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Weld inspection is significant in manufacturing to improve productivity and ensure safety. During the welding process, steel microstructures experience complex transformations depending on welding conditions. Examining weld microstructures can reveal valuable information on its metallurgical, mechanical and electromagnetic properties. In this research, a novel weld imaging method has been proposed based on EM testing. The newly designed electromagnetic (EM) sensor has a feature that at certain lift-off, the effect of a sample's conductivity on the sensor's response is reduced. Hence, the permeability can be estimated in a reasonable accuracy without the influence of its conductivity for dual phase steels. This sensor design enables better spatial resolution for weld imaging with reduced lift-off effect. A cross-section of an X70 steel submerged arc welding (SAW) sample has been imaged using impedance and a novel frequency feature in multi-frequency EM testing. It is derived that this frequency feature is closely related to the permeability of the sample. Therefore, the imaging results obtained from this feature reflects better permeability map of the sample. These images show good correlation with the hardness testing and metallurgical information of the weld sample. An approximate linear relationship was found between the EM signal and the hardness of the weld. The novel method significantly reduces scanning time with respect to hardness test and requires less surface preparation. And the operation frequency range can be adjusted to suit a particular instrument capability.

This thesis presents research towards the realisation of new eddy current-based multi-frequency techniques for non-destructive testing of the graphite bricks within the core of an advanced gas-cooled nuclear reactor (AGR). The graphite core suffers from two main ageing mechanisms (density reduction and cracking) and both were investigated in this thesis. There is a close relationship between the density of graphite and its electrical conductivity, which allows density changes to be monitored using eddy current techniques. A novel eddy current sensor configuration was specifically optimised for inspection of the graphite bricks, which contain the AGR fuel elements. The new sensor was designed to operate within the fuel channel bricks and its performance was validated using both Finite Element modelling and experimental studies. Results show that the new sensor offers an approximately 43 % improvement in sensitivity with depth, compared with the existing eddy current sensor currently used for routine reactor core inspections. The thesis also considers the novel solutions to the problem of determining the depth profile of the electrical conductivity of the bricks. The depth profile of electrical conductivity could allow the density profile to be inferred. This study has focused on the formulation of a suitable non-linear inversion algorithm that can accurately estimate the depth profile of the electrical conductivity based on the measurements collected from a real reactor core, through the implementation of a new forward model calibration and constraining techniques. Two different Tikhonov based inversion algorithms have been studied, namely regularised Gauss Newton and regularised Levenberg Marquardt. The former algorithm was already implemented for this application in a previous PhD project. This research has now extended the algorithm with new constraining techniques to improve its performance. The results from the reconstructed profiles using a data from a representative laboratory sample show that the graphite cross-sectional profile can be reproduced with 98.7 % accuracy when the new constraining technique is applied within the algorithm. A new application of the regularised Levenberg Marquardt algorithm was adapted for the conductivity profiling as part of this work, and the performance of this algorithm was compared with the regularised Gauss Newton method. The two algorithms were compared based on the reactor data inverse solutions. Slightly faster convergence rate was observed from the regularised Gauss Newton algorithm, but it suffers from inaccuracies with increasing depth from the graphite bore. The regularised Levenberg Marquardt algorithm tends to produce much more accurate profiles, although it takes longer to arrive at the final solution. Comparisons between the reconstructed profiles of the fuel channel bricks using RLM and the measurements from the trepanned sample also showed reasonable agreements between one another, with mean profile errors ranging between 1.5 % and 16.9 %. This is the first time that such good agreement has been obtained from reactor data. Experimental studies concerning with realistic sub-surface crack have been carried out using the new sensor. During these studies an attempt was made to extend the existing method of determining crack location. The results from these studies show that the new sensor along with the extended data processing method allows a detection of realistic crack that was 34 % of the entire graphite wall, whereas the existing sensor was only detected a crack that is 48 % of the entire graphite wall.

