Abstract
We studied the relationship between microstructural change and the evolutions of two nonlinear acoustic characterizations with electromagnetic acoustic resonance (EMAR) during creep in Cr-Mo-V steels and the correlation between two nonlinear acoustic characterizations. One was higher harmonic generation and other resonant frequency shift. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used axial-shear-wave EMAT, which transmits and receives SH wave propagating in the circumferential direction of a cylindrical specimen. The EMAT had a capability for easily detecting two nonlinear acoustic characterizations for cylindrical one. To measure higher harmonics, excitation of the EMAT at half or one third of the resonance frequency caused the standing wave to contain only the second or third-harmonic component, which was received by the same EMAT to determine the second or third-harmonic amplitudes. In measurement of resonant frequency shift, as the excitation level increases, resonance frequency was shifted and the shift manifested the elastic nonlinearity. Measured two nonlinear acoustic characterizations showed the peaks at 30% and the minimum at 50% of the total life, respectively. We interpreted these phenomena in terms of dislocation mobility and restructuring, with support from the SEM and TEM observation. This noncontact resonance-EMAT measurement can monitor the evolution of the surface-shear-wave nonlinearity throughout the creep life and has a potential to assess the damage evolution and to predict the creep life of metals.
