XFEM in Abaqus

Hi

I am new to modelling and am trying to model a plate with a middle crack using Abaqus.

 What is the difference between modelling a cohesive and LEFM crack growth (I know cohesive involves plasticity and the traction/displacement curve and LEFM is for brittle materials).

 For cohesive, I used max principal stress for damage criteria and damage evolution based on energy criteria. 

For LEFM I also used the above and only change I made was I used (VCCT BK Law) in interaction properties tab. I cannot see any difference between the cohesive and LEFM in visualization  for example S11, S22 values near the crack-tip.

I am also trying to get the stress intensity factor (SIF) for 3D model but the job does not complete for a stationary crack (Abaqus specifies the need to model stationary crack to get SIF values). I am getting the following messaage: 

"XFEM crack growth interaction "Int-1" references stationary XFEM crack "Crack-1". Delete or supress the interaction, or change the crack to allow growth".