This thesis describes the development of an eddy current instrument and its application in detecting early-stage pitting corrosion. Eddy current testing has previously been used in Non-Destructive Testing (NDT) applications detecting large defects, like cracks. However, the challenge of detecting corrosion pits of less than 1mm³ remains unaddressed. This research involved the design of a Field Programmable Gate Array (FPGA)-based eddy current instrument, and the design and modelling of a novel differential electromagnetic sensor. The FPGA provided accurate synchronisation among the major electronic components. The firmware developed as part of this research allowed for exact interfacing to A/D and D/A converters, performed a real-time demodulation and signal generation, the instrument also supported a multi-frequency eddy current application. The firmware showed promising end-results in terms of sensitivity and stability in relation to pitting corrosion detection. In summary, this instrument offered significant improvement in sensitivity; the size of corrosion detected is improved more than 10 per cent compared to the previously reported, which enabled the detection of pits smaller than 1 mm³. For the sensor probe, a novel differential sensor was proposed to minimise the background signal for plate scanning and improve the sensitivity. The designed probe has an advantageous feature: the sensor response can be analysed using a closed form analytical solution.

The aim of this work has been to apply the techniques of non-destructive testing and evaluation to the graphite fuel channel bricks which form the core of an Advanced Gas-Cooled reactor. Two modes of graphite degradation have been studied: subsurface cracks originating from the keyway corners of the bricks and the reduction in material density caused by radiolytic oxidation. This work has focused on electromagnetic inspection techniques. Brick cracking has been studied using a multi-frequency eddy current technique with the aim of determining quantitative information. In order to accurately control the crack dimensions this work has used radially machined slots as an analogue. Two sensor geometries were studied and it was determined that slots of at least 10 mm through-wall extent could be located. A novel, empirical method of determining the slot size is presented using a brick machined with a series of reference slots. Machined slots originating from a keyway could be sized to within 2 mm using this method. A parametric 3D finite element study was also carried out on this problem. These simulations could distinguish the location of the slots and had some sensitivity to their size, however, the model was found to be overly sensitive to the specific mesh used. Two new contributions to the inverse problem are presented. The first is a minor extension to the usual adjoint problem in which one system now contains a gradiometer. The second is a proposed solution to the ambiguous nature of the inner product required by the sensitivity formulation. This solution has been validated with finite element modelling. Density reduction has been studied via its relationship with electrical conductivity using a technique based on impedance spectroscopy. An inverse eddy current problem has been solved using the regularised Gauss-Newton method to determine the conductivity of the brick over its cross section. The associated forward problem has been solved using the finite element method on a simplified geometry. Tikhonov regularisation has been employed to overcome the ill-posed nature of the inverse problem. This method has been applied to a range of sample and sensor geometries and found to produce excellent results from laboratory data provided the finite element model is well calibrated. Bore or surface conductivity values can be reproduced to better than 1% with the accuracy reducing with distance from the sensor. The sensitivity of the algorithm to the regularisation parameter has been studied using the L-curve method and the effect of two regularisation operators has also been examined. A new method of choosing the regularisation parameter a priori is proposed and tested. Data taken during reactor outages produces physically realistic profiles although the results appear off-set from electrical resistivity values measured using the four-point method. The focus of future work should be to remove this effect which will likely require improvements to the forward model.

