The number of independent invariants of an n -preferred direction anisotropic solid

Sets of classical invariants are used to characterize the mechanical behaviour of elastic solids with preferred directions. In this paper, we prove, for an n-preferred direction anisotropy, that only 2 n + 3 classical invariants are independent. For n ≥ 2 , we show that only six strain and 2 n − 3 non-strain invariants are independent.