"Strain Sensitive" Paint Highlights Flaws On Parts For Cars, Planes

October 18, 2004

GAINESVILLE, Fla. — A new technique with the potential to significantly improve the design and manufacturing process for automobile parts may benefit consumers fed up with frequent recalls and repairs.

In research sponsored by a major auto parts manufacturer, a team of University of Florida engineers has found a way to use luminescent, or light-emitting, paint to highlight weaknesses and flaws in prototype versions of drive shafts, axles and numerous other parts. The technique could make parts safer, less expensive and longer-lasting.

“The industrial significance of this technique is hard to overstate,” said Peter Ifju, a UF associate professor of mechanical and aerospace engineering and one of the inventors. “If the industry is willing and the economy allows it, this could become routine in the ground transportation and air transportation manufacturing industries.”

The research is described in papers in the most recent editions of Experimental Mechanics, and the American Institute of Aeronautics and Astronautics Journal. The technique is being tested by Visteon, a $17 billion parts company that has spent $2 million on the UF research since 1997.

A key step in designing parts is building and testing prototypes to discover flaws or problems before a part is mass produced. Whether a bicycle frame, airplane wheel or engine cam shaft, an important element of these tests is determining how much stress or strain the part sustains, so that engineers can design it to function over the lifetime of a car or machine. Stress is a measure of force intensity, or force per unit of area, while strain measures deformation, or movement.

A number of techniques are used to measure stress, including the strain gauge, a decades-old device that functions something like a miniature scale. What many techniques have in common, however, is that they take a measurement only at one specific point, have poor resolution for larger areas, or require expensive and cumbersome equipment. These shortcomings raise the potential for designers to miss a “hot spot” — a location on the part that may appear normal to the eye and to computer-aided or “CAD” designs, but that actually experiences significant and potentially failure-level stress.

“Parts makers don’t want that part to go out and fail two years down the road,” said Paul Hubner, a UF adjunct assistant professor of mechanical and aerospace engineering. “The flip side is that they don’t want to overdesign and put too much reinforcement into a part, because if they build a million parts, it’ll cost them more than it should have.”

UF’s “strain-sensitive” coating gives designers a way to accurately measure the stresses applied to large areas or the entire part. That not only highlights potential flaws in a prototype, it helps designers refine their computer-design models, which makes them better at designing the next part.

Sprayed on with a standard commercial spray gun, the coating resembles glossy reddish paint. It is actually a potent mixture of light-emitting dyes and epoxies developed by UF chemistry professor Kirk Schanze and other researchers in his laboratory.

Designers spray on the paint, dry it, then flip on a blue or ultraviolet light. As the light transmits through the epoxies, its polarization changes in direct proportion to the amount of strain the part is experiencing. The light-emitting chemicals then transmit these polarization differences, which are invisible to the human eye, to a digital camera and computer. The result is a graphic, 3-D map of the stress levels sustained over the entire coating.

Ifju and Hubner said they have tested the technique on 3-D parts ranging from a few inches in size to 2 square feet, but nothing restricts it from being used on even larger objects – potentially even an entire car frame. So far the coating has proven effective in experimental tests aimed at identifying previously recognized hot spots on prototype parts, such as a suspension control arm for a Cadillac sedan, a drive shaft for a Jeep Cherokee and a strut dome for a Porsche SUV, they said.

Another benefit is that the technique is much faster than others, Hubner said. The system can provide a nearly complete picture to designers in about three days, while it takes a month for other techniques, he said.

The result is that parts can be designed and built more quickly, which has the potential to translate into cost savings for consumers, Hubner said. The UF-patented technique may also make parts more durable, reducing the number of failures and recalls. Visteon, which holds the license on the technology, is using it on a limited basis, Ifju said.