Georg Watzl , Martin Ryzy , Johannes A. Österreicher , Aurel R. Arnoldt , Guqi Yan , Edgar Scherleitner , Martin Schagerl , Clemens Grünsteidl
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引用次数: 0
Abstract
Standard ultrasonic thickness measurements require the sound velocity of the sample to be known and vice versa. We present a method, which we have termed combined mode local acoustic spectroscopy (CoMLAS) for simultaneously determining a plate’s thickness and sound velocities without requiring such a priori knowledge. It is based on a combination of three guided wave modes sustained by a plate at discrete frequencies, which we generate and detect using laser ultrasound. We use a pulsed laser that is shaped into a periodic line pattern on the sample’s surface to generate elastic waves and measure the response at the pattern’s center with a vibrometer. The surface acoustic wave mode produces an interference peak in the response spectrum at the frequency corresponding to the wavelength matching the pattern line spacing. By limiting the total size of the excitation pattern, we can simultaneously generate two zero-group-velocity plate resonances, providing two additional peaks in the spectrum. The plate’s local thickness and longitudinal and transverse sound velocities are calculated from the peak frequencies. We demonstrate the feasibility of CoMLAS on steel and aluminum sheets with a thickness of around 2 mm by resolving thickness steps and temperature-induced changes in the sound velocities. Using numerical simulations and control experiments, we provide insights into the method’s accuracy and limitations. The choice of excitation pattern, the method’s sensitivity, and the influence of sample inhomogeneity and anisotropy are discussed. CoMLAS does not require scanning mechanics and provides local plate properties. The results shown are achieved with low-energy lasers and signal averaging. Considerations on signal-to-noise ratio indicate that a realization with available lasers of higher energy will enable single-shot measurements. This qualifies the method for use on moving samples in an industrial environment.
期刊介绍:
Ultrasonics is the only internationally established journal which covers the entire field of ultrasound research and technology and all its many applications. Ultrasonics contains a variety of sections to keep readers fully informed and up-to-date on the whole spectrum of research and development throughout the world. Ultrasonics publishes papers of exceptional quality and of relevance to both academia and industry. Manuscripts in which ultrasonics is a central issue and not simply an incidental tool or minor issue, are welcomed.
As well as top quality original research papers and review articles by world renowned experts, Ultrasonics also regularly features short communications, a calendar of forthcoming events and special issues dedicated to topical subjects.