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?
Solved! Go to Solution.
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?
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.
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).
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.