The Science of Aging Aircraft

Airplanes, like humans, deteriorate physically during their long lives. Accumulating stresses make it important to have regular checkups and, as the years go by, more frequent visits to specialists.

Like geriatric medicine, aircraft aging science has become more proactive. The aim is not only to detect and repair damage, but also to delay or even prevent the onset of certain problems.

There are two poles of aging aircraft research. At the basic research end, NASA is seeking to address aging issues by studying the fundamental physics of damage processes. At a more immediate level, FAA’s William J. Hughes Technical Center is focusing on self-healing wire, lightweight wire, and non-destructive inspection (NDI) technologies for structural, propulsion and electric systems.

NASA

About one-year-old, NASA’s Aircraft Aging and Durability Project (AADP) focuses on understanding damage processes at the fundamental physics level. The lion’s share of the work (about 75 percent) is basic research. Budgeted at about $58 million for the first five years, the program aspires, for example, to understand cracking in metallic materials at the atomic level, well before a problem would become visible to the eye or conventional NDI methods. A possible outcome of such work would be the design of more resistant materials.

Among the factors that drove program design are the expense of current procedures and the emergence of new materials. Managing degradation is very costly and time consuming, said Rick Young, the program’s principal investigator. The U.S Department of Defense in particular expends a high number of maintenance, inspection and replacement hours per flight hour. "Because of limited capability in nondestructive inspection, maintainers are routinely disassembling major components to look for corrosion and cracks. If there is uncertainty in the effect of damage on component integrity, parts which may have a lot of life left in them are replaced. Finally, the replacement parts may perform no better than the original part, which means the parts may have to be replaced again in the future," said Young.

New materials and structural concepts also are emerging and NASA hopes to identify some of the "degradation mechanisms" of complex composite materials. Instead of a single focus on the "senior citizen population," the agency aspires to a more holistic approach, considering the entire life cycle from design through sustainment, while asking questions about the interactions of materials and structures with forces and environments that may lead to problems.

The program addresses eight "challenge problems:" the damage methodology for metallic airframe structures; the integrity of integral metallic structures; the durability and integrity of composite skin-stringer fuselage structures; the durability of bonded joints, engine fan containment structures, engine super alloy disks, engine hot sections; and wiring degradation and faults.

One thread of the metallic airframe structures emphasis (challenge problem No. 1) looks at crack initiation and growth processes at the microstructural level. Historically, attention has focused on larger cracks, Young said, citing work on residual strength and widespread fatigue damage. But cracks may not become visible until as much as three-quarters of the way through an aircraft’s lifespan, he said. "Yet there are early indications and early damage processes that lead to the formation of cracks — basically, any damage you see at the structural level initiates at the microlevel."

Although a better grasp of damage processes will help maintainers working with traditional aluminum alloys, the main benefit is expected to be in developing new, tailored materials. The knowledge gained may guide the development of advanced materials, but in most cases the actual engineering of them is beyond the scope of the current project.

NASA’s approach emphasizes modeling or computer simulation. At one extreme is the development of computer-based, "atomistic" models. Although computationally intense, they cover a tiny area. A simulation including two million atoms, for example, would translate to about one-tenth the diameter of a human hair. These models simulate "the movement of atoms under load" in a material, explained Stephen Smith, a senior materials research engineer at NASA Langley. The atomistic models are then embedded into finite element models that can simulate a larger piece of material. This "multiscale modeling" is a cornerstone of the AADP program. The idea is to simulate damage behavior at a structural level but at an atomic level of granularity.

Experiments with actual metallic coupons are carried out to validate and tweak the models. While the empirical work is vital, the computer models eventually will provide faster, more flexible and cheaper tools for predicting damage behavior and life expectancy and eventually for designing tailored materials.

The work of experimental validation is starting with relatively simple metal systems, such as single-crystal aluminum, a single piece of the metal in which all the atoms are oriented in a selected direction. But the goal is to move toward more complex, standard aircraft-grade alloys. Pieces of metal composed of multiple, similar crystals with all of the atoms aligned are termed "grains," and the boundaries between areas with different atomic orientations are called "grain boundaries." Scientists are interested in discovering the behavior of cracks relative to grains and grain boundaries and the driving forces required to initiate and propagate cracks within the microstructure.

