Surface based cohesive behavior

I am working on shear failure of sandwich composites.I have modelled a sandwich composite with surface based cohesive behavior in ABAQUS 6.9.Now i have to do damage modelling. For damage initiation i have to describe values of 'peak values of contact stress' or 'peak values of contact separation'.In addn i also have to describe values of 'critical fracture energy' for damage evolution. I don't know how to find these values or where from i will get these values.