National Aeronautics and Space Administration (NASA)

Advanced NDE of reinforced carbon-carbon (RCC) components of the leading edge structural subsystem (LESS) in the space shuttle are required to ensure early detection of anomalies and to help facilitate repair procedures of this critical subsystem. To meet this objective, a team of industry, national laboratories, and NASA centers was assembled. Argonne was selected as one of the sites to assess advanced eddy-current technology for the inspection of RCC composites. Argonne’s role under this program, coordinated by NASA Langley Research Center (LaRC), was to both analytically and experimentally assess the capability of modern multifrequency eddy-current (MFEC) technology for the detection of discrete flaws at different depths within the RCC layer. Evaluation of the MFEC technology encompassed numerical electromagnetic modeling, probe design and system interface, and data collection and analysis.

Three-dimensional finite-element modeling was performed to simulate the eddy-current probe response to flaws of interest in low-conductivity RCC structures. Numerical simulations focused initially on calculation of the probe impedance variation at several excitation frequencies and for a range of flaw sizes in layered media simulating the LESS. Theoretical results were initially verified through comparison with experimental data. These limited investigations clearly demonstrated the usefulness of numerical electromagnetic techniques for modeling the interaction of induced fields with complex media. Reliable models can be an effective tool for optimizing various eddy-current test parameters of interest and, in turn, reducing costly sample preparation and testing.

The NDE facility at Argonne was used to examine various RCC samples with the MFEC inspection system. Test pieces mostly contained manufactured flaws of various sizes. Data were collected by two transmit/receive probes having different size coils. Inspections were performed in both contact (surface-riding) and noncontact manner. Results of the experimental investigations demonstrated the capability of the technique to detect flaw sizes of interest at the reaction layer, as well as those located deeper within the test piece.

Efforts also focused on data visualization, signal processing, and data analysis methods to help improve the detection of localized flaws in RCC composites. A series of MATLAB scripts has been integrated through a single graphical user interface to allow off-line manipulation of MFEC inspection data. The analysis tool contains a series of frequency and spatial domain filters that can be sequentially applied to the data. Data can be displayed in various formats, which help to better visualize potential eddy-current indications. Feasibility studies suggest that significant improvement in signal-to-noise ratio can be achieved by applying the appropriate filtering schemes.

Following the completion of the feasibility studies, a review committee recommended further development of the MFEC technique for in-situ inspection of LESS. Argonne will collaborate with the LaRC in support of various efforts under this program.