Noncontact nonlinear resonance ultrasound spectroscopy (NRUS) for small metallic specimens

The objective of this research is to use nonlinear resonance ultrasound spectroscopy (NRUS) to evaluate the sensitivity of the nonclassical hysteretic nonlinearity parameter, α to microscopic material damage in metallic specimens. The proposed NRUS procedure uses both a noncontact source and receiver - an air-coupled piezoelectric transducer source and a laser vibrometer receiver - to excite and detect the axial motion of a slender rod. This fully noncontact configuration avoids mechanical contact which can adversely influence the measurements. This NRUS procedure measures αf and αQ by calculating the amount of the shift of the resonance frequency and the change of quality factor (Q) as a function of increasing excitation amplitudes. The sensitivity of this NRUS measured α parameters to small changes in microstructure is demonstrated on a series of thermally aged 17-4 PH stainless steel samples. These hysteretic nonlinearity (αf and αQ) results are then compared to (quadratic) acoustic nonlinearity parameter (β) values previously measured on these same 17-4 PH samples to demonstrate the sensitivity of the hysteretic nonlinearity parameters to changes in microstructure due to precipitate formation.