Four-node effective strain after deformation

[hongfengtl]

I just learn the finite element theory, now I have one problem, I want to calculate the four-node effective strain after deformation (you can see the attachment) using four-node isoparametric element and Lagrangian finite deformation theory. I want to learn the relevant theory, could you please give me the detailed calculation process? Thank you.
http://i1140.photobucket.com/albums/n579/hongfengtl/1-1.jpg