Diagnostic method tests integrity of composite military materials

Researchers at Purdue University have developed a new vibration-based technique that can pinpoint damage in composite materials that will be used to fabricate future military vehicles.

Just as a spider strums specific fibres of its web and listens for returning signals to detect prey, a technique developed at Purdue University uses vibrations to pinpoint damage in composite materials for future military vehicles.

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 and are held together in a glue-like matrix. Because they are strong and lightweight, such composite materials are increasingly being used in missiles, aircraft and other weapon systems, including a new type of armour in future tanks.

Although this armour will be far more effective than metal armour, the composite material does have a significant weakness: whereas damage in metal armour 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 armour, 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 aluminium airframe fuselage skins or to transportation infrastructure such as bridges and railways for subways and trains,’ Adams said.

Adams and his fellow researcher 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 can also 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.

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, reconfiguring itself in the event of a transducer failure to make the best use of the remaining sensors.