A modified time reversal method for Lamb wave based diagnostics of composite structures

This experimental study presents Lamb wave based diagnostics of damage in a composite plate using time reversal of signals for baseline-free damage detection. A modified time reversal method (MTRM) is developed which requires only one transducer to actuate signals and the other transducer acts as a sensor for any signal path. An 8-layer symmetric cross-ply carbon-epoxy composite plate is fabricated and four PZT actuators/sensors are surface bonded at the corners of the plate. A 20 kHz, 9.5 cycle tone burst signal is used to generate fundamental asymmetric Lamb waves with high signal-to-noise ratio and low dispersion. Theoretical analysis and experimental measurements show that conventional time reversal method (TRM) and the MTRM yield identical signals at the end of time reversal. The composite plate is impacted incrementally with a steel ball to cause three levels of damage and MTRM is applied for prediction of damage severity. Results show that magnitudes of the two damage indices used are directly correlated to the severity of damage along any signal path. The development of MTRM paves the way for implementing time reversal of signals using a single actuator and multiple sensors, including non-contact sensor such as a laser vibrometer. (C) 2012 Elsevier Ltd. All rights reserved.