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Moving Toward Structural Health Monitoring For Composites

Solid structural health monitoring for composites is the next goal of the MRO industry, and advances are being made on many fronts.

The expanding role of composites in airframes and engines is spurring important advances in nondestructive testing (NDT) and perhaps the beginning of real structural health monitoring (SHM). Composites require less maintenance than aluminum parts but still need inspections and repair. Doing so economically is more critical as composite parts become bigger, more numerous and less accessible.

Carbon-fiber-reinforced polymer (CFRP) composites are used in airframes and outside the hot section of engines. NLA Diagnostics makes a handheld instrument, Cheetah, that automatically taps and tests CFRPs. CEO Rick Webster notes that in the past engineers manually tapped composites, listening for abnormal sounds indicating defects, but the procedure was subjective.

Cheetah has computerized tapping, generating up to 10 taps per sec., capturing and interpreting sound data. The portable device stores and displays data on a smartphone that can be interpreted by an entry-level technician.

Cheetah standardizes the taps in force and angle, and its software grades the returned sounds according to seven parameters. These are reduced to a simple red or green for the tech. The stored data also allows an expert to assess any damage in more detail or can aid in predicting possible future problems. For example, if the tap data flashes green because it falls within an allowable damage limit, the part may remain on wing but be listed for reexamination in six months to determine if delamination has increased.

Bill Choate, managing director of global development, says another advantage of automation is that defects can be detected by tapping only one side. Manual, subjective taps require tapping both sides, and that means expensive removal of composite structures in places like empennages and bulkheads. 

Cheetah’s one-side tap capability also comes in handy for testing composites inside engine nacelles like the inner fixed structure and inner barrel, which are susceptible to water damage. 

Cheetah can test only one side of any honeycomb composite and any solid composite with up to 20 layers of laminate. Generally, it will spot defects down to 0.8 in. on the near side and up to 0.5-1 in. on the far side.

Webster emphasizes that Cheetah is for composite maintenance, not for manufacture, where finer inspections may be necessary. It is now being used to test U.S. Army helicopter rotor blades, and one Midwest MRO tested the tool for inspecting radomes. NLA is seeking FAA approval for the device. It is designed to be easier to use than ultrasound, which requires substantial training, calibration and accessories. Webster says a tech can become competent on Cheetah in 30 min. Other NDT technologies such as thermography, X-ray and eddy current are less mobile but can have finer detection capabilities.

Instead of inspecting composites, SHM would continuously track their condition. One SHM method looks for damage to CFRPs with acousto-ultrasonic piezoelectric transducers that detect changes in CFRP materials. Amrita Kumar, executive vice president of Acellent Technologies, says the approach needs three elements: a network of transducers or sensors; diagnostic hardware to collect data; and intelligent software to interpret data.

Building on research at Stanford University, Acellent has developed a thin layer of film, “Smart Layer,” in which a network of transducers is embedded. The highly flexible film can be installed in new CFRPs or retrofitted on existing CFRP structures. Each transducer has two modes, active and passive.

To look for damage actively, one transducer is turned to active while others receive signals. This is repeated for each transducer in the network, with one actuating and the rest receiving, until a complete scan of the CFRP is complete. It is then compared with the previous scan to spot changes. Software detects damage and where it is located, then quantifies the amount and, to some extent, the type of damage.

The Acellent system can be installed in two ways. With sensors, hardware and software onboard the aircraft, it is a true SHM system that can constantly monitor composite structures. Or, if only the Smart Layer is permanently installed and interpretive elements are on the ground, the technique is like NDT for easy, periodic ground inspections. In either case, processing hardware is small-—a tablet PC for ground analysis or a tiny lightweight onboard processor.

Kumar says the system can be used for all sizes, types and shapes of CFRPs. Retrofits would be completed by laying transducer film on an external surface. For line-fits, film could be added as a layer embedded inside CFRPs during manufacturing, because the film meets pressure and temperature requirements for CFRP curing.

The system would detect damage of a certain size, specified by the user. This would determine the density of sensors in the film. “We would not pick up microscopic damage; it is designed to detect damage that is significant for the composite,” Kumar notes.

Sandia National Laboratories is now flight-testing the Acellent system for FAA certification. Senior scientist Dennis Roach says he expects to have assembled data for the agency toward year-end. “As a turnkey SHM solution for composites, Acellent is probably the leader,” he says.

The Sandia scientist says aircraft manufacturers are interested in SHM and now have dedicated groups working on both in-house and outsourced approaches. But it is still unclear when SHM might be installed. “There’s definite interest, especially in detecting an impact. And there may be an opportunity for SHM as new aircraft get older [to see] trends they may want to monitor more closely.”

Others are busy with similar approaches to SHM. Meggitt’s PiezoPaint can print transducers on many surfaces, including CFRPs, using screen printing or even spraying. PiezoPaint offers low weight and coverage of large surfaces. Chief Technology Officer Keith Jackson says transducers are printed in strategic spots, then connected by printed conductors.

As in Acellent’s approach, the technology requires hardware and algorithms to interpret results. “You could send data to a ground station and have ground staff interpret it, or you could have algorithms onboard,” he says.

The printed transducers triangulate to locate damage. Jackson says the challenge is making the business case. “Big aerospace structures are designed not to fail; they are overdesigned to allow a certain amount of damage. We think SHM use will focus on areas with real risks of damage, like areas around doors.”

Another SHM application might be for line-replaceable units, to detect how much useful life is left. 

Germany’s BeanAir has developed a Wireless Sensor Network (WSN) for space applications that could be suitable for monitoring airframe CFRPs. CEO Damon Parsy says the network is very light, weighing just 100 grams (3.5 oz.) per 50 cubic meters. And it meets aerospace requirements for reliability, low power and accurate synchronization. A power manager can exploit harvested energy, for example, from solar power or vibrations, while a lithium-polymer battery can operate WSN for up to six months. Airbus, Zodiac Data System and Snecma have begun testing the WSN for SHM and other uses. 

TAGS: Technology
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