For the simplification of plane
stress, where the stresses in the z direction are
considered to be negligible,  , the stress-strain compliance relationship
for an isotropic
material becomes,
The three zero'd stress entries in the stress vector indicate
that we can ignore their associated columns in the compliance matrix
(i.e. columns 3, 4, and 5). If we also ignore the rows associated
with the strain components with z-subscripts, the compliance matrix reduces to a simple 3x3
matrix,
The stiffness matrix for plane
stress is found by inverting the plane stress compliance matrix, and
is given by,
Note that the stiffness matrix for plane stress is NOT found by removing columns and rows from the
general isotropic
stiffness matrix. | 
