 One can still simplify the analysis by using a two-dimensional domain, but the plane stress tensor at each point must be complemented with bending terms.Mathematically, the stress at some point in the material is a plane stress if one of the three principal stresses (the eigenvalues of the Cauchy stress tensor) is zero.That is, there is Cartesian coordinate system in which the stress tensor has the form 
For example, consider a rectangular block of material measuring 10, 40 and 5 cm along the formula_2, formula_3, and formula_4, that is being stretched in the formula_2 direction and compressed in the formula_3 direction, by pairs of opposite forces with magnitudes 10 N and 20 N, respectively, uniformly distributed over the corresponding faces.