The elasticity tensor is one of the most important fourth order tensors in mechanics. Fourth order three-dimensional symmetric and traceless tensors play a crucial role in the study of the elasticity tensors. In this paper, we present two isotropic irreducible functional bases of a fourth order three-dimensional symmetric and traceless tensor. One of them is the minimal integrity basis introduced by Smith and Bao in 1997. It has nine homogeneous polynomial invariants of degrees two, three, four, five, six, seven, eight, nine and ten, respectively. We prove that it is also an irreducible functional basis. The second irreducible functional basis also has nine homogeneous polynomial invariants. It has no quartic invariant but has two sextic invariants. The other seven invariants are the same as those of the Smith-Bao basis. Hence, the second irreducible functional basis is not contained in any minimal integrity basis.
The Odd Degree Invariants in These Two BasesIn the Smith and Bao basis and the mixed functional basis, there are four odd degree invariants J 3 , J 5 , J 7 and J 9 , and six even degree invariants J 2 , J 4 , J 6 , K 6 , J 8 and J 10 . A notable property is that when D changes its sign, the odd degree invariants change their signs, while the even degree invariants are unchanged. Using this property, it is relatively easy to show that each of J 3 , J 5 , J 7 and J 9 is not a function of the other three odd degree invariants and the six even degree invariants J 2 , J 4 , J 6 , K 6 , J 8 and J 10 .Proposition 4.1. We have the following four conclusions.(a) If there is a fourth order three-dimensional symmetric and traceless tensor D such that J 5 = J 7 = J 9 = 0 but J 3 = 0, then J 3 is not a function of J 2 , J 4 , J 5 , J 6 , K 6 , J 7 , J 8 , J 9 and J 10 .(b) If there is a fourth order three-dimensional symmetric and traceless tensor D such that J 3 = J 7 = J 9 = 0 but J 5 = 0, then J 5 is not a function of J 2 , J 3 , J 4 , J 6 , K 6 , J 7 , J 8 , J 9 and J 10 .(c) If there is a fourth order three-dimensional symmetric and traceless tensor D such that J 3 = J 5 = J 9 = 0 but J 7 = 0, then J 7 is not a function of J 2 , J 3 , J 4 , J 5 , J 6 , K 6 , J 8 , J 9 and J 10 .(d) If there is a fourth order three-dimensional symmetric and traceless tensor D such that J 3 = J 5 = J 7 = 0 but J 9 = 0, then J 9 is not a function of J 2 , J 3 , J 4 , J 5 , J 6 , K 6 , J 7 , J 8 and J 10 .Proof. We now prove conclusion (a). If there is a fourth order three-dimensional symmetric and traceless tensor D such that J 5 = J 7 = J 9 = 0 but J 3 = 0, then we may consider −D.J 2 , J 4 , J 5 , J 6 , K 6 , J 7 , J 8 , J 9 and J 10 are unchanged, but J 3 changes its sign. This implies that J 3 is not a function of J 2 , J 4 , J 5 , J 6 , K 6 , J 7 , J 8 , J 9 and J 10 . The other three conclusions (b), (c) and (d) can be proved similarly.We now have the following theorem.Theorem 4.2. Each of J 3 , J 5 , J 7 and J 9 is not a function of the other three odd degree invariants and the six even degree invariants J 2 , J 4 , J 6 , ...