Detection and localization of fatigue crack with nonlinear instantaneous baseline

This article presents a fatigue crack characterization technique using a nonlinear instantaneous baseline not only to identify but also to locate fatigue damage in an aluminum plate. In conventional ultrasonic structural health monitoring approaches, damage is identified by comparing the newly measured signal with the baseline signal prerecorded when the structure is in its pristine state. The need to compare the current signals with prerecorded signals makes these methods unattractive, since the measured signals are also affected by the changing environment and load conditions of the structure. To overcome this deficiency, the technique proposed here replaces the prerecorded baseline with an instantaneous baseline which can be obtained when the structure is inspected. The measurement of instantaneous baseline utilizes the nonlinear properties of fatigue cracks when subjected to different amplitudes of excitations. Then damage is characterized in terms of a damage index value which is defined as the spectral changes in each path. The short-time Fourier transform is adopted to obtain the spectrogram. The experimental tests are presented to validate the effectiveness of this damage detection technique. A fatigue crack on an aluminum plate was located accurately with a set of surface-bounded piezoceramic (lead zirconate titanate) transducers by adopting the instantaneous baseline.