NASA is able to install a "load frame" containing small metallic coupons into a special environmental scanning electron microscope (ESEM). While the specimen is inside the microscope, users can apply up to 1,000 pounds of force, add water vapor, and expose the sample to temperatures from -20 to 1,000 degrees Celsius. Crack initiation and growth can be observed, not just in a vacuum, as is typically required by scanning electron microscopes, but in conditions which simulate a real aircraft environment. The loads and environmental conditions can also be ramped up and down to simulate flight cycles.

The ESEM is equipped with electron back-scattered diffraction capability, which enables researchers to "see" cracks develop in relation to grains and grain boundaries, as an electron beam interacts with the specimen. Cracks less than one micron can be observed, something which would not be possible with today’s NDI methods, Smith said.

In the future, a better understanding of where, under what conditions, and how fast cracks grow at the micro level may help designers to design better materials based on today’s metal alloys. For example, a structural area where greater strength is desired may require small-grained material. A sharp edge, by contrast, may require large grains in order to avoid a large number of grain boundaries, where cracks may be more prone to initiate. The research will also improve understanding of current materials, enabling a more accurate estimate of the number of tests required to assure the integrity of a material in the manufacturing process and in operational service.

Electricals

NASA is also focusing on wire faults and deterioration. The program is trying to develop technology to locate and diagnose intermittent faults, as well as identify precursors to faults. Investigators, for example, could look at indicators within the signal transmission characteristics or external indicators such as the "outgassing" of insulation that may signal changes to the properties of the insulation material, Young said. The program intends both to develop measurement devices and a wire "fault library" or database that is intended to be shared. Researchers will also develop physics-based models to try to determine how changes in material properties affect electromagnetic and transmission characteristics, to help infer the state of the wiring system from measured data, and to predict how the damage mechanisms will grow and change.

The program has picked up an existing effort, previously co-funded with FAA, and is partnering with the Naval Air Systems Command to evaluate crimp quality. The Ultrasonic Equipped Crimp Tool has been shown to be capable of measuring attenuation resulting from incomplete crimping, missing wire strands, partial insertion of wiring or partial removal of insulation, Young said. The idea is to be able to interrogate the quality of a crimp not only during installation, but also during service life, possibly detecting deterioration of the connection due to mechanical loads or corrosion. Additional research is developing similar technology to inspect multi-pin connections.

Composites

The NASA program is also looking at composites and other new materials, as there is some uncertainty regarding their long-term performance. Through lab experimentation, NASA hopes to identify possible combinations of structural configuration, material selection, mechanical loads and environmental exposure that may lead to integrity issues in the future, Young said. The agency is looking at premier materials used today, such as graphite epoxy, which are seeing increased applications in primary structures in transport aircraft and rotorcraft, including fuselage structure, wing structure and advanced engine fan-blade containment structures.

Previous research has shown some variation in composite material performance associated with water absorption. The project includes atomistic-level modeling of epoxy systems to study the effect of water, temperature and degree of cross-linking on mechanical properties, and to start to attempt to predict interfacial strength and aging mechanisms of polymers. Chemists are also looking at how modifications to the polymer chains can affect the amount of water absorption. "Some preliminary results have shown some ability to modify water pickup by changing the curing agent," Young said, "but the net effect on material performance is yet to be assessed."

The program is developing damage propagation models for composites, an area where a lot more work needs to be done. Recently, two World-Wide Failure Exercise(s) organized by researchers at the University of Manchester Institute of Science and Technology (UMIST) and QinetiQ Inc., in the U.K., were conducted to assess the accuracy of current theoretical methods to predict failure in composite laminates. Researchers from AADP plan to participate in the third World-Wide Failure Exercise that will be announced soon.

The agency is working with Alcoa to study hybrid structures that layer glass/epoxy "prepregs" and aluminum. The latest hybrid material developed by Alcoa and their partners is called CentrAl. The hope is that the substance will combine the fatigue resistance of composites and the durability of metal. NASA is developing analytic capability for predicting damage growth in such a material to aid from a design perspective.

FAA Research

The FAA Tech Center has funded maintenance-related research regarding electrical, structural and propulsion systems. In the electrical area, interesting proof-of-concept projects have been conducted by the University of Dayton Research Institute (UDRI) in areas such as self-healing wire, smart clamps, excessive vibration sensors and carbon nanofiber technology.