Electromagnetic Acoustic Transducers (EMATs) are advanced ultrasonic transducers that generate and detect acoustic waves in an electrically conducting material without making physical contact with the material unlike its counterpart, the piezoelectric transducers (PZT). The conventional EMAT consists of copper coil that generates the dynamic field when excited with a sinusoidal current, a permanent or electromagnet that provides the bias field and the conducting material specimen. The complex interaction between the bias field and the Eddy current induced within the skin depth of the conducting material by the dynamic field gives rise to the acoustic wave that then propagates within the surface of the material. Within the research a finite element EMAT model was developed using commercial software Comsol Multiphysics, to study and compare the Eddy current density and Lorentz force density generated by three EMAT configurations: The Meander-line, Spiral and Key Type EMAT configuration respectively. It was observed that apart from the ease of fabrication and simplicity of connectivity when stacked in layers, the Key Type coil EMAT showed a high tendency to generate higher amplitude of Eddy current and Lorentz force test materials especially when stacked in layers. Also, the effect of varying some key EMAT parameters was investigated to determine the optimal performance of Key Type EMAT configuration on CS70 pipe steel plate. The research further developed a coupled finite element model using the same software, Comsol Multiphysics to account for the generation, propagation and detection of acoustic wave by the Key Type EMAT configuration on CS70 grade of pipe steel. The model can solve the magnetostatic, electrodynamic and elastic equations that give rise to acoustic wave generation, propagation and detection on the test material. The developed coupled finite element model was validated both analytically and experimentally to establish the validity of the finite element model. The analytical and experimental results obtained were consistent with the numerical result with an average discrepancy less than 9 % percent. Finally, the research developed a novel modelling strategy to decouple and quantify the various transduction forces in operation when normally-biased EMAT and magnetostrictive EMAT configurations are used on various grades of pipe steel materials. The strategy established the value of the critical excitation current beyond which acoustic wave is generated solely by the dynamic Lorentz force mechanism. The critical excitation currents when Magnetostrictive EMAT configurations are used to generate acoustic wave was found to be; 268A, 274A, 279A, 290A and 305A for CS70, L80SS, L80A, TN80Cr3 and J55 respectively. While for Normally-Biased EMAT configurations, the critical excitation current was found to be 190A, 205A, 240A, 160A and 200A respectively. This work also compared the critical excitation current of the two EMAT configurations studied and established that normally-biased EMATs are more efficient in the generation of acoustic waves than their magnetostrictive counterpart due to their lower value of critical excitation current.

AbstractThis thesis describes research on the use of eddy current techniques for nondestructivetesting of carbon fibre reinforced plastic (CFRP). The research hasinvolved bulk conductivity testing, fibre direction characterization and 3D FEMmodeling of the CFPR and eddy current probes geometry. In the conductivity testing,how the sample thickness, fibre volume content and fibre conductivity affects thesignal from the eddy current has been evaluated. Eddy current testing shows gooddirectionality as CFRP is an anisotropic material, thus is very suitable to characterizethe fibre orientation. Direction sensitive probes have been developed and tested toreveal information about the fibre direction and layer. Computer FEM software hasbeen used to analyze the magnetic field inside the sample and probes. Specific probegeometries have been designed depending on the electrical properties of thecomposites and testing requirement. The experiment, simulation and analysis resultsshow very good agreement. However, when the measuring frequency increases, noisesand parasitic capacitance inevitably become significant and have a negative influenceon the results. Improvements and further research are proposed which are believed tomake eddy-current techniques a more feasible and efficient measurement method, willcontribute to the development and maintenance of light weight CFRP composites.

This work leverages recent advances in the Robot Operating System (ROS) to design aplatform capable of processing previously unknown geometries with minimal userinteraction. Industry needs inspection platforms which are adaptable to multiple partfamilies. An eddy current inspection tool was selected to prioritize design choices whichaccommodated micrometer-precision tool positioning when deployed on an 6-axisindustrial robot. These robots offer the physical reach and flexibility required by in situinspections at the refined control level necessary for accurate data collection. Surfacescanning and reconstruction was achieved using a low-cost, commercially availablestereoptic camera. Non-contact, material inspection techniques such as eddy current useelectromagnetic field propagation to measure variation in electrical conductivity andmagnetic permeability to detect defects. Geometry with sharp changes in topology causesmagnetic variations which give improper eddy current readings. To circumvent this issue,an operator is used to interactively select inspection surfaces. This manuscript will focuson the design process and a representative deployment using a Yaskawa Motoman GP7industrial robot. This study successfully implemented a high precision robotic inspectionplatform using highly configurable software controls while maintaining an easy-to-useuser interface.

