Failure Criterion in Abaqus:Displacement at failure

Hi all,
I am having trouble in specifying a failure value. I tried different damage models. But none seem to work. I have been working on JC Cook model and it asks for displacement at failure which seems to be an unknown. I read through the reference and it gives no clue about properly defining that value. Although it gives some equations, it depends on element length rather than the material property!! Please help me, if you have an idea.
Thanks