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

第一年，我们搭建了测量系统，实现了强大的超声波入射和对非线性超声波的精确测量。由于2kV左右的高电压，商用换能器与配套线圈产生热量，损坏严重。虽然制作了一种无线圈的PZT传感器，但由于电极连接部分和PZT元件本身断裂，稳定性测量困难。然后，我们采用36度Y-cutLiNbO_3单晶作为压电元件，采用耐热银膏作为电极结部分。因此，在高达2kV的高压下稳定传输8MHz超声波成为可能。为了实现非线性超声的稳定测量，需要高度控制10 nm以下的小裂纹开度。在三点弯曲引入疲劳裂纹过程中，控制断裂韧性恒定，采用超声尖端衍射法监测裂纹尖端回波振幅。利用该方法制备具有足够裂纹开度和稳定非线性超声测量的铝合金疲劳裂纹成为可能。去年我们对核电机组容器中存在严重应力腐蚀开裂问题的SUS316不锈钢进行了应用。采用相同的方法引入疲劳裂纹，也得到了随裂纹扩展而变化的非线性超声行为。特别是，次谐波行为在很大程度上取决于裂纹的性质。为了研究次谐波产生的原因，提出了强超声作用下裂纹面接触模型。基于该模型的模拟结果与实验结果吻合较好，为了建立更精确的模型，需要对裂纹的尺寸和形状进行精确的观测。虽然使用了扫描电镜和激光显微镜来估计裂纹的开度，但还需要更多的研究来进行定量估计。少

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