CURVED, UNIFORMLY BIASED STRUCTURAL FIBER FORMS

Abstract: A process for forming a dry fiber form susceptible of being infused with a resin and molded to a fiber reinforced plastic structural article, wherein the fiber form is comprised of a structural fabric (104) bearing plys of biased yarns (106, 108), the bias of the yarns being maintained along the cur­ vature of the fiber form, without wrinkling or buckling of the fiber form. The structural fabric is attached to a form (100) bearing a shape-defining opening (102), and threads (114) are sewn within the opening. The fabric is then removed from the form, and the ends of the fabrics are then sewn with more threads parallel to the first set of threads. Thereafter, the ex­ cess is trimmed away, and the resulting sewn fiber form may be infused with resin and molded into a structural article.

Description: Technical Field This invention pertains to the art of structural composite materials, or elements of those materials, in particular, fiber forms prepared from non-woven structural fabric. In particular, this pertains to fiber forms wherein at least some of the yarns present are biased at an angle other than 90° with respect to the axis of the fabric, the fiber forms being curved, and the bias of the yarns being maintained over that curve. Background Art : Increasing demands in the transportation industry, including the rail, marine and aerospace fields, coupled with the increasing cost of energy, has placed a high demand on structural materials that are of extreme strength, durability, and at the same time, light weight. Thus, replacements for conventional alloys, and even lighter weight alloys such as aluminum, are constantly being sought. One such group of substitutes includes the composites field, generally including fiber reinforced plastic. One important type of product within this field is the structural article made by infusing or impregnating a non-woven, stitched structural fabric comprised of a plurality of plys of structure yarns (tensile modulus of elasticity generally in excess of about 40 million kPa (6 million p.s.i.), including fiber glass, Kevlar, boron and graphite) with a curable resin, and thereafter molding the product to provide a stiff, light weight finished product. Such non-woven stitched fabrics, and products that may be produced therefrom, are disclosed in U.S. Patents 4,416,929, 4,484,459 and 4,550,045, among others. The shaped article to be resin infused is generally known as a fiber form. The disclosure of these references, in particular U.S. Patent 4484459, teaches how to prepare articles of simple shape, having straight edges, and no complex configuration or curve. However, where a curved shape is sought, those prior art processes used wet, preimpregnated, "tapes", or unitapes, because of the well developed technology concerning the application and lay down of such tapes. Again, in most circumstances, the production of a shape of complex curvature is easily accomplished using such tapes. However, in certain critical applications, including aerospace applications, it is necessary to include many layers of structural yarn wherein the yarn is biased with respect to the axis of the fabric, generally at angles greater than 30° and in particular ±45°. when preimpregnated tapes as described in US Patent 4484459, or infused fabrics, are employed to prepare curved articles using such biased fabrics, at least two critical problems are encountered. First, owing to the curve in the alignment of the fabric, the bias of the structural yarns is frequently distorted, or destroyed entirely. Although the yarns may end at a given angle, it is not constant along the radius of curvature. Particular applications, such as the preparation of "stringers" and stiffening members for airplanes and the like, require that the bias be constant, uniform and reproducible. The second problem encountered, particularly when using tapes or other wet products, is the tendency of these tapes to buckle or wrinkle when distorted around a curve. Such distortion and wrinkles frequently give rise to voids and flaws in the formed article, rendering them structurally unsound, and unfit for many applications. Accordingly, there continued to be a pressing need for a curved fiber form with biased yarns, wherein the bias is maintained around the curve at a constant angle, suitable for impregnation with resin and subsequent molding. Similarly, there currently exists no known process for preparing the same. Applicants have now discovered that these problems in the product/process of US-A-4484459 can be effectively overcome by sewing holding threads along the line of curvature, thereby maintaining the bias of the yarns at a constant angle along said curvature. Accordingly, in one embodiment, the present invention provides a dry fiber form having a desired curvature comprised of a non-woven, stitched structural fabric comprised of a plurality of plys of structural yarns , the yarns of at least one ply being biased with respect to the axis of said fabric, characterised in that said biased yarns are maintained at a constant angle of bias along said curvature by holding threads sewn through said fabric along said curvature. In more detail, a form is made having an opening which corresponds to the desired shape of the fiber form, in terms of width, length and curvature. A non-woven, stitched structural fabric having the number of plys desired in the final product and having the appropriate orientation within those plys to meet the biasing requirements of the final project is attached to the form, completely occluding the shape-defining opening therein. Thereafter, holding threads are sewn in parallel lines within that opening, conforming to the shape of that