The expression you're using is valid only for small deformation. For Large deformation,

curvature = w"/(1+y'2)3/2

Now depending upon your element type used in the analysis, and whether non linear geometry was accounted for or not, you can calculate curvature by many ways:

Also make sure to use quadratic elements, not linear elements (which will lead to errors in bending unless you are very careful).

Thank you for your answers , I juste have a question about how can I cut section in mesh to calculate the curvature from displacement?

Can You explain to me with more details?

Thank You.

To create cut, check out abaqus cae user's manual.

To calculate rotation/curvature, remember that these are calculated at the beam centerline. Therefore, if you've displacements at all nodes at a section, you can easily calculate rotation as u=-r * theta (small deformation regime)

ah Ok thank you so much ^^ . Just I have other question if my model constitutes of two layers and I want to calculate or to define by Finite Elements the stress existing in the underlayer between the two layers.  Do you have any ideas how I can calculate?

Thank you ^^ .