Lamb wave assessment of fatigue damage in aluminium plates

In this paper an experimental analysis of Lamb waves interaction with riveted aluminium plates representative of aircraft space joints submitted to fatigue tests is given. In this joint evaluation technique Lamb waves are excited and received outside the joint area using piezoelectric transducers bonded onto the plates. Detected damages are clacks in joint resulting from fatigue loading. These cracks lead to waveform transformations. This phenomenon is studied in this paper by considering the first derivative of the envelope of the time domain signal. The position of the first derivative curve maximum during cyclical loading gives information about crack development. It suggests, that monitoring the change in the delay of this maximum may provide a means of sizing the defects. By using X-rays, it was possible to measure the size of the cracks and compare it with delay evolution. Results are obtained for two types of fatigue sequences. It is experimentally shown that the relative delay measured is very sensitive to crack development. With the continued progress in the field of damage assessment techniques such as methods relying on Lamb waves, the safety of such structures can be ensured.