constructed from polymer composites might be built with a continuous glass FRP or carbon FRP
structure made by liquid molding techniques, with chopped fiber composite skin and closure
panels made by stamping methods.Liquid molding can be used to make entire body structures in large, integrated sections: as few
as five moldings could be used to construct the body compared with the conventional steel
construction involving several hundred pieces. However, a number of manufacturing issues must
be resolved, especially demonstrating that liquid molding can be accomplished with fast cycle
times (ideally 1 per minute) and showing that highly reliable integrated parts can be produced that
meet performance specifications. Suitable processes have yet to be invented, which is the principal
reason that the composite vehicle is used in the 2015 “optimistic” scenario.^11 At present,
manufacturing rates for liquid molding processes are much slower than steel stamping rates
(roughly 15 minutes per part for liquid molding, 17 seconds for steel), so that order of magnitude
improvements in the speed of liquid molding will be necessary for it to be competitive.While advocates of automotive composites point to the General Motors (GM) Ultralite as an
example of what can be achieved with composites, in some ways this example is misleading. First,
the Ultralite was manufactured using the painstaking composite lay-up methods borrowed from
the aerospace industry, which are far too slow to be acceptable in the automotive industry.
Second, the Ultralite body cost $30 per pound in direct materials alone (excluding manufacturing
costs). This is at least an order of magnitude too high for an automotive structural material.
Performance
Sometimes a new material offers a degree of engineering performance that cannot be met using
a conventional material. For instance, the high strength of advanced composite materials may be
essential to fabricate flywheels for power storage that must spin at up to 100,000 rpm without
rupturing. Gas turbines may only be economical for vehicles if they operate at temperatures of
1,300° Centigrade or above--temperatures that can only be achieved with advanced ceramic
materials. Similarly, the unique formability of plastics and composites make some complex body
designs feasible that simply cannot be executed in steel.Among the most important performance criteria affecting the choice of materials are yield
strength, elastic modulus, thermal expansion coefficient, fatigue resistance, vibration damping,
corrosion resistance, and density. The most critical engineering characteristics of automobile
design over the past 15 years have been specific stiffness (the elastic modulus of a material divided
by its density) and specific strength (the strength of a material divided by its density). Strength and
stiffness of the car’s structural members directly affect the driving performance, ride
characteristics, and safety. The emphasis on “specific” properties reflects the automakers’ desire to
achieve better performance with less weight.
Specific strength and stiffness properties are an area where new materials excel by comparison
to traditional steel alloys, however, this superior performance must be balanced against their
11 Anotherreason is the current inability to model the crash performance of composite vehicle strictures (see the safety discussion below).