PhD Thesis Defense
Date: 15 September 2015
Ultrasonic Guided Wave based Models, Devices and Methods for Integrated Structural Health Monitoring
Vivek T Rathod
Supervisor: D. Roy Mahapatra
Venue: AE Conf. Hall, AE Department
Abstract
Efficient design of Structural Health Monitoring (SHM) system involves
detailed understanding of the wave field localization in the structures,
related transducers and associated signal processing circuits. In order to
achieve these in context of guided wave based SHM, in this thesis, first a
time-frequency spectral finite element for guided wave propagation in thick
and layered solid with higher-order dynamic field variables is formulated.
The normal and shear tractions on the surfaces of the thick beam/plate are
satisfied in closed form. The developed spectral finite element is validated
using two-dimensional h-p finite element based detailed modeling of wave
field. Using the developed spectral finite element, scattering model of a
notch or machined slots is derived. The scattering models for more
complicated features such as bonded and bolted stiffener are derived using
ultrasonic ray tracing based approach applied to 2D plate type structures.
Piezoelectric wafer type transducers are considered in the SHM problems of
interest. An analytical model of actuator is presented to determine the
launched guided wave characteristic in a plate structure. This model is
validated with a detailed finite element modeling and simulation.
Applications of this actuator model to generate guided waves in plates with
single and phased array element(s) are validated experimentally using
scanning Laser Doppler Vibrometer (LDV). Using the guided wave principle, a
technique is developed to characterize piezoelectric thin film sensors and
actuators at ultrasonic frequencies in situ or in integrated configuration,
and including frequency and directional sensitivity. Performance of the
piezoelectric thin film under quasi-static, dynamic and transient impact
loadings are analyzed. This characterization study enables one to select
optimal frequency bands. The thin film actuators are further designed with
inter-digital electrodes and their characteristics are analyzed. The ability
of frequency tuning and directional sensitivity is discussed. Detailed
characterization of transducers is done to determine their sensitivity with
thermal degradation and fatigue. A novel reusable bonding and calibration
techniques are developed and related advantages are discussed. Toward SHM
methodology development, a detailed study is carried out on the effect of
crack opening and specimen size on the guided wave reflection and
transmission. The effect of plastic zone on the guided wave reflection and
transmission characteristics is also studied. The feasibility to determine
plastic zone and fatigue crack propagation with integrated piezoelectric
transducers is demonstrated experimentally and the results are verified
analytically. The same method is also proven to be effective for the
detection of fatigue crack tip plastic zone. An approach to estimate fatigue
life is also proposed. Using these transducers, a concept of compact
circular array is presented to rapidly localize various types of damages in
plate type structures. An algorithm is developed that uses wavelet
transforms to localize and estimate the severity of damages like stiffener
delamination, hole enlargement, bolt failure and corrosion. A ray-tracing
algorithm is proposed to simulate guided waves in structures using
piezoelectric wafer transducers. The results obtained from the ray-tracing
algorithm are validated with experiments.
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