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09-18-2017 07:09 AM

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!

Could you please help me?

Many thanks

Ionut

Solved! Go to Solution.

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15 REPLIES

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09-18-2017 01:56 PM

XTCURVE is only applicable for the multilinear-plastic material model

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09-21-2017 10:06 AM

Thank you for your answer. Sorry for replying so late.

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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09-21-2017 10:40 AM

That is correct.

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09-26-2017 09:52 AM

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

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09-26-2017 10:52 AM

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.

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09-27-2017 11:56 AM

Siemens NX, in the Results section, gives the equivalent plastic strain

(among other strains), while you mentioned, in your reply, the effective

plastic strain. Could you please enlighten me?

Many thanks

Ionut

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09-27-2017 01:34 PM

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).

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09-27-2017 01:37 PM

Thank you

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09-28-2017 12:47 PM

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

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