Although conventional methods such as the short-time Fourier transform (STFT) and the continuous wavelet transform (CWT) have been effectively used for the analysis of dispersive elastic waves, rapidly varying wave signals may not be accurately analyzed by these methods. Because the time-frequency tiling of conventional methods do not take into account dispersion phenomena, it is often difficult to trace accurately the time-frequency varying feature of the signals. The objective of this work is to develop advanced adaptive time-frequency analysis methods whose time-frequency tiling is varying to the dispersion characteristics of the signal to be analyzed. More specifically, a method called, "the dispersion-based short-time Fourier transform (D-STFT) and the dispersion-based continuous wavelet transform (DCWT)" are newly developed. In these methods, each time-frequency tiling is adaptively rotated in the time-frequency plane depending on the estimated local dispersion rate of a wave signal. In the proposed approach, the dispersion
relationship is estimated iteratively from the ridge analysis of the result by the proposed adaptive methods where the initial estimation is based on the result by the standard methods. To verify the validity of the present approach, the Lamb waves measured in an aluminum plate were considered.
The coupling phenomenon between stress and magnetic induction, known as magnetostriction, has been successfully applied to generate and measure elastic waves. Most applications of this phenomenon thus far, however, are rather limited to cylindrical ferromagnetic waveguides. The main objective of this work is to develop a new patch-type, orientation-adjustable magnetostrictive transducer that is applicable for non-cylindrical, non-ferromagnetic waveguides. The existing patch-type transducer consisting of a ferromagnetic patch and a racetrack coil is useful to generate elastic waves only in one specific direction once the patch is bonded to a test specimen. However, the proposed transducer can transmit and receive elastic waves in any direction only with one patch at a given location. The proposed magnetostrictive transducer consists of a circular nickel patch, a figure-of-eight coil, and a couple of bias permanent magnets. Because of the unique configuration of the transducer, the propagating direction of the generated waves can be freely controlled since the set of bias magnets and the coil is not bonded to the magnetostrictive patch. In this work, the characteristics of the proposed transducer were investigated experimentally