MANUFACTURE OF ARBITRARY CROSS-SECTION COMPOSITE HONEYCOMB CORES BASED ON ORIGAMI TECHNIQUES

In recent years, as space structures have become large and require higher accuracy, composite honeycombs, which can reduce weight and have low thermal expansion, are in increasing demand. As observed in the design of antenna reflectors and rocket bodies, both flat and 3D-shaped cores are used in this field. However, these special honeycombs have high manufacturing costs and limited applications. This study illustrates a new strategy to fabricate arbitrary cross-section honeycombs with applications of advanced composite materials. These types of honeycombs are usually manufactured from normal flat honeycombs by curving or carving, but the proposed method enables us to construct objective shaped honeycombs directly. The authors first introduce the concept of the kirigami honeycomb, which is made from single flat sheets and has periodical slits resembling origami. In previous studies, honeycombs having various shapes were made using this method, and were realized by only changing folding line diagrams (FLDs). In this study, these 3D kirigami honeycombs are generalized by numerical parameters and fabricated using a newly proposed FLD design method, which enables us to draw the FLD of arbitrary cross-section honeycombs. Next, the authors describe a method of applying this technique to advanced composite materials. Applying the partially soft composite techniques, folding lines are materialized by silicon rubber hinges on carbon fiber reinforced plastic. Complex FLD patterns are then printed using masks on carbon fabrics. Finally, these foldable composites that are cured in corrugated shapes in autoclaves are folded into honeycomb shapes, and some typical samples are shown with their FLDs.  In the construction of aerospace components, lightweight, rigid, and strong honeycomb sandwich panels are required. In recent years, the use of composite materials has drastically increased in this field. In the case of sandwich panels, carbon fiber reinforced plastic (CFRP) face sheets are typically combined with an aluminum honeycomb. Currently, space structures are increasing in size and require greater degrees of accuracy; hence, the use of composites as a core material is a natural progression. It is promising for further reducing the weight of the body of satellites and enhancing the accuracy of their on-board equipment because of their low coefficients of thermal expansion (CTE). Furthermore, use of the same material for both the face sheet and the core prevents certain problems that would otherwise arise when combining materials that have different CTEs, such as CFRP-face aluminum honeycomb . Various types of composite honeycombs [2-4] are commercially available, in addition to aluminum or nomex honeycombs. A comparable product is the CFRP honeycomb that has recently been used in antenna reflectors for high frequencies [5,6]. Other composites such as Kevlar honeycombs are also manufactured and used in floor panels of the latest airplane model. Quartz fiber, which has superior electrical properties has found the application in radome. 1  August 4-7, 2013, Portland, Oregon, USA Many studies on various types of core configurations that do not include hexagonal honeycombs have been reported. The main examples include lattice materials consisting of webs and struts [7-9]. Several configurations have already been proposed in this research area: octet-truss, the 2D and 3D Kagome structure, and the tetrahedral lattice. Square composite honeycombs, which are expected to show high in-plane stretching rigidity, have also been researched [10]. These square honeycombs are fabricated by assembling slotted rectangular composite sheets. The Z-fiber and X-Cor are truss structures that are fabricated by angled carbon fiber rods embedded in polymeric foam, and they are typical examples of composite sandwich panels [11] that have potential commercial applications. However, these composite core materials are not regularly used in sandwich construction. Compared to standard aluminum or nomex honeycombs, their manufacturing costs are very high and they have limited applications. Another problem is difficulty of machining. In the manufacture of complex-shaped parts, the cores must have some degree of curvature. For aluminum honeycombs, this can be done using a contour cutter, a 3-D tracer, and numerically controlled machines. However, burrs and buckling of cell walls present a difficult problem for surface accuracy. It is clear that the machining of composite cores requires more expensive and sophisticated systems. Realizing curvatures in honeycombs is also difficult because they deform a saddle shape when bent. It requires special cell shapes such as flexcore  or cells having auxetic behavior [12,13] for a large and accurate curvature. This study proposes a novel method to construct arbitrary cross-section composite honeycombs. The basic idea originates from the fold-made paper honeycombs proposed by Nojima and Saito in 2007 , in which they attempted to apply origami and kirigami techniques to the creation of sandwich structures. Origami is the traditional Japanese art of paper folding and has received widespread attention from artists, architects, and mathematicians. Kirigami is a variation of origami. While traditional origami prohibits the cutting of paper, it is permissible in kirigami. Kirigami artists create remarkable patterns on paper using a combination of cutting and folding. Figure 1 shows the basic concept of kirigami (or origami) honeycombs. The advantage of this method is that it can be extended to manufacture 3D (non-flat) honeycomb. This is achieved only by changing the folding line patterns. Some paper samples and their folding line diagrams (FLDs) are shown in Fig. 2. However, these previous studies  have not included non-convex-shaped cross-sections. Because of geometrical restrictions, their FLDs cannot be drawn on single sheets of flat paper. Generalized FLD design methods have also remained a challenge. In this study, these 3D kirigami honeycombs are generalized by numerical parameters, and a new FLD design method is devised. This study also includes non-convex-shaped crosssections that have not been possible to realize in previous studies. The outline of the paper is as follows. First, the design method of the FLD is devised. This involves the calculation of the position of the folding lines and slits from given crosssectional shapes. The second part describes the condition pertaining to foldability and propose a modified method for unfoldable cross-sections. This approach enabled us to fold arbitrary cross-section honeycombs (including non-convex honeycombs) by folding single sheets. The third part describes a method of applying this technique to advanced composite materials. As an exapmle of kirigami composite honeycombs, a wingbox was already manufactured in autoclave-cured woven Kevlar fabric using the FLD around an airfoil profile[15]. In Ref.15, a dashed line cutting is used on a folding line. This study proposes a new method to introduce folding lines on CFRP; Silicon rubber is used as the matrix for hinge areas. Finally, these foldable composites that are cured in corrugated shapes in autoclaves are folded into honeycomb shapes, and some typical samples are shown with their FLDs. Fig. 1 Concept of kirigami honeycomb core. (a) Basic folding lines diagram. Thick lines: Slits. Fine lines: mountain folding lines.  The folding process for realizing a honeycomb shape. Fig. 2 Examples of 3D folded honeycombs and their folding line diagrams (FLDs).  Tapered honeycomb.  Convex curved honeycomb. Black lines and areas: Slits or cutouts. Gray lines: Folding lines. (a) (d) c'