Abstract
In this study, we monitored the change of acoustic nonlinearity with electromagnetic acoustic resonance (EMAR) during creep damage in 9Cr steel welded joint, under lOOMPa, 923K. Resonant frequency shift by nonlinear ultrasound spectroscopy (NRUS) and second and third harmonics were measured as the nonlinear acoustic parameters. These were resonance-base techniques exploiting the significant nonlinear behavior of damaged materials. EMAR was combination to electromagnetic acoustic transducer (EMAT) with resonant acoustic technique. We used axial shear wave EMAT to monitor NRUS and harmonic amplitude of SH wave propagating in the circumferential direction of sample surface. Axial shear wave EMAT was operated with the magnetic strain effect. Furthermore, use of EMAT makes non-contact transduction possibility. NRUS detected damage by using resonant frequency shift. In measurement of resonant frequency shift, as the excitation level increase, resonant frequency was shifted and manifested the elastic nonlinearity. Measurement of harmonics with EMAR, drive of EMAT at half or one third the resonance frequency, the standing wave generate the second or third-harmonics by nonlinear effect. In addition, ultrasonic attenuation measured simultaneously. Measured nonlinear acoustic parameter showed the peak at 25% of creep life. We assumed that these trends were resulted from the increase of void density and the change of dislocation structure.
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