Abstract
Conventional ultrasonic mensuration of sample thickness from one side only requires the bulk wave reverberation time and a calibration speed. This speed changes with temperature, stress, and microstructure, limiting thickness measurement accuracy. Often, only one side of a sample is accessible, making in situ calibration impossible. Non-contact ultrasound can generate multiple shear horizontal guided wave modes on one side of a metal plate. Measuring propagation times of each mode at different transducer separations, allows sheet thickness to be calculated to better than 1% accuracy for sheets of at least 1.5 mm thickness, without any calibration.
Highlights
In conventional ultrasonic thickness gauging of metals, a piezoelectric transducer is placed on a sample surface, and is used to generate and detect ultrasonic bulk waves within the sample [1]
The thickness is calculated from the transit time of a wave that reverberates through the thickness of the sample, using a calibration bulk wave speed
Ultrasonic speed will vary with temperature, stress and microstructure, so that even where the sample composition is known, it is not possible to guarantee the accuracy of the calibration speed that would be required for conventional ultrasonic thickness gauging
Summary
In conventional ultrasonic thickness gauging of metals, a piezoelectric transducer is placed on a sample surface, and is used to generate and detect ultrasonic bulk waves within the sample [1]. This paper describes a method that uses ultrasonic shear horizontal (SH) guided wave modes [2] to measure thickness, with no a priori knowledge of the bulk wave ultrasonic speeds, and where it is only possible to obtain access to one side of the sample (a plate or sheet). This is achieved using the SH guided wave mode dispersion curves. Researchers have reported single sided, metal plate thickness measurements, using electromagnetic acoustic transducers (EMATs) [3, 4] generating and detecting bulk waves [5] and other modes, including an alternative SH guided wave method [6]. This considers the generator only; when using a detector with the same spatial profile, the total system response is the function squared, shown in figure 4
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