Emerging Technologies For Nde Of Aging Aircraft Structures
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R.E. GREEN, JR. Center for Nondestructive Evaluation and Materials Science & Engineering Department, The Johns Hopkins University, Baltimore, MD 21218
ABSTRACT Several emerging technologies affording new opportunities for the nondestructive evaluation of aging aircraft structures. Among these technologies are ultrasonic attenuation detection of fatigue and corrosion damage; non-contact ultrasound including laser-laser, laser-EMAT, laser-air, and air-air; laser based acoustic emission source identification; full-field double-pulse holographic imaging; non-linear assessment of adhesive bond quality; x-ray diffraction topographic imaging of the quality of nickel based alloy single crystal turbine blades. FATIGUE AND CORROSION DAMAGE MONITOR The first ultrasonic technique used to study the development of fatigue damage during fatigue cycling was ultrasonic attenuation. Although, for over 40 years, this technique has been proven to be the optimum one to detect early fatigue damage, it has not proven useful for field use because of the problem of acoustically coupling the transducer to the structure in a reproducible fashion that does not influence the measured attenuation values. Experiments conducted in the early seventies to monitor the development of fatigue damage during cycling of an aircraft aluminum alloy show that ultrasonic attenuation can detect fatigue damage much earlier than conventional ultrasound reflection from a crack (Figure 1). Although detection of corrosion damage has increased in priority with the necessity of flying many aircraft well past their original design life, none of the techniques currently under development involve ultrasonic attenuation measurements. Moreover, since any corrosion on the surface of aluminum alloys will result in an increase in ultrasonic attenuation, this technique also has a very high probability of detecting even small amounts of hidden corrosion. NON-CONTACT
ULTRASOUND
Although piezoelectric ceramics have been predominantly used as transducer materials, a major problem associated with their use is the requirement that the piezoelectric transducers be acoustically bonded to the test material with an acoustical impedance matching coupling medium such as water, oil, or grease. Often more harmful, is the necessity of immersing the structures to be tested in tanks of water or using water squirter systems. For velocity measurements, which are necessary for material thickness measurements and to locate the depth of defects, the couplant can cause transit time errors. It can also lead to serious errors in absolute attenuation measurements which is the reason that so few reliable absolute measurements of attenuation are reported in the literature.
Mat. Res. Soc. Symp. Proc. Vol. 503 0 1998 Materials Research Society Downloaded from https://www.cambridge.org/core. UNSW Library, on 25 Jul 2020 at 21:49:47, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-503-3
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