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 dam-age. 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, electro-magnetics, 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 devel-op the electromagnetic-based NDE methods into useful engineering tools, it is necessary to understand the theoretical dielectric properties of advanced composites. The develop-ment of a micromechanical model for composite dielectric properties will support the de-velopment 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 under-standing of composite material theory and develop a micromechanical model that relates composite dielectric behavior to constituent properties, fiber volume fraction, and fiber orientation. In the Phase I effort, a micromechanical model was developed and exercised to compute composite dielectric properties. The composite consists of fibers, up to two discrete fiber/matrix interface layers, and matrix materials. The dielectric properties and volume fractions of the fibers, interface layers, and matrix can be varied independently. The model can be used to compute properties of unidirectional layers and arbitrary two dimensional laminates. The results of the model include orthotropic, frequency depen-dent dielectric properties including permittivities and conductivities (or dielectric con-stants and loss tangents). The theory was tested for feasibility by comparison to NDE signals measured on CMCs of interest. The model showed excellent agreement with measured dielectric properties. This Phase II program will be performed in conjunction with a compatible Phase II effort that is being proposed by Jentek. The Jentek Phase II will focus on the develop-ment of dielectric sensors and data acquisition. The MR&D Phase II will expand the mi-cromechanical theory and integrate the resulting software with Jentek’s sensors and data acquisition systems. The overall goal of both Phase II programs is to create an integrated NDE tool that can provide “go-no go” decisions for CMCs related to incoming material inspection, fabricated component quality, an in service degradation