**A.2 Out-of-plane normal component for the plane strain condition**

The following MATLAB® instructions allow to find an explicit expression of the *S*<sup>33</sup> component, from which *σ*<sup>33</sup> ¼ *J* �1 *S*33.

syms lam mi 'real' syms C [3 3] *'*real' C(l, 3) = 0; C(3, l) = 0; C(3, 2) = 0; C(2, 3) = 0; I1 = trace(C); I3 = det(C); Psi = mi/2\*(I1–3)–mi\*log(sqrt (I3))+ … lam/2\*log (sqrt (13))^2; S33 = simplify (2\*diff (Psi ,C(3, 3))); S33 = subs(S33, C(3, 3), 1);
