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A new computational method for characterizing the relationship between surface crystallography and electrical conductivity in anisotropic materials with low frequency electromagnetic techniques is presented. The method is discussed from the standpoint of characterizing the orientation of a single grain, as well as characterizing statistical information about grain ensembles in the microstructure. Large-area electron backscatter diffraction (EBSD) data was obtained and used in conjunction with a synthetic aperture approach to simulate the eddy current response of beta annealed Ti-6Al-4V. Experimental eddy current results are compared to the computed eddy current approximations based on electron backscatter diffraction (EBSD) data, demonstrating good agreement. The detectability of notches in the presence of noise from microstructure is analyzed with the described simulation method and advantages and limitations of this method are discussed relative to other NDE techniques for such analysis., Prepared in cooperation with the University of Dayton Research Institute, Dayton, OH, and the Southwest Ohio Council for Higher Education, Dayton, OH.

A new computational method for characterizing the relationship between surface crystallography and electrical conductivity in anisotropic materials with low frequency electromagnetic techniques is presented. The method is discussed from the standpoint of characterizing the orientation of a single grain, as well as characterizing statistical information about grain ensembles in the microstructure. Large-area electron backscatter diffraction (EBSD) data was obtained and used in conjunction with a synthetic aperture approach to simulate the eddy current response of beta annealed Ti-6Al-4V. Experimental eddy current results are compared to the computed eddy current approximations based on electron backscatter diffraction (EBSD) data, demonstrating good agreement. The detectability of notches in the presence of noise from microstructure is analyzed with the described simulation method and advantages and limitations of this method are discussed relative to other NDE techniques for such analysis., Published in AIP Conf. Proc., v1581 p1456-1462, 2014. Prepared in collaboration with the University of Dayton Research Institute, Dayton, OH, and the Southwestern Ohio Council for Higher Education, Dayton.

Generalized polynomical chaos (gPC) and the probabilistic collocation method (PCM) are finding considerable application to problems of interest to engineers in which random parameters are an essential feature of the mathematical model. So far the applications have been mainly to stochastic partial differential equations, but we extend the method to volume-integral equations, which have met great success in electromagnetic nondestructive evaluation (NDE), especially with eddy-currents. The problems of main interest to the NDE community in this connection are concerned with the issue of 'propagation of uncertainty' when the relevant parameters are not well characterized, or are known simply as random variables. We demonstrate the ideas by considering a metallic surface that has undergone a shot-peening treatment to reduce residual stresses, and has, therefore, become a random conductivity field.

The primary objective of this research program is to improve the characterization of small discontinuities (cracks, corrosion) using eddy current and ultrasonic methods for complex structures using computational methods in NDE. The research approach concerns the development and validation of efficient 3D models that accurately represent complex eddy current (EC) and ultrasonic (UT) inspection problems. Simulated studies can then be used to gain a fundamental understanding of the NDE characterization problem for differentiating and sizing small discontinuities in the presence of coherent noise features present in complex structures. With this understanding and modeling capability, research on novel data analysis and classification methods is conducted exploring multi-dimensional signal processing using data from scanning systems and array sensors and model-based classifiers via inverse methods. The benefits of this program address minimizing disassembly, improving reliability, and maximizing availability of aircraft., Prepared in collaboration with Computational Tools, Gurnee, IL. The U.S. Government is joint author of the work and has the right to use, modify, reproduce, release, perform, display, or disclose the work. PA Case Number and clearance date: 88ABW-2010-0180, 15 Jan 2010. Published in E-Journal of Advanced Maintenance Vol.1: 99-108; 2010. 2009 Japan Society of Maintenology. This document contains color.

Eddy current models have matured to such a degree that it is now possible to simulate realistic nondestructive inspection (NDI) scenarios. Models have been used in the design and analysis of NDI systems and to a limited extent, model-based inverse methods for Nondestructive Evaluation (NDE).The science base is also being established to quantify the reliability systems via Model-Assisted Probability of Detection (MAPOD), In realistic situations, it is more accurate to treat the input model variables as random variables rather than deterministic quantities. Typically a Monte- Carlo simulation is conducted to predict the output of a model when the inputs are random variables. This is a reasonable approach as long as computational time is not to long; however, in most applications, introducing a flaw into the model results in extensive computational time ranging from hours to days, prohibiting Monte-Carlo simulations. Even methods such as Latin-Hypercube sampling do not reduce the number of simulations enough for reasonable use., Prepared in collaboration with Computational Tools, Gurnee, IL.

To support an effort to estimate characteristics of surface-connected damage (i.e., cracks) in aerospace and propulsion structures, a computer-controlled scanning capability that acquires experimental eddy current data was developed based on an Agilent 4294A Impedance Analyzer. This instrument is being used to assess performance of different computational electromagnetic codes in terms of determining how well they predict the complex response from simulated flaws in conducting plates. The particular benchmark problem of interest in this paper measures the real and imaginary impedance due to Electrical-Discharge Machined (EDM) notches in aluminum plates that are approximately 13 mm thick. These EDM notches are 5mm deep and have the same surface length and width. A range of notches have been generated for this study, which includes vertically oriented notches and angled notches of 10, 20 and 30 degrees relative to the surface normal., Prepared in collaboration with Iowa State University, Ames, IA.

This paper presents preliminary modeling work of split D differential probes, in both regular differential and reflection modes. This work is a prelude for a more in-depth model validation study using split D type probes. A modeling comparison is made for both air and ferrite cores. Lastly, numerical and experimental results are compared., Prepared in collaboration with Dayton Univ Research Inst ., OH.

