Aging and Higher-Order Grade Materials

This chapter investigates some aspects of non-simple materials. First, aging materials are examined. Aging is a general problem of the modelling of materials. Mathematically the problem is made formal by letting constitutive parameters depend on time. It is pointed out that the second law of thermodynamics might select physically admissible evolutions of the parameters. Next constitutive equations are considered which involve higher-order derivatives, in time or in space (materials of differential type). The dependence on time derivatives is an alternative to the functionals of histories when memory effects are of interest. Yet the time derivative is not objective and hence recourse is needed to objective time derivatives.