG. 2) formed by the reinforcement pins40.

As previously mentioned, the ability of the core32to be formed into a variety of shapes, including those having curvature, permits fabrication of composite sandwich structures having a wide variety of shapes. For example, referring toFIGS. 31 and 32, the fabrication techniques described above may be employed to produce a reinforced core126having the shape of a truncated cone128. The core126may be covered with composite facesheets (not shown) to form a completed lightweight, high strength composite sandwich structure that may be employed in a variety of applications, such as a nose cone for an aerospace vehicle (not shown).FIG. 32illustrates a portion of the outer surface of the core126which reveals groups of the reinforcement pins40held in a layer of carrier foam46in which distal ends40cof the pins40are splayed and folded over onto the core126.

The core128illustrated inFIG. 31may be fabricated using a suitable tool130such as that shown inFIG. 33which likewise has the shape of a truncated cone. The core132is formed by a plurality of core segments132which are spliced together and formed onto the tool130, as shown inFIG. 34. The core segments132are produced using a core blank134as shown inFIG. 35which, in the illustrated example, is generally rectangular. As shown inFIG. 36, the core blank134may be cut along pattern lines136to form the shaped core segment132shown inFIG. 37. The individual core segments132, having been cut to the desired shape, are then placed on and formed onto the contour of the tool130, thereby imparting a curvature to the core segment132in the shape of the tool130. The core segments132may be spliced together along their mutual edges (FIGS. 34 and 37) using a suitable bonding adhesive.FIG. 38illustrates the core segments132having been formed onto the tool130and spliced together.

Attention is now directed toFIG. 39which broadly illustrates the steps of a method for fabricating shaped composite sandwich structures, including shaped reinforced cores. Beginning at140, a core layer46is formed, using structural foam and conventional fabrication techniques. At142, partially cured composite pins40are formed using protrusion or other techniques. At144, the partially cured pins40are inserted into the foam core layer46using automated equipment or other techniques, as previously described. Next, at146, the distal ends40b,40cof the pins40are flattened and splayed onto the outer faces of the foam core layer46.

In those applications where it is necessary to fabricate the core32from multiple core segments132, step150is performed which consists of cutting the core segments132to the required size and shape. Next, the core32or core segments132are formed to the desired shape and configuration, typically using a shaping tool (120,130). Next, in those applications where the core32is formed from segments132, it may be necessary to splice the core segments132together as shown at step154. At156, a suitable adhesive is applied to the opposite faces of the core32, following which the facesheets34,36are applied to the core32in order to form a sandwich structure. Finally, at160the assembled sandwich structure comprising the core32and facesheets34,36is cured using conventional techniques such as autoclaving.

Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace and automotive applications. Thus, referring now toFIGS. 40 and 41, embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method162as shown inFIG. 40and an aircraft164as shown inFIG. 41. Aircraft applications of the disclosed embodiments may include, for example, without limitation, composite stiffened members such as fuselage skins, wing skins, control surfaces, hatches, floor panels, door panels, access panels and empennages, to name a few. During pre-production, exemplary method162may include specification and design166of the aircraft164and material procurement168. During production, component and subassembly manufacturing170and system integration172of the aircraft164takes place. Thereafter, the aircraft164may go through certification and delivery174in order to be placed in service176. While in service by a customer, the aircraft164is scheduled for routine maintenance and service178(which may also include modification, reconfiguration, refurbishment, and so on.

The preferred method of the invention is well suited for forming thermoplastic composite stiffened members in the supporting framework of an aircraft fuselage. Potential examples of thermoplastic composite stiffened members include but are not limited to fuselage skins, wing skins, control surfaces, door panels and access panels. Stiffening members include but are not limited to keel beams, floor beams, and deck beams. For illustrative purposes only, the invention will initially be described in reference to forming a thermoplastic composite floor beam20for use in a commercial aircraft fuselage. However, while an I-section is shown, other stiffened member geometries such as Z-section, U-section, T-section, etc. will also be later described, including those having curvature along their length.

As shown inFIG. 41, the aircraft164produced by exemplary method162may include an airframe180with a plurality of systems182and an interior184. Examples of high-level systems182include one or more of a propulsion system186, an electrical system190, a hydraulic system188, and an environmental system192. Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry.

The apparatus embodied herein may be employed during any one or more of the stages of the production and service method162. For example, components or subassemblies corresponding to production process170may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft164is in service. Also, one or more apparatus embodiments may be utilized during the production stages170and172, for example, by substantially expediting assembly of or reducing the cost of an aircraft164. Similarly, one or more apparatus embodiments may be utilized while the aircraft164is in service, for example and without limitation, to maintenance and service178.