Negative Internal Energy in Foam

Altair Forum User
Altair Forum User
Altair Employee
edited November 2020 in Community Q&A

Hi,

 

I am trying to model a problem involving crushable foam modeled using LAW33 and coarse brick elements with property as ISOLID=14, ISMSTR=1, ICPRE=2. I have defined a relative velocity between foam and a metal part, so that the metal part comes in contact with the foam. As soon as the metal part comes in contact with the foam, the foam elements encounter negative internal energy leading to 'zero or negative volume' error. Please help me resolve the instability. I have attached the model for reference.

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Answers

  • Rahul Rajan_21763
    Rahul Rajan_21763 New Altair Community Member
    edited November 2015

    Hi,

    since you have significant variation in two material youngs modulus(.10342 for foam & 73.1for Al 2024 ).It is recommended to use Istiff=4 in Type 7 contact which is set to 0 rightnow.

    Please update this and run.

     

    Regards

    Rahul R

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited November 2015

    Hi,

     

    I have tried using IStiff = 4 as suggested but still I am getting the same error. I believe the problem is with the formulation of foam elements itself and not with the contact definition as the foam elements are distorting without any contact penetration. Can you please check the foam formulation I used and suggest any correct approach?

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited November 2015

    Hi SaiPhani,

     

    Have you refered the RADIOSS FAQ on how handle such cases?

    If not please go ahead and read RADIOSS FAQs.

     

    Also, let us know if this is happening on a particular element or a bunch or elements

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  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited November 2015

    Hi Prakash,

     

    I have referred to the RADIOSS FAQ and have followed the recommendations: I have used ISMSTR=1 (Small strain from t=0) to avoid excessive compression. However, the zero volume error in my model is not due to excessive compression as I have not defined any compressive load. According to the defined load, the foam should attain the same velocity as leg guide and move along with the guide with very little compression. I think a bunch of elements are failing as soon as the guide comes in contact with the foam without any compression whatsoever. So, I believe there is some instability is the foam itself which is coming into play upon contact. Please let me know in case you need any more details.

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited November 2015

    If this is not to do with the element formulation then it has to do with material.

     

    Can you check the material stress strain data?

     

    What type of data have you given?

     

    also try without the stress strain function in the material and check whether you the same error. 

     

    also, please check the unit consistency.

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited November 2015

    Hi Prakash,

     

    Thanks for your suggestion. I checked the material model. The problem was with the definition of stress-strain curve. I had previously defined Stress vs. total strain (elastic + plastic) but now I have changed it to Stress vs. Plastic strain and the model is now working fine. Thanks again for your help... You are awesome man. Keep rocking . :)/emoticons/default_smile.png' srcset='/emoticons/smile@2x.png 2x' title=':)' width='20'> 

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