Detection of Disbond in a Honeycomb Composite Sandwich Structure using Ultrasonic Guided Waves and Bonded PZT Sensors

"Honeycomb Composite Sandwich Structure" (HCSS), is widely used to form major structural components of aerospace, marine and automotive vehicles due to its high strength to weight ratio and high energy-absorption capabilities. In service, the bond between the honeycomb core and skin may degrade with aging and continuous cyclic loadings leading to separation of the load bearing skin from the honeycomb core (called "disbonds"), and compromising the safety of the structure. Guided wave techniques have the potential to provide information on the presence of disbond since they can travel long distance and interact with localized defects. In this study, a baseline free technique has been developed for health monitoring of HCSS plates using ultrasonic guided Lamb waves and embedded piezoelectric transducer (PZT) wafer array. The PZT wafers were arranged in a grid manner on the top surface of the HCSS plate in which one of the PZT wafer acts as a transmitter and the other wafers act as receivers. A five cycle sine pulse modulated by Hanning window is transmitted and the effect of disbond on the response signal interrogated first. The directional group velocities in the HCSS plate are obtained for both primary symmetric (S0) and antisymmetric (A0) modes. A baseline free damage index algorithm is developed by taking into account the time-frequency information of the received signals. Damage index (DI) maps are plotted using the experimentally obtained time response data to detect the location of disbond region in the HCSS plate. The DI maps clearly show the higher values of DI at disbond location with high degree of accuracy.