Nonlinear resonance ultrasonic spectroscopy for characterizing thermal damage in 17-4PH stainless steel
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Nonlinear resonance ultrasonic spectroscopy (NRUS) has been widely used to determine the hysteretic nonlinearity parameter, alpha for mesoscopic elastic materials. The current research adopts the NRUS to characterize microscopic material damage in metallic specimens with small dimensions. The geometric limitations in these small samples greatly restrict applications of other nonlinear ultrasonic techniques such as the higher harmonic generation technique. Two opposite edges of a rectangular plate specimen (10x10x2mm) are clamped by a specially designed fixture and an excitation piezoelectric transducer is attached to one of the other edges. The excitation transducer provides a frequency sweep around a resonance repsonse of the specimen while the out of plane velocity of the specimen is detected by a laser vibrometer. To simulate the copper-rich precipitate (CRP) formation and the associated irradiation damage in reactor pressure vessel steels, thermally aged 17-4 PH stainless steel in different precipitation stages is used to study the effects of CRPs on the measured hysteretic nonlinearity parameter, alpha. These results are compared to propagating wave nonlinear ultrasonic technique results on larger specimens and it is shown that the NRUS method is quite sensitive to the microstructure changes due to copper precipitates in this material.