Jeffrey P. Seebo

Terahertz NDE application for corrosion detection and evaluation under shuttle tiles

Pulsed Terahertz NDE is being examined as a method to inspect for possible corrosion under Space Shuttle Tiles. Other methods such as ultrasonics, infrared, eddy current and microwave technologies have demonstrable shortcomings for tile NDE. This work applies Terahertz NDE, in the frequency range between 50 GHz and 1 THz, for the inspection of manufactured corrosion samples. The samples consist of induced corrosion spots that range in diameter (2.54 to 15.2 mm) and depth (0.036 to 0.787 mm) in an aluminum substrate material covered with tiles. Results of these measurements are presented for known corrosion flaws both covered and uncovered and for blind tests with unknown corrosion flaws covered with attached tiles. The Terahertz NDE system is shown to detect all artificially manufactured corrosion regions under a Shuttle tile with a depth greater than 0.13 mm.

Modeling and Processing of Terahertz Imaging in Space Shuttle External Tank Foam Inspection

Recently, terahertz ray (T‐ray) imaging emerged as one of the most promising techniques to inspect the space shuttle external tank foam insulation. This paper demonstrates the application of state‐of‐the‐art computer processing and modeling technologies to assist in further refinement of this new technology. The current protocol of T‐ray inspection and its limitations are first reviewed. New strategies of using signal processing and modeling are then proposed to improve on the flaw detection. Preliminary results are presented on a series of multi‐dimensional signal processing operations of T‐ray signals. The use of models and their comparisons with experimental data from foam samples are also included.

Protocol and Assessment of Signal Processing and Feature Extraction Methods for Terahertz NDE for Spray‐On Foam Insulation

A quantitative assessment protocol was developed to evaluate potential signal processing methods for THz NDE of spray‐on foam insulation. Automated feature extraction methods were developed to replicate the important visual classification characteristics used by inspectors and a new merit assessment approach was developed to identify the most promising combination of signal processing algorithms. Through application of the protocol to test data, several signal processing methods were found to improve the discrimination of defects.

The Potential for Imaging In-Situ Damage in Inflatable Space Structures

NASA is investigating the use of inflatable habitat structures for orbital transfer and planetary applications. Since space structures are vulnerable to damage from micrometeoroid and orbital debris, it is important to investigate means of detecting such damage. This study is an investigation into methods for performing non‐destructive evaluation (NDE) on inflatable habitat modules. Results of this work showed that various electromagnetic imaging modalities from microwaves to terahertz imaging have the greatest potential for a viable, portable, NDE tool which could possibly be deployed aboard an inflatable habitat module.

Processing Terahertz Ray Data in Space Shuttle Inspection

Terahertz ray imaging is one of the most capable techniques to inspect the space shuttle external tank foam insulation. This technique, however, is limited in its current inspection protocol using indirect substrate reflections. An alternate signal processing approach, working directly on the flaw responses, was lately demonstrated to be able to overcome some of these limitations. In this paper, we report recent progresses made by utilizing this alternate signal processing procedure in additional samples to detect flaws that were missed by the current protocol. We also present a new detection approach using the probabilistic neural network in the context of Bayesian classification. Preliminary results showed that the new Bayesian classification approach can achieve even greater improvement over the alternate signal processing approach.

Application of Terahertz Radiation to the Detection of Corrosion under the Shuttle's Thermal Protection System

There is currently no method for detecting corrosion under Shuttle tiles except for the expensive process of tile removal and replacement; hence NASA is investigating new NDE methods for detecting hidden corrosion. Time domain terahertz radiation has been applied to corrosion detection under tiles in samples ranging from small lab samples to a Shuttle with positive results. Terahertz imaging methods have been able to detect corrosion at thicknesses of 5 mils or greater under 1” thick Shuttle tiles and 7‐12 mils or greater under 2” thick Shuttle tiles.