Micromechanical Assessment of Thermochemically Induced NDE Changes in Advanced Composites

The Joint Strike Fighter is considering the use of organic and ceramic matrix composites for high temperature engine components. Material coupons and flight tests have shown that when these materials are exposed to high temperatures in the presence of air, salt fog, and humidity they can react chemically. The chemical reactions may cause a significant loss in composite strength even though there is no physical evidence of damage. Standard non destructive evaluation techniques such as ultrasonics, radiography, and thermography have not been able to accurately identify and monitor the chemical changes. Fortunately emerging NDE techniques based on electrical resistivity, electromagnetics, and dielectric properties have shown promise in relating chemical changes to NDE signal. These techniques, however, are in their infancy and the measurements are sensitive to sensor design, scanning technique, and data interpretation. In order to develop the electromagnetic-based NDE methods into useful engineering tools, it is necessary to understand the theoretical dielectric properties of advanced composites. The development of a micromechanical model for composite dielectric properties will support the development of NDE techniques and standards that can monitor chemically induced changes in advanced composite engine components. Materials Research & Design proposes to build upon their demonstrated understanding of composite material theory and develop a micromechanical model that relates composite dielectric behavior to constituent properties, fiber volume fraction, and fiber orientation. The theory will be tested for feasibility by comparison to NDE signals measured on CMCs of interest. The program will be undertaken by a team of MR&D and Jentek Sensors, Inc. MR&D will develop the theory, debug the computer code, procure CMC specimens, define the test plan, correlate the data, and prepare the reports. Jentek, as a subcontractor to MR&D, will consult on the theoretical assumptions, measure the NDE response, and reduce the raw data.