## Nodal stresses vs. Element stresses in Structural FEA

## Nodal stresses vs. Element stresses in Structural FEA

(OP)

Hi,

I am running into a structural verification criterion for an aerospace component, and as we know FEA packages offer the ability to show either nodal stresses and element stresses (at the centroid for instance).

I realized that the nodal stresses values are much higher than the element ones. If we use nodal stresses because they are extrapolated to the "free surface" of the part being analyzed, aren't we being too much conservative?

What is the standard procedure in the FEA industry? Which is the correct variable to look at when verifying the structural MoS? Pros and cons of each output? When is used each variable?

I am not talking about averaging, since I understand it is a matter of the mesh discretization and FEM convergence principle, but on the nodal vs. element stress output.

BTW, I am using NX Nastran FEA solver with Simcenter Pre/Post GUI.

Thank you!

I am running into a structural verification criterion for an aerospace component, and as we know FEA packages offer the ability to show either nodal stresses and element stresses (at the centroid for instance).

I realized that the nodal stresses values are much higher than the element ones. If we use nodal stresses because they are extrapolated to the "free surface" of the part being analyzed, aren't we being too much conservative?

What is the standard procedure in the FEA industry? Which is the correct variable to look at when verifying the structural MoS? Pros and cons of each output? When is used each variable?

I am not talking about averaging, since I understand it is a matter of the mesh discretization and FEM convergence principle, but on the nodal vs. element stress output.

BTW, I am using NX Nastran FEA solver with Simcenter Pre/Post GUI.

Thank you!

## RE: Nodal stresses vs. Element stresses in Structural FEA

In a typical structure (an axially loaded rod, a beam in bending) there should be only small differences between nodal and centroidal stresses ... in the tension rod none at all (if no area change), in the beam in bending only small (predictable) changes (due to the change in moment , or M/I).

another day in paradise, or is paradise one day closer ?

## RE: Nodal stresses vs. Element stresses in Structural FEA

Only in special cases can "superconvergent" points (points that give "more accurate than expected" stress values) be found, and those are located at integration points (Gauss integration points) inside the element, not at the nodes. In addition, for frames, deflections can be "exact" at frame corners for suitably simple loading. In general (for conforming element formulations and suitably regular geometry, loading and boundary conditions), accuracy is gained by reducing element size or changing the polynomial degree of the element displacement (and possibly rotation) interpolant.

What relative differences are you witnessing in the solutions?

## RE: Nodal stresses vs. Element stresses in Structural FEA

You need to run representative test cases at varying mesh densities to sort out the appropriate results to use.

## RE: Nodal stresses vs. Element stresses in Structural FEA

## RE: Nodal stresses vs. Element stresses in Structural FEA

how do we do that ? I've never seen stress output at integration points ?

why are nodal stresses so "bad" ? whether we average across elements (from centroid to centroid) or use nodal stresses (averaging across the different element results at each node) ... is it really that different ?

FEMs are not Truth, but are (sometimes) a really good approximation of reality. To argue which approximation is better is IMHO somewhat pedantic.

another day in paradise, or is paradise one day closer ?

## RE: Nodal stresses vs. Element stresses in Structural FEA

## RE: Nodal stresses vs. Element stresses in Structural FEA

still surprised that you think that integration point stresses are so much more reliable than nodal (or centroidal).

another day in paradise, or is paradise one day closer ?

## RE: Nodal stresses vs. Element stresses in Structural FEA

Nodal results represent smooth "flow" of result (which is actual case since material is somewhat homogeneous) and elemental results represents "discontinuous flow" of result.

All result information is still "approximate" since FEA is approximate and no information is reliable unless GIGO is avoided. If the behavior is representing the actual physics, checking the nodal results is most appropriate. Most of the time nodal stresses are sufficient. But yes the amount of discretization error can be pointed out by comparing nodal results to elemental results.

## RE: Nodal stresses vs. Element stresses in Structural FEA

another day in paradise, or is paradise one day closer ?

## RE: Nodal stresses vs. Element stresses in Structural FEA

I guess rolling back to my original question, it is not standard in the industry which stress value to look at for MoS computation (except NRP99 who mentioned nodal stress as the ultime value to take, assuming the FEA model has a proper mesh representation and the results correlate with the real problem).

I was asking as a standard procedure, best-practice, that some guru folks may add to the discussion.

## RE: Nodal stresses vs. Element stresses in Structural FEA

This has some real sense for large scale structures where FEMs can't (couldn't ?) account for things like diagonal tension or effective width (in compression) or for coarse grid models (like a fuselage with a single element frame-to-frame, stringer-to-stringer) when pressure loads are not properly reacted by the skin.

Of course, if you have a 3D solid FEM of a machined part I would use FEM stresses (but then this wouldn't be "best practice" as above).

another day in paradise, or is paradise one day closer ?

## RE: Nodal stresses vs. Element stresses in Structural FEA

## RE: Nodal stresses vs. Element stresses in Structural FEA

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