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crack propagation and cohesive elements in Abaqus

Hello,

I have been reading up a lot  on "crack propagation" and "cohesive elements" in Abaqus. But i am still struggling to make it run and frankly i am running out of time. I have got only two more days to get the model running for a kind of "Introducing modern science to the public" exposition... I am trying to simulate crack propagation in rubber material in the case of uniaxial tension.

I recall the problem that i want to simulate:

A piece of rubber (hyperelastic neo-hokean) material is subjected to static uniaxial tension untill it fractures. As you can see in the attached picture, there is initially no crack in the geometry. However there is a singularity that is likely to be the crack initiation point. And the crack is going to propagate along the horizontal dotted line.

I am using Abaqus Standard to simulate static crack propagation.

To begin with i sketched the geometry in the Part module and then i partitioned the geometry to create an interface that is going to play  the role of the crack propagation line. My idea is that by making use of "cohesive elements" in the interface i could be able to get a crack propagation.

So in the Property Module i created a "hyperelastic neo-hookean material" to account for the rubber material outside of the interface. I then created two sections:
- a "solid homogeneous" section to account for the hyperelastic material outside of the interface (top and bottom regions)
- a "cohesive section" with the same material properties (hyperelastic material) as  rubber to account for the crack path...

In the Interaction Module i didn't impose any constraint on the interface faces. I don't really understand whether or not i should modify or create something in this Module.

Indeed, i should probably try to impose a "bond" constraint on the interface faces and then set a fracture criterion to progressivly debond the top and bottom regions throughout the deformation of the model...??? But how could and shoud il do that..?

Anyway, then i got to the Mesh Module and  tried to mesh the interface. But when i tried to run the analysis i got an error message like "some elements have no property". And it turns out these elements correspond exactly to the elements lying in the interface.

Please let me know what was wrong in my model and should i proceed in the different Modules (Part, Property, Assembly, Step etc...).

Regards,

Malik

PS: you will find below the link to the pictures

http://files.engineering.com/getfile.aspx?folder=37865e7e-cee0-472c-b982...

http://files.engineering.com/getfile.aspx?folder=37865e7e-cee0-472c-b982...

Hello,

I have been working for two days on my model and i made some progress in the process of understanding "crack propagation" modeling in Abaqus but i didn't get what i was looking for. Indeed no crack propagation occured ...Frown

 

The idea is to define the crack tip as the zone where  σ22  (stress component in the y-direction) is maximum (see attached picture).

I expected the crack to propagate along the horizontal line made up of cohevsive elements by defining the normal nominal stress...

The "upper" and "bottom" material is linear elastic (isotropic) and accounts for a rubberlike material in the case of small strain.

 The "interface" is defined as linear elastic (traction) associated with a Damage for Traction-Separation law

 I am trying to figure out why no crack propagation occurs.

Here are the values that i set for in the Material Module

- the elastic properties of rubber: Young modulus = 2 , Poison's ratio = 0.48

- the elastic properties of the cohesive zone: E=1000, G1=1 and G2=1

- the damage for Traction Separation law (cohesive zone):  Nominal stress _normal mode =1

Nominal stress _first direction =1000, Nominal stress _2nd direction =1000.

I am doing it in a hurry so i probably missed some important points. Moreover i am not familiar with FEM crack propagation so my questions might seem silly to some of you...

I am looking forward to your comments and remarks.

Regards,

Malik 

 http://files.engineering.com/getfile.aspx?folder=07401925-2e28-4cb9-9eb2...

Hello,

It looks like nobody wants to make remarks on these issues ...but i am not going to give up on it  Smile

I am making use of a Quads damage criterion (Traction-Separation law). It is based on nominal stress. 

I am still wondering why i am not observing any crack propagation. What i did is that a sketched a single part and partitioned it to create the interface as you can see in the pictures. I check the damage in the the cohesive elements and it turns out it is equal to zero. I beleive the damage criterion is not met. This is very surptising cauce i set the nominal strains so that at the crack tip, the criterion is met...

I am wondering if i should create different parts instead and tie them in the Interaction module instead of creating one single part like i did...

Feel free to post your comments and remarks

 

Malik

Hi,

 

Can you please let us know all the properties of cohesive elements you are using? You seem to be using bilinear (triangular) cohesive law.  You have told us about the nominal stress value and stiffness of cohesive elements. But what about the damage evolution? In Abaqus, you need to specify damage evolution (path after which nominal stress reaches in the cohesive element). 

If you don't specify damage evolution, Abaqus will not know what to do.....in other words, your crack will never form.

 

Let us know about that. Also, it might help if you can post the input file for your job. Maybe some of us can spot something there.

Hope this may give you a lead...

-Srikant

 

 

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