creating edge crack

Hi to all :blush:
i want to create an initial crack in my model.
my model is periodic in y direction.

I defined two region in the upper side of my model and using neigh_modify commend to create crack. this command works well when i use it to create a center crack but in the edge crack, both sides still have crack tip and it’s not really a edge crack.
Please look at this photo in the link

In the photo i showed how is my ideal edge crack, but as you see, the created edge crack is not like that. I think the problem comes from the periodic direction, but i dont know how i can solve it. could anyone help me? i would highly appreciate for your help.

Best regards