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Why Larger Nodal than Element Values of Stress?

Question asked by John Willett on Dec 3, 2015
Latest reply on Dec 9, 2015 by Jim Riddell

In some studies I'm getting maximum von Mises stress values that are much larger in the nodal presentation than in the element presentation  For example, the following images are from a linear, static analysis of a rod in a cylindrical hole, modeled in half symmetry.  The stress due to a (hidden) rod is being concentrated on an outside (non-reentrant) edge of the part with the hole, looking from below:

Nodal Stress:

Stud Test Bushing End 5_16 Even More Res Block Nodes.jpg

Element Stress:

Stud Test Bushing End 5_16 Even More Res Block Elements.jpg

I thought I understood from SW Help and elsewhere that, although element stresses are computed from an average of the 4 Gauss within the (second-order, tetrahedral) element, they are not considered "smoothed."  Nodal stresses, on the other hand, are supposed to be smoothed by averaging the element values in all contiguous elements.  But an average cannot be larger than any of the terms that go into it!

 

I started looking at element values because I read that it was a good way to assess the adequacy of the mesh:  Too much variability among contiguous elements (as illustrated above) means that the mesh is too coarse.  But the comparison above suggests that I'm misunderstanding how nodal stresses are computed and also maybe underestimating the maximum stresses by using element values.  Any clarification would be appreciated. -- John Willett

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