Phased array approach to generate guided waves and extract phase velocities using the V(z) method

The aim of this paper is to investigate the possibility to use a phased array transducer instead of a classical single-element transducer when applying the V(z) method to obtain the guided mode velocities of a sample, from which elastic constants can de deduced. Thus it studies the advantages and precautions that should be taken for generating and detecting acoustic guided waves by the V(z) method with a phased array transducer. By defining a delay law allowing the generated acoustic field to be focused, it is shown that it is possible to obtain V(z) curves in a similar way as when using geometrically focused (i.e. spherical or cylindrical shape) transducers. A detailed analysis is presented to illustrate how the choices of the number of activated array elements, the focal distance and the frequency values, influence the generation and detection of guided modes on an anisotropic substrate. First, a model of the V(z) acoustic signature obtained when using a phased array transducer is developed. Then, in order to illustrate the influence of the phased array parameters, the generation and detection of guided modes on a single-crystal anisotropic silicon substrate are discussed. The interest of the V(z) method coupled with a phased array transducer to generate and detect the surface acoustic wave (SAW), pseudo-surface wave (PSAW), and even the surface skimming quasi-longitudinal wave (SSQLW) is thus demonstrated. In this study, we also highlight that careful attention must be paid to the choice of the parameters of the phased array transducer in combination with the working frequency to avoid instabilities on V(z) curves.