Date of Award

2018

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Victor Giurgiutiu

Abstract

The engineering infrastructures have a growing demand for damage monitoring systems to avoid any potential risk of failure. Proper damage monitoring solutions are of a great interest to this growing demand. The structural health monitoring (SHM) and nondestructive evaluation (NDE) offer appropriate online and offline damage monitoring solutions for a variety of mechanical and civil infrastructures that includes unmanned aerial vehicles (UAV), spaceships, commercial aircraft, ground transportation, wind turbines, nuclear spent fuel storage tanks, bridges, naval ships, and submarines. The fundamentals of the ultrasonic SHM and NDE consist of multi-disciplinary fields. The dissertation addresses SHM and NDE using ultrasonic guided waves, with an emphasis on the development of an analytical solution for non-axisymmetric guided wave propagation, multiphysics simulation, and experimental study of acoustic emission from the structural fatigue damage.

An analytical solution for non-axisymmetric coupled guided wave propagation in plate-like structures was developed based on the equations of motion and elasticity relations. A general non-axisymmetric solution of guided wave propagation inplateis needed to analyze the guided wave-scatter from non-axisymmetric damage as encountered in practice. Under non-axisymmetric conditions, the problem is highly coupled and no potential based analytical solution has been reported in the literature so far. Helmholtz decomposition theorem was applied to the Navier-Lame equations that yielded a set of four coupled partial differential equations in four unknowns, the scalar potential Φ and the

three components of the vector potential Hr, Hz, HΘ. A fourth equation, the ‘gauge condition’ was then added to the decomposition. A particular interpretation of the gauge condition is proposed. Our proposed approach decouples the governing equations and reduced the number of unknowns from four to three thus allowing one to express the solution in an elegant straight-forward way. The Rayleigh-Lamb characteristic equations were recovered and a general normal-modes expression for the solution was obtained.

A hybrid global analytical and local finite element method was used to solve a practical aerospace rivet hole crack detection. The scatter cube of complex-valued wave damage interaction coefficients (WDICs) was developed to analyze any rivet hole of a multiple-rivet-hole lap joint system. It had been shown that not all parameters such as actuator-sensor locations, and frequencies were equally sensitive to the damage scatter. The optimum combination of parameters could better detect the crack in the rivet hole. The simulated time domain signals were produced for the optimum combination of parameters.

Multiphysics simulations for fatigue crack generated acoustic emission (AE) were performed and the results were validated by the experiments. A novel application of inexpensive piezoelectric wafer active sensors (PWAS) has been explored. It has been shown that PWAS transducers successfully captured the fatigue-crack generated acoustic emissions in the thin plate-like aerospace materials. PWAS performance was compared with existing commercial AE sensors. It was found that PWAS captured richer frequency content than the existing AE sensors. Various AE waveform signatures were found from the fatigue crack advancement during the fatigue load evolution. Some AE waveform signatures were found to be related to the fatigue-crack extension while some of them were related to the fatigue-crack fretting, rubbing, and clapping. This observation was confirmed viii by synchronizing the fatigue loading with AE measurement by the same AE instrument. The in-situ microscopic measurement was performed during fatigue loading in MTS which provided the insights of the AE waveform evolution. It was hypothesized that the crack length estimation could be related the AE waveform signatures. FEM simulations and experiments were conducted using laser Doppler vibrometer (LDV) to verify our hypothesis.

Two case studies are discussed showing the implementation of SHM and NDE approach in practical applications: (1) horizontal crack detection, size, and shape estimation in stiffened structures, (2) impact damage detection in manufactured aerospace composite structures.

The dissertation finishes with conclusions, major contributions, and suggestions for future work.

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