NX Nastran: How to constrain an edge to be straight?

Experimenter
Experimenter

I am trying to solve a plane strain problem, where a large flat plate bends into a circular section. To avoid having to model the whole plate, I want to model a small 'pie'-section. To this end, I want to constrain the edges of the section to stay completely straight, but allow the strain to vary along the edge.

 

I've tried RBAR elements along the edge (all dependent DOFs set to On at one end, all independent On at the other end), but they constrain the strain along the edge as well, introducing an artificial stiffness. So, I would like to add a constraint something like RBAR, but allowing the length of the RBAR elements to change. Just setting the dependent Z DOF to Off doesn't work, because the edge rotates, such that the Z direction is no longer in line with the elements after deformation.

 

How can I make this work?