Harold A. Sabbagh

Recent advances in modeling eddy-current probes

The ability of VIC-3D©, a proprietary volume-integral code, to model eddy-current probes has been significantly enhanced to include coils of virtually any shape, such as racetrack, D-shaped, and square, and oriented in virtually any direction. This allows the ability to model such well-known probes as the split-core differential probe, even when tilted relative to the workpiece, and the plus-point probe, that is often used in the nuclear power industry.

Numerical and Experimental Study of Eddy Current Crack Detection around Fasteners in Multi‐Layer Structures

Previous work on the detection of cracks around fastener holes in multi‐layer structures using eddy current NDE has met with some success; however, challenges remain in detecting smaller corner cracks while minimizing false call rates. To address this issue, a 3D model was developed for this problem and successfully validated with experimental data. Using this model, a series of numerical studies were performed to identify potential features for defect classification.

Methodology Using Inverse Methods for Pit Characterization in Multilayer Structures

This paper presents a methodology incorporating ultrasonic and eddy current data and NDE models to characterize pits in first and second layers. Approaches such as equivalent pit dimensions, approximate probe models, and iterative inversion schemes were designed to improve the reliability and speed of inverse methods for second layer pit characterization. A novel clutter removal algorithm was developed to compensate for coherent background noise. Validation was achieved using artificial and real pitting corrosion samples.

Efficient Solution of Electromagnetic Scattering Problems Using Spatial Decomposition Algorithms

This paper introduces spatial decomposition algorithms to the volume integral method, which will enable the integral equation method to efficiently solve electromagnetic scattering problems where the scatterer extends across several layers or multiple scatterers exist in multiple layers. Model comparisons with the finite element methods are used validate the approach. Studies of typical aging aircraft eddy current inspection problems are conducted using the new formulation.

Assessing thermal barrier coatings by eddy-current inversion

Nondestructive testing (NDT) of high temperature coatings is one of the important factors in achieving a high level of structural integrity in advanced gas turbines. In this paper, we demonstrate that sophisticated eddy current techniques can be utilized to measure the thickness and remaining life of high temperature coatings. Some research has been conducted to apply such techniques to the preservice case, for which the coating has one nicely defined layer and nothing of consequence has diffused into the base metal that would create additional layers of anomalous material. We discuss the much more difficult inservice case, in which the time temperature exposure of the combustion turbine blade has created a four layered system, in addition to the base metal.

Ferrite Core Eddy Current Probe Model: Description and Verification

A model of eddy current probes with ferrite cores has been developed. The model is applicable to axisymmetric cores in the vicinity of a conducting half-space (the work piece). The model is based on modern methods of computational electromagnetics, and is intended to provide a systematic and rational basis for the design and characterization of probe coils. Eddy current responses of four probes on an aluminum workpiece were predicted by the model and compared with experimentally measured responses.

Reconstruction of semielliptical and epicyclic cracks by eddy-current inversion

We describe the application of VIC-3D©, a proprietary volume-integral code, to the inversion of benchmark eddy-current data, and the reconstruction of semielliptical and epicyclic cracks. These are modeled as relatively thin surface-breaking slots, whose depth into the workpiece is described by either a semiellipse, or a linear combination of semiellipses. The latter curves, called epicyclic, have a rather complex, double-peaked shape, and are well suited to model a crack with multiple initiation sites. VIC-3D© is used to provide model data for the probe-flaw interaction, which are then fitted to the measured data. The process is quite efficient, and provides accurate reconstructions.

Validation of a volume-integral eddy-current code by bench-mark experiments

Model computations of the response of eddy-current probes to a variety of flaws are compared to data provided by benchmark experiments. VIC-3D©, a proprietary volume-integral code, is used to provide the model results, which are then compared to experimental data for (a) cracks in a double-plate system (suggestive of airframes), (b) cracks with discontinuous profiles, and (c) cracks with complex, double-peaked shapes (suggestive of multi-site damage). The model calculations agree closely with the experimental data in all cases.