NONDESTRUCTIVE EVALUATION OF MATERIAL DAMAGE WITH NONLINEAR ULTRASONIC CT

本课题的目的是建立一套二次谐波超声CT系统，并将其应用于工程材料异常的评价与诊断，主要研究成果如下：1）作为非线性超声CT的基础，研制了水浸超声衰减CT系统。通过使用已知衰减系数的参考样品和与测试样品相同的几何形状，用ART算法重建了钢块中的高衰减系数区域。重建后的高衰减区图像和衰减系数值与真实的图像吻合较好。在适当的惩罚因子下，利用超声衰减CT重建了直径为2mm的钻孔。2）采用水浸超声法，通过适当的带通滤波器降低基波的振幅，成功地构建了二次谐波振幅图像。这种二次谐波图像被应用于不同材料扩散连接界面的无损表征。3）通过结合上述两种方法，我们可以构建损伤固体的非线性（二次谐波）图像，例如，具有纳米级间隙的微裂纹的金属。

The aim of this prqject is to establish an ultrasonic CT system with the second harmonic and to apply the system to evaluation and diagnosis of engineering material anomaly.Main results in this project are summarized in the followings.1) As a basis of nonlinear ultrasonic CT, an ultrasonic attenuation CT system has been developed in water immersion. By using a reference sample of known attenuation coefficient and the identical geometry with samples tested, the region of high attenuation coefficient in a steel block has been reconstructed by an ART algorithm. The reconstructed image of high attenuation region as well as the value of attenuation coefficient agree well the real ones. With proper penalty factors, we reconstructed drill holes of 2mm in diameter by ultrasonic attenuation CT.2) With water immersion ultrasonic method, we succeeded to construct the second harmonic amplitude image with proper bandpass filters which reduce the fundamental wave amplitude. This second harmonic image are applied to nondestructive characterization of diffusion-bonded interfaces of different materials.3)By combining the above two approach, we can construct nonlinear (the second harmonic) images of damaged solids, e.g., metals with micro cracks with gaps of nm order.