Wave propagation through engineering materials; assessment and monitoring of structures through non-destructive techniques

Elastic wave propagation has been used for decades for assessment of the structural integrity of
engineering materials. The advantage it offers is the direct connection to elastic properties, the relatively easy application through commercial equipment as well as numerous empirical correlations between pulse velocity and material strength or quality in general. Advanced features like frequency dependence of wave parameters may further improve the characterization capacity. Concrete materials due to their inherent
microstructure, which is enhanced by the existence of damage-induced cracking, exhibit a complicated behavior concerning the propagation of pulses of different frequencies. The different wave lengths interact with inhomogeneities according to their size and therefore, leave their signature on the phase velocity and attenuation versus frequency curves. Although experimental measurements are troublesome in concrete structures, mainly due to attenuation of high frequencies, it is suggested that, whenever possible, application of different frequencies can provide amore detailed insight on the internal condition of the structure. Apart from classical elastic wave studies, the scattering microstructure of concrete exercises strong influence on the elastic signals emitted after cracking events, distorting therefore crucial acoustic emission parameters used for the characterization of the structural integrity. In the present
paper experimental evidence of dispersion and examples on howit can be utilized in concrete non destructive
inspection are presented and discussed.

Keywords:Elastic wave propagation, Dispersion, Acoustic emission, Cracks, Repair