Evaluation of Dispersion Modes for Layered Structures Using Focused Acoustic Waves

The evolution of Lamb wave dispersion has a great significance for the characterization of layered materials such as composite plates, bonded joints, and coating materials. The method traditionally used to evaluate locally the dispersion Lamb modes refers to the use of two wide-band transducer in “pitch-catch” disposition. By varying the emitter-receiver at the opposite incident-reflection angle ø, and scanning the emission tone-burst frequency f, one distinguishes the minima in the reflection coefficientR(f)of the test sample at each ¸, the dispersion relation of group velocity v g versus frequency f can be established by v g = v 0/sin ¸ where v 0 is the wave velocity in coupling liquid [1–3]. However, there are some inconveniences for this method. Firstly, it needs an ultrasonic goniometer to adjust two probes at a varying oblique incident angle, which is not practical for NDE utilization. Secondly, with finite-sized transducers where the incident beam is not really a plane wave, the angular and spectra resolution will be limited, and in addition there will be a dead angle for small incident angle. Thirdly, there exists the so called “non-specular reflection” occurring at Lamb critical angles and it causes perturbations for reflection wave detection [1,2]. To overcome these disadvantages, we employ a focused acoustic beam of large angular aperture at normal incidence to evaluate the reflection coefficient R(¸, f) for layered structures. This method demands only one probe and the problem of the non-specular reflection can be avoided as the wave reflection at the different angle is all captured at the same time. The principle is based on registration of Acoustic Material Signature or V (z)curve of the sample. An inversion algorithm of Fourier transform permits us to reconstruct R(¸) if the V (z) is measured in amplitude and in phase. By scanning f for each V (z) measurement, we obtain the reflection coefficient function R(¸,f). The Lamb wave modes is then be evaluated from the minima appearing in the magnitude of R(¸,f)data