基于TOFD周向扫查图像特征的管道缺陷超声检测

利用超声衍射时差法(Time-of-flight diffraction,TOFD)对管道进行周向扫查检测时，直达纵波传播路径与管道表面不平行，需分别对直达纵波上下区域内缺陷进行识别和定位检测。特别地，管道结构导致TOFD近表面盲区增大，缺陷衍射波与直达纵波发生混叠。针对上述问题，理论分析极坐标系下的管道TOFD周向扫查图像中缺陷衍射波特征，借此确定缺陷与直达纵波的相对位置。在此基础上，对盲区外缺陷进行定位检测，并结合自回归谱外推方法计算盲区内缺陷深度。试验结果显示，对于外壁半径100.0 mm，壁厚30.0 mm碳钢管道，在5 MHz中心频率和不同探头中心距(Probe center separation,PCS)的检测条件下，埋深4.0 mm的上表面开口槽与埋深16.0 mm的下表面开口槽均可检出，且定位误差不超过0.36 mm。缺陷衍射波特征与PCS直接相关，对管道进行周向扫查检测时应合理调整参数。

Ultrasonic Testing of Defects in Pipeline Based on Image Characteristics of TOFD Circumferential Scan

When the ultrasonic time-of-flight diffraction(TOFD) circumferential scan is implemented on the pipeline, the propagation direction of the direct longitudinal wave(DLW) is not parallel to the pipeline surface. It is necessary to identify and locate the defects in the range above or below DLW. In particular, the range of the near-surface dead zone in TOFD is enhanced by pipe structure, and the defect diffracted wave is coupled with the DLW. In view of these problems, the characteristics of the diffracted waves in circumferential scan image are analyzed theoretically in polar coordinate, determining the relative position between flaw tips and DLW. On this basis, the defect beyond dead zone is located, and the depths of the defects in dead zone are obtained with the autoregressive spectral extrapolation method. The experimental results indicated that the upper surface slit with 4.0 mm depth and lower surface slit with 16.0 mm depth in the carbon steel pipeline with 30.0 mm thickness and 100.0 mm outer wall radius were located by using the TOFD probes with 5 MHz central frequency and different probe center separation (PCS). The measurement errors of slit depths were within 0.36 mm. The characteristics of diffracted waves are related to PCS, which should be adjusted reasonably for the TOFD circumferential scan of pipeline.