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I have problem by definition of strain and stress. From Gockenbach's book that our reference for FEM, we have $$\epsilon=\frac{\nabla u+ \nabla u^T}{2},$$

that $u$ is vector displacement, and $\nabla u$ is the Jacobian of $u$. So we have $\epsilon$ is symmetric and also $\sigma$, that is

$$2\mu \epsilon+\lambda tr(\epsilon)I$$

My problem is that I see everywhere this statement: if $\epsilon$ is symmetric or if $\sigma$ is symmetric we have... why? I can not see the case that they not be symmetric,

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migrated from math.stackexchange.com May 26 '13 at 15:27

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This is more of a physics question (see Tharsis' answer below). I am migrating accordingly. –  Willie Wong May 26 '13 at 15:27
    
Related: physics.stackexchange.com/q/65169/2451 –  Qmechanic May 26 '13 at 15:37
    
Symmetry of $\epsilon$ comes from its definition, symmetry of $\sigma$ comes from the absence of internal torque. –  Learning is a mess May 26 '13 at 15:53
    
Thanks, but what are they defenition when they are not symmetric? sorry , I did not see the icon add comment so I have to right it here –  user24996 May 26 '13 at 18:48
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1 Answer 1

Indeed, both the strain tensor

$$\epsilon_{ij}=\frac{1}{2}\left(\frac{\partial u_i}{\partial x_j}+\frac{\partial u_j}{\partial x_i}\right) \tag{1}$$

and the stress tensor

$$\sigma_{ij}=2\mu\epsilon_{ij}+\lambda\epsilon_{kk}\delta_{ij} \tag{2}$$

are symmetric by definition.

However, bear in mind that these definitions are not always valid; $(1)$ assumes that the deformations are infinitesimal and $(2)$ assumes that the solid is elastic (obeys Hooke's law) and isotropic.

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