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

在第一年，我们建立了测量系统，其中实现了超声波的强大入射和非线性超声波的精确测量。商业化的配线线圈变送器由于电压高达2KV左右，产生了热量，损坏严重。虽然制作了无线圈的PZT换能器，但电极结合部和PZT元件本身断裂，难以稳定测量。在此基础上，采用了36度Y-CuLiNbO_3单晶作为压电体，耐热银浆作为电极结合部。实现了高达2KV的8 MHz超声波的稳定传输。为了稳定测量非线性超声，需要高度可控的小于10 nm的小裂纹开口。在三点弯曲疲劳裂纹引入过程中，控制断裂韧度不变，并用超声尖端衍射法监测裂纹尖端回波的幅度。U…在此基础上，为制备具有足够张开度的疲劳裂纹和稳定的非线性超声测量提供了可能。去年我们将这些方法应用于存在严重应力腐蚀开裂问题的SUS316不锈钢上。用同样的方法引入疲劳裂纹，还得到了随裂纹扩展而变化的非线性超声行为。特别是次谐波行为与裂纹的性质有很强的相关性，为了研究次谐波产生的原因，提出了强超声波作用下的裂纹面接触模型。基于该模型的模拟结果与实验结果吻合较好。此外，为了建立更准确的模型，需要对裂纹的大小和形状进行准确的观测。虽然已经使用扫描电子显微镜和激光显微镜来估计裂纹张开程度，但还需要进行更多的研究来进行定量估计。较少

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