Ryan D. Mooers

Model the effects of core/coil size and defect length on eddy current response

Prior split D differential probe validation studies have consistently shown model discrepancy in transition regions between the main peaks of the probe response. These discrepancies may be the result of differences in probe input dimensions, numerical error in the simulation code, or unforeseen variables in the experimental setup and data collection. This paper will investigate the effect of improved dimensional modeling, core size, and defect length have on reducing the discrepancy between model and experiment. A complete description of the experimental and modeling study will be presented. Comparisons will be made between the experimental data and the simulated results and conclusions will be presented. The study should provide insight into how probe and defect dimensions affect the simulated response of a probe. Also it will detail how the inclusion of asymmetry into model could improve agreement.

Effects of angular variation on split D differential eddy current probe response

Abstract : The complexity of the models used in validation studies over the last few years has increased. Complexity has been included in the probe operation, the core/coil shape, the inclusion of ferrite cores, and the inclusion of dimension and orientation variation. An issue with these validation studies is there is no quantitative understanding of how a small change in a parameter affects the probe response. This study will look at the variation of probe response over a range of variations in multiple orientations. In addition, a sensitivity study will be performed to determine which orientations produce the greatest variations in response and if there are parameter levels where the response is more affected by the variation.

Model-based probe state estimation and crack inverse methods addressing eddy current probe variability

A model-based calibration process is introduced that estimates the state of the eddy current probe. First, a carefully designed surrogate model was built using VIC-3D® simulations covering the critical range of probe rotation angles, tilt in two directions, and probe offset (liftoff) for both transverse and longitudinal flaw orientations. Some approximations and numerical compromises in the model were made to represent tilt in two directions and reduce simulation time; however, this surrogate model was found to represent the key trends in the eddy current response for each of the four probe properties in experimental verification studies well. Next, this model was incorporated into an iterative inversion scheme during the calibration process, to estimate the probe state while also addressing the amplitude/phase fit and centering the calibration notch indication. Results are presented showing several examples of the blind estimation of tilt and rotation angle for known experimental cases with reasonable ag...

Realistic split D differential probe model validation

Past validation studies with split D differential probes have modeled the cores and coils as symmetric objects; however, commercial inspection probes often have inherent asymmetries that cause imbalance in the probe response. To properly validate a model with data from a commercial coil, all departures from the nominal design must be considered. This paper presents model validation results for an unbalanced probe. Complete details of the experimental data collection and the computational modeling are presented. Results for various types of probe asymmetry are presented.

Progress in model development for eddy current response in the presence of small conductivity changes

In this paper, an approximation technique for predicting the response of an eddy current coil in the presence of small changes in conductivity is discussed. The small changes in conductivity that are considered in this work are changes in the orientation of single crystals in polycrystalline, anisotropic materials. Data from electron backscatter imaging techniques is presented and used for the analysis. The models were run for the microstructure data and an approximation to the eddy current response is shown. This image is compared with images from actual eddy current probes and the approximations are shown to be relatively accurate compared with a previously presented model.

Model-based inverse methods for sizing cracks of varying shape and location in bolt-hole eddy current (BHEC) inspections

A comprehensive approach is presented to perform model-based inversion of crack characteristics using bolt hole eddy current (BHEC) techniques. Data was acquired for a wide range of crack sizes and shapes, including mid-bore, corner and through-thickness crack types, and from both standard eddy current hardware and a prototype BHEC system with z-axis position encoding. Signal processing algorithms were developed to process and extract features from the 2D data sets, and inversion algorithms using VIC-3D generated surrogate models were used for inverting crack size. New model results are presented, which now address the effect of having a corner crack at an edge and a through crack adjacent to two edges. A two-step inversion process was implemented that first evaluates the material layer thickness, crack type and location, in order to select the most appropriate VIC-3D surrogate model for subsequent crack sizing inversion step. Inversion results for select mid-bore, through and corner crack specimens are p...

Split D differential probe model validation using an impedance analyzer

Benchmark and validation studies are presented that quantify the accuracy of computational models. An important factor in these studies is the ability to compare simulated impedance results with experimental data. In a majority of differential benchmark studies the data acquisition is handled by a commercial eddy current instrument which allow for only a relative comparison of the data. In this study a novel data acquisition system allows for the collection of impedance data for differential probes. Details about the data collection, experimental procedure, model construction, and data comparison will be presented.

Modeling of the Change of Impedance of an Eddy Current Probe Due to Small Changes in Host Conductivity

Two different approximation techniques for predicting the response of an eddy current coil in the presence of small changes in conductivity were developed. The small changes in conductivity are the result of changes in the orientation of individual anisotropic crystals in a polycrystalline aggregate. Orientation information from electron backscatter diffraction was imported directly into the modeling domain and the simulations were run to map orientations into an approximated eddy current response. These approximated responses were compared with experimental data obtained with commercially available eddy current equipment, and the approximations were found to be in good agreement with experiment. Further verification was performed with other existing numerical and analytical models to demonstrate the accuracy of the approximations made in deriving the eddy current response. This paper shows these results and demonstrates the viability of using low-fidelity approximations in predicting the eddy current response when the change in conductivity is low.

Continued developments in the modeling of complex dimensions and orientation variation in split D differential eddy current probes

In this paper, model benchmarking for differential eddy current probes is presented, addressing increasing probe complexity including asymmetry, positional, and orientation variations. In prior work, results were presented showing that current models are capable of modeling, with a high degree of accuracy, the variations that come from dimensional, position, and orientation variations; however, one challenge concerned addressing some inherent irregularity and uncertainty in the dimensions of commercial eddy current probes. This current effort addresses these challenges using larger differential coil assembly with coil construction that is more precise and accurate, and through improved model construction and increased mesh resolution. A model validation study is presented that verifies the capability of modeling software to accurately represent probes with varying angle and for different probe designs. Results are also presented highlighting efforts to reduce modeling error observed in prior work in hopes...

Role of interface conditions in low frequency electromagnetic testing of multilayer structures

When developing simulations of NDE testing methods it is imperative to use an accurate physical representation. Many inaccuracies in simulations can be traced to faulty or incomplete assumptions made for specific problem geometries, parameter constraints, or boundary conditions. An example of this is the assumed eddy current flaw response in layered media. A critical assumption often made is that when the plates are in contact, they are in electrical contact and behave as a single conductor. The purpose of this study is to dissect this assumption and propose alternate representations of the problem. A series of experiments were conducted alongside numerical models to provide evidence of the inadequacy of the assumption. Simulations were also used to predict the effects of varying interface conditions on eddy current signals due to surface cracks in sub-layer metals.