The self-healing wire concept is based on water solutions of polyvinyl alcohol (PVA), a synthetic polymer substance that is used in eye drops and pill coatings, said Bob Kauffman, UDRI distinguished research chemist. The PVA solution optimized by the lab can be thinned down and applied to damaged wire harnesses as a spray or thickened to produce a dip coating for application between the metal conductor and the insulator during wire manufacture. Whenever the polymer/water solution comes in contact with current at cracked insulation or an exposed conductor, the PVA forms a water-insoluble insulator. (The spray has water in it; the treated wire would rely on condensed water that is frequently present on aircraft wiring.)

The self-repair process is quick, according to UDRI. When insulator cracking was simulated by cuts with a razor blade, the healing process was so rapid that the resulting current flow could not be captured on a laboratory strip chart or multimeter. When a 1-2-millimeter gouge exposing the conductor — simulating abrasion — was made in the insulation, the maximum current flow never exceeded 20 milliamps and the conductor sealed itself in about 5 seconds. Lab tests on fairly small wire runs have ranged from 27 volts (dc)/1 amp to 115 volts (ac)/20 amp.

The process is repeatable. Kauffman said UDRI has proven five repair cycles in the lab, but estimates that the material might withstand 20 to 30 damage cycles in the same spot.

UDRI has patented its technology and is talking to "a couple of companies" about commercializing it. Kauffman foresees early adopters in automotive and industrial areas — more work is required for use in aviation applications. It is necessary, for example, to understand the long-term ramifications of the material, which "may not be as stable as the original installation," he said. He envisions the possibility of using it on a short-term basis, to repair wire breaches in the field in order to get a plane back to a maintenance facility where traditional repairs could be made. The concept has been proved to be viable, and the lab is planning to have a major aircraft wiring manufacturer make 500 feet of PVA-treated wire for arc and abrasion testing.

UDRI is also exploring methods to monitor the stability of wire bundles and determine when clamps are about to break, to prevent wires from swinging free and causing chafing and wire damage. One approach is a "smart clamp" that would alert aircraft maintainers when pressure is too tight, too loose or when the clamp has broken. At this point the lab envisions a tiny sensor, "like a postage stamp," which would be placed on a wire bundle where it will be clamped to the aircraft sidewall, Kauffman said.

This sensor would read the pressure when a clamp is closed and "talks to you like an RFID tag," Kauffman said. At this stage, UDRI is investigating RFID tags, but the eventual device will probably use much lower frequencies than current RFID tags to allow penetration of metal aircraft structures.

A parallel, FAA-funded project would use piezoelectric cable to monitor wire bundle status. Piezoelectric sensors detect vibration and output a corresponding electrical signature. A single piezoelectric cable attached along the bundle with ties could detect excessive vibration when clamps are about to break or have broken. Different levels of vibration would produce different signals. Eventually, the lab will choose between the smart clamp and the piezoelectric approaches based on factors such as weight, accuracy and reliability, Kauffman said.

Another area of research involves the development of lightweight, flexible wire. The laboratory initially applied carbon nanofiber material inside Kevlar-like, polymer wire, hoping that the nanofibers would "line up," forming a good conductor. But the nanofibers did not line up to form a conductive layer, so the material was not useful as an electrical wire.

On the next pass at the problem, the lab used a silver "ink" made by Parelec of Rocky Hill, N.J. When heated for a couple of minutes to 200 degrees Fahrenheit, this ink turns into pure silver, an excellent conductor. But when the ink was applied onto the polymer wire and heated, the silver film flaked off as the wires were flexed. Then the lab mixed carbon nanofibers into the silver ink and applied the mixture onto the original wire/nanofiber material. This time, the silver coating did not flake off. The nanofibers in the ink "tangled" with the nanofibers extending from the polymer wire surface, acting like Velcro to help the silver coating to adhere to the inner wire. This wire so far has survived 35 bends without breaking, Kauffman said.

Although this approach is not mature, it is being used in a sensor development program as a convenient means to get electricity to a sensor attached to a nonconductive composite surface. Instead of having to set up an elaborate wire harness, developers can simply "draw" a line with the carbon nanofiber-filled silver ink, heat it with a heat gun, and create a functional conductive trace.

NDI

One approach to detecting insulation breaches was developed by FAA and Sandia and is being commercialized by Astronics. Known as Pulse Arrested Spark Discharge (PASD), the technique transmits a high-voltage spark along the wire. (Because the pulse is a mere 30 nanoseconds — billionths of a second wide — the energy level is very low.) When the pulse encounters an insulation breach, a "harmless spark jumps to ground or other wires... creating momentary short circuits whose locations can be traced," according to Sandia.

