Considerations for measuring acoustic birefringence in thin sheets using electromagnetic acoustic transducers

In aluminium or steel sheets it is generally accepted that the energy of through thickness shear waves is steered into two orthogonal polarisations, parallel and perpendicular to the rolling direction of the sheet. Ultrasonic velocity measurements used to determine the orientation distribution coefficients (ODCs) in thin sheets can be obtained from the last Fourier Transform of the time domain signal. In the case of thin birefringent samples, data from radially polarised shear waves can be analysed in the frequency domain allowing peaks to be resolved that correspond to the slightly different velocities associated with each shear wave polarisation. By rotating a linearly polarised electromagnetic acoustic transducer through 360 degrees in small increments it is observed that the calculated velocities are not constant and are dependent on the signal processing used with the fast Fourier Transform. Further investigation using simulated waveforms show that this inconsistency in calculated velocity was not due to any physical effect of the ultrasound in the test sample but rather an artefact of the data analysis itself.