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Ultrasonic testing uses beams of high-frequency sound waves to detect flaws, measure thickness and evaluate material properties. When used to inspect aerospace structures, ultrasonic testing plays an important role in safety, quality assurance and cost. Its flaw-resolving capabilities can be the difference between life and death. Using ultrasonics, inspectors can examine areas of aerospace structure that would otherwise be uninspectable without dismantling them to gain access to the internal areas. Ultrasonics can inspect and detect damage that is too small to be discovered by visual means.
To protect public safety and security, it is imperative to test aerospace parts efficiently and ensure their quality. So much so, the aerospace industry is regulated by governmental authorities such as the Federal Aviation Administration. Responsibilities for identifying potential problems and finding solutions are shared by the aircraft manufacturers, the airlines and the maintenance centers.
The thin layers used in an aircraft fuselage require a high-frequency, highly focused imaging system to be used. "For aerospace metals, the material is thinner and the tolerances require high-precision measurements, high frequency transducers and wide bandwidth instruments," says Neil Hankinson, applications manager at NDT Solutions Ltd. (Chesterfield, England). "For aerospace composites, the material is usually higher quality than most composite industries, such as marine wind energy, making the inspection easier because of less problems with porosity."
Surprisingly, aerospace testing has similarities with the testing used in the medical industry. "Both often deal with complex geometries," says François Mainguy, vice president of technology at Harfang Microtechniques Inc. (Quebec, Canada). "Both are challenged by laminar structures of subtle acoustic impedance difference-fiber layers vs. tissue layers. Both work with a lot of diffusion-fat/water/muscle vs. fiber/glue. Both suffer from extreme sound attenuation and both require good resolution, otherwise life is in danger. What's probably unique in aerospace is the honeycomb structure which can't be compared to anything else in other industries."
Common defectsUltrasonics can find aerospace defects such as debonding in honeycomb structures and surface-initiated stress cracking in landing gear. It also can detect a lack of fusion in the electron-beam welding geometries and inspect porosity, inclusions and lack of fusion in aluminum friction-stir welding.
"Skin corrosion and engine blade erosion can be quantified with ultrasound," says Andre Lamarre, business development manager of aerospace at Olympus NDT (Waltham, MA) "Airplane windshield thickness also can be successfully measured.
"On the manufacturing side, as composites increasingly become employed in commercial aircraft, the search for delaminations and porosity becomes prominent. Also, the use of new materials and welding methods such as friction stir weld increases the need for ultrasonic inspection. For engines, ultrasound can be used to inspect the titanium billet for hard-alpha particles before final machining."
As fleets get older, fatigue cracks are the most common defect and often the most critical defect on the flying surface of aircraft. Because aircraft are cycled-loaded and unloaded-as they fly, land, taxi and pressurize the cabin, many components are prone to fatigue cracking over time. Even parts that are loaded below the level that cause them to deform can develop fatigue cracks after being cycled for a long time. "The common location and direction of (most) fatigue cracks is known to the manufacturers who specify inspection procedures," Hankinson says. "Fatigue cracks are detected using ultrasonics by observing direct reflections off corners." Fatigue cracks also can occur because of lightning strikes.
The use of focused transducers to inspect fastener holes and lap joints increases detectabilty of fatigue cracks. Sometimes this test requires scanners but most of the time inspection can be done manually. "The use of ultrasonic phased-array technology also improves this inspection by using the steering capabilities of the probe to reach different areas of the fastener holes reliably," Lamarre says. "The use of dimensional images as B-Scan, C-scan and S-scan greatly improves the operator's ability to interpret the size of the defect."
Mainguy says that diffraction is the key to successful ultrasonic fatigue crack testing. "When enough acoustic energy impinges on a crack edge, there is a ‘tip' diffraction echo, a sharp signal with typically 10X less energy than a normal reflection echo," he says. "It is a reliable technique used in many other industries such as nuclear, petrochemical and other structural welding inspections. Phased array is excellent to grasp sharp diffraction because it's covering a lot of the region of interest-the region in which you want to check for defect-from only one probe location. The probe doesn't have to be positioned perfectly to catch diffraction, so it removes the requirement for an operator's dexterity."
In spite of ultrasonic testing's many advantages, it is not the perfect or only inspection tool for aerospace applications. It cannot successfully inspect any material where it cannot penetrate or provide resolution. For components that are riveted and not bonded, ultrasonics cannot pass through the air gap to inspect below it. Carbon fiber in excess of 50 millimeters thick cannot always be tested with generic ultrasound apparatus-special focusing techniques may be required. For sandwich panels, ultrasonics cannot reach the far-side bond except in specific circumstances, and depending on the material combination, is often not reliable for inspecting the near-side bond.
X-ray inspection is still a powerful NDT method when the aerospace geometry is too complex or when the attenuation is too high. "Some honeycomb materials are very difficult to penetrate at conventional ultrasound frequencies, but other technologies such as resonance-based bond testing with low frequency ultrasound can be used successfully," Lamarre says. "Very small and critical surface cracks in engine blades and around fasteners can more reliably be detected with eddy current technologies."
Ultrasonics evolvingToday's ultrasonic processes and tools have become more sophisticated and desirable for the aerospace industry. Probe sensitivity has been greatly improved by the introduction of composite transducers. New ultrasonic systems are doing a better job finding inhomogeneous resin distribution, high-resolution delamination, sandwich structures defects, metal skins bonded to each other or foam cores, and general misalignment.
"Handheld, battery-operated equipment such as thickness gages and flaw detectors feature greater operator convenience through improvements like highly visible color displays," Lamarre says. "The democratization in the use of ultrasonic phased-array technology has totally changed the picture as now the operator can benefit from the higher accuracy and speed provided by that technology. Instruments based on digital signal processing can provide greater measurement accuracy, resolution and setup repeatability than old analog designs. More sensitive probes allow detection of smaller indications. The use of imaging technology permits clear visualization of cracks and similar internal features of a part."
New scanning equipment allows storage of full data for logging and future review. New flaw detectors are beginning to introduce the concept of storing the procedures on the instrument so that the inspector can use them as a manual as well as an inspection device. Also, "newer ultrasound systems produce images that are much easier to interpret than the A-scans of old," says Dr. Lawrence Busse, vice president of engineering at USUT Labs (Tulsa, OK). "The ability to see a clear image increases inspection speed and reduces the incidence of false-calls."
Buyers and operatorsAs advanced as ultrasonics has become, wisely purchasing and correctly operating the equipment is still requisite. Milman says that buyers should first determine their requirements and ask themselves what is it that they need. Other factors such as ease-of-use, portability, after-sales support, applications support, technological expertise, instrument lifetime and the manufacturer's reputation are all important as well.
"Have the instrument and technique been shown to be effective for the type of defect you need to detect?" is a question that Busse says buyers need to be ask. "Have the relevant probability of detection (POD) studies been done to show that critical flaw sizes can be detected? Does it provide results that are easy to interpret? Can my staff be trained to operate and handle this new system in a reasonable length of time?"
Correct training of ultrasonics aerospace equipment is surprisingly routine. "For aircraft maintenance, the predefined procedures usually give all the required information on how to set up the ultrasonic instrument and interpret test results," Lamarre says. "The airlines provide training for their inspectors. Equipment now has the capability to save setups, so that all inspection parameters are saved. The operator can then use these predefined setups."
Standard ultrasound digital flaw detectors are relatively easy to use. "They're pretty much all equivalent in functionality," Mainguy says. "Not much can be done more to optimize them. Phased-array instruments are not yet as mature and in that respect, they radiate complexity to the operator-while in fact the produced images are easier to understand."
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