The geometrically nonlinear Cosserat micropolar shear–stretch energy. Part II: Non‐classical energy‐minimizing microrotations in 3D and their computational validation ***

In any geometrically nonlinear, isotropic and quadratic Cosserat micropolar extended continuum model formulated in the deformation gradient field F ≔ ∇ φ : Ω →GL + ( n )and the microrotation field R : Ω → SO ( n ) , the shear–stretch energy is necessarily of the formW μ , μ c( R ; F )≔ μsym ( R T F − 1 ) 2 + μ cskew ( R T F − 1 ) 2 .We aim at the derivation of closed form expressions for the minimizers ofW μ , μ c( R ; F )in SO(3), i.e., for the set of optimal Cosserat microrotations in dimension n = 3 , as a function of F ∈GL + ( 3 ) . In a previous contribution (Part I), we have first shown that, for all n ≥ 2 , the full range of weights μ > 0 andμ c ≥ 0 can be reduced to either a classical or a non‐classical limit case. We have then derived the associated closed form expressions for the optimal planar rotations in SO(2) and proved their global optimality. In the present contribution (Part II), we characterize the non‐classical optimal rotations in dimension n = 3 . After a lift of the minimization problem to the unit quaternions, the Euler–Lagrange equations can be symbolically solved by the computer algebra system Mathematica . Among the symbolic expressions for the critical points, we single out two candidatesrpolarμ , μ c ± ( F ) ∈ SO ( 3 )which we analyze and for which we can computationally validate their global optimality by Monte Carlo statistical sampling of SO(3). Geometrically, our proposed optimal Cosserat rotationsrpolarμ , μ c ± ( F )act in the plane of maximal stretch. Our previously obtained explicit formulae for planar optimal Cosserat rotations in SO(2) reveal themselves as a simple special case. Further, we derive the associated reduced energy levels of the Cosserat shear–stretch energy and criteria for the existence of non‐classical optimal rotations.