Time Reversal Health Monitoring of a Composite Structure using Lamb Waves

Abstract

Time reversal active sensing using Lamb waves is investigated for health monitoring of a composite structure. In the present work experiments were conducted on a glass/epoxy composite plates to study the time reversal behavior of A0 and S0 Lamb wave modes. PZT wafer sensors were surface bonded to the structure for generation and reception of the Lamb wave modes. Time reversal experiments were further carried out on carbon/epoxy composite t-pull specimen. The specimen was subjected to a tensile loading in a Universal testing machine. The PZT sensor measurements were carried out for healthy and also during different stages of delamination due to tensile loading. Time reversal of A0 and S0 mode was applied for both healthy and delaminated composite structures. In this work, the shape of the time reversed Lamb wave in the presence of delamination was studied experimentally. This study aims in showing the effectiveness of Lamb wave time reversal for damage detection in health monitoring applications. Lamb wave based NDE methods are widely used for damage detection in engineering structures, especially aerospace, civil and marine structures. The existence of damage in a structure is generally traced by comparing the time-domain traveling wave response of the structure at its present state with a base-line response. Any fluctuation from the base-line response is correlated to the damage location through the time of arrival of the new peaks (scattered waves). Therefore by employing the wave based methods, the presence of damage in a structure is detected by looking at the wave parameters affected by the damage. The wave parameters that are commonly used for damage detection are the parameters representing attenuation and reflection of waves due to damage, mode conversion etc. In addition the ability of Lamb waves to interrogate large and complex structure quickly and the generation (and reception) of Lamb waves using embedded or surface bonded PZT wafers makes them suitable for Structural Health Monitoring (SHM) applications. The major problem of employing Lamb waves for SHM is being multimodal and dispersive in nature which makes it more difficult to analyze and interpret the experimental signals. The other major concern is the effect of damage on the Lamb waves are small comparable to the effects due to geometry of the finite structure which causes dispersion and scattering of waves. To overcome the above problems, advanced signal processing techniques based on time-frequency analysis have been employed to extract the useful information from the Lamb waves. But signal processing techniques post process the wave response and remove the dispersion effects. Therefore a new approach is required to reduce the dispersion effects of Lamb waves before applying signal processing techniques and one such method is the time reversal method.