Watzl Georg, Ryzy Martin, Österreicher Johannes A, Arnoldt Aurel R, Yan Guqi, Scherleitner Edgar, Schagerl Martin, Grünsteidl Clemens
Research Center for Non-Destructive Testing GmbH (RECENDT), Altenberger Straße 69, 4040 Linz, Austria.
LKR Light Metals Technologies, Austrian Institute of Technology, Lamprechtshausenerstraße 61, Ranshofen, 5282, Austria.
Ultrasonics. 2025 Jan;145:107453. doi: 10.1016/j.ultras.2024.107453. Epub 2024 Sep 7.
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 2mm 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.
标准超声厚度测量需要知道样品的声速，反之亦然。我们提出了一种方法，我们称之为组合模式局部声学光谱法（CoMLAS），用于同时确定板材的厚度和声速，而无需此类先验知识。它基于板材在离散频率下维持的三种导波模式的组合，我们使用激光超声来产生和检测这些模式。我们使用脉冲激光，它在样品表面被整形为周期性线图案，以产生弹性波，并使用振动计测量图案中心处的响应。表面声波模式在响应光谱中对应于与图案线间距匹配的波长的频率处产生一个干涉峰。通过限制激励图案的总尺寸，我们可以同时产生两个零群速度板材共振，在光谱中提供另外两个峰值。板材的局部厚度以及纵向和横向声速由峰值频率计算得出。我们通过分辨厚度步长和温度引起的声速变化，证明了CoMLAS在厚度约为2mm的钢板和铝板上的可行性。通过数值模拟和对照实验，我们深入了解了该方法的准确性和局限性。讨论了激励图案的选择、该方法的灵敏度以及样品不均匀性和各向异性的影响。CoMLAS不需要扫描机械装置，并能提供板材的局部特性。所示结果是使用低能量激光和信号平均实现的。对信噪比的考虑表明，使用能量更高的现有激光进行实现将能够进行单次测量。这使该方法适用于工业环境中的移动样品。
