Next, we choose a general elastic law in the stress-free placement with a material stress tensor
\({\mathop {\varvec{ T}}\limits ^{\tiny {\text {mat}}}}\), the stiffness tetrad of the material as a tensor of fourth-order
\({\mathbb { K}}\) and a strain measure. We use the Seth strain tensors
$$\begin{aligned} \varvec{ E}_{\text {m}}=\frac{1}{\text {m}}(\varvec{ C}_{\text {e}}^{{\text {m}}/2} - \varvec{ I}) \end{aligned}$$
(8)
with “m” being a real parameter and “e” marking the elastic part of a variable [
26]. A general framework can simply be set up by postulating quadratic elastic strain energy in terms of the Seth strain tensors,
$$\begin{aligned} w=\frac{1}{2} {\mathop {{\mathbb { K}}}\limits ^{\tiny {\text {4}}}} : : \varvec{ E}_{\text {m}} \otimes \varvec{ E}_{\text {m}} \ , \end{aligned}$$
(9)
We assumed small elastic deformations, such that it is sufficient to account only for the quadratic term in the strain energy. It would be possible to present our proposal with a general potential. A generalization could be obtained by a series expansion of the strain energy:
$$\begin{aligned} w&= \frac{1}{2}{\mathop {{\mathbb { K}}}\limits ^{\tiny {\text {4}}}} : : \varvec{ E}_{\text {m}} \otimes \varvec{ E}_{\text {m}} \nonumber \\&\quad +\, \frac{1}{3}{\mathop {{\mathbb { K}}}\limits ^{\tiny {\text {6}}}} : : : \varvec{ E}_{\text {m}} \otimes \varvec{ E}_{\text {m}} \otimes \varvec{ E}_{\text {m}} \nonumber \\&\quad +\, \frac{1}{4}{\mathop {{\mathbb { K}}}\limits ^{\tiny {\text {8}}}} : : : : \varvec{ E}_{\text {m}} \otimes \varvec{ E}_{\text {m}} \otimes \varvec{ E}_{\text {m}} \otimes \varvec{ E}_{\text {m}} \nonumber \\&\quad +\,\cdots . \end{aligned}$$
(10)
One could then approach the change of all higher-order
\({\mathbb { K}}\) tensors similar to our strategy for
\({\mathbb { K}}\) (4th order) given below. It would be interesting to assess the quality of such an approach, but it this beyond the scope of our work. The elastic law is
$$\begin{aligned} {\mathop {\varvec{ T}}\limits ^{\tiny {\text {mat}}}} = {\mathbb { K}}: \varvec{ E}^m \ . \end{aligned}$$
(11)
We write the elastic part
\(\varvec{ C}_e\) as
$$\begin{aligned} \varvec{ C}_e = \varvec{ F}_e^{\text {T}} \varvec{ F}_e = \varvec{ F}_{\text {p}}^{-\text {T}} \varvec{ F}^{\text {T}} \varvec{ F}\varvec{ F}_{\text {p}}^{-1} = \varvec{ F}_{\text {p}}^{-\text {T}} \varvec{ C}\varvec{ F}_{\text {p}}^{-1} \ . \end{aligned}$$
(12)
We now insert this equation into Eq. (
8) and expand the
\(\varvec{ I}\) with
\(\varvec{ F}_{\text {p}}\) to
$$\begin{aligned} \varvec{ E}_{\text {m}}=\frac{1}{\text {m}}((\varvec{ F}_{\text {p}}^{-\text {T}} \varvec{ C}\varvec{ F}_{\text {p}}^{-1})^{{\text {m}}/2} - \varvec{ F}_{\text {p}}^{-\text {T}} \varvec{ F}_{\text {p}}^{\text {T}} \varvec{ F}_{\text {p}} \varvec{ F}_{\text {p}}^{-1}) \ . \end{aligned}$$
(13)
If we confine to the special case
\(\text {m}=2\), it is possible to factor out
\(\varvec{ F}_{\text {p}}\), and thus, we can further simplify to
$$\begin{aligned} \varvec{ E}_{\text {m}}=\frac{1}{2} \varvec{ F}_{\text {p}}^{-\text {T}} (\varvec{ C}- \varvec{ F}_{\text {p}}^{\text {T}} \varvec{ F}_{\text {p}}) \varvec{ F}_{\text {p}}^{-1} \ . \end{aligned}$$
(14)
The choice m=2 results in further simplifications. The stress tensor
\({\mathop {\varvec{ T}}\limits ^{\tiny {\text {mat}}}}\) becomes
\({\mathop {\mathbf {T}}\limits ^{\tiny {\text {2PK}}}}_{\text {SF}}\) in the stress-free placement. This also allows for a simple conversion to the Cauchy stresses
