Design of a carbon fiber isogrid-­‐s1ffened automo1ve suspension arm A carbon fiber suspension arm is demonstrated, showing an impressive 40% weight savings compared to its aluminum version. An isogrid rib paAern was employed to increase part s1ffness and stability. The composite system is comprised of vinyl ester and chopped carbon fiber molding compound, and unidirec1onal prepreg in a toughened epoxy matrix. A rubber isolator bushing is co-­‐molded with the arm, reducing the need for secondary opera1ons. Ball joints are inserted into bores machined directly from the arm. 1 Introduc1on Demand for overall weight reduc:on of automobiles is ever-­‐increasing; lower weight allows lower emissions, be?er performance, longer range, and less costly ba?eries in the case of electric vehicles. Manufacturers are willing to pay for these weight savings. Aggressive CO2 emission targets set for 2025 can only be reached through increasing electrifica:on, and BEV’s benefit most from light weight structures [1]. Wheel Side Coil spring/ Damper Chassis Side Fig 1: Automobile suspension lower control arm Suspension arms have been evolving from cast iron, through steel stamped and welded assemblies, to aluminum cas:ngs and forgings, and recently to composite construc:on (Fig. 2). The only material capable of replacing aluminum in this applica:on is carbon fiber, whose cost premium now fits into the accepted cost of lightweigh:ng. It has already been shown that composite arms can be 20-­‐30% lighter than aluminum [2], and the present work has pushed the weight saving to 40% over the baseline aluminum part. Cast Iron
Stamped/welded steel Aluminum Composite Δ30-­‐50%
Δ30-­‐50%
Δ20-­‐40% Fig 2: Evolu9on of the suspension arm. Delta values indicate typical mass savings between the alterna9ves. While their tensile strengths are similar, the modulus of the composite is lower than aluminum (however the UD content ameliorates this deficiency). The task of the SMC is to form webs, ribs and bushing bosses by flowing into mold cavi:es that unidirec:onal material could not fill. It also imparts a level of orthotropy to these features required by the mul:axial loading condi:ons, which would be very difficult to achieve with con:nuous fiber materials. Unidirec:onal plies are placed in the web of the arm where axial forces dominate. The design of the aluminum arm was modified to suit the lower modulus of the composite. In order to minimize the weight of the arm, a truncated 25mm cell isogrid structure is used (Fig. Ball joint 3), which stabilizes the thin (~1.5mm) Isogrid unidirec:onal component in the web of the arm, and Rubber isolator bushing provides shear flow paths between flanges, the web and the Fig 3: Lower control arm assembly (Patent Pending) bushing housings. The top of the isogrid ribs are cut to eliminate material with low contribu:on. FEA analysis of the composite arm led to a design with peak stress of 221MPa, and peak displacement of 7.8mm (see Fig. 5). The weight of the as-­‐molded composite arm is 795g, including the threaded steel insert. The front lower control arm uses one rubber isolator bushing and two ball joints. The bushing is insert-­‐molded during the press molding opera:on to avoid secondary bonding opera:ons. Bushings are designed in a way to aid mold seal-­‐off. Note that as an alterna:ve, bushings may be pressed into the net-­‐shape bores of the arm as a secondary process. The outer and inner front ball joint bores are machined directly from the composite for the present applica:on, as the joint assembly is pressed in at low interference and located with internal retaining rings. Threaded steel inserts are placed in the tooling manually (this process will later be automated). These are required at load input loca:ons, such as damper and an:-­‐roll bar mounts. The suspension of this vehicle connects to a steel pushrod, so only one insert was required. 2 Aluminum reference design The reference arm is machined from 6061-­‐T6 aluminum, and is the front lower control arm of the vehicle. The part was analyzed for a combina:on of the vehicle’s worst case load condi:ons, based on 1g cornering, 3.5g bump and 2g braking forces. At the :re contact patch these accelera:ons result in a transverse, longitudinal and ver:cal loading of 7500N, 7500N and 10000N, respec:vely. Due to the very low projected occurrence of this combina:on loadcase, its main purpose is to compare ul:mate strengths of the arm designs, not to establish fa:gue life. As a result of this loading, the aluminum arm shows a peak stress of 203MPa and a peak displacement of 6.7mm (see Fig 4). The weight of the arm is 1281g. Fig 4: Baseline aluminum arm FEA
