## Need advice on FEA settings. Result is not satisfied of required safety factor.

## Need advice on FEA settings. Result is not satisfied of required safety factor.

(OP)

Hi everyone .

I am making stress analysis of an electric motor.

The problem is i have gotten the higest stress near the weld connection which is the reason why my construction is not satisfied the required safety factor.

The highest stress is situated at one fourth of the part depth (third picture) so i suppose if i calculate this cut by hand i will get much lower result.

I have no idea how i can try to reduce the stress. Currently i am changing the grid and if it wont help and i dont know what to do else.

Can someone advice me on what else i can do to reduce the stress at this spot?

Thanks in advance.

I am making stress analysis of an electric motor.

The problem is i have gotten the higest stress near the weld connection which is the reason why my construction is not satisfied the required safety factor.

The highest stress is situated at one fourth of the part depth (third picture) so i suppose if i calculate this cut by hand i will get much lower result.

I have no idea how i can try to reduce the stress. Currently i am changing the grid and if it wont help and i dont know what to do else.

Can someone advice me on what else i can do to reduce the stress at this spot?

Thanks in advance.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

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

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

Can you show the whole model with boundary condition and load symbols ?

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

Edit: I would also add that is a fairly good sized chunk of red you have there, could be something to this more than inaccurate weld modeling. Have you considered your part is yielding and you might need a bigger weld / better geometry? I would be uncomfortable using non-linear analysis to prove a weld good unless you are really an expert. Analyzing welds in FEA is an advanced topic. Another consideration, will you have repeated loads on this part? Fatigue could be of concern as well depending on use.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

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

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

You don't change the mesh until you get the result you want, you change the mesh until it's accurate. I'm confused why others are giving advice on non-linear FEA with this level of misunderstanding of the finite element method in the original question.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

There's a lot going on in his model … clearly area contact, modelling a weld (the weld material) is something I wouldn't do (I'd do some hand calc).

Presumably the material properties of the weld material are right ? and the local heat affected zone ?

There so many things we can't really see. Constraints seem to be a couple of arrows … which makes me think of fixed constraints, which is scary.

But the usual answer to localised stress peaks is NL (ok, material nonlinearity). Sure, you could do some "neuber" analysis with linear stresses (can you apply neuber to a weld ?).

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

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

To decide if it's acceptable, you need to elaborate on what you are trying to prevent. Are you designing for a single overload failure (collapse) or fatigue? If overload failure and you have a ductile material, plastic analysis is definitely the way to go. Do you have a safety factor against collapse you need to meet? Is there a deflection limit? These things are straightforward with plastic analysis. If you are designing for fatigue with welds, I'd stick with elastic and look into a structural stress method.

I'll also second the comment above about mesh insensitivity. The results must be mesh insensitive or else they are meaningless. Personally, I find rules of thumb like <10% change to be unreliable unless the analyst has significant experience. For a novice, I'd shoot for a much greater degree of insensitivity.

-mskds545

## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.

How many points will it take to fit a

continuoussinusoid (one frequency/one period)? Two? Four? Ten? Look up Nyquist criterion if you do not know the answer. Intuitively, you know you will need "sufficient" points for thediscretesinusoid. In other words, the more points you sample the sinusoid, the closer the discrete sinusoid will look to the continuous one. If you have too few points, then the discrete version will look strange. This phenomenon known as aliasing is observed in real life when spinning objects like ceiling fans, tires, etc. appear to moving slow or even in the opposite direction. It happens because the sampling of our eye is too slow in time.When you solve a discrete system of equations numerically, you are trying to compute various spatial fields of interest (displacement, strain, stress, etc.) over time. These fields may be represented as a combination of sinusoids so, as in the example above, you want to be sure you have sufficient number of sampling points (nodes) so that the quantity of interest is effectively independent of the mesh i.e., it does not vary "by much". You want to do the same thing over time as well, if time is taken under consideration. Note that just because you got a converged solution for one quantity of interest in one model, you cannot say the same about other fields from the same model or a "slightly tweaked" model.

Unfortunately, the more you dig into it, the messier it becomes but the discussion above is sufficient for a majority of practical purposes especially when the risk profile (due to modeling prediction) is low. If you are mathematically inclined and interested to know more, papers by Roache, Oberkampf and Roy might be good to look at.

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## RE: Need advice on FEA settings. Result is not satisfied of required safety factor.