English: Illustration of stress forces acting on a particle (greenish sphere) in a homogeneous continuous medium under a uniform linear stress, as a function of the orientation of a surface element on the particle's boundary. The arrows show the intensity and direction of the stress acting across each surface element. A red arrow indicates that the normal component of the stress is directed outwards, a blue arrow that it is directed inwards. In its eigenvector basis, the stress tensor has normal components with strength +5, +2 and -3.

The picture could be used to illustrate any other symmetric purely linear second-order tensor in 3-space, such as strain, strain rate, polarization, etc.

Image created with POV-Ray by J.Stolfi on 2013-02-02.

Source code available at http://www.ic.unicamp.br/~stolfi/misc/2013-02-02-stress-strain-pics.tgz