3 Carbon fiber suspension arm design The materials used for the suspension arm were procured from the Advanced Interna:onal Mul:tech’s PPG Division. The part is composed of two dis:nct phases: a carbon fiber SMC material, and a unidirec:onal prepreg. The SMC material consists of 25.4mm long PAN based 12K carbon fiber tows in a vinyl ester matrix. Fiber content is 53%w/w. The unidirec:onal prepreg is a 200gsm 12K standard modulus carbon fiber fabric in a toughened epoxy matrix. Fiber content is 60%w/w. The toughening agents, including carbon nanotubes, were selected to provide transverse toughness for the unidirec:onal stack, as no off-­‐axis reinforcement is used. Chris:an Jansen1, CTO; Anthony Wei, CEO Gaius Automo:ve, Inc., Taiwan 1: chris:[email protected] 4 Manufacturing Both the prepreg and the SMC materials are supplied in a sheet, and are cut on a CNC cuong machine to minimize waste. The composite charge consists of ten flat unidirec:onal prepreg layers sandwiched between stacked and shaped carbon fiber SMC meso-­‐charges. These charges are inserted into both the top and lower mold halves, achieving 80-­‐90% mold coverage. Mold halves are prepared off-­‐line so charge placement does not impact the overall cycle :me. When ready for inser:on, the bushing is clamped between two cylindrical mold inserts. This eliminates the intrusion of the molding compound into the bushing cavity during ini:al mold closure. While the inner bushings for the lower Fig 6: Prototype tooling control arm are posi:oned with their axes perpendicular to the control arm plane, upper control arms have bushings parallel to this plane. For such arms, the bushing assemblies are rotated by 90 degrees but otherwise the inser:on process is iden:cal. Molding opera:on is carried out at 140°C. Mold closure speed is 5-­‐8mm/s. The press is switched to force control mode for final compac:on, through which a target compac:on pressure of 100bar is reached. Cure :me is 10 minutes, driven by the epoxy matrix of the unidirec:onal layers. Overall process dura:on is driven primarily by the cure :me requirement of the epoxy resin used in the unidirec:onal prepreg. Charge placement is currently 4-­‐5 minutes, bushing inser:on takes only seconds. Together with the 10 minute cure :me and material loading and unloading, the complete cure cycle is under 11 minutes. This work is easily scalable and transferrable to other suspension elements, and other highly loaded structural parts of the vehicle. Such parts include currently cast aluminum door and window frames, suspension and subframe mounts and composite chassis inserts. As always, subs:tu:ng more expensive materials have to be evaluated in terms of overall vehicle cost, emissions and performance benefits. About Gaius Gaius is a design house capable of product, material and process development from concept through produc:on to disposal/recycling (cradle to grave). Gaius is experienced with composite structural laminate design and analysis, process development and verifica:on, and has a processing porqolio that includes hand layup, prepreg compression molding, RTM, resin infusion, bladder molding, braiding, joining, machining and assembly technologies. Gaius has established a posi:on for leveraging Taiwan’s mature composite manufacturing industry for high quality, high volume produc:on. References [1] McKinsey & Company, "Lightweight, Heavy Impact," Allianz Faserbasierte Werkstoffe Baden-­‐Wür?emberg, Baden-­‐Wür?emberg, 2012. [2] P. Feraboli, "Lamborghini Forged Composite Technology for the Suspension of the Sesto Elemento," Automobili Lamborghini Advanced Composite Structures Laboratory, Sea?le, WA, 2011. [3] C. Gomez, "Global Auto Report," Sco:abank Economics, Toronto, Dec. 27, 2012. Fig 5: Carbon fiber isogrid arm FEA
An aluminum suspension control arm was successfully redesigned for composite material subs1tu1on. The weight of the composite arm is 40% lower than the baseline, with only marginal increase in deflec1on. A weight saving of this magnitude is made possible by the effec1ve placement of unidirec1onal fibers, and the isogrid-­‐s1ffened topology. www.JECcomposites.com