Composite analysis for Ford GT

Automotive Design & Production, Feb, 2005

The Ford GT has a curb weight of just 3,485 lb. Which means that there are a whole lot of lightweight components used in the assembly of the vehicle. One such component is the deck lid inner. Unlike other cars, the deck lid inner is a rather large component, measuring approximately 7 X 5 ft, as it encompasses the engine and rear wheel wells because of the clamshell-cover configuration. Initially, the deck lid inner was to be a four-piece aluminum assembly. But based on an analysis of weight and costs (manufacturing, tooling, assembly), an alternative was sought. The selected alternative is a unidirectional carbon fiber-epoxy composite component produced by SPARTA Composites (San Diego; www.composites.sparta.com). SPARTA is better known as a defense contractor than an automotive supplier. In the case of the GT, it was a subcontractor to Mayflower Vehicle Systems, which is responsible for the body-in-white.

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Initially it was proposed that SPARTA use the process deployed to make the hood for the 2004 Commemorative Edition 206 Corvette: laying up unidirectional prepreg tape on an Invar alloy mold, then curing it in an autoclave (see: http://www.autofieldguide.com/articles/080306.html). "Unidirectional tape is more expensive than aluminum and more difficult to lay up than a woven prepreg," says Gary Lum, SPARTA engineer. So given that the aluminum assembly was deemed to be too expensive, another alternative had to be found. Which was to figure out how to effectively and efficiently deploy the unidirectional tape, especially as regards the lay up. One of the things the company did was deploy FiberSIM software from VISTAGY (Waltham, MA; www.vistagy.com). This software works within a CAD system (in this case CATIA v5). It permits the user to perform part and process analysis for composite components. So one of the things SPARTA engineers used it for was to simulate the manufacturability of the decklid component, to determine how the layers would need to be applied, and how many of them in which sections. Also, it was used for strength verification; says JT Lyons, SPARTA program engineer, "With a part this thin--1.5 mm--it was crucial that we avoid splice stack-up. If, for example, three splice ends met on top of each other, that point would be considerably weaker than the rest of the part." Also, they used it to be assured that the part as processed wouldn't have wrinkles or buckles, because the deck lid inner isn't painted. In addition, the software was used to determine how the flat patterns for the material needed to be cut before the tooling was available, which saved time in this time-constrained program.

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The part as produced, incidentally, weighs just over 14 lb.