The local elastic response of plates to excitation with a laser source is dominated by zero-group-velocity (ZGV) resonances and thickness resonances. In the time-spectrum of the response, these show as distinct peaks at frequencies determined by the plate’s thickness and sound velocities. The ratio of the frequencies of two independent resonances, e.g. a ZGV mode and a longitudinal thickness mode, suffices to determine the Poisson’s ratio of the plate, with high sensitivity [1,2].We used this principle to monitor the Poisson’s number of a 0.8 mm steel plate sample while applying a temperature profile to it in a thermal simulator. The plate was heated from room temperature up to 700°C in this process. The local responses obtained from single shot measurements showed ZGV modes and a multitude of longitudinal thickness modes. We traced the lowest order ZGV mode (2-3 MHz) and the 2nd order thickness mode (5-6 MHz) and solved for the Poisson’s ratio. The temperature was logged with a thermo-couple attached to the sample. During the measurement, the Poisson’s ratio changes significantly. Additional ZGV modes, that only exist for certain ranges of Poisson’s ratio [3] appear and disappear accordingly in the spectrum.The method proved to be practical, as it provides high signal-to-noise ratio and requires a bandwidth &lt;20MHz for mm-scale plates. First results show a large difference for the detected Poisson’s ratio curves for different steel compositions. A possible application is the inline monitoring of the Poisson’s ratio for quality control.[1] Clorennec D, Prada C, Royer D. Local and noncontact measurements of bulk acoustic wave velocities in thin isotropic plates and shells using zero group velocity Lamb modes. J Appl Phys. 2007;101(3).[2] Grünsteidl C, Murray TW, Berer T, Veres IA. Inverse characterization of plates using zero group velocity Lamb modes. Ultrasonics. 2016;65.[3] Prada C, Clorennec D, Royer D. Local vibration of an elastic plate and zero-group velocity Lamb modes. J Acoust Soc Am. 2008;124(1):203–12.

In Situ Measurement of Poisson's Ratio of Steel Plates During Thermal Processes Using Resonant Modes

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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