how does ABAQUS design the cohesive element

Dear all,

I recently need calculate the stiffness matrix of the cohesive elements manually via mathematica. I used the finite element method to fulfill the assignment, but the result I got didn't match the stiffness matrix I extracted from the Abaqus. Neither did my supervisor.

knn, ktt and kss seem to be so obvious for the traction-seperation law, as also it measioned in Abaqus.  Could you please help me out by offering the way to find how the ABAQUS exactly design the cohesive elements or how ABAQUS calculate the stiffness matrix.

Any little help is so much appreciated. Thank you in advance.

Sincerely,

Chen