Periodic Boundary Conditions and Contact Elements with Cohesive Zone Law in Ansys

Hi all,

I have a problem with periodic boundary conditions using contact elements with a cohesive zone law in Ansys.

In my micromechanical simulations of an RVE with periodic boundary conditions I would like to have a predefined fracture path. My aim is to realize the fracture path with voronoi cells. There I get problems at the boundary of the periodic domain.

A picture of the simplified problem is attached here:

http://dl.dropbox.com/u/4920002/PBC_CZM.png

The domain here is periodic and square. The red lines symbolize the cohesive zone law which connects the coincident nodes.  I have no problem with the red nodes.

But how can I couple the green points with periodic boundary connections. In previous simulations I did this with constraint equations but there I did not have fracture path so I had well defined pairs of nodes on each side of the domain. But now I have two nodes on the right side an one on´the left. I can not figure out how to define constraint equations between these three points that are physically correct.

Anybody any idea.

Thanks for help.

Andreas