# Derivative of deformation gradient with respect to Green-Lagrangian strain?

For hyperelastic material, the elastic energy $$\Psi$$ is related to the deformation gradient $$F$$ and other internal variables (e.g. temperature $$\theta$$).

However, in many literatures (including Malvern's and Belytchko's) the derivatives (especially Hessian) are usually derived in terms of left Cauchy-Green tensor $$C = F^t F$$. For example, 2nd PK stress $${S_{ij}} = \frac{{\partial \Psi }}{{\partial {E_{ij}}}} \qquad \text{and} \qquad C_{ijkl}^{SE} = \frac{{{\partial ^2}\Psi }}{{\partial {E_{ij}}\partial {E_{kl}}}} \, .$$ I can convince myself that such derivation may help simplify the steps as the materials are usually represented by tensor $$C$$, but what I'm having truble now is a possibility of other ways, such as:

$$S = {F^{ - 1}}\frac{{\partial \Psi }}{{\partial F}} \qquad D = \frac{{\partial S}}{{\partial F}}\frac{{\partial F}}{{\partial E}} \, .$$

To me it looks $$\frac{{\partial F}}{{\partial E}}$$ should be straitforward (as both $$\frac{{\partial S}}{{\partial F}}$$ and $$\frac{{\partial S}}{{\partial E}}$$ is attainable), but it makes me perplexed is that its inverse $$\frac{{\partial E}}{{\partial F}}$$ is not invertible as:

$$\frac{{\partial E_{ij}}}{{\partial F_{kl}}}= \frac{{\partial \left( {{F_{pi}}{F_{pj}}} \right)}}{{\partial {F_{kl}}}} = \left( {{F_{ki}}{\delta _{lj}} + {F_{kj}}{\delta _{li}}} \right) \, ,$$

which is a kind of Sylvestre equations. I think there is an alternative way to bridge these two equations using tensor manipulation, but I'm at a loss.

In short, my question is whether it is possible to compute $$\frac{\partial F}{\partial E}$$.

It might help getting $$C_{ijkl}^{SE}$$ from $$\frac{\partial S_{ij}}{\partial F_{ij}}$$, which is sometimes conveinient when compared to $$\frac{\partial S_{ij}}{\partial E_{ij}}$$.

• It's not very clear what you're asking. Could you rephrase your question? – Tyler Olsen Aug 7 '15 at 16:52
• @TylerOlsen please check the edit. I summerized the question into bolded sentense. Thanks for your comment. – Hayoung.Chung Aug 8 '15 at 1:32
• Can you clear up the line that says: $S = F^{-1}\frac{\partial \Psi}{\partial F} D = \frac{\partial S}{\partial F} \frac{\partial F}{\partial E}$? It appears to me that this was supposed to be 2 different statements. Namely: $S = F^{-1}\frac{\partial \Psi}{\partial F}$ (this is true), and $D = \frac{\partial S}{\partial F} \frac{\partial F}{\partial E}$. Can you confirm this? – Tyler Olsen Aug 8 '15 at 18:00
• It should be noted, also, that most constitutive laws are not written directly in terms of $F$. In fact, they are much more commonly written in terms of (invariants of) the left and right Cauchy-Green deformation tensors $B$ and $C$. For this reason, $\frac{\partial S}{\partial E}$ is usually computed as $2\frac{\partial S}{\partial C}$. Can you provide a concrete example of why $\frac{\partial S}{\partial F}$ makes sense? – Tyler Olsen Aug 8 '15 at 18:10
• @TylerOlsen As you have indicated in the first comment, it was a 2 different statements in single line. I edited it accordingly. – Hayoung.Chung Aug 9 '15 at 1:51

To my knowledge, I'm afraid it is not generally possible to compute $\frac{\partial\mathbf{F}}{\partial\mathbf{E}}$. Here's the reason:

Usually we compute the Green-Lagrange strain tensor from the deformation gradient with its definition $$\mathbf{E}(\mathbf{F})=\frac{1}{2}(\mathbf{F}^T\mathbf{F}-\mathbf{I}) \tag{1}$$ It is easy to verify that $\mathbf{E}$ is symmetric, but $\mathbf{F}$ is not necessarily symmetric. Take the 3D space as example, one would have 9 independent components for $\mathbf{F}$, but only 6 independent components for $\mathbf{E}$. That is to say, one may not possible to obtain the inverse of Eq. (1), namely $\mathbf{F}(\mathbf{E})$, not even $\frac{\partial\mathbf{F}}{\partial\mathbf{E}}$.

Being symmetric is also the reason that people prefer to use the right Cauchy-Green tensor $\mathbf{C}$ as well as the 2nd Piola-Kirchhoff stress $\mathbf{S}$.

However, there are still formulations using the 1st Piola-Kirchhoff stress $\mathbf{P}$, which is computed as $$\mathbf{P}=\frac{\partial\Psi}{\partial\mathbf{F}} \tag{2}$$ and the corresponding tangent modulus $$\mathbb{A}=\frac{\partial\mathbf{P}}{\partial\mathbf{F}} \tag{3}$$ which is usually called the 1st elasticity tensor.

Maybe Eq.(2) and (3) are what you are looking for.

The derivative $\partial\mathbf{E}/\partial\mathbf{F}$ maps from a nine-dimensional space (the differentials d$\mathbf{F}$) to a six-dimensional space (the differentials d$\mathbf{E}$). That said, it is clear that two different d$\mathbf{F}$ can be mapped to the same d$\mathbf{E}$. So the mapping is surjective, which means it is not invertible. This image is taken from this website 