CompositesWorld
Issue link: https://cw.epubxp.com/i/930524
FEBRUARY 2018 62 CompositesWorld FOCUS ON DESIGN low tooling costs that was scalable to keep expenses manageable as production volumes increased. From a design standpoint, still another issue was that the basic geometry of the strut itself was a solid rectangular billet, but mounting hardware (force-transmis- sion points) on each end of the bars needed to be added to facili- tate connections and transfer loads. After an internal review, researchers determined that one process would best meet project goals. "We considered all relevant processes right from the beginning," recalls Dr. Karl- Heinz Füller, Daimler's manager – hybrid material and concepts. "However, with our demanding performance requirements and tight cost situation, only pultrusion could provide feasible economics." Team research, which included a complete lifecycle analysis (LCA) that is done for all new production processes introduced at the company, showed that not only did pultru- sion operate with virtually no waste — important with high-value CFRP materials — but that it also was the most cost-competitive composites process because it requires relatively little energy (owing to low process forces) and has negligible tooling costs. ese features would be important at the start of the project and later, should the CFRP strut program expand to other, higher- volume vehicles. "Our strategy at Daimler for using CFRP is to start our applica- tions with more exclusive, lower-volume vehicles, like Mercedes- AMG cars," adds Füller. "at gives the CFRP supply chain — fiber and resin producers as well as processors — the time they need to further develop the cost position to meet the demands of high- volume productions. We felt the pultrusion process had the best opportunities of all composite technologies to get developed into this direction." e team began the hard work of optimizing the pultruded strut's design with epoxy and carbon fiber, running numerous static and dynamic simulations on complete vehicle models in NX Nastran (from Siemens PLM Software Inc., Munich, Germany), including noise/vibration/harshness (NVH), crash, fatigue and delta-alpha simulations (coefficient of linear thermal expansion (CLTE) differentials) using different ambient tempera- tures. Significant effort was expended to define the best layup of rovings for different carbon fiber types — from 12-50K — to achieve the part's high mechanical performance. Although initial commercial designs featured pure CFRP composite, several years of simulation and physical testing of numerous material and design variants taught the team several things and shifted the focus to a hybrid-composite design with the following features: First, more economical 48-50K carbon fiber tow would work and the CFRP part was likely to be 45-60% lighter than steel benchmarks. Multiplied by two or four struts, depending on the vehicle, the savings would increase. Second, to address concerns about galvanic corrosion between the carbon fiber and metal connection hardware, as well as surrounding metallic structures, and to prevent damage from stone chips kicked up by tires, the team decided to add a Struts stiffen the chassis, improve vehicle handling Chassis struts are designed to resist compression longitudinally, thereby holding two components apart to prevent them from flexing or collapsing. Correctly designed and installed, strut bars stiffen substructures, reducing chassis flex as vehicles cross uneven surfaces or round tight corners. This improves the driver's experience of vehicle handling. Source | Daimler AG A "simple" part with more than its share of complexity To balance mechanical and production requirements for the strut bars, researchers selected a high fiber-volume fraction of aligned, continuous carbon fibers, for compressive strength, a high-temperature epoxy to handle close proximity to the vehicle exhaust system, adopted a solid rectangular billet geometry that lent itself to variations in mounting hardware, and also made it possible to use the pultrusion process with its low tooling cost and scalable production speed. Finally, researchers added a thin core of high-strength steel (visible on the end in photo) to improve crash performance and ductility, and face layers of fiberglass to reduce risk of galvanic corrosion. Source | Daimler AG