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Plastic strain output from UMAT

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  • Plastic strain output from UMAT

    Dear Dr Jorgen,

    I have written and verified a UMAT for ABAQUS. However I have one bug...

    I want to request an output of the plastic strain components (*ELEMENT OUTPUT \cr PE) at integration points over my model, but when I do this and view the the results in CAE the plastic strain components are zero everywhere. I know this has to be impossible as the material is clearly yielding. Does ABAQUS not generate a store of plastic strain when a UMAT is used?

    Any help would be very appreciative.

  • #2
    ... ps: id be grateful from help from anyone, not just Dr Jorgen although he seems to be something of the guru here.

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    • #3
      I believe that Abaqus does not allow you to output plastic strain as you normally would when you use a UMAT. Instead you need to save the plastic strain components that you are interested in as state variables. You can then plot the state variables using CAE.

      -Jorgen
      Jorgen Bergstrom, Ph.D. PolymerFEM Administrator

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      • #4
        There is an example cylbillet_slow_usr_umat_umatht.for that demonstrates the definition of plastic strain as a state variable.

        Frank

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        • #5
          Originally posted by Jorgen View Post
          I believe that Abaqus does not allow you to output plastic strain as you normally would when you use a UMAT. Instead you need to save the plastic strain components that you are interested in as state variables. You can then plot the state variables using CAE.

          -Jorgen
          Hi Jorgen,

          I have learnt a lot from your replies and hope you can help me with this case.

          I am currently simulating spring back behavior of a elastic-plastic material and its material properties are characterized through a user-defined material routine. Here is what I am really confused of:
          Say, if I define elastic strain (or plastic strain) as a function of total strain and saved these data as state variables successfully. However, these values have no influence to the actual strain values I read from the odb file.
          For instance,
          statev(1)=eelas(1), statev(2)=eplas(1) //define elastic strain and plastic strain as state variables.
          eelas(1) = stran(1) * 0.5. //elastic strain equals to half of the total strain in 1 direction.
          eplas(1) = stran(1) - eelas(1) //plastic strain equals to the total strain minus elastic strain.
          eelas(1) = statev(1), eplas(1) = statev(2) //update state variables.

          In the odb file, statev(1), and statev(2) are half of LE as defined in the umat. But the total strain is fully recovered during the unloading process, suggesting no plastic deformation.

          I wish to know how to modify my umat.

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          • #6
            I don't quite understand. If you already know what changes that you like, can you not implement those changes...

            -Jorgen
            Jorgen Bergstrom, Ph.D. PolymerFEM Administrator

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