Structural Health Monitoring Using Ultrasonic Guided Waves And Piezoelectric Wafer Active Sensors

This dissertation is organized in three major parts covering ardent and challenging research topics in structural health monitoring (SHM). Part I focuses on the theoretical and numerical analysis of acoustic emission (AE) guided waves in both Cartesian coordinates (2D) and cylindrical coordinates (3D) system using energy approach, which, to the best of our knowledge, has not been yet reported elsewhere. AE guided waves appear due to sudden energy release during incremental crack propagation in a plate. The Helmholtz decomposition approach is applied to the inhomogeneous elastodynamic Navier-Lame equation for both the displacement field and body forces. For the displacement field, we use the usual decomposition in terms of unknown scalar and vector potentials. For the body forces, we hypothesize that they can be also expressed in terms of known scalar and vector excitation potentials. It is shown that these excitation potentials can be traced to the energy released during an incremental crack growth. Thus, the inhomogeneous Navier-Lame equation has been transformed into a system of inhomogeneous wave equations in terms of known excitation potentials and unknown solution potentials. The solution is readily obtained through integral transforms and application of the residue theorem. The resulting solution is a series expansion containing the superposition of all the Lamb waves modes existing for the particular frequency-thickness combination under consideration as well as the bulk waves. A numerical study of the AE guided wave propagation is conducted for a 6 mm thick 304-steel plate. A Gaussian pulse is used to model the growth of the excitation potentials during the AE event; as a result, the actual excitation potential follows the error vi function in the time domain. The effect of plate thickness, source depth, rise time, higher propagation modes and propagating distance on guided waves are investigated. Part II focuses on, an efficient analytical predictive simulation of scattered wave field based on physics of Lamb wave propagation for detecting a crack in a plate with stiffener. In this method, the scattered wave field is expanded in terms of complex Lamb wave modes with unknown amplitudes. These unknown amplitudes are obtained from the boundary conditions using a vector projection utilizing the power expression. An analytical tool (analytical global-local software) is developed to understand the effect of a cracked stiffener on Lamb wave propagation. The simulation results are verified with the finite element modeling and experimental results. Part III focuses an experimental and analytical study of irreversible changes in the piezoelectric wafer active sensor (PWAS) E/M impedance and admittance signature under high temperature and radiation exposure. For temperature dependent study, circular PWAS transducers were exposed to temperatures between 50oC and 250oC at 50oC intervals. The material properties of PWAS transducer were measured from experimental data taken at room temperature before and after high-temperature exposure. Change in material properties of PWAS transducers may be explained by depinning of domains or by domain wall motion without affecting the microstructure of PWAS transducer material. The degraded PWAS material properties were also determined by matching impedance and admittance spectrums from experimental results with a closed form circular PWAS transducer analytical model. For irradiation test, PWAS were exposed to gamma radiation (a) slow radiation test 100 Gy/hr rate for 20 hours and (b) accelerated irradiation test 1200Gy/hr for 192 hours. Electro-mechanical (E/M) impedance-admittance signatures and vii electrical capacitance were measured to evaluate the PWAS performance before and after gamma radiation exposure. The piezoelectric material was investigated microstructurally and crystallographically by using a scanning electron microscope, energy-dispersive X-ray spectroscopy, and X-ray diffraction methods. No noticeable changes in microstructure, crystal structure, unit cell dimension, or symmetry could be observed. This proposed dissertation ends with summary, conclusions, and suggestions for future work