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# NX Solution 601

Builder

Dear world,

I am having the following problem (in NX11, solution 601):

I want to model the flattening of a pipe segment.

The material has a different behaviour for stress and compression. This is proved by the fact that the experiment shows that the pipe continues to deform even when the strain goes beyond the last value on a  stress-strain curve (XTCURVE is set for Not extended), with only a branch for positive strain/stress.

I presume the solution will be to use a Nonlinear elastic material. This will allow me to define a stress-strain curve with different positive and negative branches. Is it correct?

Even if this is the case, I still have a question. When will NX consider that elements where strain goes beyond the last values on the negative branch of the strain-stress curve fail?  I am asking that, because the last value on the stress-strain curve, for the negative branch should be negative, but I presume that NX will use a Von Mises equivalent strain, that is positive!

Many thanks

Ionut

15 REPLIES

# Re: NX Solution 601

Siemens Phenom

XTCURVE is only applicable for the multilinear-plastic material model

# Re: NX Solution 601

Builder
Hello JimB,
Should I understand that If I am using a nonlinear-elastic material and the
analysis tends to go beyond the last point of the stress-strain curve, the
curve is automatically extended? If so, this will happen on both ends
(positive and negative branches)? As a consequence I will not have dead
elements?
Many thanks
Ionut

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Siemens Phenom

That is correct.

# Re: NX Solution 601

Builder

Dear JimB,

Thank you again. In order to be perfectly clear I will ask you a final question:

If I choose not the extend the stress-strain curve, then elements will die when the strain will go beyon a certain limit.

What preciselly is this limit:

Von misses strain, or other strain? If other, what?

What strain (whatever would it be), elemental, or nodal?

Averaged or not?

Many thanks

Ionut

# Re: NX Solution 601

Siemens Phenom

From section 3.4.1 of the Advanced Nonlinear Theory and Modeling Guide:

The rupture plastic strain corresponds to the effective plastic strain at the last point input for the stress-strain curve. [...]. When rupture is reached at a given element integration point, the corresponding element is removed from the model.

The criteria is the effective plastic strain at an element integration point.

# Re: NX Solution 601

Builder
Thank you JimB,
Siemens NX, in the Results section, gives the equivalent plastic strain
plastic strain. Could you please enlighten me?
Many thanks
Ionut

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# Re: NX Solution 601

Siemens Phenom

Equivalent plastic strain is computed from tensoral plastic strains using the Von Mises formula

Effective plastic strain computation for SOL 601 is defined in section 3.4.1 of the Advanced Nonlinear Theory and Modeling Guide (link in original post above).

Builder
Thank you

# Re: NX Solution 601

Builder

Dear JimB,

Regarding the death of elements, I am having the following problem:

1. When I used a bilinear stress-strain curve, the analysis finished with a number of supressed (dead)  elements (although the literature states that elements are not supressed in this case!).

2. When, for the same problem, I changed the stress-strain curve into a multilinear one (this is the only change), the analysis did not finish (it stopped at approx. 75% of the imposed displacement).

I do not understant what is the cause of this behaviour.

Many thanks

Ionut