Abstract
In the case of non-destructive testing and evaluation (NDT & NDE) of plate materials and structures ultrasonic Lamb waves are used. Very often air-coupled ultrasonic transducers are used for non-contact Lamb wave excitation and reception. This method has a very serious drawback: global insertion losses for an air-coupled system may be 120 dB–160 dB typically. In order to improve this NDT method a better understanding of Lamb wave excitation process is needed. A numerical investigation was performed in order to evaluate air-coupled Lamb wave excitation in an isotropic single layer plate. It is shown how Lamb wave normal displacement amplitude depends on an incidence angle and the distance between the plate and the transducer. DOI: http://dx.doi.org/10.5755/j01.eee.20.1.6162
Highlights
Many ultrasonic NDT methods use couplants between the material under a test and ultrasonic transducers
After the jump the phase difference decreases significantly. It was shown how A0 Lamb wave mode normal displacement maximum amplitude depends on the ultrasonic wave incidence angle and the signal frequency
Different curves were obtained when the distance between the plate and the transducer corresponds to the transducer near field, intermediate field and far field zones
Summary
Many ultrasonic NDT methods use couplants between the material under a test and ultrasonic transducers. Finite Element Method (FEM) based models are used to calculate Lamb wave propagation in plate structures and Impulse Response Method (IRM) based models are used to calculate acoustic pressure, radiated by air-coupled transducer [12], [13]. Those hybrid models are exact enough, but they still require a lot of computational time and storage for output data. Reference [14] shows that in the case of an isotropic single layer plate entire air-coupled Lamb wave system can be modelled analytically. The objective of this research was to investigate Lamb wave amplitude dependency on the incidence angle and signal frequency at the different distances between the plate and the emitting transducer, corresponding to transducer pressure signal near field, intermediate field and far field zones
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