Define crack when using cohesive surfaces in abaqus

Hello,

Im trying to model Delamination in abaqus using cohesive surfaces. I did the following :

-created a cohesive intertaction property, defined damage initialisation and evolution in this and then used general contact between 2 surface and used the interaction property for it.

-For the initial crack i left a certain region unbounded and used a double cantilever model.

The crack isnt propagating but the unbounded region opens up. I assume i havent defined the crack and therefore this is happening but i have no clue how to define a crack. Any help or tips would be highly appreciated

Thanks

Abhishekaran