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I want to calculate stress matrix in a cube with two faces parallel to x axis and perpendicular to z axis (sorry I don't know how can I put a picture in this post).

There are two force uniform distributions (that we'll indicate with p) over this two surfaces: the superior is in the x direction, the inferior in the -x direction.

So, I'll have only shear stress, and a shear-modulus $\mu$ dipendence.

We suppose uniform stress tensor in the cube, because every infinitesimal dV of medium is in statical equilibrium with +pdS force with x direction by the superior infinitesimal dV and -pdS force by the inferior infintesimal dV for 3rd Newton's law.

Remembering that any stress over a surface is $t_{ij}n{j}$, where $n_j$ is the normal versor to the surface, we must write:

$T_{ij}n_1=0$ because we haven't any force on the surfaces perpendicular to the x-axis; so the first column is composed from three 0;

$T_{ij}n_2=0$ because we haven't any force on the surfaces perpendicular to the y-axis;so the second column is composed from three 0 too;

$T_{ij}n_3=p n_1$, because we have the force-distribution p over the surfaces that are perpendicular to z axis.

$n_1$ is $\begin{pmatrix} 1 \\ 0 \\ 0 \end{pmatrix}$, $n_2$ is $\begin{pmatrix} 0 \\ 1\\ 0 \end{pmatrix}$, $n_3$ is $\begin{pmatrix} 0 \\ 0 \\ 1 \end{pmatrix}$. So the matrix $T_{ij}$ becomes:

$$\begin{pmatrix} 0 & 0 & p\\ 0 & 0 & 0 \\ 0 & 0 & 0 \\ \end{pmatrix} $$

But this haven't any sense, because stress-tensor must be simmetric for conservation of angular momentum. Where is the mistake?

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Boy, the right stress tensor for similar static situations is symmetric, indeed. It's not hard to see why: the stress tensor knows about the density of forces and an asymmetry would destroy the equilibrium. See

The last paragraph of the section above contains a proof why the stress tensor is symmetric. You may say that the antisymmetric part would act as a torque.

In your example, both $T_{xz}$ and $T_{zx}$ are equal to the shear stress you denoted $p$. In particular, it is not true that $T\cdot n_x=0$, as you wrote in the first step (with confusing extra indices), because the stress tensor measures the internal forces and not just forces that you actively add by your hand.

The internal forces respect the symmetry between the $x$ and $z$ axes. The only asymmetric tensor was one you wanted to "prescribe" to the system. But you can't prescribe arbitrary properties and behavior to the physical system: the objects' properties and behavior obey the laws of physics rather than your expectations. In particular, laws of physics guarantee that the shear stress will treat any pair of axes symmetrically.

You should imagine that the squares in an atomic grid in the $xz$ plane are deformed to rhombi. But the rhombic curvature exists relatively to both axes $x,z$. If you insert a probe into the solid that will measure the internal tension, be sure that you will get totally identical results when you push the $z=0$ plane in the $x$ direction as if you push the $x=0$ plane in the $z$ direction. The result of both things is to change the angle between the (former) $x=0$, $z=0$ planes in the solid. There's no difference.

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The stress tensor is only symmetric if either the spin density describing the density of internal angular momentum is zero. For most materials, this is the case, because we can't align most of the spins of the constituent atoms or molecules in the same direction.

Imagine the cube you mentioned embedded as a subsystem of a much larger material system. With an asymmetric stress-tensor, the forces will cause the cube to spin relative to the material it is embedded in, which for most materials, will be prevented by molecular binding forces between the molecules or atoms on both sides of the boundary. In other words, the cube can't spin relative to the material just outside the boundary. This is due to come counterforce due to molecular binding and shows up as a counterforce on the surfaces normal to the $x$-direction.

You also have to realize this stress analysis only applies in the limit as the cube size becomes infinitesimal. Otherwise, we can have unbalanced forces on the different surfaces, which only means the shape of the cube becomes deformed in time.

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Boy, I agree with QGR that at least in some definitions, the stress tensor doesn't have to be symmetric. However: It's very strange how you write your equations, such as $T_{ij}n_z=pn_x$. Because of the $ij$ indices, it looks like a set of 9 equations, but because of $n_z$ and $p_x$, it looks like the indices are fixed and it's only one equation. Moreover, it's strange to use the letter $p$ (like "pressure") for forces that are completely non-uniform and they behave as vectors. While the non-symmetricity of the stress tensor would be a contradiction, I wouldn't trust your derivation. – Luboš Motl Feb 10 '11 at 7:04
In particular, Boy, I don't trust your equation $T_{ij}n_x=0$. You just made it up, you don't have any evidence. I think that the right answer to your problem is a symmetric stress tensor. If you tilt a solid, deforming the grid in the $xz$ plane, then the squares in the grid become rhombi, and the rhombi feel the very same tension in the $xz$ double-direction as in the $zx$ double-direction. You need two indices. What's the problem if you conclude that both components $xz$ and $zx$ are equal to the shear stress? – Luboš Motl Feb 10 '11 at 7:09
@Luboš Motl I correct the simbology that you have told in the question.I put $T_{ij}n_x=0$ because my hands exercite forces only on the surfaces that are perpendicular to z axis, so surfaces that are parpendicular to x and y are free and the stress on them (that is fixed with boundary conditions) is $T_{ij}n_j=0$,with j=1,2 (as x, y).This was my derivation.I think my result it's wrong too, because the stress that I have obtained I asimmetric, that isn't possible in classical linear elasticity theory... – Boy Simone Feb 10 '11 at 11:57
...I agree with you, there's a mistake, but I can't find it,I would see the correct derivation for stress tensor in this physical situation (that's statical). – Boy Simone Feb 10 '11 at 11:58

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