Elasto Plastic Curve and Multilinear Isotropic Hardening

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Fabricio.Urquhart posted this 1 weeks ago

Hello, now I am trying to understand the application of plasticity in Ansys, so I am modeling a simple bar, fixed on both sides. But I am suing a plane of symmetry in the middle, because is the same plane of symmetry which I am using in my master thesis.

The question is about the multilinear isotropic hardening. I am using some curves from the literature, and in the reality we have a gap between the end of elasticity and the start of plasticity, where the stress is constant and there is a variation of the strain. I considered it in the multilinear material curve:

I would like some opinion. Because I have read some articles, and the people do not consider this "constant" gap. I think the because is difficult to know the strain when this gap starts. What do you think about it?

 

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SandeepMedikonda posted this 6 days ago

Hi Fabricio,

  It looks like this is the engineering stress, is that correct? If so, you would have to convert this to true stress-strain data. Also, it looks like you don't have enough data points there. The reason why it is not recommended is since stiffness is zero it will introduce instability into your analysis.

  Check out this article, you might find useful.

Regards,

Sandeep

Fabricio.Urquhart posted this yesterday

Thank you Sandeep. I have read the articles you said. It was very useful. 

The model attached is not converging. Do you think that the reason is that I am using engineering stress?

 

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SandeepMedikonda posted this yesterday

Hi Fabricio,

  It's possible, but more importantly, the metal plasticity material models are anticipating a true stress-strain data. So when you input Engineering stress-strain data you are getting a wrong result.

  Some suggestions:

  • Keep an eye on the plastic strain in your simulation is it may be exceeding the max. value you might have provided.
  • Remove the initial points in your simulation (0 & 0.0001) and just start from 0.02 plastic strain and see if it helps.
  • Increase the maximum sub-steps to about 1000. 

~Sandeep

 

Fabricio.Urquhart posted this yesterday

Sandeep,

- I will keep an eye on the plastic strain

- Why remove?It is the elastic strain. And if I start with 0,02 strain, Ansys does not accept Mthe multilinear tabular. I did not agree with it.

- I will increase

 

Thank you

SandeepMedikonda posted this yesterday

Fabricio,

When we define the data for Multilinear Isotropic Hardening, we are defining the plastic strain vs true stress right? But from looking at your data it looks like you had the complete engineering stress-strain profile input. So, I was suggesting you to separate the elastic strain data as discussed here.

Also, check out section 4.4.2.2.2 in the manual.

~Sandeep

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