Friction stir welding (FSW) is an advantageous solid-state joining process, suitable for many materials in the energy, aerospace, naval and automotive industries. Like all other welding processes, friction stir welding requires non-destructive evaluation (NDE). The time and resources to preform NDE is expensive. To reduce these costs, nontraditional NDE methods are being developed for FSW. Spectral based defect recognition uses the forces during the welding process to validate weld quality. Although spectral NDE methods have shown promise as an alternative NDE processes, many research welding speeds do not correspond to manufacturing speeds, nor do they explain the relationship between the spectral data and the process. The purpose of this work is to explore the possibility of acquiring additional information about the defect. Namely the defect’s type, location, and magnitude. In this study, welds with “wormhole” defects were produced at 2000, 2500 and 3000 mmpm in 5754 aluminum. The welding process forces and torque were measured and analyzed spectrally. The welded plates were then imaged with x-ray photography, a validated NDE method. It was found that low frequencies (0 – 4 Hz) in the y & z force signals correlate with defect presence in high speed FSW. In addition, the strong correlation between the spectral data and the presence of a defect allowed for defect magnitude predictions. Linear fits were applied to the defect measurements and the spectral data. Large error inhibits the wide use of this prediction method.

Nowadays, metamaterials have found their places in different branches of wave physics ranging from electromagnetics to acoustic waves. Acoustic metamaterials are sub-wavelength structures in which their effective acoustic properties are dominated by their structural shape rather than their constitutive materials. In recent years, acoustic metamaterials have gained increasing interest due to numerous promising applications such as sub-wavelength imaging, perfect absorption, acoustic cloaking, etc. The focus of the work herein is to leverage acoustic metamaterial/metasurface structures to manipulate the acoustic wavefront to pave the road for future applications of the metamaterials. In the first part of the work, the metamaterial structure is introduced, which can be leveraged for better manipulation of the transmitted wave by modulating both phase and amplitude. Initially, a general bound on the transmission phase/amplitude space for the case of arbitrary metasurface has been presented and subsequently, the necessary condition for the complete modulation of the transmitted wave is investigated. Next, a horn-like space coiling metamaterial is introduced, which satisfied the aforementioned condition and enabled us to simultaneously modulate both the phase and amplitude of the transmitted wave. Furthermore, our initial efforts toward designing a metamaterial capable of real-time phase modulation with relatively constant amplitude will be discussed. In the second part of this work, a novel metamaterial-based methodology is presented for the design of the air-permeable acoustic silencer. In this work, the concept of the bilayer-transverse metamaterial is introduced, and its functionality for silencing the acoustic wave is demonstrated. Furthermore, it is shown that the methodology presented herein essentially does not limit the ratio of the open area, and ultra-open metamaterial silencers may be designed. Eventually, based on the presented methodology, the ultra-open metamaterial featuring nearly 60% open area is designed, and silencing capacity of about 94% at the targeted frequency is experimentally realized. In the last part of this work, the behavior of a locally resonant class of acoustic metamaterial in the non-Rayleigh regime has been explored. Elaborately, it is demonstrated that in the case of spherical inclusion in a matrix material, large variation in the effective acoustic impedance emerges near the inclusion’s Eigenmode. Eventually, the potential application of this novel phenomenon in the non-destructive evaluation (NDE) and ultrasound imaging is discussed. / 2020-08-09T00:00:00Z

This thesis describes the design of a robotic inspection tool which is based on a differential track-drive platform. The robotic inspection tool is a one man-portable UGV that has been developed for the purpose of non-destructive evaluation (NDE) and internal inspection of environments where human penetration may be difficult or hazardous. Various NDE and sensing techniques are described in this paper but the focus is on the mechanical and electrical design of the platform itself. The platform is a versatile device for mobile robotics research and development that supports a wide variety of instrumentation and payloads. Variable height control of the payload is achieved with a scissor lift assembly that provides accurate positioning of equipped sensors and instrumentation. The architecture of the platform was designed to support future autonomous implementations. / Master of Science

