The present invention relates to an improvement in the field of composite laminate structures known as sandwich structures formed with outside skins of a polymer matrix composite and an internal core of either foam, end-grain balsa wood, or honeycomb, and more specifically to the field of these sandwich structures which additionally have some type of Z-axis fiber reinforcement through the composite laminate and normal to the plane of the polymer matrix composite skins.
There is extensive use in the transportation industry of composite laminate structures due to their lightweight and attractive performance. These industries include aerospace, marine, rail, and land-based vehicular. The composite laminate structures are made primarily from skins of a polymer matrix fiber composite, where the matrix is either a thermoset or thermoplastic resin and the fiber is formed from groupings of fiber filaments of glass, carbon, aramid, or the like. The core is formed from end-grain balsa wood, honeycomb of metallic foil or aramid paper, or of a wide variety of urethane, PVC, or phenolic foams, or the like.
Typical failures in laminate structure can result from core failure under compressive forces or in shear or, most commonly, from a failure of the bond or adhesive capability between the core and the composite skins (also known as face sheets). Other failures, depending on loading may include crimpling of one or both skins, bending failure of the laminate structure, or failure of the edge attachment means from which certain loads are transferred to the laminate structure.
Certain patents have been granted for an art of introducing reinforcements that are normal to the planes of the skins, or at angles to the normal (perpendicular) direction. This is sometimes called the “Z” direction as it is common to refer to the coordinates of the laminate skins as falling in a plane that includes the X and Y coordinates. Thus the X and Y coordinates are sometimes referred to as two-dimensional composite or 2-D composite. This is especially appropriate as the skins are many times made up of fiber fabrics that are stitched or woven and each one is laid on top of each other forming plies or layers of a composite in a 2-D fashion. Once cured these 2-D layers are 2-D laminates and when failure occurs in this cured composite, the layers typically fail and this is known as interlaminar failure.
The patents that have been granted that introduce reinforcements that are normal to the X and Y plane, or in the generally Z-direction, are said to be introducing reinforcements in the third dimension or are 3-D reinforcements. The purpose of the 3-D reinforcement is to improve the physical performance of the sandwich structure by their presence, generally improving all of the failure mechanisms outlined earlier, and some by a wide margin. For example, we have shown that the compressive strength of a foam core laminate structure with glass and vinyl ester cured skins can be as low as 30 psi. By adding 16 3-D reinforcements per square inch, that compressive strength can exceed 2500 psi. This is an 83 times improvement.
Childress in U.S. Pat. No. 5,935,680, Boyce et al in U.S. Pat. No. 5,741,574 as well as Boyce et al in U.S. Pat. No. 5,624,622 describe Z-directional reinforcements that are deposited in foam by an initial process and then secondarily placed between plies of fiber fabric and through heat and pressure, the foam crushes or partially crushes forcing the reinforcements into the skin. Practically, these reinforcements are pins or rods and require a certain stiffness to be forced into the skin or face layers. Although Boyce et al describes “tow members” as the Z-directional reinforcement, practically, these are cured tow members, or partially cured tow members that have stiffness. As Boyce et al describes in U.S. Pat. No. 5,624,622, compressing the foam core will “drive” the tow members into the face sheets. This cannot be possible unless the Z-directional or 3-D reinforcements are cured composite or metallic pins.
A standard roll of fiberglass roving from Owens Corning, typically comes in various yields (of yards per pound weight) and a yield of 113 would contain on a roll or doft 40 lbs. of 113 yield rovings. In the uncured state, these rovings are multiple filaments of glass fiber, each with a diameter of less than 0.0005 inches. The roving, uncured as it comes from Owens Corning, is sometimes called a “tow”, contains hundreds of these extremely small diameter filaments. These hundreds of filaments shall be referred to as a “grouping of fiber filaments.” These groupings of fiber filaments can sometimes be referred to, by those skilled in the art, as tows. It is impossible to drive a virgin glass fiber tow, or grouping of fiber filaments, as it is shipped from a glass manufacturer such as Owens Coming, through a face sheet. The grouping of fiber filaments will bend and kink and not be driven from the foam carrier into the skin or face sheets as described by Boyce et al. Therefore, the “tow” described by Boyce et al must be a rigid pin or rod in order for the process to work as described. It will be shown that the present invention allows easily for the deposition of these groupings of fiber filaments, completely through the skin-core-skin laminate structure, a new improvement in this field of 3-D reinforced laminate structures.
