Subharmonic ultrasonic measurement and imaging for quantitative evaluation of stress corrosion and fatigue cracks

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To establish the basis of subharmonic ultrasonic measurement and imaging of the fatigue and stress corrosion cracks, we first derived an analytical solution considering the adhesion force, and realized the relation between the threshold of subharmonic eave generation and the crack closure stress. Nest, we developed the elastc body-oscillator model since detailed information is needed to develop practical apparatus. Through numerical analysis of this model, we investigated the dependence of subharmonic wave upon the input wave amplitude and upon the closure stress, and comparison between measured and calculated waveform in experiments of fatigue cracks in an aluminum alloy (A7075).As the next step we developed nonlinear ultrasonic imaging system for detailed measurement of the depth of closed cracks in thickness direction. As a transmitter, we employed a LiNbO3 single crystal oscillator that has excellent durability for high voltage electric signal and employed an array sensor for producing image by the received signal focusing. Moreover, the delay law within the specimen was formulated and designed a IIR digital filter as a preprocess before imaging to construct a system that can be used to obtain linear and nonlinear images of cracks.In addition, we applied the developed system above described to open and closed cracks in aluminum alloy and showed that the subharmonic image is useful for imaging closed cracks. Moreover, we verified the system to closed crack in stainless steel (SUS316L) specimen with comparable thickness to that used in recirculator piping of atomic power plants. Then we proved that it has high accuracy in crack sizing. Finally, we applied this system to cracks in steel specimen generated under processing, and we showed that the system is useful for oblique and branched cracks.