1.1 WHAT IS DIMENSIONAL STABILITY?

Dimensional stability is a general property of a material, component, or structure which enables it to maintain or retain its shape, size, or any dimension. Conversely, dimensional instability is the total of all distortions, deformations, or strains undergone by a material when subjected to changes in external conditions, or as observed over time. We shall emphasize the components of use, or load parameters such as stress, temperature and specific environmental effects in order to give the engineer the maximum ability to apply this knowledge to a specific application. A simplified version of the basic Duhamel-Newman relation [1], which sums the (linear) strains ( ?) due to various influences, is a good starting point to describe dimensional instability. Extending it suggests:

(1.1)

We note that this is a combination of external influences such as applied stress ( ?), temperature ( T), absorbed moisture ( M), time ( t), and fluence or radiation ( Q). Material properties included here are:

• S = compliance = 1/ E where E = Young's modulus or material stiffness

• ? = CTE = coefficient of thermal expansion

• ? = CME = coefficient of moisture expansion

• ? = CTE = coefficient of temporal expansion

• ? = CRE = coefficient of radiation expansion

These terms are further defined in the relevant chapters that follow. Cross-coupling or combined effects will be addressed in Chapter 10. In isotropic materials, such as most conventional...