Diagnostic Method Tests Integrity Of Composite Military Materials

The vibration approach, developed by Douglas E. Adams, an assistant professor of mechanical engineering at Purdue, can automatically diagnose the structural integrity of composite materials. These materials are made of layers of ceramics, plastics, metal alloys and fabrics, all held together in a glue-like matrix. Because they are strong, yet lightweight, such composite materials are increasingly being used in missiles, aircraft and other weapon systems, including a new type of armor in future tanks.

Although this armor will be far more effective than the metal armor in today's vehicles, the composite material does have one Achilles heel: whereas damage in metal armor is easy to spot, composite materials sometimes appear to be fine on the outside when there is serious damage on the inside, Adams said.

The new vibration-based technique he has developed could be used to constantly check the integrity of the composite armor, and then could issue a warning if the material is about to fail. The technique has proved to be sensitive enough to detect damage caused even by small impacts like those that might be incurred in the field when a wrench hits the material.

"The method could apply equally well to commercial aluminum airframe fuselage skins or to transportation infrastructure such as bridges and railways for subways and trains," Adams said.

Adams will announce his findings detailing the technique on May 4 at the 16th U.S. Army Symposium on Solid Mechanics in Charleston, S.C. Adams' co-authors are graduate students Shankar Sundararaman and Timothy Johnson and mechanical engineer Elias Rigas, from the Army Research Laboratory at the Aberdeen Proving Ground in Maryland.

The researchers have found that significant damage can be caused inadvertently in the field during transport when "heterogeneous structures" made of composite materials are dropped or struck with on object.

"This impact damage can cause the part to catastrophically fail," Adams said.

Damage from impacts or wear also can cause layers of the composite materials to "delaminate," essentially separating from each other and weakening the affected area.

The diagnostic system the researchers have developed uses a series of vibrating "actuators" and sensors placed around the edges of a part. The actuators transmit high-frequency acoustical waves that hit defects in the material and scatter back toward the transmitting source, where the sensors pick them up.

"Depending on how that scatter is distributed we can tell how big the damage site is, and we can tell where it is, which is precisely what spiders do to locate prey," Adams said. "They send out propagating waves that bounce off the prey. Their tactile sensitivity is extraordinarily fine."

The diagnostic method also has been shown to be very sensitive, he said.

"We haven't been able to hit a structure with anything below a foot-pound of energy and not see the effects. That's what you would get if you dropped a wrench from, say, three or four feet onto one of these parts."

Other researchers have used similar techniques, but they have embedded a large number of sensors and actuators throughout the composite material, which weakens the material.

"What we are doing is using a relatively sparse array of actuators and sensors on the perimeter of a structure," Adams said. "Our sparse arrays do no harm, which is the first requirement for any structural health monitoring system and are much easier to maintain than a widely embedded array if a transducer happens to fail."

Purdue researchers are working on ways to integrate the method into military weapon systems. One possible application will be to detect damage in ramps used to transfer equipment from one ship to another in high seas. Another possible application is in the fan assembly that will enable a future jet called the Joint Strike Fighter to take off and land vertically.

Adams has previously developed a radar-like diagnostic technique in which sensors were placed at specific structural locations that are prone to damage, such as the sites of rivets or bolts. That method was limited to detecting damage only at those locations.

The new technique can detect damage no matter where it is located.

"We can cover a much larger area this way," Adams said.

The method also can be tuned to look for damage in specific directions and to cancel out interference from other vibration and energy sources, such as engines or rotating parts. In addition, the software algorithm used in the method is "adaptive," meaning it can reconfigure itself in the event of a transducer failure to make the best use of the remaining sensors.

The research has been funded by the U.S. Army and through a Presidential Early Career Award for Scientists and Engineers, issued by the White House Office of Science and Technology Policy.