CompositesWorld

SEP 2017

CompositesWorld

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SEPTEMBER 2017 100 CompositesWorld FOCUS ON DESIGN Variable-axial composites open path to lighter composite structures CFRP recurve bow riser demonstrates design and manufacturing approach with potential to cut weight vs. aluminum by 50-75% while increasing strength and stiffness. » Tailored fiber placement (TFP) is an automated preforming technology that uses modified CNC embroidery machines to create preforms for lightweight, load-optimized composite structures with practically zero waste. Equipped with multiple stitching heads, these indus- trial machines can perform 1,000 zigzag stitches per minute per head at a speed of 5 m/min. According to licensed supplier TAJIMA GmbH/ FilaCon Systems (Winter- lingen, Germany), more than 100 TFP machines have been installed for producing composites in Europe and Asia. Parts manufactured include Airbus A350 window frames, helicopter longerons, bicycle brake levers and other sporting goods parts, as well as structural automotive components (see Learn More, p. 103). A TFP strength is its ability to produce fiber patterns to meet practically any load path, including curvilinear shapes with radii as tight as 5 mm. Because it can deposit continuous fiber with such a high degree of freedom, TFP is described as filling a gap between additive manufacturing and automated fiber placement (AFP). "TFP enables variable-axial composites," says Dr. Axel Spick- enheuer, head of the complex structures group in the Compos- ites Department at Leibniz Institute of Polymer Research (IPF, Dresden, Germany), where TFP was developed. "Multiaxial composites are made of unidirectional layers, with each discrete layer having constant thickness and fiber orientation," he explains. "In variable-axial composites, both thickness and fiber orien- tation may be locally adjusted to meet stiffness or bearing load requirements." Spickenheuer's group demonstrated computer- aided modeling and efficient manufacturing of a variable-axial composite via the rec16 recurve bow riser. A recurve bow is identified by curved tips at either end, which increase the bow's speed and the smoothness of its release. Widely used in target archery, it is the only style of bow permitted in the Olympics. Higher poundage recurve bows are used in field archery and bowhunting. e bow's riser is the center structure that connects the top and bottom limbs, and where the bow grip and sight are attached (see drawing, next page). e effort was so successful, the riser captured a 2017 JEC Innovation Award. But Spickenheuer points out, "Our objective was not to become a manufacturer of bow risers, but instead to demonstrate this design and manufacturing approach as a valid, cost-effective means of achieving next-generation composite parts." Five-step design process e recurve bow was, however, an excellent choice for a demon- stration. Although risers for such bows are most often made from aluminum, and weigh 950-1,400g, they also are made from wood and carbon fiber-reinforced plastic (CFRP). However, most CFRP risers, weighing in at 1,000-1,350g, aren't any lighter than aluminum. Speckenheuer's team at IPF knew that, with TFP, they could do better. e design approach for the rec16 recurve bow riser began with identification of the structure's load cases. A linear process was then followed, comprising five basic steps. e first step was completing topology optimization, using finite element analysis (FEA) software tools, including TOSCA (Dassault Systèmes, Vélizy- Villacoublay, France). Topology optimization (TO) uses numerical analysis to find the best distribution of material given an optimization goal — in this case, lightest weight — and set of design constraints (for the riser, 200N lateral load, 635 mm height). Applied to any material, not FIG. 1 Besting both aluminum and legacy CFRP bows This uniquely shaped composite riser, the central section of the rec16 recurve bow, uses four sets of two complex TFP preforms (see drawing, next page) and resin infusion to optimize riser weight and mechanical performance and bring bow response to levels not previously attainable. Source (All images) | Leibniz Institut für Polymerforschung By Ginger Gardiner / Senior Editor

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