Computational model of the non-contact surface wave NDT scanner and its experimental validation

LI, JI; Ecole Centrale de Lille; France

Li, J.; Ecole Centrale de Lille; France
Piwakowski, B.; Ecole Centrale de Lille; France

ID: ECNDT-0322-2018
Session: Mathematical Modelling - UT
Room: J2
Date: 2018-06-11
Time: 16:40 - 17:00

Surface waves (SW) have been widely used in non-destructive testing. With the development of air-coupled ultrasonic transducers which employ air as a coupling medium, a rapid automatic non-contact testing can be achieved. The example of the application of such technique is the non-contact SW scanner. The measurement consists in the SW signal recording by means of the non-contact receiver moving along measured sample. The signal is radiated by a non-contact emitter
Our experience shows that the operation of the scanner is considerably influenced by its operating parameters such as transducers dimensions, transducers inclination, transducer-sample distance, excitation signal shape and its bandwidth, positions and number of reception points, etc. In order to clarify the role of above parameters and to find the recommendations for optimal operation, a tool enabling to model the entire system is required.
The paper presents a time domain model of the non-contact (SW) ultrasonic scanner. The model is based on Rayleigh’s integral employing time domain causal Green’s functions for lossy medium which enables to take into account the attenuation of waves both in air and in the tested sample and the associated velocity dispersion.
Computations are divided into three steps: wave propagation from air-coupled emitter to the inspected sample, the SW propagation along the inspected sample surface, leaky wave re-propagation from sample to air-coupled receiver. The finite aperture of receiver and the electrical response of the emitter-receiver set are also taken into account.
The above computational tool is used to predict the output signals for the scanner employing 350 kHz air-coupled emitter and wide-band air coupled receiver. The scanned solid sample is the plexiglas bloc. The model validated experimentally and a very good agreement is obtained for the predicted SW signal waveforms, on-axis field distribution, and for radiation directivity