The objective of this study is to develop a nondestructive damage evaluation methodology that can identify simultaneously both stiffness and damping changes in a structure. Two approaches are used to meet the stated objectives. First, a method is developed on the basis of the conservation of total energy; second, the other method utilizes the acceleration-structural parameters (stiffness and damping) sensitivities. The total energy in a system consists of the sum of the kinetic energy, the potential energy, and the dissipated energy. In the second approach, a baseline structure is first identified. A baseline structure is defined to be a structural system having a similar dynamic response to the existing structure with no damage. In this study, natural frequencies and modal damping values are used to identify the baseline structure. The performance of the developed methodology is validated using several numerical experiments; Two classes of structures are considered here: (1) a high-rise building modeled as shear beams and (2) a two-span continuous beam structure. In the shear beam model of the structure, the damping damage is simulated by increasing the Newtonian dash pot constant which models the dissipation at the damaged story. For the two-span continuous beam structure, it is assumed that damping of the undamaged structure can be modeled using a proportional damping matrix. The damping matrix of the damaged structure is modeled as the combination of a proportional damping matrix of the undamaged structure and a stiffness proportional damping matrix of the damaged element. Three damage cases are investigated for each of the two structures considered here. Only one element experiences damping damage for the first damage scenario. In the second damage scenario, both stiffness damage and damping damage are simulated with different severities in one element of the model. In the third damage scenario, two elements are simulated with stiffness damage and damping damage, to verify whether or not the developed methodology works for multi-damage cases. The proposed method is modified to use mode shapes and the modified proposed method is applied to experimental data to identify stiffness damage in a R/C structure.

No description available.

No description available.

Infrared thermography was used to evaluate Carbon Fiber Reinforced Panels (CFRP) and optimize the inspection process so that a set of guidelines can be established in order to be efficient and effective. It has been shown in previous work that when a material is heated up the heat will diffuse through the material at a constant rate. However, if there is a defect in the material, such as a delamination, this defect will act like an insulator. When this happens the heat cannot penetrate as quickly as the rest of the material so the surface above the defect shows up hotter than the rest of the material. An operator looks for inconsistent temperatures in the sample to determine the quality of the piece. Samples with simulated defects were made and modeled using a finite element program. Heat will be applied to the models and the temperature profiles analyzed. Along with changing the heat and time, different post-processing techniques were used to improve the method in determining defects in the sample. Once this has been optimized, actual CFRP with the same simulated defects was experimentally tested using the conditions from the analytical model. The analytical and experimental data was compared to insure that the testing process has been optimized. A standardized process was developed for evaluating the CFRPs using infrared thermography.

No description available.

No description available.

An investigation was conducted of the existence of seismic surface pulses (SSP) on crack faces in near-failure fatigue. An SSP has components of various modes of wave propagation. The component with the largest amplitude is a Rayleigh surface wave pulse. The possibility that these surface modes have much higher amplitudes than bulk modes of acoustic emission (AE) was illustrated by an idealized thought experiment relating an SSP on a half-space to the response of crack faces to crack extension. A number of aspects of AE monitoring in finite objects were investigated. Attributes of surface wave propagation on the edge of a specimen were found to be easier to monitor than other modes of wave propagation. Wavelet analysis was used to compare the characteristics of brittle AE with other sources. A new testing paradigm was developed to reduce interference from secondary sources of AE and enhance the investigation of AE from critical crack behavior. Unique specimen design features were developed, data acquisition features sought and validated, a dead weight load frame was modified, and data analysis procedures were developed. Criteria based on velocity, frequency content, amplitude and shape were devised to determine if an AE event is an SSP. The tests were designed to mimic load conditions on structures such as bridges and hence investigate the difference between AE generated in field conditions and that of typical laboratory conditions. Varieties of steel, from very ductile to very brittle, were tested. It was concluded that plastic zone formation, considered a secondary source of AE, was found not to interfere with the SSP activity. The SSP was found experimentally to have 2-3 times the amplitude of the bulk wave AE. The lack of sufficient AE did not allow for determination of conclusive changes in the AE as the specimens approached failure. However, it was found that brittle crack extension in fatigue and ductile failure can produce wave propagation resembling the SSP. / Ph. D.

A laser-based ultrasonic technique using a pulsed laser for stimulating ultrasonic waves in fiber-reinforced composite materials is the subject of investigation. For convenience, the material is chosen to be homogeneous transversely isotropic. The study is strictly limited to the laser power regimes that are suitable for nondestructive evaluation. An elastodynamic methodology is presented based on integral formulation in order to develop a representation for the dynamic responses in terms of the characteristics of the source that originated the motion. This requires a computation of elastodynamic Green function which represents the displacement field from the idealized synthetic sources localized precisely in both space and time. A two-dimensional numerical analysis utilizing a finite difference method for computation of the Green function in a finite plate is developed which provides the basis for quantitative nondestructive evaluation of fiber reinforced composite materials. Numerical results are presented for the surface displacement at the epicenter. Prediction based on numerical simulations are compared with experimental results. / Ph. D.

A method for acquiring and analyzing ultrasonic wavefields to characterize scattering from defects is described. A laser vibrometer and XY scanner are used to record high resolution wavefield data for angle-beam waves propagating in both a defect-free plate and a plate containing crack-like defects emanating from a through-hole. The properties of angle-beam wave propagation are first studied, which include wave generation mechanisms, propagation trajectories, and apparent phase velocities on the measurement surface. Scattering from a defect of interest is then analyzed by subtracting wavefields recorded before and after introduction of the defect. Wavefield subtraction is very sensitive to unavoidable spatial misalignment, which must be corrected prior to subtraction. Two methods for aligning wavefield data sets prior to subtraction are described and their performance is assessed. Several methods for characterizing scattering, including radial energy plots and scattering patterns, are described and used to quantify scattering from the introduced defects. Finally, efficacy of the scattering characterization methods is discussed and recommendations are made for future work.

The research described in this thesis involves the design, development, and implementation of an automated positioning system for fiber-optic interferometric sensors. The Fiber-Tip Interferometer (FTI) is an essential component in the proven Thermo-Acousto-Photonic NDE technique for characterizing a wide range of engineering materials including polymers, semiconductors and composites. The need to adapt the fiber-optic interferometric system to an industrial environment and to achieve precision control for optimizing interferometric contrast motivated the development of an automated, self-aligning FTI system design. The design enables high-resolution positioning and alignment by eliminating manual subjectivity and allows significantly improved repeatability and accuracy to be attained. Opto-electronic and electromechanical devices including a GRIN lens, 2x2 fused bi-conical taper couplers, photodiodes, motor-controlled tip/tilt stages, oscilloscopes, and a PCI card, constitute a closed-loop system with a feedback controller. The system is controlled by and communicates with a computer console using LabVIEW, a graphical language developed by National Instruments. Specifically, alignment is quantified by scanning the voltage readings at various orientations of the GRIN lens. The experimental setup specific to achieving maximum interferometric contrast intensity when interrogating silicon wafers with various surface depositions is discussed. Results corresponding to the interferometric contrast data obtained at several different standoff distances (Fizeau Cavity magnitudes) demonstrate the robustness of the novel design.