CompositesWorld

FEB 2018

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63 CompositesWorld.com protective top layer of slightly heavier but less costly fiberglass. e glass also improved damping values. ird, simulations showed that it would be beneficial to add a thin steel core to the pultruded profile to increase its ductile crash behavior and offer new cost-efficient mounting designs. In the end, use of the metallic core — which is formed using high- strength steel (0.8-1.2mm/0.03-0.5 inches in diameter, depending on application) and fed directly into the pultrusion die along with glass and carbon fibers — also made it easier to insert attachment bushings into the struts. All metallic hardware directly touching the struts is protected by either coatings or adhesives or both. Initially an all-CFRP and, later, a hybrid (steel-cored, carbon and glass fiber-reinforced epoxy) composite, the struts were produced by Secar Technologies GmbH (Mürzzuschlag-Hönigs- berg, Austria). e strut body is cut from a pultruded profile. Typical length is 0.8-1.3m (2.6-4.3 ft), typical width is 35 mm/1.4 inches, and typical thickness is 8.0 mm/0.31 inch). Holes are drilled and bushings installed. Using standard surface prep, attachment hardware is bonded to each end of the bar via struc- tural epoxy adhesives. Bonding currently is the rate-limiting step in the production process; pultrusion is quite capable of main- taining a high-volume production rate. "anks to the dedi- cated mix of various glass and carbon fibers, a high-performance resin and a high- strength metal band at the core, the function and cost of our material mix was optimized," notes Ralf Bernhardt, Mercedes- AMG's manager – body-in-white development. "at made it possible for us to optimize other component functions, such as damping and corrosion characteristics, temperature resistance and crash performance. In combination with the impressive economic efficiency of the manufacturing process, this provides the ideal conditions for larger production volumes." Award-winning design expands to more vehicles Initially, pure CFRP strut bars were introduced on Mercedes-Benz SLS AMG Coupé Black Series vehicles in 2013. Since then, further design and process developments were applied cost-effectively to vehicles with much greater volumes, such as the Mercedes-AMG C-Class C63/C63S coupés/cabriolets (2016), AMG GT R (2017), and AMG Mercedes S-Class (2017). e application also won the 2016 Innovation Award from the Verstärkte Kunststoffe e.V. (AVK) industry association at the Composites Europe conference. Although the CFRP and hybrid struts are more costly than earlier steel struts, they meet Daimler's ambitious lightweighting requirements and achieve other benefits as well. "Local stiffness improvements in the right areas lead to a more precise, harmonic, trustful driving experience," notes Bernhardt. "at means that less-experienced drivers can drive more safely and faster, and experienced drivers can drive much faster because it takes less time to understand the character and behavior of the car." He adds that final handling evaluations are subjective and measurable benefits are vehicle-specific. "For an automotive development engineer, the effort to develop a CFRP part compared to a metal one is significantly higher," Füller reminds us. "Finding a partner that is capable to handle a large automotive series with all boundary conditions, including a satis- factory profit margin, has been a quite challenging journey." None- theless, he adds that while the opportunity to convert metal struts to CFRP on passenger cars is limited, still the lessons learned in terms of design and bonding can be applied to other composite structures. A quick look at CFRP innovation on other Mercedes- AMG vehicles shows the team has done just that. Hybrid Composite Struts Read this article online | short.compositesworld.com/AMGstruts Contributing writer Peggy Malnati covers the automotive and infrastructure beats for CW and provides communications services for plastics- and composites-industry clients. peggy@compositesworld.com Final preparation for underbody installation After pultrusion, the profile for the hybrid composite strut body is cut to size, holes are drilled on either end and bushings are installed. Next, the surface is prepared and mounting hardware (force-transmission points) is bonded (via structural epoxy adhesive) to both ends of the strut. All metallic hardware that directly touches the struts is protected by either coatings or adhesives or both. Source | SPE Automotive Div. (top) and Daimler AG (bottom)

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