Solid Mechanics: Hooke's Law 
Hooke's Law for Plane Stress 
For the simplification of plane
stress, where the stresses in the z direction are
considered to be negligible, , the stressstrain 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 zsubscripts, 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. 
Plane Stress Hooke's Law via Engineering Strain 
Some reference books incorporate the shear
modulus G and the engineering
shear strain g_{xy}, related to
the shear strain e_{xy} via,
The stressstrain compliance matrix using G and g_{xy} are,
The stiffness matrix is,
The shear modulus G is related to E and n via,

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