There is a growing need to determine the residual stress profiles in surface-treated components to accurately and reliably determine the remaining service life of the component and to reduce the risk of failure. To acquire depth-dependent residual stress profiles, the XRD technique requires multiple layers of material to be removed with electro-etching, which renders the tested component unsuitable for service use. The nondestructive evaluation community has been in search of a method that can determine the residual stress profiles in surface-treated aerospace engine alloy components nondestructively. A promising nondestructive technique using high-frequency eddy current spectroscopy has been investigated with much potential. However, the two principal effects of shot peening, namely residual stress and cold work, are opposite in sign in Ti-6Al-4V, resulting in almost complete cancellation between the two competing effects, causing the eddy spectroscopy technique to be incapable of determining the residual stress profile of shot-peened components. This study investigates an alternative electromagnetic residual stress profiling technique using a dual-mode inspection with Hall impedance and eddy current spectroscopy to separate the components of shot-peening. The effect of residual stress (elastic strain) and cold work (plastic strain) will be investigated on Ti-6Al-4V (36 HRC) using both Hall coefficient and eddy current techniques. Experimental results, relative Hall impedance and AECC change spectra, were 3 obtained on shot-peened Ti-6Al-4V (36 HRC) with Almen intensities of 4A, 8A, and 12A. Destructive XRD residual stress and cold work depth profiles were obtained from the literature on shot-peened Ti-6Al-4V (32 HRC) with Almen intensities of 4A, 8A, and 12A for comparison. Relative Hall coefficient and electric conductivity depth profiles were calculated from the XRD residual stress and cold work depth profiles on the shot-peened specimens, using electromagnetic and galvanomagnetic gauge factors that were obtained experimentally. Relative Hall impedance and AECC spectrums were simulated, which are used for comparison purposes. Next, the simulated relative Hall impedance and AECC change profiles of Ti-6Al-4V (32 HRC) were compared to experimental relative Hall impedance and AECC change profiles of shot-peened Ti-6Al-4V (36 HRC). It was found that the simulated relative Hall impedance change profiles are capable of predicting the general trend caused by shot-peening, but are unable to predict the magnitude of change. The simulated relative AECC change profiles are capable of predicting the trend caused by shot-peening and the magnitude of change fairly well in comparison to the relative Hall impedance relative change. In this study, the separation of the two competing effects caused by shot-peening was concluded to be not feasible at this time. Although the separation is currently not possible, the study shows evidence illustrating the possibility of separating the residual stress and cold work using the dual-mode inspection technique with Hall impedance and eddy current spectroscopy with further efforts. Empirical calibrations will need to be completed to obtain accurate galvanomagnetic and electromagnetic gauge factors that extend over a range of residual 4 stress and cold work levels that are comparable with the values in common shot-peened components.

Eddy currents induce velocity errors (veddy) that influence PC-MRI derived parameters; static tissue corrections aim to minimize these veddy influences. Local and global static tissue corrections have been proposed; however, these separate strategies have yet to be thoroughly characterized. The dependence of local and global static tissue corrections on SNR and the amount of static tissue used for correction is not well understood. To characterize these relationships, corrections were analyzed within a static phantom and flow phantom experiment. Corrections were repeated for a range of SNR and for increasing amounts of static tissue used for polynomial fitting. Local and global corrections were separately applied to reduce velocity offset within ROIs within a static tissue phantom. Global correction reduced velocity offset in 100% of ROIs and local correction reduced velocity offset in ~96% of ROIs. Local correction introduced error in ~4% of ROIs (always when SNR < 30). Correction differences between local and global strategies in the static phantom experiment were on the order of 0.9 cm/s for low SNR protocols and 0.2 cm/s for high SNR protocols. Velocity offset within static ROIs was reduced by up to an additional ~0.6 cm/s by increasing static tissue coverage for low SNR protocols (~0.05 cm/s for high SNR protocols). Local and global corrections were separately applied to correct total volumetric flow estimates within a flow phantom. Veddy errors induce differences between total flows measured at different locations along a flow phantom. Static tissue corrections reduced these differences between total flow estimates in 90% of corrections. Of these cases, global correction provided a larger reduction in flow difference ~67% of the time. Local fitting introduced error in ~27% of total corrections, and errors were observed for a range of SNR. Global fitting introduced one instance of error (for lowest SNR). Flow differences were reduced by up to an additional ~2 mL by increasing static tissue coverage. This study indicates that: 1) Local static tissue correction is suitable only for higher SNR PC-MRI applications and 2) Maximizing the amount of static tissue used during polynomial fitting improves performance of applied corrections for low SNR applications.