Eddy current detection of flaws in edges presents challenges in experimental procedures during benchmark studies in the laboratory for model validation as well as practical implementation of a real world detection system. These difficulties result in distortions to the signal that mask the effects from the flawed region itself. Rather than attempting to perfect the experimental setup, we propose to make the numerical models more robust by incorporating randomness in the experimental procedure with uncertainty quantification methods. We present the motivation for the specific method chosen, the probabilistic collocation method (PCM), and the mathematical development behind the method, and then present the results from numerical simulations with a validation measure., Prepared in collaboration with Dayton Univ Reasearch Inst., OH and Wright State Univ., OH.

In order to quantify the reliability of NDE systems, large amounts of experiments are performed to develop a probability of detection (POD) curve for the system. These POD studies require a substantial amount of experimentation which can sometimes be cost prohibitive. To expedite the process of developing these curves, highly precise numerical models are used in conjunction with NDE sensors to understand the uncertainties associated with the inspections. Numerical models are also used in stochastic inversion methods such as Bayesian inversion, which provide a means of characterizing system properties with uncertainties. A strong basis has been developed in the modeling and simulation community for deterministic forward models in NDE, but to fully incorporate these models in model-assisted probability of detection (MAPOD) studies or stochastic inversion schemes, the models must be treated in a stochastic sense. A method of taking random inputs to a black box forward model and developing the full probability distribution function (PDF) of the response has been proposed.

Recent research results indicate that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stresses in surfacetreated nickel-base superalloy components. According to this approach, first the depth-dependent electric conductivity profile is calculated from the measured frequency-dependent apparent eddy current conductivity spectrum. Then, the residual stress depth profile is calculated from the conductivity profile based on the piezoresistivity coefficient of the material, which is determined separately from calibration measurements using known external applied stresses. This paper presents new results that indicate that in some popular nickel-base superalloys the relationship between the electric conductivity profile and the sought residual stress profile is more tenuous than previously thought. It is shown that in delta-processed IN718 the relationship is very sensitive to the state of precipitation hardening and, if left uncorrected could render the eddy current technique unsuitable for residual stress profiling in components of 36 HRC or harder, i.e., in most critical engine applications. The presented experimental results show that the observed dramatic change in the eddy current response of hardened IN718 to surface treatment is caused by very fine nanometer-scale features of the microstructure, such as &#947 and &#947 precipitates, rather than micrometer-scale features, such as changing grain size or &#948 phase and carbide precipitates., Published in Journal of Nondestructive Evaluation, v29 p143-153, 2010.

Recent research results indicate that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stresses in surface-treated nickel-base superalloy components. According to this approach, the depth-dependent electric conductivity profile is calculated from the measured frequency-dependent apparent eddy current conductivity spectrum. Then, the residual stress depth profile is calculated from the conductivity profile based on the piezoresistivity coefficient of the material, which is determined separately from calibration measurements using the known external applied stresses. This paper reviews the basic principles, measurement procedures, advantages, and limitations of eddy current residual stress profiling., Published in Nondestructive Testing and Evaluation, v24 n1-2 p209-232, Mar-Jun 2009. Sponsored in part by Contract no. F09650-00-D-0018.

The shot peening intensity of nickel base materials has been examined with an innovative eddy current measurement. The goal is to provide a nondestructive tool to quantitatively evaluate the surface conditions after shot peening. Traditionally, the residual stress caused by the shot peening process can be examined by X-ray diffraction. Recent eddy current works have shown promising results in evaluating the small conductivity variation due to the residual stress. This study explores the feasibility of utilizing the cable which connects to a network analyzer and a conventional eddy current probe to monitor the surface conditions due to the shot peening., Submitted for presentation at the Review of Progress in Quantitative NDE Conference. Prepared in collaboration with Air Force Research Laboratory (AFRL), Wright-Patterson AFB, OH. The original document contains color images. All DTIC reproductions will be in black and white.

Atomic Force Microscope (AFM) based eddy current imaging technique has been used to characterize carbon fiber reinforced composites and carbon nanofibers nanocomposite. The surface topography and eddy current images of the same region of the sample acquired at the same time are presented. While the contrast in AFM images is due to surface topography variations, the contrast in the eddy current images is due to the local variation in the electrical conductivity of the sample. The results show that the combined techniques of AFM and eddy current imaging can be used effectively to investigate the distribution, dispersion of the carbon fibers in the polymer matrix and the fiber matrix interphase. An enhanced contrast at the interface between the fiber and the matrix has been observed in the eddy current images. The implications of the improved contrast in eddy current images and its application to investigation of fiber-matrix interface/interphase in carbon fiber polymer matrix composites is discussed., Conference paper submitted to the Proceedings of the 2007 SAMPE Conference.

Recently, it has been shown that shot-peened nickel-base superalloys exhibit an approximately 1% increase in apparent eddy current conductivity at high inspection frequencies, which can be exploited for nondestructive subsurface residual stress assessment. Unfortunately, microstructural inhomogeneity in certain as-forged and precipitation hardened nickel-base superalloys, like Waspaloy, can lead to significantly larger electrical conductivity variations of as much as 4 6%. This intrinsic conductivity variation adversely affects the accuracy of residual stress evaluation in shot-peened and subsequently thermal relaxed specimens, but does not completely prevent it. Experimental results are presented to demonstrate that the conductivity variation resulting from volumetric inhomogeneities in as-forged engine alloys do not display significant frequency dependence. This characteristic independence of frequency can be exploited to distinguish these inhomogeneities from near surface residual stress and cold work effects caused by surface treatment, which, in contrast, are strongly frequency-dependent., Pub. in Journal of Nondestructive Evaluation, v25 n1, Mar 2006. Dept. of the Air Force contract nr F33615-003-D-5204.