4.1 INTRODUCTION

Temperature is the most common non-mechanical parameter responsible for dimensional instability, since it affects all materials (ref: Chapter 2). Since fiber reinforced composite properties vary, in general, in different directions, we are primarily interested in linear thermal expansion, defined below. Micromechanics relates the thermal expansion behavior of constituent materials (Section 4.2) to that of the composite (Section 4.3). Macromechanics extends these predictions to laminates (Section 4.4). The volumetric CTE (or expansivity) has practical consequences in the use of composite adhesives and fillers and is discussed in Section 4.5. The specific thermal behavior of resin, metal and ceramic composites is introduced in Sections 4.6, 4.7 and 4.8, respectively. Problems of interest to optomechanical or composite design engineers include the uniformity of CTE (Section 4.9-especially important for first surface mirror substrates). Special forms of composites are discussed in Section 4.10, including asymmetrical laminates, fabrics, multidimensionally reinforced composites, sandwich structures, thin films, tubes and whisker or nanoparticle-reinforced composites.

"Thermal stability" is a term often used to reassure the user when presenting material, component or structure specifications, such as electrical or optical properties. The use of this term is not recommended, since it is inherently imprecise, and may refer to a low CTE, aging, phase transformations, recrystallization, grain growth, etc. For example, the parameter of time influences the thermal expansion behavior of metal matrix composites, where annealing, plastic flow, work hardening and other temperature-dependent processes occur. Combined temperature and time-dependent behavior such as thermal cycling and thermal spiking are reviewed in Chapter...