This issue is further verified by an earlier patent of Boyce et al, U.S. Pat. No. 4,808,461, in which the following statement is made: “The material of the reinforcing elements preferably has sufficient rigidity to penetrate the composite structure without buckling and may be an elemental material such as aluminum, boron, graphite, titanium, or tungsten.” This particular referenced patent depends upon the core being a “thermally decomposable material”. Other US Patents that are included herein by reference are: Boyce et al, U.S. Pat. No. 5,186,776; Boyce et al U.S. Pat. No. 5,667,859; Campbell et al U.S. Pat. No. 5,827,383; Campbell et al U.S. Pat. No. 5,789,061; Fusco et al. 5,589,051.
None of the referenced patents indicate that the referenced processes can be automatic and synchronous with pultrusion, nor do they state that the processes could be synchronous and in-line with pultrusion. Day describes in U.S. Pat. Nos. 5,589,243 and 5,834,082 a process to make a combination foam and uncured glass fabric core that is later molded. The glass fiber in the core never penetrates the skins of the laminate and instead fillets are suggested at the interface of the interior fiber fabric and the skins to create a larger resin fillet. This is a poor way to attempt to tie the core to the skins, as the fillet will be significantly weaker than if the interior fiber penetrated the skins. Day has the same problem that Boyce et al have as discussed earlier. That is, the interior uncured fabric in Day's patent is limp and cannot be “driven” into the skins or face sheets without being rigid. Thus the only way to take preinstalled reinforcements in foam, and then later mold these to face sheets under pressure, and further have the interior fiber forced into the skins, is to have rigid reinforcements, such as rigid pins or rods or, as in Day's case, rigid sheets.
Boyce et al in U.S. Pat. No. 5,186,776 depends on ultrasound to insert a fiber through a solid laminate that is not a sandwich structure. This would only be possible with a thermoplastic composite that is already cured and certain weaknesses develop from remelting a thermoplastic matrix after the first solidification. Ultrasound is not a requirement of the instant invention as new and improved means for depositing groups of fiber filaments are disclosed. U.S. Pat No. 5,869,165 describes “barbed” 3-D reinforcements to help prevent pullout. The instant invention has superior performance in that the 3-D groups of fiber filaments are extended beyond the skins on both sides of the composite laminate, such that a riveting, or clinching, of the ends of the filaments occurs when the ends of the filaments are entered into the pultrusion die and cured “on-the-fly.” The clinching provides improved pull-out performance, much in the same way as a metallic rivet in sheet metal, that is clinched or bent over on the ends, improves the “pull-out” of that rivet versus a pin or a bonded pin in sheet metal. This is different from the current state-of-the-art. Fiber through the core is either terminated at the skins, unable to penetrate the skins, or as pure rods penetrates part or all of the skin, but is not riveted or clinched. And many of the techniques referenced will not work with cores that don't crush like foam. For example, the instant invention will also work with a core such as balsa wood, which will not crush and thus cannot “drive” cured rods or pins into a skin or face sheet. Furthermore, the difficult, transition from a composite laminate structure to an edge can easily be accommodated with the instant invention. As will be shown later, a composite laminate structure can be pultruded with clinched 3-D groupings of fiber filaments and at the same time the edges of the pultruded composite laminate can consist of solid composite with the same type and quantity of 3-D grouping of fiber filaments penetrating the entire skin-central composite-skin interface. As will be shown, the skins can remain continuous and the interior foam can transition to solid composite laminate without interrupting the pultrusion process.
