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400 Commonwealth Drive Warrendale PA 15096 0001 U S A Tel 724 776 4841 Fax 724 776 5760 Web www sae org 2005 01 0469 2006 Corvette Z06 Carbon Fiber Structural Composite Panels Design Manufacturing and Material Development Considerations John Remy David Hamilton Edward D Moss Boyd Pukalo and Gary Zu General Motors Body Structure therefore there is minimal investment cost for floor panel tooling The carbon fiber floor panel interfaces to the vehicle frame and other components exactly the same as the fiberglass version of the floorpanels making the installation seamless to the assembly process Design Requirements Four explicit loading conditions were considered when developing the carbon fiber floorpanel These included 1 Lateral in plane loading that may be seen during a side impact crash 2 Bending and torsional loading floorpanel contribution to vehicle stiffness 3 Occupant seat loading localized stiffness 4 Impact strength These loading conditions were evaluated using a variety of testing and analysis methods The crashworthiness aspects were investigated by installing the floorpanel into actual vehicles crash tested at the Milford Proving Grounds The contribution of the floorpanel to the vehicle level bending and torsional stiffness and the localized stiffness was investigated using finite element models see Figure 3 The impact strength of the carbon fiber samples was evaluated initially simply by dropping a pointed object weighted dart from a fixed height This testing evolved into more controlled impact testing using a Dynatup Details of Carbon Fiber Mat Development The development of the floor panels led by MFG was started shortly after defining the process and material for the C5 Commemorative Edition Z06 hood Many materials including the same UD pre preg as used in the hood were considered in the floorpanel design However the material utilized for the hood development was not practical when applied to floorpanel Various epoxy resin based carbon proposals were considered although the expense of these types of materials and the extended cure times for epoxy resin systems would not allow the floorpanels to provide a mass saving at a cost that met objectives In the case of the UD tape the pre THIS DOCUMENT IS PROTECTED BY U S AND INTERNATIONAL COPYRIGHT It may not be reproduced stored in a retrieval system distributed or transmitted in whole or in part in any form or by any means Downloaded from SAE International by Guangxi University of Tech Saturday December 29 2012 06 14 46 AM preg contains insufficient epoxy resin to provide sufficient attachment between the carbon fiber and the balsawood core Failure modes observed were weak face sheet bonds and dry mold surfaces Several additional versions of the carbon fiber design were investigated These included unidirectional fiber mats multi directional and multi layered woven mats and random chopped fiber mats Several issues with the various carbon fiber mats were quickly realized The unidirectional mats were prone to splitting along the fibers Woven mats were seen to offer impressive strength and stiffness along the directions of the fibers but were seen to be lacking in other directions Additional layers of carbon fiber mats laid in multiple directions were able to address this shortfall but quickly became cost prohibitive Also considered was a random pattern like that of the fiberglass mat used in the existing design A continuous strand carbon mat was sought but was not available A method to cut the carbon fibers while randomizing their orientation had to be developed MFG manufactures glass SMC using a chopper which simultaneously cuts many glass tows into typically 2 lengths Carbon fiber SMC from other suppliers show that the chopping process works with carbon Experiments with carbon show that the fine nature of the carbon fibers and their lower mass meant that the same process used for the fiberglass mats could not be used for the carbon fiber design The conductive nature of the fibers also raised environmental and hazardous material concerns One experiment for a randomizing process for a carbon fiber was to evaluate a directed fiber preform DFP This is also a process that MFG has developed with chopped glass fiber A screen the shape of the part is mounted onto a vacuum plenum and with a vacuum present the fibers along with a liquid binder system are projected onto the screen The preform is removed after a binder cure stage and placed into the mold Liquid resin is added and the mold is closed and the part molded Some parts were molded with DFP Much development work was conducted for a carbon version of this process The resin pour method appears to saturate a glass perform more evenly than a carbon one Shaped performs are attractive for complex parts but the C6 program timing made DFP impractical The cost of carbon roving is affected by tow size a 3K product is more expensive than a 12K increasing the tow size can further reduce cost Unfortunately the preform trials showed that structural performance and reinforcement uniformity improve with smaller tow sizes MFG continued the experiments and eventually developed a method for acheiving acceptable