or other state variables that change in a measurable way as degradation alters properties such as stiffness and strength. This change in properties, in turn, determines the performance of the material element, and when that performance falls below acceptable levels, it delimits life. In certain copolymers, for example, the microstructure can change dramatically (the matrix and distributed phases can be inverted, for example) as a function of hydration and temperature, greatly altering stiffness, fracture strain and stress, creep rates, and failure times. The precise (and robust) correlation of that microstructure with the resulting full complement of properties and performance is a continuing challenge.