Abstract: The 90° elbow can generate complex mode conversions when ultrasonic guided wave passing through, thus makes it difficult to inspect defects in elbow and beyond elbow. The detection of circumferential cracks of different areas in elbow pipes are investigated in experiments and simulations using the guided waves in mode of L(0, 2) which are excited by a piezoelectric transducer. The effect of elbow to guided wave inspection is researched by comparing the result of straight pipe inspection. The signal is denoised by using three-level discrete wavelet decomposition and reconstruction which mother wavelet is db1. The experimental results show that the circumferential cracks can be determined in axial position for both elbow and bend straight side by the reflected echo of defects in the pipes. The detection sensitivity depends on the location of the cracks in bend areas of the pipes. It is easier to detect cracks in the extradose of the bend while harder to detect those in the other locations of the bend. There is a certain relationship between the detection sensitivity and the frequency of guided waves. The signal-to-noise ratio of detecting the crack before and beyond elbows is highest in the frequency ranges from 120 kHz to 130 kHz; the crack in the intradose of elbow can be inspected accurately in the frequency nearby 80 kHz. Therefore, the combination of high-frequency and low frequency is used to detect the bended pipes. The propagation behaviors in elbows are investigated by using numerical simulation. The results of simulation intuitively explain the experimental phenomenon.

Key words: cracks, elbow pipe, frequency, guided wave, wavelet de-noising