TITLE: An Infrared Fiber Optic NDE/I Sensor for Galvanic Degradation of Imide Based Composites

Abstract:
This paper presents the results of a study undertaken by Foster-Miller, Inc. to demonstrate a novel fiber optic infrared spectroscopic nondestructive inspection technique (NDI) to detect and quantify galvanic induced degradation of graphite/imide based composites used in naval aircraft. When coupled to a fiber optic endoscope and robotic manipulators, this NDI sensor could be capable of making inspections of difficult to access components such as in-wing fuel tanks under field and depot level conditions. This study was conducted because imide based composites are the only thermosetting organic matrix materials that can withstand operating temperatures >300°F (1), and recent laboratory experiments have demonstrated that graphite fiber/imide based composites can degrade via a galvanic process that produces hydroxyl groups (2). The hydroxyl groups attack the imide ring in the matrix resin producing amic acid salts and amines (3). The breakdown of the resin leads to resin loss in the plies of the composite and a degradation of the mechanical properties. Environmental factors such as moisture concentration, pH, temperature, and stress can affect the degradation rate. The NDI infrared spectroscopic technique demonstrated in this work can detect the onset and track the progress of galvanic degradation. The spectral data show that the onset of corrosion can be detected before there is a significant reduction in mechanical properties of the composites, thus providing the Navy with a tool for performing appropriate preventative maintenance actions. The NDI technique used in this effort was based on diffuse reflectance (DRIFT) infrared spectroscopy.

Source: Proceedings of the 'NDE applied to Process Control of Composite Fabrication' - Conference, 4-5 Oct 1994 St. Louis, Missouri. Publisher and Organizer: Nondestructive Testing Information Analysis Center (NTIAC) Texas Research Institute Austin, Inc. Austin, Texas [http://www.ntiac.com] [Buying the Proceedings]