Thermal Expansion Coefficients of Composite Materials Based on Energy Principles

Abstract

Bounds on effective thermal expansion coefficients of isotropic and anisotropic composite materials consisting of isotropic phases are derived by employing extremum principles of thermoelasticity. Inequalities between certain approximate and exact forms of the potential and complementary energy functionals are first estab lished. These inequalities are then used in conjunction with a new method for minimizing the difference between upper and lower bounds in order to derive volumetric and linear thermal expansion coefficients. Application is made to two- and three-phase isotropic composites and a fiber-reinforced material. It is found for some important cases that the solutions are exact, and take a very simple form. We also show conditions under which the rule-of-mixtures and Turner's equation can be used for thermal expansion coeffi cients. Finally, simple methods for extending all results to viscoelastic composites are indicated.