Antiresonances of Cut and Cracked Bars: Experimental Results

An experimental study was carried out to evaluate the possibility of using the antiresonances of flexural vibrating bars to detect cracks or cuts. Two groups of bars, cracked bars and cut bars, were considered. The flexural vibrations of each free-free constrained bar, excited using the impulsive technique, were experimentally analysed measuring the accelerations in each excitation point. The distribution of the first antiresonance frequencies vs the position of impact/measurement points was then plotted, and the correlation with the corresponding n-th mode shape was found. The defect position corresponded to the areas in the plot where this distribution is discontinuous and it is determined with an uncertainty of ±2% of the bar length. These results confirm that, in laboratory experiments, the flexural behaviour of cut bars is similar to that of cracked ones. From a diagnostic point of view, this method can be used together with other methods that use resonances to detect defects.