Jay L. Fisher

Multifrequency eddy current image processing techniques for nondestructive evaluation

This paper describes a new technique for processing eddy current images as used in nondestructive evaluation of materials. A simple description of the image formation physics is given followed by a description of the current state of the art in image processing. Eddy current testing in 2-D results in a sequence of complex valued images that can be linearly combined to enhance the signal-to-noise ratio (SNR) of features of interest. A new approach that selects weights for a linear combination of the images based on SNR maximization is presented. Results on experimental data show SNR improvements up to 1100 percent over traditional two-frequency techniques. The new algorithm presented is general to any number of frequencies, and results of four-frequency processing are given.

Prediction of the Probability of Eddy Current Flaw Detection

The validity of a nondestructive inspection method can be measured by two quantities, the probability that a given flaw size will be detected, and the probability that background noise will give rise to a false indication of the presence of a flaw. Unfortunately, experimental determination of the probability of detection (POD) and the probability of false alarm (PFA) requires a rather extensive set of measurements to obtain statistically sound estimates. Furthermore, if one or more of the parameters that define the method, such as the scan track spacing or probe configuration, are changed, then the full set of measurements must be repeated to obtain new estimates of the POD and PFA, thus adding to the time and expense of test validation. In some situations, where one is designing an inspection for a part or flaw size that is not yet available for testing, there is no way that the validity of a proposed inspection can be evaluated beforehand.

Eddy Current Probe Design for Second-Later Cracks under Installed Fasteners

The United States Air Force has an operational need to reliably detect second-layer cracks around fastener holes in two-layer airframe structures with the fasteners in place. Because access to the second layer is usually not available, the inspection must be performed by placing a probe on the outer surface of the structure and detecting cracks through the first layer. Eddy current methods have been applied to this inspection problem [1–6], and have met with some success; however, much improvement is still needed to achieve the desired sensitivity to cracks and rejection of signals caused by the geometry of the structure under inspection.

NDE OF AGING AIRCRAFT STRUCTURE USING ORTHOGONAL-AXIS EDDY CURRENT PROBES

Detection of subsurface cracks around fasteners and hidden corrosion in multi-layer aluminum structure is a critical requirement for aging aircraft inspection. Of particular concern is the Navy P-3 aircraft which is currently undergoing refurbishment In this aircraft, multi-layer structure with thicknesses up to 0.3 inch must be inspected nondestructively to identify areas in need of repair. In order to meet productivity requirements, eddy current testing (ECT) using the McDonnell Douglas MAUS III system will be performed.

Eddy Current Measurement of Fiber Volume Fraction in Metal Matrix Composites

The objective of this work was to develop an eddy current method for measuring fiber volume fraction in continuous-fiber metal matrix composites. Because an eddy current measurement can be affected by the spatial distribution of fibers as well as the overall fiber density, the measurement method had to be tolerant of possible variations in spatial distribution that might be encountered in practice. For this reason, the work began with the development of models of the effective resistivity tensor for a composite with an arbitrary fiber distribution and the resulting eddy current probe response [1,2]. The intent was to use these models to help design a measurement method and to test the method for ordered and disordered arrangements of fibers.

A Cryogenic Eddy Current Microprobe

In nondestructive eddy current testing (ET), wire coils are excited to induce electric currents in conducting test specimens. The distribution of these eddy currents is altered by the presence of flaws in the material or by changes in material properties. The distribution changes are then sensed by one or more detector coils.

Eddy Current Probe Performance Variability

Eddy current testing is used extensively by the Air Force for nondestructive inspection of many aircraft structural components. Although the reliability and consistency of inspections depends to a large extent on the characteristics of the eddy current probes used, no adequate specifications or certification methods presently exist for assuring probe performance. Because of the variability in probe performance, a need exists for establishing a means to control probe performance characteristics which will eventually lead to improved test results.