| There has been an increasing demand in using Structural Health Monitoring (SHM) and nondestructive tesing (NDT) techniques for monitoring damage within civil infrastructure, aerospace, and mechanical systems. This thesis aims to propose robust Lamb wave based NDT methods that enhance conventional guided wave approaches. Conventional guided wave studies have focused on damage detection schemes using baseline data for damage classification. However, there are significant technical challenges to realizing this pattern comparison. As alternatives that can overcome the challenges of the conventional methods, new NDT techniques which do not rely on baseline data are proposed. In this thesis, three different types of reference-free damage detection schemes are proposed.;The first scheme is the application of a time reversal process (TRP) for detecting nonlinear damage. According to the TRP, the shape of a reconstructed signal should be identical to that of the original input signal unless there is nonlinearity induced by damage. However, the application of the TRP to Lamb wave propagation is complicated due to dispersive characteristics of Lamb waves and reflections from structures. In this thesis, the effect of Lamb wave dispersion, multimodes, and reflections on the TRP are investigated theoretically and experimentally. Furthermore, practical issues for implementation and the effect of a linear defect on the TRP are investigated.;The second scheme is the use of PZT transducers attached on both sides of a plate. In a thin elastic medium, the presence of a crack or a thickness variation causes Lamb wave mode conversion. In this thesis, it has been shown that converted Lamb wave modes due to the crack can be extracted from instantaneously measured signals. Furthermore, the applicability of the technique to more complex structures has been demonstrated.;The third scheme is the use of Dual PZT transducers. The dual PZT transducers are developed to enhance the second scheme so that PZT transducers can be placed only on a single side of the specimen. Numerical and experimental results are presented to demonstrate the applicability of the proposed technique to instantaneous crack detection in a plate with a uniform thickness. |