It is an object of this invention to provide a low cost alternative to the current approaches such that the composite laminate structure can find its way into many transportation applications that are cost sensitive. All prior art processes referenced have a degree of manual labor involved and have been only successful to date where aerospace is willing to pay the costs for this manual labor. The instant invention is fully automatic and thus will have extremely low selling prices. For example, earlier it was mentioned that by adding a certain number of groups of fiber filaments to a foam core composite laminate that the compressive strength improved from 30 psi to over 2500 psi. This can be achieved for only $0.30 per square foot cost. None of the existing processing techniques referenced can compare to that performance-to-cost ratio. This can be achieved due the automated method of forming the composite laminate structure. Other differences and improvements will become apparent as further descriptions of the instant invention are given.
SUMMARY OF INVENTION
The method and apparatus for forming an improved pultruded and clinched Z-axis fiber reinforced composite structure starts with a plurality of upper and lower spools that supply raw material fibers that are formed respectively into upper and lower skins that are fed into a primary wet-out station within a resin tank. A core material is fed into the primary wet-out station between the respective upper and lower skins to form a composite laminate preform. The upper and lower skins and the core are pulled automatically through tooling where the skin material is wetted-out with resin and the entire composite laminate is preformed in nearly its final thickness. The composite laminate preform continues to be pulled into an automatic 3-dimensional Z-axis fiber deposition machine that deposits “groupings of fiber filaments” at multiple locations normal to the plane of the composite laminate structure and cuts individual groups such that an extension of each “grouping of fiber filaments” remains above the upper skin and below the lower skin.
The preformed composite laminate then continues to be pulled into a secondary wet-out station. Next the preformed composite laminate is pulled through a pultrusion die where the extended “groupings of fiber filaments” are all bent over above the top skin and below the bottom skin producing a superior clinched Z-axis fiber reinforcement as the composite laminate continues to be pulled, catalyzed and cured at a back section of the pultrusion die. The composite laminate continues to be pulled by grippers that then feed it into a gantry CNC machine that is synchronous with the pull speed of the grippers and where computerized machining, drilling and cutting operations take place. The entire process is accomplished automatically without the need for human operators.
It is an object of the invention to provide a novel improved composite laminate structure that has riveted or clinched 3-D groupings of fiber filaments as part of the structure to provide improved resistance to delaminating of the skins or delaminating of the skins to core structure.
It is also an object of the invention to provide a novel method of forming the composite laminate structure wherein an automatic synchronous pultrusion process is utilized, having raw material, for example glass fabric such as woven roving or stitched glass along with resin and core material pulled in at the front of a pultrusion line and then an automatic deposition station places 3-D Z-axis groupings of fiber filaments through a nearly net-shape sandwich preform and intentionally leaves these groupings longer than the thickness of the sandwich structure, with an extra egress. This is then followed by an additional wet-out station to compliment an earlier wet-out station. The preform then is pulled into a pultrusion die and is cured on the fly and the 3-D Z-axis groupings of fiber filaments are riveted, or clinched, in the die to provide a superior reinforcement over the prior art. The cured composite laminate structure is then fed into a traveling CNC work center where final fabrication! machining operations, milling, drilling, and cut-off occur. This entire operation is achieved with no human intervention.
It is another object of the invention to utilize core materials that do not require dissolving or crushing as previous prior art methods require.
It is a further an object of the invention to provide a novel pultruded panel that can be continuous in length, capable of 100 feet in length or more and with widths as great as 12 feet or more.
It is an additional object of the invention to produce a 3-D Z-axis reinforced composite laminate structure wherein the edges are solid 3-D composite to allow forming of an attachment shape or the machining of a connection.
It is another object of the invention to provide a preferred embodiment of a temporary runway, taxiway, or ramp for military aircraft. This composite laminate structure would replace current heavier aluminum structure, (known as matting) and could easily be deployed and assembled. The 3-D Z-axis reinforcements ensure the panels can withstand the full weight of aircraft tire loads, yet be light enough for easy handling.