Measurement of non-linear elastic properties of metals using non-contact methods

• Main Author: Arnaudov, Yavor Emilov
• Published: University of Warwick 2016
• Subjects: QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.731325

Nonlinear ultrasonic techniques have the potential to allow for earlier detection of material degradation. Recent advances in the area show promising results, but the methods used are often impractical outside laboratory conditions. Non-contact methods have the potential to address some of the current measurement limitations. Electromagnetic acoustic transducers (EMAT) are one potential candidate for noncontact, nonlinear measurements in metals. The EMAT ultrasonic wave generation process is complex and requires an in-depth understanding of the electro-mechanical coupling phenomena. The potential of EMAT transducers as generators and detectors has been evaluated for multiple nonlinear measurement techniques. The ultrasonic wave, generated by the interaction of the dynamically generated magnetic field and the eddy currents within the sample surface, called the self-field Lorentz force, have a significant impact on generated nonlinearity and must always be considered. For longitudinal wave measurements, the ultrasonic wave generated by the self-field Lorentz force has a similar amplitude and behaviour to the second harmonic wave generated by the material nonlinearity, thus making the measurements impractical. For shear waves, the third harmonic nonlinear ultrasonic generation shows inconclusive results with regards to microstructural change. Additionally, shear wave, nonclassical nonlinear ultrasonics show no significant change during the fatigue lifetime for aluminium alloy. Ultrasonic measurements of the temperature dependence of the sound velocity and of the coefficient of temperature expansion show a promising new field of study where EMAT application is well suited. Meanderline EMATs have a great potential as receivers, but not as ultrasonic transmitters for nonlinear Rayleigh waves. The ultrasonic Rayleigh waves generated by the self-field Lorentz force, have similar amplitude and behaviour to the nonlinear ultrasonic Rayleigh waves. Optimised PPM (periodic permanent magnet) EMATs are used for entirely non-contact, nonlinear measurement of third harmonic shear horizontal waves and show great potential. A new method to study and minimise experimental system nonlinearity is proposed. Measuring the ultrasonic waves generated from an artificially added harmonic distortion to the generation signal can provide a method to separate the effects of the system and the material nonlinearities.