Propagation behaviour of guided waves in tapered sandwich structures and debonding identification using time reversal

Fundamental symmetric and anti-symmetric Lamb wave modes are characterised in terms of their velocity and magnitude variation as they encounter a gradual change in the thickness of a composite sandwich plate with a high density foam core (Dyvinicell HP100). Wave mode selection is optimised to enhance the sensitivity of guided waves for debonding inspection of tapered sandwich structures, and the conversion in wave modes after interaction with the taper is also studied. A taper change in the sandwiched structure can induce a clear change in the wave magnitude when the guided waves are excited at relatively low frequency (below around 150 kHz), while there is less effect on the velocity of the propagating waves. The effect of the taper on the signal magnitude and velocity is reduced when the excitation frequency is increased beyond 200 kHz. An imaging algorithm based on time reversal is developed to detect multiple debonding in tapered composite sandwich panels, using guided waves at different excitation frequencies ranging from 150 to 200 kHz, from an active sensor network. Correlation coefficients between the original and reconstructed time reversal signals are calculated to define a damage index for individual sensing paths, which are subsequently used in a fusion process to identify the presence of multiple debonded areas.