Wenfeng Xiao

Date of Award

Fall 2020

Document Type

Open Access Dissertation

First Advisor

Lingyu Yu


Ultrasonic Lamb waves have been proved as an effective nondestructive evaluation (NDE) method due to their ability to propagate a long distance with less energy loss as well as their sensitivity to various defects on the surface or inside the structure. However, there are still challenges towards using them as a rapid inspection method for complex structural geometries, various damage types, and harsh environments, such as efficiency in Lamb wave actuation and sensing, understanding of the complicated multi-modal Lamb wave propagation, and robust detection algorithms for damage quantification and evaluation. To address these challenges, this dissertation focuses on developing a fully non-contact Lamb wave inspection system and appropriate damage detection algorithms and their applications to various structural components. Toward it, fundamental studies of the Lamb wave propagation are first conducted to understand the complicated Lamb waves (part I). Next, the single-mode Lamb wave inspection system with non-contact ACT actuation and SLDV sensing is configurated and damage detection algorithms are developed for quantitative damage detection (part Ⅱ). Finally, the applications for damage detection are applied to both isotropic metallic structures and anisotropic composite structures (part Ⅲ).

In Part I, the fundamental Lamb wave dispersion curves are calculated by solving the Rayleigh-Lamb wave equations. Besides, the plate structure and material influence on the dispersion curves are studied and the Lamb wave modeshapes are also theoretically calculated and studied. Next, to understand the Lamb wave propagation, the 1D and 2D analytically modeling of Lamb waves with both in-plane actuation and out-of-plane actuation are performed. To validate our analytical modeling with in-plane actuation, the Lamb wave modeling in the case of PWAS excitation is conducted. Then experimental work using the PWAS for in-plane actuation and SLDV for out-of-plane velocity is carried out for analytical modeling validation. Similarly, to validate our analytical modeling with out-of-plane actuation, the Lamb wave modeling in the case of pulsed laser excitation is performed, then experiments are conducted using the pulsed laser for out-of-plane excitation and SLDV for out-of-plane velocity measurement for the analytical modeling validation. After that, the Lamb wave interaction with a discontinuity is then conducted using the finite element method to understand the wave behavior when interacting with discontinuity.

In Part Ⅱ, the fully non-contact system is constructed by using a non-contact air-coupled piezoelectric transducer (ACT) for actuation and remote scanning laser Doppler vibrometer (SLDV) for sensing. Extensive studies on ACT configuration, actuation, and calibration are conducted to operate ACT actuation with optimal parameters. Based on Snell’s law, single-mode ACT Lamb wave actuation is conducted with incident angle θ. The result shows that a single fundamental antisymmetric Lamb wave mode (A0) can be obtained. To obtain the optimal Lamb wave signal, the ACT actuation setup is optimized by incident angle tuning. Various sensing schemes, line scan, or area scan, are investigated to obtain adequate information regarding wave propagation. The multi-dimensional Fourier transform method is adopted to analyze the multi-dimensional Lamb wave data giving the wave information in either time, space, frequency, or wavenumber domain for characterization analysis. The characterization result verifies the single A0 mode and shows the ACT actuated Lamb wave propagation is highly directional with its strongest wave intensity along the ACT axis direction. Other than that, to quantitatively evaluate the damage, and an improved cross-correlation principle-based imaging method using the scattered waves of all directions is proposed for damage imaging inspection.

In Part Ⅲ, applications of the ACT-SLDV system and single A0 mode method are explored for both nuclear-spent fuel dry cask structures and composite structures. To address the complex multilayered structures in spent fuel casks, systematic studies of detections of machined crack, simulated damage growth monitoring are preliminary conducted. Then the detections of fatigue-induced crack inspection as well as crack growth monitoring are implemented. Towards the complicated multi-layered dry cask structure application, crack inspection in multilayered plate structures is conducted. Other than that, the noncontact acoustic emission testing by the ACT is conducted on various metallic structures as well as composite structures. In composite structures, the most typical delamination and impact damage are inspected using the ACT-SLDV Lamb wave method. Other than that, composite manufacturing defects, such as weak-bond quality and composite wrinkle defects are inspected.