# Help applying summation convention to tensors(generalised Hooke's law)

I understand the simplest application of the summation convention.

$x_{i}y_{i}$

I create a sum of terms such that in each term the subscripts are the same i.e.
$x_{1}y_{1}+x_{2}y_{2}+x_{3}y_{3}+...$

But now when I look at understanding summation convention applied to the generalised Hooke's law:

$\tau _{ik}=C_{ikrs}E_{rs}$

I'm a bit unsure what this means. I understand that C is a tensor with 81 components and that by applying the summation convention I should get linear combinations for every $\tau$ component (I think). But as for applying summation convention to see what exactly those sums look like I'm unsure.

The statement below doesn't seem to help my understanding either

"$\tau _{ik}$ is a linear combination of all strain components $E_{ik}$"

Any assistance would be appreciated.

## The Attempt at a Solution

I could hazard I guess and say I only sum when r or s equals i or k, and when r=s and i=k. Still unsure if that is correct though.

cepheid
Staff Emeritus
Gold Member
From your statement about C_ikrs having 81 components, I am assuming that each index ranges over 3 possible values i.e. i = {1,2,3}, and likewise for k, r and s. since only repeated indices are summed over, this means that i and k are not summed over, and you actually have nine different equations represented here, one for each component of tau. So just cycle through all the components of tau (ie all possible values of i and k ):

$$\tau_{11} = C_{1111}E_{11} + C_{1112}E_{12} + C_{1113}E_{13} + C_{1121}E_{21} + C_{1122}E_{22} + C_{1123}E_{23} + C_{1131}E_{31} + C_{1132}E_{32} + C_{1133}E_{33}$$

And that's just ONE of the nine equations. You still have to do the same for tau_12, tau_13, tau_21 et cetera.

cepheid
Staff Emeritus
If it helps, the NON shorthand way of writing your tensor expression would have been: $$\tau_{ik} = \sum_{r=1}^3\sum_{s=1}^3 C_{ikrs}E_{rs}$$