Advanced Ultrasonic Structural Monitoring of Waveguides

Ultrasonic Guided Waves (UGWs) are a useful tool in those structural health monitoring applications that canbenefit from built-in transduction, moderately large inspection ranges and high sensitivity to small flaws. Thispaper describes two methods, based on linear and nonlinear acoustics for structural damage detection based onUGWs. The linear method combine the advantages of UGW inspection with the outcomes of the DiscreteWavelet Transform (DWT) that is used for extracting defect-sensitive features that can be combined to perform amultivariate diagnosis of damage. In particular, the DWT is exploited to generate a set of relevant waveletcoefficients to construct a uni-dimensional or multi-dimensional damage index that, in turn is fed to an outlieralgorithm to detect anomalous structural states. The nonlinear acoustics method exploits the circumstance that acracked medium exhibits high acoustic nonlinearity which is manifested as harmonics in the power spectrum ofthe received signal. Experimental results also indicate that the harmonic components increase non-linearly inmagnitude with increasing amplitude of the input signal. The proposed nonlinear technique identifies thepresence of cracks by looking at the harmonics and their nonlinear relationship to the input amplitude. Thegeneral framework presented in this paper is applied to the detection of fatigue cracks in an I-shaped steel beam.The probing hardware consists of Lead Zirconate Titanate (PZT) materials used for both ultrasound generationand detection at chosen frequency. The effectiveness of the proposed methods for the structural diagnosis ofdefects that are small compared to the waveguide cross-sectional area is discussed.