Transversely isotropic materials are commonly described by one characteristic direction in the material configuration, which commonly is assumed to be constant. However, for some applications it makes sense to consider a reorientation of the characteristic direction, as for instance the orientation of biological materials adapts to the mechanical loading, see e.g. KUHL ET AL. [4]. Other examples are piezoelectric materials, which reorient due to electric or mechanical loading, as described in SMITH [7], and liquid crystals changing their orientation on account of electric or magnetic fields, see ERICKSEN [1]. Furthermore simulations with a reorientation of the characteristic direction can be used in the context of optimisation of composites. In this contribution we restrict ourselves to the modelling of hyper-elasticity and assume a time dependent reorientation of the characteristic direction with respect to the principal strain directions, see for instance MENZEL [5]. Thereby we concentrate on the numerical implementation into a finite element code.
Constitutive EquationsTransverse isotropy can be described by the characteristic direction n A or respectively by the structural tensor A = n A ⊗ n A . In this contribution we assume a reorienting characteristic direction n A . Thus, insertion of the standard free energy density ψ 0 = ψ 0 (C, A) and the Piola-Kirchhoff stresses S = 2∂ C ψ 0 into the dissipation inequality yields the reduced dissipation inequalitywith the extra entropy source S 0 satisfying the second law of thermodynamics, see for instance HIMPEL ET AL. [2]. Analogous to the director in the theory of shells a rotation of the characteristic direction n A is assumed, so that its evolution becomeṡFor the definition of the angular velocity ω an alignment with the principal strain directions, see e.g. MENZEL [5], the principal stress directions, see e.g. HARITON ET AL.[3], the gradients of the electric or magnetic field or other characteristic directions is thinkable. In this contribution we concentrate on a reorientation in terms of the principal strain direction. It can be shown that the critical state of free energy can be reached for coaxial stress and strain tensors. The Piola-Kirchhoff stresses for transverse isotropy can be depicted asdepending on the scalars S i=1,...5 = ∂ Ii ψ. Thus it can easily be observed, that for an alignment of n A with one of the principal strain directions, the Piola-Kirchhoff stresses S and the right Cauchy-Green tensor C become coaxial. Hence, following to MENZEL [5] we assume an alignment of n A with the maximum principal strain directionTo avoid drilling rotation the angular velocity is defined orthogonal to n A and n Cwith the material parameter t acting as time relaxation parameter.
Implementation into a Finite Element CodeFor the implementation into a finite element code the characteristic direction has been introduced as internal variable. Due to the EULER-theorem exp(−ε · ω ∆t) is a rotation about ω by the angle ω ∆t. Thus we apply an implicit exponential map fo...