Determination of absolute material nonlinearity using noncontact detection
Torello, David Edward
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Quantitative evaluation of the microstructural state of a specimen can be deduced from knowledge of the sample's absolute acoustic nonlinearity parameter, beta, making the measurement of beta a powerful tool in the NDE toolbox. However, the various methods used in the past to measure beta each suffer from significant limitations. Piezoelectric contact transducers are sensitive to nonlinear signals, cheap, and simple to use, but they are hindered by the variability of the interfacial contact between transducer and specimen surface. Laser interferometry provides non-contact detection, but requires carefully prepared specimens or complicated optics to maximize sensitivity to the higher harmonic components of a received waveform. Additionally, laser interferometry is expensive and relatively difficult to use in the field. Air-coupled piezoelectric transducers offer the strengths of both of these technologies and the weaknesses of neither, but are notoriously difficult to calibrate for use in nonlinear measurements. This work proposes a hybrid modeling and experimental approach to air-coupled transducer calibration and the use of this calibration in a model-based optimization to determine the absolute beta parameter of engineering materials. This approach is used to measure beta in longitudinal wave second harmonic generation experiments by testing samples of aluminum and fused silica, which are both well-documented materials and provide a strong reference for comparison of experimental and modeling results. This work also proposes adaptations to current measurement techniques using Rayleigh waves by accounting for diffraction and attenuation affects, as well as source nonlinearity and their effects on the measured acoustic nonlinearity parameter beta_11. Ultimately, this thesis provides a much needed tool in the NDE toolbox for measuring absolute material nonlinearity using a receiver type that has the potential to take NLU measurements from the lab to the field.