| Summary: | <p> Composite materials are receiving increasing attention and broadly used in aerospace industry due to their superior strength-to-weight ratio, corrosion resistance and design flexibility. The need for rapid nondestructive evaluation (NDE) techniques for composites is growing rapidly as the complexity and dimensions of the structures are increasing significantly. Structural health monitoring (SHM) has been attracting much attention as a means of providing in-service and in-situ monitoring of various critical structures. Due to their capability of long-range and through-the-thickness interrogation of the structures for small defects, guided waves have been studied extensively in damage detection for plate-like structures. </p><p> However, a few challenges exist when Lamb wave-based SHM/NDE techniques are employed. For example, the dispersion effect decreases the accuracy of many damage imaging algorithms; damage severity quantification is always a difficult problem. To provide possible solutions to above challenges, two damage imaging algorithms were developed and utilized for Lamb-wave based damage imaging. </p><p> The first algorithm is reverse-time migration (RTM), which was first used in geophysics to provide proper solutions to complex wave phenomena. The traditional imaging condition utilized in SHM is called excitation-time imaging condition, which used ray tracing and group velocity corresponding to the center frequency of the input signal. Due to the dispersion effect, the time-of-flight (ToF) estimation cannot always be accurate, especially for the situations that the Lamb waves propagate for a long distance. In this thesis, new imaging conditions are proposed to form enhanced zero-lag cross-correlation reverse-time migration (E-CCRTM) techniques. The proposed damage imaging technique takes into account the amplitude, phase, and all the frequency content of the Lamb waves propagating in the plate; thus, the severity of multiple sites of damage can be non-biasedly imaged regardless of the damage locations in comparison with using existing imaging conditions. The other imaging algorithm is called ‘DORT-MUSIC’. A Lamb wave-based, subwavelength imaging algorithm is developed for damage imaging in large-scale, plate-like structures based on a decomposition of the time-reversal operator (DORT) method combined with the multiple signal classification (MUSIC) algorithm in the space-frequency domain. The physics of wave propagation, reflection, and scattering that underlies the response matrix in the DORT method is mathematically formulated in the context of guided waves. Singular value decomposition (SVD) is then employed to decompose the experimentally measured response matrix into three matrices, detailing the incident wave propagation from the linear actuator array, reflection from the damage, and followed by scattering waves toward the linear sensing array for each small damage. The SVD and MUSIC-based imaging condition enable quantifying the damage severity by a ‘reflectivity’ parameter and super-resolution imaging. </p><p> The two algorithms were also integrated with a hybrid system mainly comprised piezoelectric actuators mounted onto the structure and a laser Doppler vibrometer (LDV) for reception. The flexibility of the proposed system was used for inspection of various plate-like structures. The experimental results show that the 2-D E-CCRTM has robust performance to image and quantify multiple sites of damage in large area of the plate using a single PZT actuator with a nearby areal scan using LDV, and the DORT-MUSIC (TR-MUSIC) imaging technique can provide rapid, highly accurate imaging results as well as damage quantification with unknown material properties.</p>
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