Technical structure components like airplane fuselage, often consist of thin aluminum face sheets, connected through rivets or containing holes which are sources of stress concentration and crack formation at their boundaries. We propose an experimental method for the detection of such cracks based on low frequency structural wave propagation with wavelengths larger than the structure (plate) thickness, and hence much larger than the typical crack dimensions. The waves are excited by means of a piezoelectric or electromagnetic acoustical transducer producing a transverse force with a time function corresponding to a narrow-band signal. Due to the fact that only large wavelengths are represented in the narrow-band spectrum, the main direction of propagation is parallel to the plate faces. Furthermore, the strong dispersive character of structural waves is an advantage, since useful information can be collected from the frequency dependency of the signal without losing too much amplitude in the propagating narrow-band signal.When the wave hits a discontinuity like a hole, a typical scattered displacement field is obtained and can be analyzed quite accurately both in theory and in experiment. A crack at the boundary of the hole changes the scattered field corresponding to an undamaged plate. In our laboratory experiment, a laser-interferometer has been used to measure the scattered field around a hole before and after a crack was artificially introduced at the boundary. Experimental constraints like the adequate choice of the narrow frequency band and the minimal detectable crack length are discussed. The experimental results are compared to theoretical calculations and the practical applicability of the method is discussed.
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