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Where is my neutral axis? Position differs in first and third principle.

Where is my neutral axis? Position differs in first and third principle.

Where is my neutral axis? Position differs in first and third principle.

(OP)
Hi,

We have like a banana shaped plastic part that is being pulled at its ends. The cross section is a T-shape.
I have been trying to find out what area is under tensile and what area under compression.

You would expect this to show up in any first- or third-principle plot, but the problem is that this differs A LOT.
When looking at first principle (tensile) it looks as if most of the part is under tension, whereas when I look at third principle most of the part seems to be under compression!
Of course this can't be.
Because our plastic has different strengths in compression and tensile, we never use Von Mises (because Von Mises assumes equal strength for tensile and compression).

So I might look at von Mises to know the exact position of the neutral axis (if this would be correct), but I need to know the exact magnitude of the tensile and compressive stress too, and this is now totally unclear (different height and position in first and third principle) .

In another test case where I just did some quick bending FEA, you'd expect the compressive and tensile area to be about symmetrical, but no compression shows up in the first principle plot, even though I chose a symmetrical tension scale.

Thanks in advance for the help!

RE: Where is my neutral axis? Position differs in first and third principle.

Hi.
Not sure if I understands your problem.
It would be easier if you uploaded a picture.

But anyway, I believe that your problem is due to scale settings when you post your results.
See the attached pictures.
If you ignore the stress concentrations in the corners due to my bad BC:s, does this make sense for you?


RE: Where is my neutral axis? Position differs in first and third principle.

Hello there. I believe if you pull on each end of a banana section along its curve, that would be like hoop stress in a section of a pressure vessel and hence would have no bending in it or compression unless I am missing something? It should be pure tension I think. Sort of like pulling on a rope around a cylinder.
Regards

Neil

RE: Where is my neutral axis? Position differs in first and third principle.

Hi Avantibngrant,

A more severe banana would be a C-clamp, and when over taxed they bend.

RE: Where is my neutral axis? Position differs in first and third principle.

Tmoose, I would agree. I took the post to mean "pulling on each end" as a tangential pull. Anything different would give a moment on the banana and resulting bending with a neutral axis like a c clamp you mentioned.
Regards
Neil

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