在检测高衰减的大厚度构件时，相控阵超声声束在非聚焦区域的声能量损失严重，采用线阵或矩阵超声换能器的检测精度和最大检测深度常难以满足要求。首先，建立了合成声束三维声场分布模型，分析了不同阵列超声换能器的空间声场能量分布特点，发现环阵超声换能器在相同聚焦深度处的声场特性更好，同时具有阵元数量少、声束焦斑沿轴线中心完全对称等特点。然后，建立了一种采用环阵超声换能器的全聚焦方法（TFM，Total Focusing Method），并基于试样群速度测量结果对各向异性材料中的聚焦算法进行了优化。在这种方法中将所采集到的全矩阵数据沿换能器中心轴线进行重建，可实现沿深度方向的逐点无穷聚焦。最后，利用所研制的阵列超声C扫描自动检测系统对预埋有缺陷的3D打印钛合金试样进行了对比检测实验，结果表明采用全聚焦成像算法的环阵超声换能器能实现55 mm试样内部直径0.8 mm、深度5.0 mm平底孔和横孔缺陷的精确检测，相对于常规的动态聚焦算法有更高的检测信噪比和定量精度。
Detection accuracy and maximum detection depth in large thickness components with high attenuation often pose a challenge for conventional linear ultrasonic arrays or planar ultrasonic arrays because of the serious loss of acoustic energy of phased array ultrasonic beams in the non-focused area. In this study, a three-dimensional acoustic field distribution model of synthetic beams is first established to analyze the characteristics of acoustic energy distribution of different ultrasonic array transducers. It is found that the annular ultrasonic transducer has a better acoustic field at the same focus depth and a fully symmetric beam focal spot with fewer elements. Then, a Total Focusing Method (TFM) applied to annular ultrasonic arrays and a group velocity optimization method of anisotropic material detection are proposed. In this method, the full matrix data is reconstructed along the axial line of the transducer to realize pointwise infinity focusing along the depth. Finally, experiments on a 3D printed titanium alloy specimen with prefabricated defects are conducted based on the developed ultrasonic array immersion C-scan system. The experimental results show that the annular array transducer based on the TFM algorithm can achieve accurate detection of the flat bottom hole and the transverse hole defects with a diameter of 0.8 mm and depth of 5 mm in the 55 mm thick specimen. Compared with the conventional dynamic depth focusing method, the C-scan images of the TFM have a better signal to noise ratio and quantitative accuracy.
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