Abaqus cohesive elements/cohesive surface

For my
graduation project I’m working on a model to describe cracking behavior. For
this model I want to model a fibre-epoxy interface, see the attached figure 1
(due to symmetry only a quarter is modeled). The fibre part is modeled with
lineair elastic material. The epoxy part is modeled with CPS4 elements, with
elastic behavior and MAXPS damage (damaged evolution included). The connection
between the fibre and epoxy interface can be modeled with both a cohesive
surface and cohesive elements.  However
cohesive surface will give inaccurate results due to peak stresses (see
attached image 2).

For the
attached image 2 I tested single cohesive elements, and two lineair elastic
elements (very high E-module, thus these approach infinite stiffnes) connected
with a cohesive surface.

Therefor I
want to model the fibre-epoxy interface with cohesive elements instead of
cohesive surfaces. I previously used a cohesive surface to model the
interaction (before I found out about the inaccurate results). The cracking in
the epoxy interface was modeled with XFEM. The results of the model where
reasonable, however not completely correct. The normalized traction separation
diagram is added in figure 3. However this normalized traction separation
diagram should look as attached figure 4 as found by Alfaro and Suiker (2010).

Alfaro, M.
Suiker, A. (2010) Transverse Failure Behavior of Fibre-epoxy systems, Journal
of Composite Materials, Vol. 44, No. 12/2010.

So I tried
to adjust the model, and change the fibre-epoxy interface from a cohesive
surface to cohesive elements. When these cohesive elements are modeled the
behavior of the model is however completely incorrect.

The input
(.inp) files for both the cohesive surface and the cohesive element fibre-epoxy
interaction can be found here:

Cohesive element.inp

Cohesive surface.inp 

Why does
Abaqus give completely different answers to the model when cohesive elements
are used to model a fibre-epoxy interface.

And how can
the model be adjusted to behave correctly.

Thank you
in advance,

Any help
would be very helpful!

Steven