This study develops a technique to decompose a multi-mode, transient Lamb wave signal into individual Lamb mode signals. The previously-proposed technique (presented at the SPIE NDE conference 2005) showed an encouraging efficiency for numerically-simulated signals, but suffered when evaluating real experimental signals due to its high sensitivity to noises and experimental errors. The improved technique starts with the same assumption of known Lamb wave propagation characteristics (known propagation dispersion curves). However, a highly-strict signal model governing the development of the previously-proposed technique is relaxed to tolerate unavoidable presence noises and errors, and the problem is re-formulated. For actual experimental signals, some additional signal processing techniques are introduced in signal pre-conditioning. The entire implementation of the improved technique is first tested with simulated signals, and then applied to actual experimental signals. The final results with real experimental are presented and discussed.
This research proposes a technique to decompose a transient, multi-mode, Lamb wave, time-domain signal into its individual Lamb wave modes. The technique is derived for a Lamb wave signal consisting of two Lamb wave modes (double-mode Lamb wave signal). The extension to the general multi-mode signal is straightforward, but requires additional computations. The proposed technique assumes knowledge of the dispersion characteristics of Lamb waves, which can be theoretically calculated or obtained by other signal processing techniques. The proposed technique is verified for simulated (by eigen-expansion) signals to demonstrate the method's accuracy in a well-controlled environment. The use of eigen-expansion signals for the simulation allows for the calculation of the total response, as a combination of responses due to all existing Lamb modes. The comparison between the decomposed and simulated signals shows good agreement and demonstrates the validity of the proposed technique. The paper concludes with a discussion of the extension of this technique to the more general multimode, Lamb wave signal in leaky conditions, and its use in attenuation calculations.
This research uses a combination of laser ultrasonics, signal
processing and analytical modeling techniques to examine the
propagation of transient Lamb waves in absorbing plates -- in particular an isotropic plate with a lossless fluid on one side.
The motivation is to develop a non-contact, point source-receiver
technique capable of measuring attenuation of Lamb waves in lossy
situations in general. The theoretical model enables an understanding
of loss mechanisms and enables prediction of attenuation. The laser
ultrasonic techniques enable broadband, point source-receiver
idealization. The proposed signal processing technique, the
short-time Fourier transform, resolves a signal into the
time-frequency domain and enables Lamb mode separation and
calculation of energy distribution at specific frequency-velocity
points on the dispersion curves. The experimental procedure uses a stress-free plate as a reference for system variation and other loss mechanisms. Then, a plate with fluid on one side is tested to validate the signal processing algorithm. There is satisfactory agreement between the attenuation measured and predicted in the frequency range of interest.
This research examines the propagation of guided Lamb waves in
bonded components, establishing the effectiveness of combining
laser ultrasonic techniques with a time-frequency representation
(TFR) to experimentally measure the dispersion curves of a layered
medium. The specific layered medium examined is a fiber-reinforced
polymer (FRP) plate bonded (with an adhesive layer) to concrete. A
TFR is used to operate on experimentally measured, guided Lamb
waves to resolve individual Lamb wave modes and to generate the
system's dispersion curves. The objective of this research is to
demonstrate that it is possible to develop the dispersion curves
of FRP bonded components from a single, experimentally measured
guided wave signal. The experimental results show that, by
examining the characteristics of the system's dispersion curves,
the stiffer the bond, the more deviation from the behavior of a
free plate case, and the less modes that are present.