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Current methods of non-destructive ultrasonic material testing are based on the analysis of elastic wave reflection, absorption and interference. These methods are difficult to apply to inhomogeneous building materials showing tiny cracks and defects distributed throughout the specimen bulk, or in the cases where the defect size is comparable with the wavelength. Analysis of these phenomena occurring in intricate shapes is also difficult. To cope with such problems, application of wave propagation related nonlinear effects and higher harmonic signal generation in the defect vicinity is advisable. Due to the presence of defects, the atomic potential energy ceases to be exactly harmonic. Second and third harmonic frequencies arise. In this domain, methods employing the nonlinear acoustic spectroscopy apply. These novel defectoscopic methods are based on the nonlinear behaviour of current defects and inhomogeneities regarding the elastic wave propagation processes. Unlike the electromagnetic and acoustic emission methods, which only allow the localization of currently emerging cracks and defects, the non-linear ultrasonic defectoscopy is all-defect-sensitive, thus constituting a method applicable to characterizing the quality and reliability of materials.

The Possibilities of Nonlinear Ultrasonic Spectroscopy for the NDT in Civil Engineering