Node vs. integration point result?

General Questions
1

Greetings, everyone!

I started changing positions of recently as a mechanical engineer to a FEA analyst and focused on reading more about the nitty-gritty of PrePoMax. I stumbled across the topic of nodes and integration points, which I admit, I still don’t fully understand.
Appropriately, soon afterwards I noticed that, when I use the Show max value and location option and switch the result output from Von Mises to Max. abs. principal stress the maximum stress is once displayed at a node position and then onto an … integration point??

Max VM stress
Max VM stress1585×892 115 KB

Max abs pln stress
Max abs pln stress1625×889 110 KB

I figured that I can not use the query tool to catch the integration points unfortunately (if that is the case).
My question is should I rely on the higher result of 52MPa or sticking to the 47MPa is a more realistic option? Or the VM result interpretation of 42MPa makes the most sense?
In case you wonder:

  1. I am using a non-linear plastic material and
  2. the point is next to an edge with no rounding.

I apologize, if this was already discussed before or if my question is obviously stupid. :slight_smile:

Have a good day!

2

CalculiX always provides averaged nodal stresses in .frd files. You can only get the integration point values from .dat files (text files filled with optionally requested history output). So what you see is just another node (its number is also provided). So the max value just switched from node 142727 to node 300779 for this different stress measure. Of course that new node can be a midside node or belong to a different element.

Check this topic too if you want to know how to obtain more accurate stress values (although in practice it’s rarely done with CalculiX results): Is Zienkiewicz-Zhu Stress better than "regular" Stress?

In fact, you can get the coordinates of all integration points of a selected element if you add this card via Keyword Editor:

*EL PRINT, ELSET=elset_name
COORD

You just need to create Element set whose name is referenced as elset_name. Then check the .dat file in the current work directory after the analysis. You can also add S to COORD to also get the integration point stresses (they will be shown in a table in PrePoMax too).

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3

In short words, integration (or Gauss) points are locations inside an element (whose relative positions are specified for each element type in the literature) where stresses and strains are calculated because they can be most accurately evaluated there. Then these results have to be extrapolated to nodes and averaged for nodes sharing multiple elements to provide a contour plot.

Note that there are reduced integration elements (with R) that have less integration points for efficiency. Of course, this has a negative impact on the accuracy and some other consequences too.

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