Because it uses pulses from hundreds of volts to more than 10 kilovolts, "airlines are a little leery about using it until they understand it," said Larry Schneider, deputy director of Sandia’s Pulsed Power Sciences Center. But the energy levels are extremely low, he said. "It’s similar to the spark you might get walking across a carpet."

Schneider maintains PASD is the best available method of detecting insulation defects. Sandia manufactured a wire harness with built-in defects and invited instrumentation experts to try to locate the faults, he recalled. "The only one that succeeded for insulation breaches was PASD."

FAA and Sandia also have been working with comparative vacuum monitoring sensors developed by an Australian company, Structural Monitoring Systems. The rubberized, postage-stamp-sized, self-adhesive devices contain alternating channels maintained at vacuum and atmospheric pressure. A crack propagating under the sensor would break a vacuum channel and be detected by the subsequent change in air pressure. FAA panel tests showed that CVM could detect cracks 0.02 inch in size. This approach is promising enough for Boeing to have added it to its Common Methods NDT (nondestructive test) Manual. Efforts are ongoing to have CVM approved as an alternate means of inspection in place of eddy current technology.

One advantage over eddy current is that sensors could be permanently mounted in remote locations and then be periodically queried. Another advantage over eddy current involves sealant removal. Use of eddy current inspection requires sealant to be removed and replaced for each examination. But if CVM were used, the sealant could be removed once to allow sensor mounting and then the sealant would be reapplied over the mounted sensor and structure, with vacuum lines routed from under the sealant to an accessible location for later query during periodic scheduled inspections.

Because CVM sensors don’t use electricity, it could be possible to mount them inside fuel tanks, said Dave Galella, inspection systems research manager at the FAA Tech Center. As it is very difficult to put a person into the fuel tank to do an inspection, the technology opens the possibility of inspecting the area remotely. Lightweight plastic vacuum lines, extended from the sensor leads to a readily accessible area, can be plugged into a handheld monitor that takes pressure readings.

Approximately 26 CVM sensors have been tested in trial installations on Delta and Northwest airplanes to prove they could provide a valid signal in airline operating environments, according to Dennis Roach, distinguished member of the technical staff at Sandia. Extensive lab tests were also conducted to validate the technology.

Engine inspection is also a priority at the Tech Center. One promising approach for detecting small surface cracks (0.03-inch long) on critical rotating parts is thermo-acoustic technology. This involves the application of ultrasound energy to the surface of a part. This energy causes the mating surfaces of a crack to vibrate against each other, generating a small amount of heat. A highly sensitive infrared camera is used to detect the presence of any heat source, thus locating the surface cracks. Although this approach might seem problematic, FAA has studied whether it causes any localized damage. An initial report indicates that it has "no impact on the life of a component or crack growth" if the energy applied is kept within established parameters, said Cu Nguyen, project manager for the Tech Center’s Engine Inspection program.

The next step in validating thermo-acoustic inspection technology will be to evaluate it in the lab on a critical rotating part such as an engine disk. The results would then be compared with those from fluorescent penetrant inspection (FPI), a method commonly used to detect cracks. FPI involves the application of a liquid penetrant to the surface of a part, followed by inspection under an ultraviolet light. The penetrant trapped in the surface cracks is visible to the inspector. FPI is widely used because it is relatively inexpensive, but is subject to human error, resulting in poor reliability and reproducibility. Thermo-acoustic inspection avoids most of these shortcomings, according to Nguyen, thereby making it a possible replacement for the current FPI process, provided that it can be adapted to the production floor.

Another promising surface inspection technology is the "magnetic carpet probe" — an array of eddy current (EC) coils capable of detecting very small cracks. The primary advantage is the relatively large surface area it inspects in a single pass compared with eddy current "pencil probes," which inspect a very small area. FAA is working with a contractor to develop an EC array containing 93 coil elements. Although the present prototype covers only a one-by-one inch surface area, the contractor has proved that the technology is feasible. Current carpet probes have demonstrated the ability to detect a 0.03-inch crack in a one-by-one inch area of titanium material. The FAA plans to fabricate a larger carpet probe and to test it on the web and bore areas of an engine disk. This approach could be more reliable and reproducible than FPI.