As existing bridge structures age, they are susceptible to the effects of deterioration, damage and other deleterious processes. These effects hamper the capacity and efficiency of transportation networks and adversely impact local, regional and national economic growth. As a result, bridge authorities and other professionals have become more sensitive to maintenance issues related to this aging infrastructure. While highway bridge condition have been monitored by visual inspection, non-destructive evaluation (NDE) technologies have also been developing and they are expected to be utilized for effective management of highway bridges or other civil infrastructure systems. Efficient use of these technologies saves time spent or bridge inspections, and also helps the bridge authorities for management decision-making. One of the NDE technologies is the image-based technology. In this thesis research, image-based technologies using high resolution digital images (HRDI) and infrared thermography image (IRTI) are introduced, described and implemented. First, a review of the mechanisms of these technologies is presented. Due to the specific engineering utilization and recent technological development, there is a need to validate effectiveness of HRDI and IRTI for their practical use for engineering purpose. For this reason, a pilot project using these technologies was conducted at an in-service bridge and a parking structure with the support of Florida Department of Transportation District 5 and the results are presented in this thesis. Secondly, in order to explore and enhance the usability of infrared thermography technology (IRTI), experiments on campus and on another bridge were conducted to determine the best time to test bridges and the sensitivity of IRTI to delamination volume. Since the iv accuracy of damage detection using infrared thermography technology is greatly affected by daily temperature variation, it is quite important to estimate an appropriate duration for infrared thermography inspection prior to the inspection. However, in current practice, the way to estimate the duration is to monitor the temperature of the concrete surface. Since the temperature varies depending on the area or region, there is a need to visit the bridge before the actual test and monitor the temperature variation. This requires additional visits to the bridge site and also access to the bridge for measuring concrete temperature. Sometimes, this can be a practical issue. In this research, in order to estimate an appropriate duration without visiting bridges, a practical method is explored by monitoring and analyzing variation of concrete surface temperature at one location and projected to another location by also incorporating other factors that affect the concrete temperature, such as air temperature and humidity. For this analysis, specially-designed concrete plates of a few types of thickness and shapes are used and the regression analysis is employed to establish a relationship between environmental effects and temperature variation between two different sites. The results have been promising for this research study and it is shown that HRDI and IRTI are excellent technologies for assessing concrete structures in a very practical manner.

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Corrosion begins as moisture penetrates the protective barrier of a surface, starting an electrochemical process which over time leads to surface pitting. The combined action of mechanical stresses and corrosion induced pitting reduces structural integrity as the pits enlarge to form nucleation sites for surface cracks, which propagate into through-thickness cracks. In most cases, the total mass loss due to corrosion within the structure is small; however, significant reductions in mechanical strength and fatigue life can occur in the corroded material leading to advanced crack growth rates or fast fracture. Since the structural damage due to localized corrosion pitting is small and the crack growth rates may be large, traditional inspections methods and "find it and fix it" maintenance approaches may lead to catastrophic mechanical failures. Therefore, precise structural health monitoring of pre-crack surface corrosion is paramount to understanding and predicting the effect corrosion has on the fatigue life and integrity of a structure. In this first third of this study, the impedance method was experimentally tested to detect and the onset and growth of the earliest stages of pre-crack surface corrosion in beam and plate like structures. Experimental results indicate the impedance method is an effective detection tool for corrosion induced structural damage in plates and beams. For corrosion surface coverages less than 1.5% and pit depths of less than 25 microns (light corrosion), the impedance method could successfully detect corrosion on plates and beams at distances up to 150 cm from the sensor location. Since the impedance method is a proven tool for corrosion detection, it makes sense to determine how well the method can quantify and track key corrosion variables like location, pit depth, and surface coverage. In order to make fatigue life adjustments for corroded structures it is necessary to quantify those variables. Thus, the second portion of this study uses the impedance method to quantify corrosion location, pit depth, and location. Three separate tests are conducted on beam-like structures to determine how well the damage metrics from the impedance method correlate to the key corrosion variables. From the three tests, it is found that the impedance method correlates best with the changes in corrosion pit depth, so if combined with data from routine maintenance it would be possible to use the impedance method data in a predictive or tracking manner. The impedance method can be correlated to location and surface coverage changes, but the relationship is not as strong. Other NDE techniques like Lamb Waves could use the same sensors to quantify corrosion location, and perhaps surface coverage. The impedance method can detect and quantify pre-crack surface corrosion which leads to shortened fatigue life in structures; however, the sensors must be robust enough to withstand corrosive environments. The last portion of this study tests the following: corrosive effect on Lead Zirconate Titnate (PZT) and Macro Fiber Composites (MFC) sensors, Kapton protected MFC actuators for corrosion detection, and determines if corrosion damage can be sensed on the side of the structure opposite the damage. Sensor recommendations regarding the use of piezoelectric sensors in corrosive environments are made. / Master of Science

