RESUMO
A baseline-free defect localization method in thin plates is proposed and tested. In this proof-of-concept work, a steel ball pressed against an aluminum plate is used to mimic a surface contact defect. The technique takes benefit of a repetitive nonlinear pump-probe interaction with a backpropagation imaging algorithm. High-frequency probe waves are periodically emitted by a piezoelectric patch transducer glued to the plate. Propagated flexural waves are recorded using a distributed array of transducers. At the same time, a continuous low-frequency pump vibration provided by a shaker fixed to the plate modulates the contact state. By combining multiple probe signals, the contact can be successfully localized. Contrast of the localization images is finally improved by a factor of 3 to 5 by implementing a modified version based on synchronous detection of the imaging algorithm.
RESUMO
In this paper, a new approach is proposed for the detection of ultrasonic guided waves using a LiNbO3 single crystal-based micro-transducer matrix. This matrix was designed, manufactured, and then used to detect Lamb and Pochhammer-Chree guided waves in plate- and cylinder-like structures. This study highlights the identification of the first flexural mode F(1,1) in cylinders at low frequencies. A network analyser and a laser Doppler vibrometer (LDV) were used to characterise and study the behaviour of the micro-transducer matrix. An experimental device was designed and used to acquire electrical measurements of the micro-transducer vibrations. Then, an original experimental device was developed to generate a selected flexural guided mode in a solid aluminium cylinder. The emitter comprised two semicircular piezoelectric transducers excited with only one phased signal thanks to the inverse position of polarisation. Finally, the results prove that the flexural mode F(1,1) is selected and generated by the emitter, then detected and identified by the micro-transducer matrix.