uniformity resin wetting and mechanical properties with large carbon tows in chopped form Various carbon fiber configurations were evaluated A 24K tow was selected because it is theoretically less expensive than a 12K tow The process offsets the coarseness observed right after the chopper and in the molded parts Without this process a more random mat would require lower tow size i e 3K resulting in higher material costs Molding cycle time is a significant component of part cost Several resins systems were experimented with to improve cure times with the goal of having an equivalent cycle to the fiberglass reinforced version of the floor Epoxy resins would not fit those criteria with the faster curing resins taking 10 minutes to cure Epoxy prepregs also require refrigeration another cost driver The challenge was defining a vinylester or polyester resin that would retain the properties that could be achieved with the epoxy systems The finer nature of the carbon fibers and much smoother surface of fibers on a microscopic level proved to be very challenging in finding a resin which would adhere satisfactorily Most carbon fiber sizing development work to date has been for epoxy resin systems As volumes and demand increase faster epoxy and vinylester resins will need to be developed or carbon sizings that are optimized for polyester The pressure molding process in existing tooling including heat to cure the resin in an acceptable cycle time also was a significant constraint in the choice of the resin material MFG calls the final material solution Carbon Molding Compound CMC The chopped carbon mat is processed first through a needling stage and then is impregnated with a polyester resin The compound is made into 100 rolls which are transported to molding operation after 5 days in a maturation room There are currently two versions of CMC The floor uses an unfilled resin and sufficient carbon to produce a 0 9 mm face sheet The wheelhouse outer uses a compound with a filled resin and is molded at 2 0 mm As new parts are considered with specific tradeoffs required there may be requirements for new variants of CMC The floor panels produced with CMC weigh 7 lbs compared to the 10 45 lbs for the fiberglass floors This gives the Z06 2 82 kg per vehicle mass saving for the two floors combined Carbon Fiber Wheelhouses The Z06 front wheelhouse outer panels was the other structural panel selected for design The wheelhouse outers were to be produced with the UD pre preg epoxy tape and the vacuum bag and autoclave cure process As the design progressed the part and tooling cost estimates were more expensive than planned Labor intensive lay up costs and long autoclave cure cycles caused the wheelhouses to become unaffordable relative to the amount of mass being saved putting the project in jeopardy of reverting to a fiberglass reinforced component design See Figure 7 THIS DOCUMENT IS PROTECTED BY U S AND INTERNATIONAL COPYRIGHT It may not be reproduced stored in a retrieval system distributed or transmitted in whole or in part in any form or by any means Downloaded from SAE International by Guangxi University of Tech Saturday December 29 2012 06 14 46 AM Figure 7 The C6 Coupe and convertible Front Wheelhouse Outers are non cored panels which are produced by MFG utilizing a compression molded chopped fiberglass preform and a low density polyester resin system that has a 1 4 specific gravity They are 2 5 mm thick MFG came to the Corvette Engineering Team with another creative proposal The CMC developed for the floors showed some capability of molding parts with shape Reducing part thickness the primary strategy for the floors doesn t work as well with non cored parts Molds are more difficult to build for thinner parts Stiffness in bending is greatly influenced by thickness Random reinforcement is more likely to be crushed as part thickness is reduced The glass reinforced parts considered for carbon were already using lightweight materials MFG had been experimenting with a low density version of their CMC MFG demonstrated with parts made using CMC and existing tools that they could produce CMC parts with a 1 1 specific gravity and 2 0 mm thick This combination of a slightly thinner gauge part made with a light material met the project requirements Structurally the design intent was to match the stiffness and ride frequency characteristics of the fiberglass version of the C6 coupe and convertible models MFG developed the low density version of CMC by adding glass microspheres to the resin mix Microspheres are used in SMC Lite as well as in a lightweight resin mix developed by MFG for C5 parts such as the rear tub Strength is a trade off with the microspheres but the stiffness is the major characteristic to maintain Validation testing determines sufficiency of strength Review of the projected mass savings along with part and tooling costs for the low density CMC proposal put the part back in line with program guidelines saving 73 kg per wheelhouse outer for a total 1 46 kg savings to the vehicle The Corvette Team agreed to continue with the CMC proposal but were concerned that all ride and durability could be met with the thin low density CMC material MFG offered a tooling approach tha