Hello to everyone

I have found that my 2D plane strain model shows much stiffer behaviour than it should be (comparing to the experimental data and 3D model).

So I checked the simple case: elastic square (one element 1x1 mm, Young modulus = 8 GPa, poisson ratio=0.4), bottom fixed, small displacement applied on the top (5%). Analytically you expect to get 400MPa stress in the element. Solving the problem with Plane Strain element gives me 476 MPa. Solving with Plane Stress - right answer (400 MPa).

Why Plane Strain works wrong?

I am using Abaqus 6.11-2