Covetics are hybrid materials fabricated by fusing carbon with metals in an induction furnace. There is an indication that covetics have better thermal and electrical proper-ties in comparison with pure metals. The main goal of this research is to study thermal transport in covetics measured by specific absorption rate using calorimetry. In addition, temperature distribution has been measured along the both metal and covetic wires carrying currents of 5 A and 10 A, respectively. The first set of copper covetic (Cu cov) samples with nominal content of 36 at% carbon were prepared by Third Millennium Materials Company (TMMC) in the induction furnace using ceramic mold. After cooling, these samples were subjected to cold rolling to make wires. To remove contamination, defused from ceramic molds into covetics, the samples were subjected to numerous remelting steps in the induction furnace. In addition to remove stresses, the samples were annealed at elevated temperature in argon. Overall four Cu cov samples: as-received, and 8 remelt, including annealing, were pre-pared by TMMC. The second set of six Cu cov including annealing samples with nominal contents of 0 at%, 5 at%, and 10 at% carbon were prepared in AFRL facilities using graphite mold. For comparison, four other silver covetic (Ag cov) samples with nominal con-tents of 0 at% and 36 at% carbon were prepared under three different cooling processes by the TMMC. The SAR measurements of all covetic samples were done in a calorimetric system at currents of 5 A, 10 A, and 15 A, and at 177 kHz of AC magnetic field. The experimental data show that covetic samples have mixed heating rates in comparison to the pure metals. For instance, all the silver and Cu cov samples show lower specific absorption rates than the corresponding host metals, except the annealed Cu cov and druzzy Ag cov (a molten Ag cov was slowly cooled and stirred with graphite) samples. Additionally, SAR was higher for all the samples when the temperature sensor was touching top of the samples than for the sensor 2 mm above the samples. Finally, we observed the temperature distribution along Cu cov wire which was significantly lower than pure copper wire. Our research have showed that Cu cov samples provided by AFRL with nominal atomic percent of C increased from 0 to 10 at% indicate gradual decrease in average values of SAR and heating rate. For example, average value of SAR including annealed samples changes from 348 W/g for pure Cu (5N) to 77 W/g for Cu cov with 10 at% C. This de-crease of SAR (or heating rate) can be easily explained by changes in heat carriers from electrons (metal) to phonons (carbon). In contrast to this result, the samples provided by TMMC characterized by heavy cold rolling (Cu cov wire or Cu cov as received (AR)) with initial value of SAR very low and related to that low thermal conductivity. It can explain the observed high ampacity in this system. However, the same samples subjected to annealing process show significant increase in the thermal conductivity or SAR value as a result of stress removal in the presence of impurities or voids that are uniformly distributed inside of Cu or Ag grains.

