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-   -   crack growth modelling implementing Cohesive Zone Model (CZM) (http://www.cfd-online.com/Forums/ansys/88033-crack-growth-modelling-implementing-cohesive-zone-model-czm.html)

_Denis_ May 6, 2011 07:24

crack growth modelling implementing Cohesive Zone Model (CZM)
 
Hello, everyone!
Could you, please, help me with the following issue?
Iím trying to simulate a crack growth using cohesive zone model (CZM), andthe results I get do not agree with my expectations, based on a common senseand simple estimations.



In short, the problem is: the cracking starts only after reaching the normal stress that is million times greater than the maximum normal contact stress parameter.



As I donít yet feel myself on a safe ground with APDL, I make use of AnsysMechanical, implementing command objects where needed.
At the moment the model is looking like a two beams (meshed with 20-nodebricks) bonded together with a surface-to-surface contact, beingseparated from each other.
In the Contacts branch I create a bonded Solid to Solid contact, and attacha 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 mymodel Mechanical automatically creates two contact sets, with real constant ID3 and 4, respectively. Thus, I believe, it makes sense to delete one of thesesets.
The elastic properties of the material in use are: linear anisotropicelasticity with 11, 22, 33 components of the elasticity matrix around 1e11 Pa.All faces of the beams are perpendicular to the relevant coordinate axis, theseparation traction is applied along one of coordinate axis, normal to thecontact plane (to be precise, I impose on two faces of the different beams notthe 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. Iestimate that if I set this value to 1e10 Pa, the model should crack afterdeformation of less than 15 %. On the practice, if this value is greater than1e4, the body deforms until the deformation causes instabilities withoutcracking, while the normal stress result branch clearly shows that the actualstress at the interface is much greater than the current normal contact stressparameter value. 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!

imnull April 20, 2012 15:51

Max_Stress=50 !MPa
Max_Udl=0.05 !/100%
t=0.1 !mm

Gap=Max_Udl*t

tb,czm,cid,,,cbdd
tbdata,1,Max_Stress,Gap,Max_Stress,Gap,0.001,1

Stiffness=-Max_Stress/(0.10*Gap)

rmodif,cid,3,Stiffness
rmodif,cid,12,Stiffness




PLUS


The pinball radius should be set less than your mesh size!!
and dot forget PURE PENALTY formulation in Contact Region

_Denis_ April 20, 2012 16:04

Thank you very much for answering.
The only problem is that my post is 1 year old, and I've alread got over that problem. :)

imnull April 20, 2012 18:25

Quote:

Originally Posted by _Denis_ (Post 355906)
Thank you very much for answering.
The only problem is that my post is 1 year old, and I've alread got over that problem. :)

have u done a model validation with real experiment?

_Denis_ April 22, 2012 03:24

I haven't conducted any experiments myself, but I've compared the speed of crack propagation resulting from my simulation with known experimental results.

alizadeh July 23, 2013 10:41

cohesive mateial
 
hi dear Denis
i m trying to model delamination via contact element and cohesive material which involve defining cohesive material between two planes meshed by shell99
would you let me know how can i define cohesive material between two planes

all the best
alizadeh

NithyaRamesh April 14, 2015 08:41

Hi
i have also one doubt regarding cohesive zone modeling. That, is it possible to give 1000mm shear separation for 100mm Double cantilever beam?

omgukguk April 19, 2015 04:01

Thanks a lot! :)


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