A high frequency impedance-based qualitative non-destructive evaluation (NDE) technique has been successfully applied for structural health monitoring at the Center for Intelligent Material Systems and Structures (CIMSS) [1-3]. This new technique uses piezoceramic (PZT) patches as actuator-sensors to provide a low-power driven constant voltage dynamic excitation, and to record the modulated current flow through the structure. Therefore, it relies on tracking the electrical point impedance to identify incipient level damage. The high frequency excitation provided by the PZT, ensures the detection of minor changes in the monitored structure. It also limits the sensing area to a region close to the PZT source, therefore only changes in the near field of the PZT are detected, enhancing the ability of this technique to localize incipient damage. The phenomena of the PZT's sensing region localization has been the driving motivation for this research. More fundamental analytical research should be performed before full application of this technique is possible. Thereby, a wave propagation continuum mechanics based approach has been applied to model the high frequency vibrations of one dimensional structures. Energy dissipation mechanisms, such as bolted connections and internal friction, are considered to have a major role in the attenuation of the PZT's induced wave, therefore these mechanisms has been extensively studied. To analyzed bolted connections, linear and nonlinear joint models have been used to describe the wave interaction with such nonconservative discontinuities. Also, with the use of an impedance based model, the electromechanical coupling of the PZT and the host structure is added into the formulation. The wave interaction and energy dissipated at the bolted discontinuity has been assessed with energy flux computations of the incident, transmitted, and reflected waves. The effect of loosening the bolted joint has been also analyzed by reducing the spring stiffness and increasing the damping in the dash pots for the linear joint model, and reducing the Coulomb stiffness and shearing force at the interface for the nonlinear case. A scheme based on the correspondence principle has been applied to calculate the specific damping capacity of a system, at any given frequency, as a quantification of the energy dissipated through the system. The material damping was added into the formulation assuming the modulus to have a complex representation, and therefore the corresponding loss factors were found with active measurement of the material properties of the specimen via a wave propagation method, that monitories the wave's speed at two locations. Once the bases of the analytical model have been set up and corroborated with experiments, a parametric study has been developed to account for the various factors that can affect the sensing range of the PZT’s induced wave, and therefore to have a “rule of thumb on how to go about” when bonding PZTs to structures to monitor them. Apart from the energy dissipation mechanisms, other parameters responsible for the reflection of the incoming wave, and its consequent attenuation, has also been reconstructed. With the extensive analysis of these parameters, an impedance damage metric, based on the undamaged and damaged impedance, has been developed for various factors that can be the source of incipient damage. An attenuation metric has also been introduced to identify the degree of transmission of the propagating wave at certain discontinuities. The analysis of the case scenarios reproduced in this parametric study will aid in the knowledge about the number of PZTs needed to be placed in the monitored structure, the most critical locations, and when a monitored member in a system need to be replaced. / Ph. D.

The ultrasonic NDE technique to characterize the ultrasonic nonlinear response of the cyclically load 7075-T6 aluminum is described in this thesis. In order to estimate the nonlinear relation of the ultrasonic waves due to material fatigue damage or degradation, the spectral analysis techniques such as the power spectrum, bispectrum, and bicoherence spectrum are applied. The ultrasonic nonlinearity parameters by Cantrell and Jhang are introduced and presented as a function of the material fatigue growth, the number of fatigue cycles. This thesis presents the effectiveness of the bispectral analysis for evaluating the nonlinear aspects of the ultrasonic wave propagation. The results show that the nonlinearity parameters by Cantrell and Jhang are responsive to the output amplitude of the received signal and vary for the various materials, and independent of the input frequency and the ultrasonic wave propagation distance. By using the bispectral analysis tools, particularly the bicoherence spectrum, the increase of the coupling levels between the fundamental, its harmonic, and subharmonic frequency components is presented as the number of fatigue cycles is increased. This thesis suggests that the application of the bicoherence spectrum based on the nonlinear wave coupling relations be more effective for estimating the level of the material fatigue life. / Master of Science

An estimated 15 million people in the US have had a near‐death experience (NDE), an experience of usually lucid consciousness during a close brush with death. Following an NDE, experiencers (NDErs) sometimes feel challenged and seek counseling to integrate the experience into their subsequent lives. They have reported psychologically harmful experiences disclosing their NDEs to healthcare professionals, including counselors. Counselors' knowledge and attitude about NDEs appear to be critical variables in their ability to uphold the ethical imperative to do no harm to clients. The recent development of a psychometrically sound instrument to assess these variables, coupled with online availability of a three-part NDE educational program for health professionals, made possible for the first time a large‐scale pre‐post study of the effect of the program on counselors' knowledge and attitude about NDEs. Participants were 212 licensed professional counselors (LPCs) aged 23 to 71 years old (M = 44.93, SD = 12.69); sex self-identified as 12.3% male, 87.3% female, and .5% other; racially/ethnically self-identified as 84% White and 17% non-White and as 6.6% Latino-Hispanic and 92.5% non-Latino/Hispanic; and representing four regions of the US. Results revealed that, compared to control group, composed of LPCs who completed topically unrelated online programs (n = 112), those who completed the NDE program (n = 100) showed significantly more accurate knowledge and more positive attitude about NDEs. Participants also overwhelmingly expressed enjoyment of the programs. These results support the use of online training to increase counselors' knowledge and improve their attitude about NDEs so they can provide clinically and ethically sound treatment to NDErs. Limitations of the study and future research are addressed.

This thesis investigates current scientific literature on near-death experiences (NDE), focusing on two common types of models used to explain the NDE: biological models and spiritual models. Four biological models and two spiritual models are presented and evaluated. During the evaluation, particular emphasis is placed on how these models accommodate veridical evidence. I found that, of the biological models, only one could account for veridical evidence, whereas both of the spiritual models provided explanations of these strange occurrences. From this conclusion, the implications of NDE research on human survival are explored.