The stress vectors are shown in the Figure1 below. The sign convention for direct stresses and strains is that tension is positive and compression is negative. For shears, positive is when the two applicable positive axes rotate toward each other.
Figure 1 Three-dimensional Stress Components
The elasticity matrix 
is symmetric. For isotropic
materials, all of the terms of depend on only two
elastic constants: the elastic modulus (
) and Poisson’s ratio (
). For the isotropic case:
(Shell
element)
(Solid
element)