Abstract
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.
Highlights
Due to their assumedly higher sensitivity and more accurate quality and reliability characterization capacity, the non-linear ultrasonic spectroscopy methods are ranked among the most promising material quality and reliability characterization tools
Detailed studies of dynamic non-linearities and hysteresis in inhomogeneous media have shown that the occurrence of mesoscopic elements in the material structure gives rise to strongly non-linear dynamic phenomena accompanying the elastic wave propagation (Van Den Abeele at al. 2000, Johnson, 1999)
These non-linear effects are observed in the course of the degradation process much sooner than any degradation-induced variations of linear parameters
Summary
Due to their assumedly higher sensitivity and more accurate quality and reliability characterization capacity, the non-linear ultrasonic spectroscopy methods are ranked among the most promising material quality and reliability characterization tools. Non-linear parameters have proved to be very sensitive to the presence of any inhomogeneities and progressing degradation of the material structure. Non-linear wave methods open new horizons to the acoustic non-destructive testing: they provide higher sensitivities, application speed and easy interpretation. One of the fields in which a wide application range of non-linear acoustic spectroscopy methods may be expected is civil engineering. Some of the non-linear acoustic defectoscopy methods are less susceptible to the aforementioned restrictions and one may expect them to contribute to a great deal to further improving the defectoscopy and material testing methods in civil engineering
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