hi, the reason for stiffer response in FEM is alright, since you are dividing the geometry into number of finite element. when you seek solution for global geometry, the stiffness matrix is going to be much stiffer. i suggest you go with larger size element or verify Drucker - Prager model parameters. i feel you also need to check material model parameters and also interface parameters to define the interface b/w two materials. ok all the best..

Hi Karthigeyan . S , Thanks for the reply. The mesh size is not having much effect on the stiffness on the model. Very minute. The material model still looks very stiff.

hi..nice to hear from you..you have not mentioned about the type of material and its properties ..if you tell all these material details then only i could verify the model..ok all the best..

Hi,

I think you should convert your engineering stress values into true stress values and input the True stress vs plastic strain values in the Drucker-Prager material module. The attached file gives very good guidelines on how to do this (go to page no. 34 for interpretation of compression test data into Drucker-Prager model parameters).

I hope it helps.

Hi,

Thank you both for the reply.

Akash Sharma , I have read through the manual that you shared and used the formula for true strain and true stress as shown in page 34. But that made the model slightly stiffer(IT-14).

Alternatively, I tried to create compression hardening values till the force vs deflection matches exactly with the test data as you can see in the DPP-13. I still dont understand what is the logical behind such compression hardening.

Karthigeyan . S I am working on a foam material, is all I can share unfortunately at this point of time.

Hi ,

Any suggestions or relevant articles that anyone can share ? It would be really great of you.

Regards,

Vaidehi

Hi,

Kindly check if you are using negative values of compression stresses and strains while calculating the true stress vs plastic strain curve as its given in the manual that sigmaC etc are negative values. Also make sure that the strain values are NOT in percentage.

Hi Akash Sharma , As you can see in the above picture., I have not used negative values for compression stress and strain. So that is not the problem. Reducing the yield stress at higher strain values is helping. But there is no logical explanation of why should do this.

Regards,

Hi Vaidehi,

I suggest that you do use the negative values and compute the yield stress vs plastic strain curve. However, while giving input to the abaqus, you use the absolute computed stress and plastic strain values.

I used to face similar problem while matching my material stress- strain curve with simulations but when I used negative values for compressive calculations, I was successful.

Can anyone share their excel for creation of drucker prager model from test data ? Especially for foam. Has anyone worked on foam for creating this kind of Drucker prager model ?