The objective of this research is to determine the ability of several Non-Destructive Inspection (NDI) methods to detect various levels of High Cycle Fatigue fretting fatigue damage induced in simulated Ti-6Al-4V dovetail engine components. To generate various levels of fretting fatigue damage; a specially designed dovetail specimen is utilized which more accuracy simulates the cyclic loading interaction between a compressor blade and disk of an aircraft turbine engine. All fretting fatigue tests were conducted with un-coated Ti-6Al-4V alloy at ambient temperature, at a load ratio of 0.1, and two 30 Hz cyclic load levels (10% and 30% of expected life). In addition, two microstructures (α+β, β-annealed) are utilized to determine their effect on fretting fatigue as well as the NDI signal response. To quantify the extent of fretting fatigue damage; mini-C specimens are extracted from the fretted dovetail specimens and “step-tested” to quantify the debit in fatigue strength. Specimens are heat-tinted after fretting fatigue to help qualify the extent of fretting fatigue damage and aid in crack initiation site identification using both optical microscope and the Scanning Electron Microscope (SEM). Three NDI techniques are used to qualify the extent of fretting fatigue damage in Ti-6Al-4V and relate this damage to the NDI signal response and the debit in fatigue strength. The NDI techniques utilized in this research include: White Light Interference Microscopy (WLIM), Wyle Lab Eddy Current Inspection System (ECIS) and the JENTEK Meandering Winding Magnetometer (MWM) Array. Note: these NDI techniques are used “as is” and were not modified for fretting fatigue detection. However, in the case of the WLIM, a fretting fatigue damage parameter methodology is utilized to specifically quantify the extent of fretting damage. In addition, the Scanning Electron Microscope (SEM), Auger Electron Spectroscopy (AES), and Knoop micro-hardness tester were utilized to investigate the compacted fretting fatigue layer beneath the fretting fatigue scar and determine the existence of either a tribologolically transformed structure (TTS) or hard alpha case (HAC). In general, all three NDI techniques were able to detect various degrees of fretting fatigue damage (i.e., fretting fatigue cracks). However, as fretting fatigue cycles increased, the white light surface measurement technique's ability to discern higher levels of crack damage is suppressed by the modification of the contact fretting surface features (i.e., particle compaction with reduced asperity heights with nano sized contact debris) to include debris filled pits and cracks that help promote the loss of surface fidelity. The WLIM damage parameter and elements of the JENTEK MWM signal did correlate with the Mode I Newman-Raju stress intensity factor. However, the detection of fretting fatigue damage beyond fretting fatigue cracking was not attempted and would require special calibration specimens of a specific damage type (i.e., TTS, HAC, multiple co-linear cracking or perpendicular, slanted or zig-zag cracking, or corrosion effects) to correlate the NDI signal response to that damage.

The research work is focused on conducting a feasibility study on a new “non-contact” single probe dual coil inductive sensor for sensing the proximity and thickness of Aluminum (Al) 3003 alloy metal sheets, which is a non-magnetic metal. A bulk of the research and development work has already been done in the area of non-destructive testing (NDT) using eddy current technology targeted to various applications like corrosion detection, material thickness, material conductivity, etc. The research work presented in this thesis uses the prior research and development work completed in NDT as a platform for conducting this study to estimate proximity and thickness of Aluminum 3003 alloy metal sheets, which is not considered a flaw detection application. Some of the current technologies in the area of eddy current NDT for proximity and thickness estimation, each with its own limitations, include single probe ‘contact' sensors for magnetic metals, single probe ‘non-contact' sensors with separation distance of less than 1 mm and dual probe sensors that requires probes on both sides of the metal sheet.A swept multi-frequency scanning technique is used together with an automated data collection system to measure and collect output voltage and phase difference data over a wide range of frequencies. The skin effect in conductors and its associated property of skin depth is used to extract proximity and thickness information from the data collected, and then correlated with reference values to validate the results. Experimental results show the output voltage and phase difference of the sensor is dependent on the metal parameters (resistivity ‘ρ', permeability ‘μ', thickness ‘T') and coil parameters (diameter ‘D', frequency ‘F', lift-off ‘L'). Further, proximity is estimated from output voltage difference, and metal thickness (single/double) is estimated from phase difference independent of lift-off, which is a novel approach for thickness detection. The test sensor provides an accurate measure of proximity and thickness of Al 3003 alloy from a single sided measurement with varying lift-off, overcoming the limitations of other sensor configurations.

Recent research results indicated that eddy current conductivity measurements can be exploited for nondestructive evaluation of near-surface residual stresses in surface-treated nickel-base superalloy components. Most of the previous experimental studies were conducted on highly peened (Almen 10-16A) specimens that exhibit harmful cold work in excess of 30% plastic strain. Such high level of cold work causes thermo-mechanical relaxation at relatively modest operational temperatures; therefore the obtained results were not directly relevant to engine manufacturers and end users. The main reason for choosing peening intensities in excess of recommended normal levels was that in low-conductivity engine alloys the eddy current penetration depth could not be forced below 0.2 mm without expanding the measurements above 10 MHz which is beyond the operational range of most commercial eddy current instruments. As for shot-peened components, it was initially felt that the residual stress effect was more difficult to separate from cold work, texture, and inhomogeneity effects in titanium alloys than in nickel-base superalloys. In addition, titanium alloys have almost 50% lower electric conductivity than nickel-base superalloys; therefore require proportionally higher inspection frequencies, which was not feasible until our recent breakthrough in instrument development. Our work has been focused on six main aspects of this continuing research, namely, (i) the development of an iterative inversion technique to better retrieve the depth-dependent conductivity profile from the measured frequency-dependent apparent eddy current conductivity (AECC), (ii) the extension of the frequency range up to 80 MHz to better capture the peak compressive residual stress in nickel-base superalloys using a new eddy current conductivity measuring system, which offers better reproducibility, accuracy and measurement speed than the previously used conventional systems, (iii) the lift-off effect on high frequency eddy current spectroscopy, (iv) the development of custom-made spiral coils to allow eddy current conductivity characterization over the whole frequency range of interest with reduced coil sensitivity to lift off, (v) the benefits of implementing a semi-quadratic system calibration in reducing the coil sensitivity to lift-off, and (vi) the feasibility of adapting high-frequency eddy current residual stress characterization for shot-peened titanium alloys.

