CompositesWorld
Issue link: https://cw.epubxp.com/i/866813
SEPTEMBER 2017 102 CompositesWorld FOCUS ON DESIGN FIG. 2 Complex, asymmetrical path to performance The basic shape of the rec16 bow riser was developed from topology optimiza- tion results (top) and then refined using FEA software with 2D and then 3D CAD models (center and bottom). The bottom image shows the 3D CAD model from the bow's front (as seen from the bow master's target), highlighting the riser's 3D, out-of-plane shape. is pattern was applied in Step 4 to derive a 3D FE model, using Advanced Optimal Path Software (AOPS) developed by IPF. AOPS features import and export functionality for several FE solvers, and can generate and export principal stress trajectory plots of an initial structural calculation to create an optimized TFP pattern. When the TFP fiber path had been refined to consider manu- facturing aspects, AOPS became an FE pre-processor, generating a 3D variable-axial composite model that would account for specific local fiber orientation as well as the local variable thickness of the intended TFP part. is enabled a design loop, where every change of the curvilinear fiber layout led to a different FE model with specific structural properties. With this tool, composites engineers can manually optimize even complex curvilinear fiber structures to achieve the overall structural objectives. In Step 5, the basic fiber pattern of the riser's 3D design was refined for the TFP manufacturing process by using EDOpath software (TAJIMA/FilaCon), which transforms the path data into a CNC-based stitch pattern that is readable by modern TFP machines. e final riser design called for eight carbon fiber preforms — four sets each split between preform A and preform B (see right side of drawing, p. 101) — which were stacked to achieve a symmetric laminate. Materials and manufacturing e fiber chosen to create the rec16 riser was 24K ultrahigh- modulus (UHM) carbon by TohoTenax Europe GmbH (Wuppertal and Heinsberg, Germany) with a nominal linear density of 800 tex. It was applied and fixed with a 10-tex polyester stitching yarn onto a 108-g/m² woven glass fabric. e preforms were produced at IPF, using a TAJIMA dual-head machine (Fig. 3). e riser was resin infused, using epoxy resin. Because the molding process did not require high pressure, but only vacuum, less costly tooling could have been used. But because the riser structure features many openings and an out-of-plane 3D shape, and requires integrated metal inserts as well as a high-quality surface finish, an unsealed but closed aluminum tool was selected, i.e., multi-part molds were matched but not designed to seal as they would be for resin injection. e manufactured preforms were placed in the tool and infused with PoxySystems EP L + EPH L room-temperature-cure epoxy resin supplied by R&G; Faserverbundwerkstoffe (Waldenbuch, Germany). No further trimming or machining was required. Redefining designs, revolutionizing properties e CFRP rec16 riser design reduces weight to 600g, a 40% weight reduction, while increasing the mass-specific stiffness by 43%. "Manufacturing waste was also reduced," notes Spickenheuer, "especially in comparison to milled aluminum risers." Further iteration of the rec16 riser design is in process. "We did not achieve sufficient bending stiffness with the first preforms, which causes too much torsion in the upper arm," Spickenheuer explains, "so we will increase the use of ±45° fibers." "TFP makes it possible to place carbon tows in any orienta- tion to match the highly stressed regions of the part architecture," he points out. "e results are complex curvilinear structured preforms that are able to utilize the full potential of anisotropic carbon fiber composites." TABLE 1 Designing for TFP The five-step design process for variable-axial composite structures using Tailored Fiber Placement. 1. Topology optimization (isotropic model) 2. Principal stress analysis (isotropic model) 3. Initial fiber layout/TFP pattern design 4. FEA including local fiber orientation and thickness distribution 5. Final optimization of fiber pattern