Simulating crack growth in Ansys implementing Cohesive Zone Model (CZM)

Hello, everyone!

Could you, please, help me with the following issue?

I’m trying to simulate a crack growth in an anisotropic media using cohesive zone model (CZM), and the results I get do not agree with my expectations, based on a common sense and simple estimations.

As I don’t yet feel myself on a safe ground with APDL, I make use of Ansys Mechanical, implementing command objects where needed.

At the moment the model is looking like a two beams (meshed with 20-node bricks)  bonded together with a surface-to-surface contact, being separated from each other.

In the Contacts branch I create a bonded Solid to Solid contact, and attach a command object to it. Here is the text from the command object:

ESEL,S,REAL,,4 

EDELE,all 

ESEL,ALL

tb,czm,3,,,cbde

tbdata,1,1e4,5e-8,,,1e-8

About the first 3 lines: if I get it right, because of the symmetry of my model Mechanical automatically creates two contact sets, with real constant ID 3 and 4, respectively. Thus, I believe, it makes sense to delete one of these sets.

The elastic properties of the material in use are: linear anisotropic elasticity with 11, 22, 33 components of the elasticity matrix around 1e11 Pa .All faces of the beams are perpendicular to the relevant coordinate axis, the separation traction is applied along one of coordinate axis, normal to the contact plane (to be precise, I impose on two faces of the different beams not the force, but ramped displacement).

You can see that the maximum normal contact stress is set to be 1e4 Pa, which is not much, considering the elastic properties of the material. I estimate that if I set this value to 1e10 Pa, the model should crack after deformation of less than 15 %. On the practice, if this value is greater than 1e4, the body deforms until the deformation causes instabilities, without cracking. How could these be? Where is my mistake?

Some additional information about my model:

The pinball radius is set to be program controlled, large deflection = 0n, Augmented Lagrangian method , Physics type: structural, Analysis type: transient.

Thank you very much for your attention!