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Stranger than Fiction

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Posts: 59
 lynx
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(@lynx)
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Joined: 15 years ago

Dear Jorgen and all,

Recently I have been working on a subroutine (VUMAT) for finite strain elastoplasticity which utilizes generalized strain and stress measures.

I have done my 1 element analysis with error-free results. Then I have started to solve more complicated ones.

For 3D analysis, although the code works quite well until certain deformation levels, there comes a point where the analysis looses stability and crashes.

As an example, for a 3D necking problem (triggered by a geometrical imperfection in the case of Simo and colleagues), I have quite good results

which preserves symmetry, however then after certain point the symmetry is lost, as far as the distribution of equivalent plastic strains are concerned.

I also see that the loss of symmetry is always in a certain direction. I mean if the circular bar has a x-section located on 1-2 axis and longitudinal axis is 3, the accumulation of equivalent plastic strains occur on axis-1 at the face.

What do I have to suspect at this point. Can you help me with solutions or can you suggest me some benchmarks?

Thank you for your time,

Kumar

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(@jorgen)
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Joined: 5 years ago

Stranger than Fiction - I kind of liked that movie 🙂

How distorted are the elements at the time you reach the instability?

Does it help to use smaller time increments?

I would try different elements types, e.g. plane strain, and displacement BCs.

Unfortunately, debugging a VUMAT can be quite frustrating 😮

-Jorgen

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Posts: 59
 lynx
Topic starter
(@lynx)
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Joined: 15 years ago

Dear Jorgen,

Thank you for your interest. I have solved the problem.

I have recently found that the error was in one of the shear component of the push-front transformation of the Second PK tensor. This error is hidden quite well that it really was frustrating to debug :eek:.

Best

Kumar

ps: Stranger than Fiction is a nice movie, I agree 😀

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