Usage of structural elements formed of composite materials is currently very popular in the aerospace industry where the lightweight, highstrength characteristics of these elements are especially desirable. The composite materials that make up these structural elements typically consist of high-strength anisotropic fibers, such as graphite, embedded within a thermosetting resin matrix. One common type of composite material used in the fabrication of these structural elements is called "prepreg." Prepreg is a form of composite material wherein the resin is mixed with the fibers and partially cured before further molding or curing takes place. Prepreg composite material is available in the form of unidirectional fibers in thin strips called "tape." Prepreg tape is available in various widths and is typically stored on spools. In order to fabricate a structure, the prepreg tape is placed layer upon layer (or "laid up") in the configuration of the desired structure, and then cured into a rigid element.
When laying up prepreg tape it is important that the layers are evenly placed so that no voids are created in the resulting structure. Also, the strips of composite material must be handled in a way that avoids alteration of either the thermosetting characteristics of the resin or the anisotropic properties of the fibers. Voids in the material or damage to the fibers can seriously weaken the strength of the final structural element.
Laying up prepreg tape to form substantially planar or curved portions of composite structural elements usually presents few problems with respect to fiber damage or the creation of voids. However, some portions of structural elements have shapes that make them difficult to form without creating voids or otherwise diminishing the structural strength of the element. For example, in forming a composite structural element that has a T-shaped cross section (for instance, a flat panel having a perpendicularly extending flat stiffener), prepreg tape is laid up to form two L-shaped segments that are positioned back-to-back to create the T shape. The abutting legs of the L form the web of the T and the other legs of the L extend away from each other to form the flange of the T. A multiple-layered skin of prepreg tape is laid across the flanges of the T to complete the structure. Since it is important that the layers of prepreg tape that are used to form the web and flange of the T do not suffer loss of strength across the 90.degree. bend in the L-shaped segments, the 90.degree. bend is formed with a somewhat smooth curve to prevent damage to the composite material that would result if a sharp 90.degree. fold were used. Forming the 90.degree. bend with a smooth curve results in a cavity being formed at the junction of the web, flange and skin. The cross-sectional shape of this cavity is generally triangular; a flat base formed by the skin, and two concave sides formed by the facing surfaces of the bent portion of the L-shaped segments. If the panel is cured without filling the above-described cavity, the final structural element will be considerably weakened and/or deformed in the area of the cavity. Therefore, it is necessary to fill the cavity with a thin, narrow fillet. It is in forming the fillet that problems arise.
In the past, the cavity has been filled with a narrow piece of prepreg tape that was folded and hand-shaped into a fillet having a cross section similar to the cross section of the cavity. These hand-formed fillets were made by folding the prepreg tape into a size roughly equal to the cross-sectional size of the cavity. Sometimes the folded tape was pulled by hand through a die to further force the tape into a shape similar to that of the cavity.
Preparing a fillet by hand is very time consuming. It is also very difficult to ensure that no voids are present in the folded tape while at the same time maintaining the structural integrity of the fillet. Specifically, handforming of the fillet commonly results in twists or bunches that, after curing, result in voids in the structure. Additionally, hand shaping of the fillet can often result in damage to the fibers. Furthermore, pulling the composite prepreg material through a die generates frictional heat which transfers to the composite material and causes some premature curing of the resin, resulting in a somewhat stiff fillet that can be difficult to work with. Additionally, hand forming--with or without subsequent pulling of the material through a die--provides very little compaction of the fillet; the resulting cured fillet having voids and thus an unacceptably low density.