非線形3波相互作用法用非接触電磁超音波探触子の開発

抄録

We developed of specifically non-contacting electromagnetic acoustic transducer for nonlinear three wave interaction method, with EMAR (Electromagnetic Acoustic Resonance), which was the combination with ultrasonic resonance and non-contacting transducer, EMAT (Electromagnetic Acoustic Transducer).In nonlinear three-wave interaction method, two intersecting ultrasonic waves produced a scattered wave when the resonance condition was satisfied. The amplitude in resonant scattering wave was measured. Nonlinear three-wave interaction method exhibited high sensitivity to micro-structural change of the damaged material but three wave interaction method needs several transducer. We contrived a newly technic with possibility detection of interaction wave by the single EMAT. We used balk-shear-wave EMAT, Which transmits and receives shear wave propagating in thickness direction of a plate specimen. We applied to investigation nonlinear ultrasonic characterization, newly nonlinear three-wave interaction, throughout the creep life in the welded joints of ASME Gr.91, one of high Cr ferritic heat resisting steels .Creep tests of thick welded joints specimens were carried out at 873 K, and 90MPa and interrupted at several time step. The Nonlinear acoustic parameters rapidly increased to 10% of creep life. After that they decreased to 40% of creep life. Nonlinearity was increased from 40% of creep life to rupture. It rapidly increased from 50% of fatigue life to the fracture. TEM (Transmission Electron Microscope) and EBSD (Electron Backscatter Diffraction) observations supported this phenomenon caused by dislocation movement. The three wave interaction method measurement by single EMAT can monitor the evolution of nonlinearity throughout the creep life and has a potential to assess the damage advance and to predict the damage of metals.