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Signal processing for non-contact NDE

Kubínyi, M. ; Docekal, A. ; Ramos, H. ; Ribeiro, A. L.

Przeglad Elektrotechniczny Vol. 86, Nº 1, pp. 249 - 254, January, 2010.

ISSN (print): 0033-2097
ISSN (online):

Journal Impact Factor: 0,244 (in 2011)

Digital Object Identifier:

The detection of defects in aerospace structures with non-destructive techniques is an important
requirement for quality checks not only during production phase but mainly during in-service maintenance
operations. Visual inspection allows only the analysis of surface characteristics of materials. A deeper analysis is
required for the characterisation of the widest defects types such as subsurface corrosion or cracks. For aerospace
structures inspection, two different methods are mainly used: eddy current and ultrasonic testing. The first method is
based on the induction of currents in the electrically conducting material to be tested. The perturbations of the eddycurrent
flow path due to physical, structural or metallurgical inhomogeneities are detected either using coils or
magnetic field sensors. The second method uses high frequency sound waves which are sent into the tested object.
A probe picks up the reflected waves and analysis of the received signal is done to locate flaws in the tested object.
Ultrasonic inspection can detect defects such as cracks and discontinuities mainly inside the studied object. In this
paper both techniques are addressed in order to conclude the different detection performances of these nondestructive
inspection methods using different industrial and experimental probes. Two different eddy current probes
were projected and implemented. One includes an excitation coil and two differential detection coils. Another having
a similar excitation coil to generate the eddy currents in the material to be tested but using a Giant Magneto-Resistor
(GMR) to asses the magnetic field induced. Both probes were tested with an aluminum standard airframe plate
sample with and a dedicated data acquisition measuring system. This sample was also inspected using a certified
industrial Eddy Current system. The ultrasonic method was tested using an Electromagnetic Acoustic Transducer
(EMAT) probe and multidimensional signal processing. The same measurements performed with the experimental
Eddy Current system were carried out with industrial piezoelectric ultrasonic transducers.