PhD Thesis Colloquium

Date: 18 August 2014

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.