Scientists from Iowa State’s Center for Nondestructive Evaluation have developed electronic instrumentation to test for aircraft defects where tapping a coin once was used.

The new device, called the instrumented tapper, is an inexpensive and reliable way to test the integrity of composite material on aircraft.

“Aircraft have a lot of parts made out of composites,” said John Hsu, senior scientist at the Center for Nondestructive Evaluation.

These materials are very lightweight and very strong, he said. They are generally honeycomb structures covered with various materials.

Inspecting the integrity of these materials often is done by tapping a coin on them and listening for a hollow or “dead” sound, Hsu said.

“They tap around to hear the sound change,” he said. “That is very subjective. Our instrument is computerized. It gives you a picture.”

This then indicates whether the material is as stiff as it is supposed to be, Hsu said.

“You will see a region which shows a reduction in the stiffness,” he said.

Without the tapper, it is hard to get a good idea of what the damage is, Hsu said.

“You don’t know the size, shape or the degree of damage,” he said.

The tapper has an accelerometer, which is a sensor with a crystal inside, and a brass tip on it to tap the material, Hsu said.

“When you use this sensor to do the tap, it experiences a sudden impulse of pressure. The crystal is such that it gives you a voltage output,” he said. “You get the picture from this output.”

The main figure to concentrate on is looking for the duration of the impact from the tapper, Hsu said.

“That is the key quantity we measure,” he said. “A long duration indicates damaged material.”

Hsu compared the procedure to dropping a bowling ball on cement instead of dropping it on a mattress.

The duration of the impact would be much longer on the “stiff” concrete than it would be on the mattress.

The data from the tapper is then analyzed with software developed by Nordica Hudelson, senior in mechanical engineering.

According to the Institute for Physical Research and Technology [IPRT], which took part in the research, “The computer displays the impact duration as an image while the data are being acquired and, with the click of a button, converts the data into an image of the local mechanical stiffness of the tapped region.”

The device already has been tested at Northwest Airlines, American Airlines, TWA and Iowa Air National Guard. Hsu said the results were positive.

The group is hoping the device will be used commercially in the future, he said.

“We have quite a bit of interest from the users and also from some vendors,” Hsu said.

The project was funded by the Federal Aviation Administration through IPRT.