\(\varvec{ T}\). The next step is to introduce our ansatz for capturing the evolution of the anisotropy axes during the plastic deformation. We know that the anisotropy axes transform by vectors. We will call the initial vector
\(\varvec{ a}_0\) and the vector of the transformed axis
\(\varvec{ a}\). Second-order tensors transform tangent vectors that represent line elements. We will name this tensor
\(\varvec{ P}_{\text {K}}\) with
$$\begin{aligned} \varvec{ a}= \varvec{ P}_{\text {K}} \varvec{ a}_0 \ . \end{aligned}$$
(15)
Let us consider a transversal isotropic material. From representation theory, we can write the material stiffness tetrad
\({\mathbb { K}}^{\text {ti}}\) of this material as
$$\begin{aligned} {\mathbb { K}}^{\text {ti}}&= c_1 {\mathbb { I}}+ c_2 \varvec{ I}\otimes \varvec{ I}+ c_3 \varvec{ a}\otimes \varvec{ a}\otimes \varvec{ a}\otimes \varvec{ a}\nonumber \\&\quad +\, c_4 (\varvec{ I}\otimes \varvec{ a}\otimes \varvec{ a})_{\text {sym}} + c_5 (\varvec{ a}\otimes \varvec{ I}\otimes \varvec{ a})_{\text {sym}}, \end{aligned}$$
(16)
where “sym” is the symmetrization according to Eq. (
1). Because we know this representation and there is only one vector
\(\varvec{ a}\), we can transform the whole stiffness tetrad by transforming the vector
\(\varvec{ a}\). In other cases, we either do not know the representation theorem or it changes during plastic deformation due to a change of the symmetry class (e.g., cubic symmetry). Because our theory has to be valid for all possible material symmetries, we have to make a simplification. In Eq. (
16), we identify the summand with
\(c_3\) as the part capturing the main anisotropy features. The structure is similar to the Rayleigh product so we decide to directly apply the transformation
\(\varvec{ P}_{\text {K}}\) to the whole stiffness tetrad
\({\mathbb { K}}\)$$\begin{aligned} {\mathbb { K}}= \varvec{ P}_{\text {K}} * {\mathbb { K}}_0. \end{aligned}$$
(17)
\(\varvec{ P}_{\text {K}}\) is defined in the stress-free placement and transforms the stiffness tetrad. It is not a change of placement. Furthermore, it is now possible to summarize
\(\varvec{ F}_{\text {p}}\) and
\(\varvec{ P}_{\text {K}}\) in the following equation and compare it with other theories. In the stress-free placement, the second Piola–Kirchhoff stresses are
$$\begin{aligned} {\mathop {\varvec{ T}}\limits ^{\tiny {\text {2PK}}}}_{\text {SF}} = \left( \varvec{ P}_{\text {K}} * {\mathbb { K}}_0 \right) : \frac{1}{2} \varvec{ F}_{\text {p}}^{-\text {T}} (\varvec{ C}- \varvec{ F}_{\text {p}}^{\text {T}} \varvec{ F}_{\text {p}}) \varvec{ F}_{\text {p}}^{-1}. \end{aligned}$$
(18)
With the transformation between the stress-free placement and the reference placement using
\(\varvec{ F}_{\text {p}}\) and its determinant
\(J_{\text {p}}\)$$\begin{aligned} {\mathop {\varvec{ T}}\limits ^{\tiny {\text {2PK}}}}_{\text {RP}} = \varvec{ F}^{-1}_{\text {p}}{\mathop {\varvec{ T}}\limits ^{\tiny {\text {2PK}}}}_{\text {SF}} \varvec{ F}^{-\text {T}}_{\text {p}} J_{\text {p}} \end{aligned}$$
(19)
we can factor out
\(\varvec{ F}_{\text {p}}\) and summarize with
\(\varvec{ P}_{\text {K}}\). We consider only isochoric plastic deformation and therefore
\(J_{\text {p}} = 1\). Finally, we end up with the following equation for the second Piola–Kirchhoff stresses in the reference placement
$$\begin{aligned} {\mathop {\varvec{ T}}\limits ^{\tiny {\text {2PK}}}}_{\text {RP}} = \frac{1}{2} \left( \varvec{ F}^{-1}_{\text {p}} \varvec{ P}_{\text {K}} \right) * {\mathbb { K}}_0 : \left( \varvec{ C}- \varvec{ F}^{T}_{\text {p}} \varvec{ F}^{-1}_{\text {p}} \right) \end{aligned}$$
(20)