6.1 INTRODUCTION

Many composite materials are used in a radiation environment, and this may result in both immediate dimensional changes and changes in related properties, such as the CTE, CME and micromechanical properties. Schulgasser [36] suggests that the irradiation expansion coefficients can be derived from the thermal or hygroscopic coefficients and that the elastic properties of either the constituents or composite need not be known. Using the nomenclature of Equation 1.1, he suggests:

(6.1)

? _ij, ? _ij and ? _ij are the effective coefficients of radiation, thermal and hygroscopic expansion, respectively, dependent on the models involving the bulk moduli ( K _i) (e.g., Section 4.3), and ? ?, ?, ? ? are the respective rule-of-mixtures coefficients. ? _ij is the Kronecker delta. This model applies to a statistically homogeneous composite consisting of two isotropic constituents and homogeneous stress and strain fields.

In general, one may expect that any environment resulting in material defects (e.g., voids) will change both the dimensions and dimensional stability properties. Composite constituents exhibit a varied response to radiation, and this results in property gradients. Equilibrium is unlikely since further exposure tends to continue the effects. Organic matrices are generally more sensitive to radiation damage than graphite or ceramic fiber reinforcements. As a result, composites such as Gr/epoxy, Gr/glass, Gr/Al or glass/epoxy exhibit a complex dimensional response to radiation [1]. Table 6.1 lists some studies of radiation effects on composite materials.