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
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
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.
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
Boeing have both invested significant time in at least partially automating
manufacturing systems for their carbon composite aircraft.
production requirements, at least in terms of speed, are much more demanding than
are those for giant aircraft. Progress, however, is being made
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
, now in production.
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.
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
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.
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.
Click here for larger image
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.
that the simulation of the complex fabric SMC deformation behavior
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.
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."
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.
Volt shown as a concept
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.