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## Autodesk Simulation Mechanical and Multiphysics

Contributor
Posts: 22
Registered: ‎09-07-2010

# Why pin joint (truss element) collapsed?

196 Views, 2 Replies
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

Distinguished Contributor
Posts: 226
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

15 years experience with Simulation Mechanical
Contributor
Posts: 22
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