opening. The sewn threads extend the length of the opening, and are repeated from one edge of the opening to the other. The sewn fiber form will now conform to the shape of the opening. However, in order to provide an article ready to be impregnated, the ends of the fiber forms, which were previously overlapping on the form, must also be sewn. To accomplish this, a vertically disposed cradle, capable of translational motion along an arc, is provided, and the fiber form clamped thereto, in a fashion such that the edges of the sewn fiber form remain free. A sewing machine is brought into position over the fiber form, and the fiber form is sewn again, in parallel with the previous sewing lines, by moving the cradle along its arc. At the end of each line, the sewing machine is displaced slightly, and another line sewn, until the entire width of the fiber form that will correspond to the finished article has been sewn. The excess is thereafter trimmed. The resulting product has the curvature of the shape-defining opening, and each bias yarn in the individual plys of the finished fiber form maintains that bias across the curve. When the dry fiber form is bent to be attached to the form, the fibers actually slide in response to the tension created, and accordingly, the bias is maintained by actually curving the yarns. Of course, because the entire fabric is now curved, this results in a maintenance of desired biased angle. The finished product may thereafter be infused or impregnated with a resin, and cured, to give the desired structural article. Brief Description of the Drawings Figure 1 is an illustration of a suitable form, provided with a shape-defining opening. Figure 2 is an illustration of a structural fabric attached to the form, completely including the shape-defining opening. Figure 3 is a perspective view of the first sewing operation within the form opening, illustrating a point nearly at the end of that sewing process. Figure 4 is an illustration of the vertically disclosed cradle used to achieve the second sewing stage of the process of this invention, together with the sewing machine associated therewith. Figure 5 is a representational illustration of the finished fiber form. Best Mode for Carrying Out the Invention The above objects, and others, can be further understood by reference to the detailed description provided below, which refers to Figures 1-5, wherein like reference characters indicate like parts in all drawings. In order to prepare the fiber form of this invention, a form, or stencil, must be provided. A suitable form is illustrated in Figure 1. The form is comprised of a frame 100, which defines an opening 102 therewithin. Opening 102 should be formed so as to correspond to the length, width, and curvature of the desired end fiber form. A structural fabric 104, containing the desired number of plys of the finished article, wherein at least some of the plys of that fabric contained biased structural yarns 106 and 108 is attached to the form, completely occluding opening 102. This attachment may be achieved by use of clamps 110, or other conventional means, including tape, glue and the like. It is critical to the claimed invention that the fabric employed be non-woven, and be "dry", that is, not impregnated with any resin at all. When such fabrics are used, the structural yarns can "slip" or "slide" to a certain degree within the fabric, when bent about the shape-defining opening 102. This bending will be uniform along the curvature, such that, when the final shape has been achieved, the bias of the yarns will remain, although the yarns themselves will curve, in a degree corresponding to the curvature of the shape, from one long edge to the other of the fiber form. Thus, one point of criticality of the claimed invention is the use of a dry, or non-impregnated structural fabric. In this respect, the claimed invention is distinct from prior art processes involving tapes and the like, which are considered "wet" products. As illustrated in Figure 3, the form 100, bearing the curved fabric 104, is placed on a support member, such as table 120, which can be brought into proximity to a sewing unit 112, which may be preferably based on an independent table 122, which may be rolled away from and towards table 120. Beginning at one side of the fiber form, and continuing in parallel lines to the other, a holding thread is sewn into the curved fabric. These parallel threads are designated 114. The sewing machine used can be of any conventional, industrial type, provided that the actual head and needle 116 are of sufficient strength and sized to penetrate the fiber form which can be 100 plys or more thick. Similarly, sewing threads 114 can be of any desired composition, saved that they must be strong enough to meet the requirements of the end product. Essentially, sewn threads 114 maintain the fiber form in its curved configuration. In prefered embodiments, these threads include polyester threads and polyamide threads such as Kevlar™, although virtually any synthetic or natural thread can be used, depending on the application. In order to facilitate the sewing of the fabric, a guide 118 may be employed. The guide should correspond to the shape of the opening 102, so as to insure that the sewn threads 114 are as parallel as possible. Of course, guide 118 may be dispensed with if the form is provided with some type of track to run in, or the form is manually moved while sewing continues. At the end of this first sewing step, the fiber form has been nearly entirely sewn, and certainly, the fabric has been converted into the desired shape. The fabric is removed from form 100, where upon the curve imparted by the sewing remains. However, the ends of the fiber form, which were attached