Surface enhancement methods, which produce beneficial compressive residual stresses and increased hardness in a shallow near-surface region, are widely used in a number of industrial applications, including gas-turbine engines. Nondestructive evaluation of residual stress gradients in surface-enhanced materials has great significance for turbine engine component life extension and their reliability in service. It has been recently found that, in sharp contrast with most other materials, shot-peened nickel-base superalloys exhibit an apparent increase in electrical conductivity at increasing inspection frequencies, which can be exploited for nondestructive residual stress assessment.The primary goal of this research is to develop a quantitative eddy current method for nondestructive residual stress profiles in surface-treated nickel-base superalloys. Our work have been focused on five different aspects of this issue, namely, (i) validating the noncontacting eddy current technique for electroelastic coefficients calibration, (ii) developing inversion procedures for determining the subsurface residual stress profiles from the measured apparent eddy current conductivity (AECC), (iii) predicting the adverse effect of surface roughness on the eddy current characterization of shot-peened metals, (iv) separating excess AECC caused by the primary residual stress effect from intrinsic conductivity variations caused by material inhomogeneity, and (v) investigating different mechanisms through which cold work could influence the AECC in surface-treated nickel-base superalloys. The results of this dissertation have led to a better understanding of the underlying physical phenomenon of the measured excess AECC on nickel-base engine alloys, and solved a few critical applied issues in eddy current nondestructive residual stress assessment in surface-treated engine components and, ultimately, contributed to the better utilization and safer operation of the Air Force’s aging aircraft fleet.

Recently planar magnetic technologies have been widely used in power electronics, due to good cooling and ease of fabrication. High frequency operation of magnetic components is a key to achieve high power density and miniaturization. However, at high frequencies, skin and proximity effect losses in the planar windings become significant, and parasitics cannot be ignored. This piece of work deals with the modeling and design of planar integrated magnetic component for power electronics applications. First, one-dimensional eddy current analysis in some simple winding strategies is discussed. Two factors are defined in order to quantify the skin and proximity effect contributions as a function of frequency. For complicated structures, 2D and 3D finite element analysis (FEA) is adopted and the accuracy of the simulation results is evaluated against exact analytical solutions. Then, a planar litz structure is presented. Some definitions and guidelines are established, which form the basis to design a planar litz conductor. It can be constructed by dividing the wide planar conductor into multiple lengthwise strands and weaving these strands in much the same manner as one would use to construct a conventional round litz wire. Each strand is subjected to the magnetic field everywhere in the winding window, thereby equalizing the flux linkage. 3D FEA is utilized to investigate the impact of the parameters on the litz performance. The experimental results verify that the planar litz structure can reduce the AC resistance of the planar windings in a specific frequency range. After that, some important issues related to the planar boost inductor design are described, including core selection, winding configuration, losses estimation, and thermal modeling. Two complete design examples targeting at volume optimization and winding parasitic capacitance minimization are provided, respectively. This work demonstrates that planar litz conductors are very promising for high frequency planar magnetic components. The optimization of a planar inductor involves a tradeoff between volumetric efficiency and low value of winding capacitance. Throughout, 2D and 3D FEA was indispensable for thermal & electromagnetic modeling.