Damage mechanics : basic variables in continuum theories

It is shown that any type of material damage that causes stiffness degradation in general requires description by an eighth order damage tensor but that the principle of strain equivalence permits a reduction to a tensor of order four. The actual number of independent damage parameters in such a tensor is related to the material and damage symmetry. For the isotropic case, there must be two independent damage parameters which can be expressed in terms of damage parameter with physical meaning.