Ultrasonic inspection of multi-wire steel strands with the aid of the wavelet transform

Abstract

The non-destructive detection of structural defects in multi-wire strands used aspost-tensioned tendons and cable stays is a challenging, yet critical task. A promisingmethod under investigation is based on the use of ultrasonic stress waves that propagatewithin the strand and interact with structural discontinuities. The waveguide-like geometryof the strands lends itself to the monitoring of long lengths at a time (long range).The topic of this paper is the enhancement of ultrasonic monitoring of strands by a jointtime–frequency analysis based on the discrete wavelet transform (DWT). The test set-upuses magnetostrictive sensors for the excitation and the detection of ultrasonic guidedwaves in the strands. The main advantage of the DWT is an unmatched de-noisingperformance. Effective de-noising becomes necessary for the detection of small defectslocated far away from the inspection probes as desirable in the field. When compared to thetraditional signal averaging, the DWT can be used in real time owing to its computationalefficiency.The theory of the DWT filter bank decomposition is first revised. The effectiveness of thewavelet processing is then demonstrated for the detection of small notches of varyingdepths located in the free portion of the strands as well as in the critical anchored areas.The study also shows the importance of selecting the proper mother wavelet function forbest performance. The DWT proves effective in eliminating the need for signal averagingand in reducing the power supply required by the monitoring system. Both outcomesmake the guided wave inspection method for strands more suitable for field use.