Automotive design is going through one of its most profound changes since the gasoline engine eclipsed steam power. Fuel prices and growing environmental concerns have made efficiency the biggest prerogative in vehicle design since the gas shocks of the early 1970s. President Obama’s 2011 agreement with 13 large automakers to increase the CAFE (Corporate Average Fuel Economy) standards from 27.5 miles per gallon for cars and light trucks today, to 54.5 miles per gallon by 2025, will affect vehicle design priorities throughout the automotive industry.

Comparison between isotropic and anisotropic approach for two load cases. The Digimat multi-scale material modeling approach correlates excellently with the experiment. (Courtesy of Ticona and ArvinMeritor)
Ford, GM, Chrysler, BMW, Honda, Hyundai, Jaguar/Land Rover, Kia, Mazda, Mitsubishi, Nissan, Toyota, and Volvo — which manufacture more than 90 percent of all vehicles sold in the United States — are all parties to the 2011 agreement. Higher mileage standards will influence their design decisions for at least a decade. The effects will also ripple out through the automotive industry supply chain that provides the major OEMs with parts and subsystems.

Manufacturers of hybrid, electric, and conventionally powered vehicles are experimenting with new designs and materials to decrease weight, or mass, and improve economy. In fact, reducing mass is where today’s efficiency battle is being fought.

Oak Ridge National Laboratory has determined that reducing a car’s mass by 10 percent increases mileage by 7 percent. The EPA says that for every 100 pounds taken out of the vehicle, the fuel economy is increased by 1-2 per - cent. There are also cost benefits to mass reduction. Using 10 to 20 percent fewer materials in a vehicle can reduce its costs 5-15 percent.

Wider use of plastics and carbon fiber composites is a major innovation for reducing weight while maintaining performance. While composites have been used in high-performance sport vehicles for decades, designers are experimenting with them in mainstream vehicle designs to achieve desired fuel efficiency improvements. They’re also expanding plastic use beyond interiors and small mechanical components.

Reducing mass by integrating plastics and composites into vehicle design affects safety, comfort, noise, and quality — all essential properties of a successful vehicle. New material use introduces a new universe of variables into what had been the straightforward exercise of using less steel to build a vehicle without affecting its performance. Reducing steel mass or steel with a lighter metal — usually aluminum — was routine for design engineers because they were working with a familiar material with familiar properties. Metals have consistent stiffness throughout their shape, or geometry, so they deflect and deform predictably.

Micromechanical-based material models are sensitive to the fiber orientation. On the foundation of injection molding simulation, this can be used to predict the local stiffness on the part. (Courtesy of Renault)
Composites and plastics do not. The shape of a part or assembly affects composite and plastic stiffness. Engineers can’t approximate how composites and plastics will behave as they can with metals. They have to validate their plastic and composite designs, ensuring the designs meet requirements using the optimal amount of material. Endless physical prototyping is prohibitively expensive, so validation must occur through simulation software to keep costs in line.

That makes simulation and analysis an even greater need in automotive design than it has been for almost 40 years. But just as vehicle design has to adjust to the times, so does simulation and analysis technology. Plastics and composites do not behave like metals. They have different properties, and for simulation technology to advance designs while keeping costs in line, it has to represent their properties and behaviors accurately.

The Plastic and Composite “Black Zone”

Composites and plastics offer stiffness comparable to metals but at lower weights. That opens up a broad new range of design options to automotive engineers, but it also presents challenges.

The density and length of fibers in a composite’s matrix and the way it is injected into a mold can make it stiffer in one direction than in another. That variation means that an engineer could optimize a composite part design to eliminate extra mass, yet end up with different problems stemming from the composite’s varying properties — reductions in durability or crash-worthiness, or higher noise and vibration levels. Composites and plastics also have lower damage tolerance than metals, which has to be factored into designs.

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