Strain energy density for a plate under uniform stretch deformation

Can anyone give a definition about strain energy density for a plate? When a plate under a uniform stretch s, some gives an equation like ek=Es2/2(1+μ)(1-2μ), some gives ek=Es2/2. What are the difference of these two definition? Please note both of the cases are for plane stress conditions. If under plane strain conditions, what is the definition for its strain energy density? Thanks for help.