Detection and Evaluation of Closed Crack for Aged Structural Materials Using Nonlinear Ultrasound

点击翻译按钮获取中文摘要

In the first year, we built the measurement system in which enable a powerful incidence of ultrasonic wave and accurate measurement of nonlinear ultrasonic wave. Due to the high voltage about 2kV, a commercial transducer with matching coil generated heat and serious damage. Although a PZT transducer without coil was made, an electrode junction part and a an PZT element itself were fractured and the stable measurement was difficult. Then, we adopted the single crystal of 36-degree Y-cutLiNbO_3 as a piezo-electric element and heat-resistant silver paste for electrode junction part. As the result stable transmission of 8MHz ultrasound with high voltage up to 2kV became possible. Highly controlled small crack opening below 10 nm was required for stable measurement of nonlinear ultrasound. During the fatigue crack introduction by three points bending, fracture toughness was controlled to be constant and the amplitude of crack tip echo was monitored using ultrasonic tip diffraction method. U … More sing this procedure, preparing the fatigue crack with adequate crack opening and stable nonlinear ultrasonic measurement for aluminum alloy became possible.In the last year we applied these procedures to SUS316 stainless steel which has serious problem about stress corrosion cracking in nuclear power generator vessel. The fatigue crack introduced with the same technique we also obtained the nonlinear ultrasonic behavior that changed as to the crack propagation. Especially, a subharmonic behavior depended strongly on properties of the crack.In order to investigate the cause of subharmonics, the crack plane contact model with powerful ultrasonic wave was proposed. The simulation based on this model agree well with the experimental result Furthermore, to establish more accurate model, accurate observation for the crack size and shape were required. Although SEM and a laser microscope were applied to estimate crack opening, some more investigation will be required for the quantitative estimation. Less