Challenges for data evaluation and interpretation of Impact Echo data for non-destructive testing of concrete structures

Impact Echo (IE) is a commonly applied non-destructive testing (NDT) method in the field of civil engineering for the thickness measurement of concrete structures, which are only accessible from one side, as well as for the detection and localization of defects. When conducting measurements, a small mechanical impact is generated, and the resulting displacement is measured a couple of centimeters away from the impact point. The method measures the frequency of the zero-group-velocity S1 Lamb wave mode, which is directly correlated to the specimen’s thickness. Problems in data interpretation arise when lateral dimensions are not much larger than the specimen’s thickness. In such cases, so-called geometry effects mask the signals of the S1 Lamb wave. However, the physical cause for these geometry effects was not fully understood. This study presents a review on different numerical studies, which have been conducted to characterize these geometry effects and to minimize their influence for data evaluation. For the latter, time-frequency techniques like spectrograms or the continuous wavelet transform have been utilized for more accurate S1 frequency estimations. Also, the possibilities for the application of f-k-filters as well as array devices have been explored.