Lightweighting Special Coverage: Optimized Mass Reduction

The use of simulation and smart engineering can lead to better products. Here’s how.

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One of the most impressive vehicle structures to be developed of late is that for the Cadillac CT6. This vehicle not only combines a variety of materials—high-strength steels and high-strength aluminums, among them (overall, it is 38 percent steel and 62 percent aluminum), and we mean literally joining them, as in using a variety of processes, as in steel and aluminum spot welds (there are 3,073 alloy spot welds), flow drill screws, self-piercing rivets, laser welding, aluminum arc welding, and 180 meters of structural adhesives—but there are also sophisticated castings, including 13 complex high-pressure die castings that make up the lower structure of the CT6.

The car is not only light for its size—3,657 pounds, which makes it even lighter than some midsize European luxury sedans and know that the CT6 is a full-size car—but it is also the most torsionally rigid Cadillac ever developed.

To create the architecture for the CT6, engineers conducted 50-million hours of computational analysis, including 200,000 structural simulations. (It is also worth noting that to develop the car, 21 patents were generated.)

This past August the Cadillac CT6 received the 4th annual Altair Enlighten Award, presented by Altair (altair.com), a company that specializes in software for simulation and development, and the Center for Automotive Research (cargroup.org). The award is presented to products that have achieved significant weight reductions.

An award was also presented to a supplier, ContiTech (contitech-usa.com), for a polyamide rear cross beam that is used for the 2016 Mercedes S-Class (coincidentally a vehicle that the CT6 competes with) which is 30 percent lighter than the aluminum component that it replaces.

S. Richard Yen is the vice president, Global Automotive, for Altair. He explains that the company started out as an engineering services company that, as time went on, came to realize the importance of deploying software for creating better products. One of the areas that the company specialized in early on was topology optimization, which Yen says they became familiar with in the mid-1990s through the work of then-University of Michigan professor Dr. Noboru Kikuchi.

Essentially, Yen explains, topology optimization, which is performed early on in the development process, considers the design space, the physical area that the part or component being developed must fit within, as well as the loads that the object will have to handle. “Through the loads and the boundary conditions,” Yen says, “our software program can find the load paths, where the loads will go through the part.”

Then, by knowing the load paths, engineers can put materials where they will be required. By being able to target the specific areas, materials can be used more efficiently, meaning that there is less wasted mass. This can lead to things like the lighter CT6.

Yen says that in some cases, the load paths may be in places that might seem counter-intuitive, which is why it is important to run the simulations.

What’s more, Yen emphasizes that topology optimization is only one step in the development, that there must be the performance of MDO, or “multi-disciplinary optimization,” looking at various other aspects of the component being developed. That is, NVH, durability, crash and other factors may play roles.
While Yen says that high-performance computing capability makes simulations faster (he notes that whereas it might have taken a week to perform a simulation of a frontal crash, now it can be done in a day), it isn’t just a matter of letting a cluster of computers do all of the work. He emphasizes that engineers must be involved in making assessments and determinations. For example, “You have to have engineering knowledge to understand your manufacturing constraints.”

And, “You can have a beautiful design, but if you want a customer to spend a fortune to fix their car, that’s not going to work. So the engineer has to work with the service engineers, manufacturers, material companies, suppliers and others—there must be a lot of discussions.”

Although lightweighting has been going on for some time in the auto industry, Yen says that he’s seen more innovative approaches to designs—as well as changes in materials and methods that allow those designs to be realized—in the last five years than in the previous 20. He thinks that progress will continue to be made in developing products that are lighter yet meet all of their functional requirements—and then some.