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.
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.
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.
Simulation is a natural solution to these problems. But just as design has had to adapt to wider plastic and composite use, so does simulation. Specifically, a large blind spot in simulation technology has to be eliminated — namely, accurately representing plastics and composites.
Most simulation solutions represent plastic as “black metal” and composite as “black aluminum.” Most vendors have incorporated some level of non-uniform material behavior into their solutions. However, it’s only at the surface level. A truly realistic model requires an intelligent handle on:
- Individuals properties of the fiber and the matrix;
- The composition of the overall materials; and
- Manufacturing processes’ influence.
Conventional simulation tools do an excellent job of modeling a party’s geometry, loading, deformation physics, etc. Incorporating detailed material behavior for composites drives further precision into the simulation lifecycle.
Unlike metal properties, plastic and composite properties can change significantly depending on the manufacturing process they go through. A metal-stamped part will behave the same way, for the most part, regardless of how it is produced. By contrast, the manufacturing process can change a fiber-reinforced plastic part’s behavior significantly because the process can affect the orientation of the fibers in the material’s epoxy resin matrix. Properties listed in a material manufacturer’s data sheet often do not take the manufacturing process’ influence on the material into account.
Design engineers must take that into account to guard against failure. Designers can approximate how the new material will perform, but that inevitably leads to over-designing to guard against failure. Over-designing undermines the purpose of designing with plastic or composite in the first place — using less material and reducing weight. It also adds unnecessary cost.
Simulation technology developers must respond with a new generation of simulation tools that accurately represent plastic and composite behaviors in real operational conditions.
Simulate, Don’t Prototype
Automotive OEMs cannot afford to produce physical models to evaluate every iteration of a part of a subsystem. We’ll use the term prototyping to describe this part- and assembly-level testing, though in the automotive industry, that term is usually reserved for complete vehicles assembled for system-level track or crash testing. At the part and assembly level, physical model testing usually consists of production part approval process (PPAP) builds or validation tests as opposed to full prototyping.
Regardless of what you call it, minimizing physical modeling and testing is a key imperative in automotive design. Composites are much more expensive than metals. Their cost is offset by properties that enable engineers to combine many functions and parts into a single assembly. However, if it takes more physical modeling to perfect composite-based designs, their advantages erode.
This is the way designing with composites should work: An automotive OEM wants to re-design a metal engine mount in composite to save weight. Design engineers develop the basic geometry for the new mount in a 3D CAD environment. It weighs 1.2 kilograms. Simulation reveals that the engine mount performs its function under normal loads and in normal operating conditions.
Through finite element analysis to analyze the composite’s behavior in that shape and function, the design team does a series of iterations, simulates the mount’s performance, and reduces its mass by 40 percent without compromising performance. The lower mass shaves 15 percent from the mount’s cost.
This is what design teams can achieve when they have the tools to model and simulate new materials with the same precision they have for modeling metals. Plastic and composite simulation is an essential process for developing the new vehicles that will deliver higher fuel economy and lower costs and carbon emissions.
This year, BMW plans to put its zero-emission i3 sedan into full production. It will be the first mass-market vehicle with a carbon composite body. The light body enables the plug-in hybrid to go farther between charges. BMW is estimated to be three to five years ahead of the industry in large-scale composite vehicle manufacturing.
With the cost of finite element analysis technology dropping and usability improving sharply, other automotive OEMs have a valuable tool for creating their own lightweight vehicles. High-end simulation and analysis will enable them to work with composites and plastics throughout the design cycle, from the idea phase to end-of-life. The key is accurately simulating the behavior of these new materials in actual working conditions on a computer screen and not through an endless string of expensive prototypes.
This article was written by Dr. Roger Assaker, founder and CEO of e-Xstream Engineering (Mont-Saint-Guibert, Belgium) and Chief Material Strategist at MSC Software (Santa Ana, CA); and Dr. Prabhakar Vallury, Director of Business Development at e-Xstream Engineering. For more information, Click Here .