to form 100, are still loose. In order to provide a product having the necessary uniform strength and characteristics, these ends must be sewn, again, in the desired curvature. In a preferred embodiment, to achieve this end, the partially sewn fabric 104 is transfered and adhered to a cradle 124, which is capable of translational movement along an arc. The cradle is supported in a conventional fashion, for example, by tripod 126, to allow free movement of the cradle. A sewing unit, which may be unit 112 on movable table 122, or any other suitable machine, is brought into proximity with the fabric 104, in position that the fabric may be sewn through from above. As sewing proceeds, the cradle is moved along its arc. At the end of each arc, the sewing machine is displaced slightly , and the cradle returns, this step being repeated until the ends of the fiber form have been sewn. At the end of this step, any excess yarn, or fabric, is trimmed away. Of course, any method of bringing the sewing unit and the fiber form into close relationship and sewing along the fiber form curve may be used. Thus, the fiber form may be held constant, and a suitable, mounted sewing machine may be displaced horizontally and vertically along a predetermined path. The resuling article is illustrated in Figure 5. The original dry fabric has now been bent into a curved shape, wherein the bias of each bias layer in the original fabric is maintained along the curvature of tie article, from points A to B. The angle theta defined by the intersection of the radius of the curve with the biased yarns is equal all along the curvature of the fabric. This is true whether this intersection is measured at the interior of the fabric, in the middle of the fabric, or at the exterior edge of the fabric. This is because the bias yarns, in being bent around the curvature, actually slide or slip into a curved configuration, so that the bias is maintained. The finished article may now be infused or impregnated with a curable resin, and subsequently molded to form a composite, structural article of exceedingly high strength, meeting any specific requirements, and being reproducible. Of course, the possible shapes that the fiber form may assume are limitless. A simple curvature has been illustrated. Other curves or shapes that may be contemplated include nacelles and "J" figures, which are frequently used as reinforcing parts. The article may include a plurality of curved and straight portions, such in a nacelle, or be a single, long curve, as illustrated. The invention is not limited or defined by the nature of the curvature, save that some curve must be present. It should also be noted that the final product produced from this fiber form need not be two dimensional. If it is desired to bend the fiber form to provide flanges, such as in a Z-cross section article, or any other desired bending, this can be easily achieved through conventional processes, such as match molding, because the fiber form remains drapeable and flexible along its width. The only direction in which the fiber form will not flex is in the direction of its curvature.

Claims: A dry fiber form having a desired curvature comprised of a non-woven, stitched structural fabric (104) comprised of a plurality of plys of structural yarns (106, 108), the yarns of at least one ply being biased with respect to the axis of said fabric, characterised in that said biased yarns are maintained at a constant angle of bias along said curvature by holding threads (114) sewn through said fabric along said curvature. The fiber form of Claim 1, wherein said structural yarns (106, 108) are comprised of a material selected from the group consisting of fiberglass, polyamide, boron, and graphite. The fiber form of Claims 1 or 2, wherein said sewn threads (114) are comprised of polyester or polyamide materials. The fiber form of Claim 1, 2 or 3 wherein said curvature is a simple curve of constant radius. The fiber form of Claim 1, 2 or 3 wherein said curvature is a complex curve of non-constant radius. The fiber form of Claim 5, wherein said complex curve includes straight portions adjoining said curved portions at either end. A process for making a dry fiber form of desired curvature comprising the steps of: attaching a dry structural fabric (104) comprised of a plurality of plys of structural yarns (106, 108), the yarns of at least one ply being biased with respect to the axis of said fabric, to a form (100) bearing an opening (102) defining said desired curvature in such fashion to completely occlude said opening by bending said fabric about said opening, sewing a plurality of first courses of holding thread (114) within that opening (102) parallel to the curvature of said opening from end to end of said opening, thereby maintaining the bias of the yarns at a constant angle along said curvature, removing said sewn fabric from said form and sewing a plurality of second courses of holding threads (114) along the edges of said fabric parallel to said first threads; trimming the excess of said fabric to said desired shape. The process of Claim 7, wherein said sewing of the edges of said fabric is achieved by attaching the fabric to a vertically disposed cradle (124) capable of translational movement along an arc, and sewing along said fabric while said cradle is displaced relative to a sewing unit. The process of Claim 8, wherein said structural yarns are selected from the group consisting of fiberglass, polyamid, boron, and graphite. The process of Claim 7, wherein said threads are comprised of polyester or polyamide fibers.

Inventors: DRUMMOND, TODD, VEES, FREDERICK
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1986
Main classification: D04H3
IPC codes: D04H3, C08J5, D04B21, D05B21