Handheld ultrasonic camera ‘gun’ finds composite cracks

  • 15-Feb-2012 09:23 EST
GrayCFRP3d0-2.jpg

3-D ultrasonic image of drill holes in CFRP shows a hidden crack leading from the lower hole.

Engineers at DolphiTech AS, a small start-up firm in Hedmark, Norway, have developed an ultrasonic camera gun that airport maintenance personnel can use to scan carbon-composite aircraft components for damage not visible to the naked eye. The easy-to-use handheld device can produce 3-D color images as well as the more common 2-D C-scans of the underlying composite structure.

EADS Innovation Works, the corporate R&D arm of Airbus-parent EADS, recently signed a cooperation agreement with DolphiTech for further development of the innovative nondestructive testing (NDT) system into a commercial product. DolphiTech is a subsidiary of Elop–DolphiScan.

More composites, more inspection

Such NDT equipment is becoming increasingly important as aircraft designers use ever more carbon-fiber composite structures—multiple layers of woven fabric plies embedded in a resin epoxy matrix—in airliners and military aircraft to save weight and cut fuel consumption. Rising adoption of composites can be problematic for maintenance specialists, however, because it is their job to find hidden damage in the lightweight, strong, but surprisingly delicate materials.

Aviation safety authorities can take no chances with even tiny cracks or flaws in carbon fiber-reinforced plastic (CFRP) and glass fiber-reinforced plastic (GFRP) composites because such damage can be the starting (initiation) points for the growth of larger cracks and possible catastrophic failure. Accidental impacts, say, by catering or luggage trucks on the runway, operational use fatigue, or even exposure to extreme heat or water intrusion can cause the materials harm by delaminating the composite plies.

Such structure-weakening damage has become of increasing concern now that entire pressurized fuselages, such as those of the Boeing 787 and the forthcoming Airbus A350 wide-body airliners, are being fabricated from CFRP.

Even when discovered, composite cracks can be costly to fix. Repairs ordered by the European Aviation Safety Agency to the "micro-fissures" that were detected in January in L-shaped brackets (rib feet) that attach the wing skins to their underlying frames in several Airbus A380 superjumbo jets, for example, will reportedly cost about a million euros ($1.3 million) per aircraft. The flaws in the rib feet, which an Airbus spokesman said are not even primary load-bearing structures, have been traced to unexpected stresses stemming from the manufacturing process.

Invisible damage

Unlike aluminum and other metals, which readily show visible effects of impact damage, resin composites can be invisible to surface inspection and therefore hard to detect, said Terje Melandsø, Managing Director of DolphiTech. As a result, airport maintenance staff must often resort to tapping on composite structures with small hammers or other metal objects while they listen for the telltale hollow sounds of flaws or voids that can be deep inside.

Such NDT techniques are steadily becoming more important because of the rising need to maintain efficient flight operations. “The airlines want an instrument that is available at the airport service shop that can inspect potential damage,” Melandsø explained. “It is extremely expensive to hold an aircraft on the ground until an expert can come to determine the extent of any damage and decide if a repair is necessary.”

Using the new technology, an operator without specific training can scan the area for possible damage and rapidly download the inspection data to experts, thus significantly reducing aircraft downtime.

DolphiTech’s new inspection tool can be used for many NDT inspections of composite materials, said Eskil Skoglund, Project Manager. The gun essentially fires ultrasonic waves at the target and deduces from the time of flight of the reflections what lies beneath. The ultrasonic unit operates in cooperation with a laptop computer that runs image-presentation software provided by EADS.

The system’s ultrasonic sensor is a 3 x 3-cm (1.2 x 1.2-in) matrix of 16,000 transmission/reception elements that produce about 10 frames/s, Skoglund said. “One of the technology’s big advantages is that it uses a dry coupling—a silicon polymer plate,” he added. “You do not need to put a gel between the instrument and the material.”

The start-up company, which was founded by Melandsø six years ago, was originally intended to commercialize a device for corporate shipping and handling departments that could read 2-D bar codes that had been painted over or otherwise obscured. “Unfortunately, the global financial crisis hit so we lost our financing,” he recalled.

Sometime later, Melandsø “bought the technology from the bank and started over again.” With the help of his son, Frank Melandsø, an engineering professor at the University of Tromsø, the firm developed the ultrasound bar-code technology for testing composites.

Enter EADS

Last year, DolphiTech began working with the EADS Innovation Center on behalf of Airbus, testing the technology’s viability for composite NDT applications. Right now, the ultrasonic gun is undergoing a stringent qualification process by Airbus specialists in Bremen, said Melandsø, who expects that the unit could be used in production by the end of the year.

“The new system will speed up NDT procedures in manufacturing and maintenance and will help the users to save time and money,” said Martin Bach, NDT specialist at EADS Innovation Works.

Melandsø claimed that the new system is more favorably priced than existing NDT ultrasonic camera systems offered by competitors such as Olympus, GE, and Imperium Inc., which typically rely on phased-array sensors.

“The adaptation of our novel ultrasound technology for impact assessment by EADS will open a very attractive global market for our small business,” he concluded.

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