A new ultrasonic technique for detection and location of defects in three-layer plastic pipes with a reinforced internal layer

Ultrasonic non-destructive testing (NDT) of the multi-layer and fibre-reinforced composites meets some specific problems caused by a high acoustic attenuation and a high structure noise due to scattering of ultrasonic waves by fibre-reinforced layer and due to multiple reflections inside the samples caused by different acoustic impedances of the layers. Objective of this study was to develop the new signal processing technique for detection of defects in three-layer plastic pipe with internal fibre-reinforced layer. The proposed method consists of the improved algorithm eliminating the signals reflected by interfaces of the pipe layers, the optimized mother wavelet analysis and manipulation by the wavelet coefficients. The elimination of the reflected signals by interfaces is based on subtracting from each analyzed signal the reference signal. Further the processed signal is decomposed into a sum of elementary wavelets. The optimal mother wavelet is chosen according to the introduced weighting coefficients and results of investigations of echo signals, reflected by different artificial defects in a multi-layer plastic pipe.