The influence of propagation path on elastic waves as measured by acoustic emission parameters

Apart from the quantitative parameters of acoustic emission testing, such as the total activity or the location of the sources, much more information can be exploited by qualitative characteristics of the signals. The shape of the waveform strongly depends on the source, supplying information on the type of cracks. Shear cracks which normally follow tensile during fracture, emit signals with longer rise time as well as lower average frequency. However, due to the inherent inhomogeneity of the media, which is enhanced by the nucleation of cracks, each pulse suffers strong dispersion which results in serious alteration of the waveform shape. Therefore, classification of cracks based on acoustic emission parameters would be probably misleading in case the separation distance of the sensors is long or the material contains many cracks. In the present study, numerical simulations were conducted in order to examine the influence of distance on the shape distortion of an excited wave inside concrete. Results are compared with actual experiments on steel fiber reinforced concrete, showing that the distance between the source crack and the acquisition point should not exceed a threshold value in order to lead to reliable crack classification.