关键词: 幅值包络成像, 厚壁管道, 连续小波变换, 周向导波, 轴向缺陷

Abstract: Circumferential guided waves are used for scanning detection of defects in thick wall pipes. Wavelet transform coefficients at a certain frequency point are extracted for imaging defect in thick wall pipe by using continuous wavelet transform. In experiments, the angle beam probe is used to scan the axial defect in the step of 2 mm in the axial direction and inspection signals are obtained when the element of angle beam probe covers the axial defect with different coverage extents. The signals containing defect echoes and circumferential echoes are intercepted and their continuous wavelet transform coefficient envelope signals at 500 kHz are extracted. According to the comparison of these envelop signals and dispersion curves, received inspection signals are confirmed to be mainly circumferential Lamb wave mode CL3. Furthermore, continuous wavelet transform coefficient envelopes of inspection signals at different scanning points are used for amplitude envelop imaging. The defect is reconstructed based on this amplitude envelope image and its axial length can be confirmed efficiently. The signal to noise ratio of amplitude envelopes at a certain frequency point is greatly better based on continuous wavelet transform and imaging results show high focus performance. Therefore, amplitude envelop imaging method can be used for enhancing inspection signal identification and defect recognition based on continuous wavelet transform.

Key words: Amplitude envelop imaging, Axial defect, Circumferential guided waves, Continuous wavelet transform, Thick wall pipe