R. Kim Murphy

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.

Demonstration of model-assisted probability of detection evaluation methodology for eddy current nondestructive evaluation

A model-assisted POD (MAPOD) evaluation approach is explored for the eddy current inspection of fatigue cracks in titanium. Experimental results are presented comparing crack and EDM notch responses and different eddy current measurement systems. The MAPOD evaluation process is performed using a simulated case study, highlighting the benefit of leveraging better models to mitigate variation in the model fit and minimize test sample requirements. The benefit of validating NDE techniques that use inverse methods to estimate uncontrolled measurement conditions is also studied.

Advances in Modeling Eddy‐Current NDE of Ferromagnetic Bodies

We apply our proprietary volume‐integral eddy‐current code, VIC‐3D©, to the solution of challenging NDE problems that involve ferromagnetic anomalies or workpieces. These include problems with “wear scars and permeable crusts” in steam‐generator tubing, characterization of coatings with electrical and magnetic layers, and flaws in ferromagnetic workpieces. Validation of the code by the use of benchmark experiments will also be described. Finally, we will explain the development of the mathematical model in terms of volume‐integral equations through the use of Amperian currents for magnetic effects.

Demonstration of model-based inversion of electromagnetic signals for crack characterization

The objective of this work is to demonstrate model-based inversion techniques to characterize the length, depth, width, and orientation of surface-breaking cracks using eddy current (EC) NDE. The paper presents experimental testing to acquire high fidelity automated eddy current data, enhancements made in VIC-3D® to improve both speed and accuracy, benchmark studies demonstrating model accuracy, improved data registration and reduction methods, and surrogate models and model calibration schemes to ensure the fastest and highest quality models are used for inversion. Initial inversion results indicate the potential to accurately size cracks and EDM notches over a wide range of flaw characteristics and probe orientations. Insight into EC variability for POD crack sets is presented using inversion results for crack length and depth.

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.

Model-based inverse methods for sizing surface-breaking discontinuities with eddy current probe variability

The objective of this work is to demonstrate and validate model-based inversion techniques to characterize length, depth, width and orientation of surface-breaking cracks using eddy current NDE under varying probe conditions. A series of parametric studies of probe characteristics are presented for a fixed set of well-characterized flaws with varying length, depth, opening width and orientation angle. Results show inversion performance differences between probes with the same design specifications. Inversion results were also evaluated for a probe that was selectively controlled for varying probe liftoff, varying tilt in two directions, and orientation. Certain levels of probe tilt and liftoff were found to degrade the performance of the inversion technique. By using a model calibration process that incorporates the matching probe calibration data, better inversion results can be achieved, to a limited degree. There is a need to more appropriately adapt the model through the calibration fit to compensate ...

Model-based inverse methods for bolt-holt eddy current (BHEC) inspections

This work presents a comprehensive approach for model-based inversion of crack characteristics using eddy current nondestructive evaluation (NDE) and includes a demonstration using a bolt-hole eddy-current (BHEC) technique. Data were acquired using standard eddy current hardware for crack and notch samples of varying size and profile. The inversion results were found to be accurate within 10% for sizing the depth of through cracks and able to accurately bin mid-bore cracks by size.

SENSITIVITY ANALYSIS OF INVERSE METHODS IN EDDY CURRENT PIT CHARACTERIZATION

A sensitivity analysis was performed for a pit characterization problem to quantify the impact of potential sources for variation on the performance of inverse methods. Certain data processing steps, including careful feature extraction, background clutter removal and compensation for variation in the scan step size through the tubing, were found to be critical to achieve good estimates of the pit depth and diameter. Variance studied in model probe dimensions did not adversely affect performance.

APPLICATION OF MODEL‐BASED INVERSION TO EDDY‐CURRENT NDE OF HEAT‐EXCHANGER TUBING

In this paper we apply model based inversion to problems relevant to the nuclear power industry. Laboratory data for heat exchanger tubes were acquired from the Electric Power Research Institute (EPRI) and the Naval Surface Warfare Center, Carderock Division, and then model based standards for the inversion of these data, using our proprietary volume-integral code, were developed. The EPRI data are from round bottom, inner diameter pit standards, and were acquired using a standard eddy current instrument. After undergoing a preprocessing step that includes feature extraction with clutter removal and scan step correction, these data were then transformed into impedance data for inversion by means of a simple linear transformation. The Carderock data, which are due to a through-wall round hole, are already impedances and do not require this processing step. Results of the inversion process that show excellent agreement with the measured data will be presented. Finally, certain estimation-theoretic metrics are introduced, and their utility in model based inversion is described.

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...