厚壁焊缝阵列超声成像方法研究

Thick walled welds are widely applied in many heavy and important equipments, which need to be 100% detected with NDT technology. The detection problems of thick walled welds with coarse grains can't be effectively solved by the usual welds detection technology, such as radiographic testing, ultrasonic pulse-echo method, ultrasonic phased array, TOFD method and etc. Hence, a new ultrasonic testing technology, named ultrasonic probes array imaging, is put forward in this project. By arranging narrow-pulse probes array to cover the whole weld zone, a group of correlative ultrasonic signals are obtained with each probe transmitting & all array receiving. Using probes array synthetic aperture focusing technology and ultrasonic signal cross-corelation analysis method, based on the geometric relationship among the transmitting probe, receiving probe and target points, as well as the features of acoustic field in thick walled welds, the acoustic image of the weld can be obtained by inversing the amplitude and phase of each ultrasonic signal to the concerned target (As is called virtual focalization). In the proposed method, the anisotropy of the weld structure and the interaction of different acoustic waves in weld zone are taken into account, which can achieve high resolution acoustic image of thick walled welds with less probes. With the technology of matching pursuit and wavelet analysis, the method can solve the ultrasonic scattering problem and defect identification issue caused by coarse grains in thick walled welds. With the phase correction technology, the method can solve the problems of acoustic propagation deflection and defect orientation error caused by the anisotropy of columnar crystals. The proposed method would be a renovation for thick walled welds detection. In addition, this technology is also expected to be used for composites detection, which is with high attenuation and strong anisotropy.

厚壁焊缝广泛存在于许多重大装备中，必须进行100％的无损检测。焊缝检测中常用的射线探伤法、超声反射法、超声相控阵和TOFD等尚不能很好解决厚壁粗晶焊缝的检测问题。为此，本项目提出了阵列超声成像新方法，通过布置阵列窄脉冲探头覆盖整个焊缝区域，利用阵列探头一发多收得到的超声信号的相关性和多探头合成孔径接收聚焦技术，根据发射探头、接收探头与探测点（靶点）的位置关系以及厚壁焊缝声场的特征，将检测信号的幅度和相位准确反演到所考虑的靶点（虚拟聚焦），得到焊缝的成像。该方法综合考虑了焊缝内部组织的各向异性和多种声波的相互作用，能以较少的探头和较高的分辨率获得焊缝的图像。该方法通过匹配追踪和小波分析技术解决厚壁焊缝粗晶组织引起的超声散射和缺陷识别问题，通过相位校正技术解决柱状晶各向异性引起的声束偏转和缺陷定位问题，对于厚壁焊缝缺陷的检测是一次革新，预期研究成果还可应用于高衰减且各向异性的复合材料的检测。

厚壁焊缝广泛存在于许多重大装备中，必须进行100％的无损检测，但目前焊缝检测中常用的无损检测手段尚不能很好解决厚壁粗晶焊缝的检测问题。为此，本项目提出了厚壁焊缝阵列超声成像新方法，通过布置阵列窄脉冲探头覆盖整个焊缝区域，通过相控阵扫查实验及全矩阵捕获实验获得具有相关关系的信号矩阵，根据各向异性材料中的超声传播模式开发声线追迹算法、声场仿真算法及缺陷信号模拟算法，将检测信号的幅度和相位准确反演到图像中的各靶点处（虚拟聚焦），最终得到焊缝成像结果。该方法综合考虑了焊缝内部组织的各向异性、不均匀性及强衰减性，通过时频分析和独立信号分析等手段解决厚壁焊缝粗晶组织引起的超声散射和缺陷识别问题，通过声线追迹、声场仿真等算法进行相位校正以解决柱状晶各向异性引起的声束偏转和缺陷定位问题，可以获得分辨率和准确率较高的焊缝图像。该方法通过步进式与最小化式结合算法实现了P2M（起始点到目标矩阵）声线追迹，可高效率、高精度地计算各成像点的传播路径及传播时间，从而利用相位校正提高成像准确率。该方法还利用多元高斯声束模型及缺陷散射模型实现声场仿真及缺陷信号模拟，为研究焊缝粗晶组织中的超声传播模式和声场分布情况提供仿真结果支持。此外，本项目中还研制了厚壁焊缝典型试件及宽带窄脉冲阵列传感器，获得了适合成像算法开发及研究的检测信号。本项目中全矩阵捕获实验获得的信号矩阵用于全聚焦成像（TFM），相控阵扇形B扫查实验获得的信号用于矫正B扫查成像（CBI），并根据声线追迹算法的结果开发了包含相位校正的两种改进成像算法iTFM与iCBI。本项目提出的成像方法有效改善了厚壁焊缝的成像效果，提高了缺陷定位精度，对于厚壁焊缝缺陷的检测是一次革新，研究成果还可应用于高衰减且各向异性的复合材料的检测。