1. The problem statement, all variables and given/known data I have an initial configuration and a final configuration for a sample that has been stressed. Both of these are trapezoidal shapes and I have exact coordinates for each point. I know how to use the shape equations as well as deriving the Jacobian. Additionally I have computed the u and v displacement vectors for between the configurations. Now I need to determine the displacement and deformation gradients so I am able to compute strain on the tissue. The issue is that I don't know how a change in configuration affects the shape equations. Relating a single shape to a parent configuration is one thing, but I don't know how to map the two configurations so that I can determine stress and strain. As far as I can find, all examples relate some shape to the double unit square rather than a pre-existing configuration. I would appreciate any sort of help with this quite a bit. For what it's worth the best classification of this problem is a 4 node isoparametric mapping. 2. Relevant equations x = Σi = 1->4Ni (s,t)xi N1=¼*(1-s)(1-t)*x1... y = Σi = 1->4Ni (s,t)yi N1=¼*(1-s)(1-t)*y1... u = x - X (lowercase = deformed configuration, uppercase = initial configuration) v = y - Y 3. The attempt at a solution I have nothing since I can't start the problem without knowing how to relate the configurations. I would prefer having someone point me in the right direction rather than putting up exact values and having my problem solved.