In situ laser-ultrasonic monitoring of Poisson’s ratio and bulk sound velocities of steel plates during thermal processes

We demonstrate the in situ measurement of zero-group-velocity Lamb waves and thickness-resonances in steel sheet samples using laser-based ultrasound, while changing the sample temperature from ambient to 1000 ∘C. The ratio of resonance frequencies yields the temperature dependent Poisson's ratio of its material without requiring knowledge on the sheets' thickness. Changes in the temperature derivative of the determined Poisson's ratio reflect changes in the microstructural constituents. We show that additional resonances present in the response spectrum can be used to cross-check the evaluation. With additional knowledge on the thickness, longitudinal and transverse sound velocities are determined as well. We find good qualitative agreement between the temperature dependence of the measured quantities and reference data obtained from dilatometry and thermodynamic material simulations. The determination of the Poisson's ratio and transverse sound velocity complements previous investigations limited to the longitudinal sound velocity. This is particularly evident above the Curie temperature during the ferrite to austenite transformation and could be beneficial to disentangle transformations with more than two microstructural constituents involved. Finally, the limitations posed by elastic anisotropy introduced by rolling texture, and experimental uncertainties due to temperature inhomgeneities are discussed. We conclude that the laser-ultrasonic measurement of plate resonances provides a robust, contact-free, non-destructive, and in situ determination of Poisson's ratio.