More Composites for Cars?

The emergence of electric cars like the Chevy Volt hikes the bar for materials requirements for automotive applications. Most importantly, cars must become much lighter to reduce power demand. A 10 percent reduction in vehicle weight permits about a 10 percent reduction in battery size.

One of the most practical short-term solutions is increased use of glass-reinforced composites, particularly for structural components such as underbodies now made of steel. Longer term, the answer may be carbon-fiber-reinforced composites, such as those used in the Boeing Dreamliner 787.

GM, Ford and Chrysler have formed a research group called the USCAR Automotive Composites Consortium (ACC) to develop automotive structural components.

"The goal of this is the development of composite automotive underbody that requires structural performance including crash loads with equivalent or superior performance to existing components and with reduced mass and acceptable cost," says Libby Berger, staff researcher at the General Motors' Chemical Sciences and Materials Lab, and co-chairman of the ACC Focal Project Four.

System requirements for the materials under study include:

• They must be manufacturable within a 150 sec cycle time. Two-shifts must be able to produce enough parts for 100,000 vehicles per year.
• Methods for joining underbodies to the steel body-in-white (BIW) must be developed. BIW refers to a bare, welded body shell prior to painting.
• Fabric reinforcements for sheet molding compound must be manufactured quickly.

Two of the three requirements refer to long-term concerns with composites: their ability to be assembled and manufactured quickly. Steel parts can be stamped rapidly, but composites require assembling of the fabric structures that form the shape of the part, and then the curing of the resin matrix system that impregnates the fabric.

Airbus and Boeing have both invested significant time in at least partially automating manufacturing systems for their carbon composite aircraft.

But automotive production requirements, at least in terms of speed, are much more demanding than are those for giant aircraft. Progress, however, is being made.

The ACC group has studied three types of composite materials: sheet molding compound (SMC) using vinyl ester as a matrix with glass fabric and chopped glass reinforcement, long-fiber thermoplastic, and urethane long-fiber injection. Carbon fiber is not being considered now because of its high cost. Existign resources are also strained by booming demand for the Dreamliner and the Airbus XWB, now in production.

"Based on the ability of each to meet program requirements and a technical cost model, glass-fabric SMC with a low-density SMC core was selected as our material and process system," Berger says.

Today, SMC component manufacturers use chopped fiberglass and resin to mold a variety of parts such as wheel housing supports, instrument panels, and appearance-grade parts, such as trunk lids and doors.

The SMC system envisioned in the ACC analysis, reinforced with ribs, replaces up to 17 steel stampings and reduces weight by 25 lb.

Adhesive Bonding
A combination of spot welding and adhesive bonding is used to join the composite underbody to the steel body-in-white. Quasi-static and dynamic tests are being performed to determine key parameters to be modeled by finite element analysis.

The ACC group and the University of Massachusetts Lowell are conducting research to see how fabric weaves and directions may allow fiberglass fabric to be used most optimally. Fiberglass fabric is the preferable reinforcing material over chopped fiberglass because it will allow a large panel to withstand structural and potential crash forces during the life of the vehicle.

One challenge is understanding how fiberglass fabric will deform as it takes the shape of a part in the molding tool. While chopped fiberglass SMC flows easily within the heated tool, fiberglass fabric performs differently.

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 UMass Lowell Professors James Sherwood and Julie Chen are using computer simulations of the sheet molding compound to study the deformation of the glass reinforcement fabric, both without resin and as compounded, into fabric SMC.

"They found that the simulation of the complex fabric SMC deformation behavior
 is possible with certain simplifying assumptions," says Berger. "We also performed experiments to determine the shearing, tensile and frictional behavior
of the material, as well as drapeability of multiple layers of the fabric SMC."

FEA Modeling
As a result of the study, design engineers will be able to better predict how the forces of molding the component will affect the glass fabric SMC, thus understanding what geometries can be used to design a lighter weight structural underbody.

"Being able to predict the behavior of the fabric SMC is extremely useful in developing the processing of the structural underbody," Berger says. "This technology will play an important role going forward with the underbody project and in the future use of structural composites in crash-critical automotive components."

Berger reviewed progress of the research at the SPE Automotive Composites Conference & Exhibition (ACCE). The goal of USCAR, founded in 1992, is to further strengthen the technology base of the domestic auto industry through cooperative research and development.

The Chevy Volt shown as a concept car in 2007 made significant use of lightweight materials, but most of those ideas were abandoned in favor of conventional materials to accelerate the launch of the vehicle, now scheduled for this month. If the Volt is a commercial success, it's expected to become a test bed for innovative materials technologies, such as increased use of composites.