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Registered: ‎09-07-2010

Why pin joint (truss element) collapsed?

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09-10-2012 06:54 AM

I use pin joint setting in my numerical model. But the solver program always shows lots of warning messages like:

 

*** Warning : Truss element collapsed!

                      Numerical exception may occur

                      Element number=51

                      Original element length =11.87

                      Current element length=NaN

 

 then the time step is reduced again and again, and the model seems difficult to converge.

 

Why and how to avoid the problems? Thanks!

                                                                                                     CCZ90

 

 

*Expert Elite*
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Registered: ‎08-30-2012

Re: Why pin joint (truss element) collapsed?

09-10-2012 10:02 AM in reply to: ccz90

It sounds like a numerical problem in the MES simulation, so I suggest trying a smaller time step just before this event occurs. (online wiki Help) I am hoping that a different calculation path will avoid the problem. (NaN stands for "Not a Number", so something is occurring mathematically.)

 

Otherwise,

  1. What material model are you using for the trusses?
  2. What is the stiffness of the trusses? (Cross-sectional area and modulus of elasticity)
  3. What step does this occur on?
  4. What is the force in the truss before it collapses? ("Results Contours > Other Results > Element Forces > Axial Force")
  5. What element type is the truss connected to?

 

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

Simulation Mechanical user since Dec 1997
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Registered: ‎09-07-2010

Re: Why pin joint (truss element) collapsed?

10-03-2012 09:00 AM in reply to: AstroJohnPE
  • What material model are you using for the trusses?

      Linear

  • What is the stiffness of the trusses? (Cross-sectional area and modulus of elasticity)

      Cross-sectional area from 7~300 mm^2 , E= the defacut material parameters from autodesk material library

  • What step does this occur on?

      Changeful. It maybe at the bigining or the process of the calculation.

  • What is the force in the truss before it collapses? ("Results Contours > Other Results > Element Forces > Axial Force")

     In some cases, the forcees are very small, 7 ~ -4.9N

  • What element type is the truss connected to?

    Brick (isotropic elastic steel)

 

I have used a smaller time step with bigining of 1~0.1 ms, and the solver might reduce the time step more. The

warning messages still display  randomly.

                                                                                 Ccz90