Author: Hwanjeong Cho

Publisher:

Published: 2017

Total Pages:

ISBN-13:

Ultrasonics provides a well-defined methodology for detecting and characterizing defects in materials. In particular, emerging ultrasonic guided waves techniques for advanced non-destructive evaluation (NDE) and structural health monitoring (SHM) applications aim for improved detection reliability and faster inspections. While the sensitivity of linear ultrasonics widely used for many conventional techniques is limited to defect sizes in the order of millimeters, nonlinear ultrasonics can provide opportunities to develop new techniques that enable detection of incipient micro-scale damage that precedes macro-scale cracks and delaminations. Thus, nonlinear ultrasonic guided waves are rapidly developing techniques that possess significant potential for early damage detection capabilities to be incorporated into emerging NDE and SHM techniques.The goal of this dissertation is to investigate several key aspects of generation and reception of nonlinear guided waves in plates based on understanding wave mechanics and nonlinear physics. This dissertation begins by discussing a multi- element PVDF (polyvinylidene difluoride) sensor able to detect guided waves polarized as either Rayleigh-Lamb (RL) or shear horizontal (SH) modes and determine their wavelength. A uniaxial polyvinylidene difluoride (PVDF) film is employed and rotated 45 to enable the coupling between in-plane shear deformation and the electric displacement in the out-of-plane direction. The multi- element configuration guarantees the wavenumber-frequency analysis for modal decomposition. Accompanying numerical and experimental studies validate the outstanding advantages such a low-profile, low-mass, conformable, inexpensive sensor ideal for SHM and other applications.The development of the new PVDF sensing method spurs investigation on its capability to detect localized fatigue damage in an aluminum plate with a nonlinear SH-SH guided wave interaction that results in another RL guided wave generation. A mode triplet (SH-SH-RL modes) is selected based on the theoretical formulation for mutual interaction of guided waves in plates and the corresponding mode selection criteria. The finite element simulations explore several numerical aspects of the associated wave propagation, interaction of the selected mode triplet, and mode identification. Laboratory experiments confirm second harmonic generation in an aluminum plate using the PVDF sensor by conducting frequency-wavenumber and other analyses, which are later used to detect localized fatigue damage in an aluminum plate with a new scanning technique.The utility of PVDF is further extended to both actuation and reception of self- interacting second harmonic guided waves to detect fatigue damage accumulation in an aluminum plate subject to cyclic mechanical loading. A comb type dual PVDF transmitter is designed to selectively actuate finite amplitude symmetric RL waves that in turn generate second harmonics. Self-interacting S1-S2 mode nonlinear guided wave measurements from an aluminum plate with progressive, but invisible, fatigue damage demonstrates the excellent robustness and sensitivity of PVDF transducers as well as early damage detection capabilities of nonlinear guided waves before catastrophic failures.Furthermore, to facilitate advancing to studies of nonlinear guided waves in anisotropic and layered media, this dissertation also discusses second harmonic generation in composite plates from theoretical and numerical standpoints. A theo- retical framework to analyze second harmonic generation in transversely isotropic plates is presented. Conditions for internal resonance are derived, followed by expanded mode pair selection strategies. Then, finite element simulations are con- ducted to confirm the predicted generation of second harmonic in a unidirectional lamina and quasi-isotropic laminate.Finally, the importance of skew angles on the generation of cumulative second harmonics in composite plates is emphasized by investigating the selection of internally resonant mode pairs under the influence of skewing guided waves. The skew angle occurs when wave velocities are directionally dependent and it causes deviation of the energy velocity vector from the intended wave vector. Taking into account the wave skewing effect, the skew angle matching condition is discussed for selecting mode pairs in a transversely isotropic plate and a quasi-isotropic laminate. Numerical simulations revealed that the effect of skew angle mismatch can be significant for second harmonic generation in anisotropic media.