FEA FOS question...again

I have a question about determining the minimum factor of safety of a part that probably doesn't have a simple answer I know.

When doing FEA reports on parts it's very important to have a factor of safety. However, due stress concentrations and various other factors, how do you fea guru's determine what this fos is? I can't believe you would follow exactly what the software gives you and list it to be that...? It seems to me in most cases with stress concentrations every part would be overdesigned. Is there a rule of thumb or other factors that I should look for when determining what the "most accurate" fos to be for a part.

Does anyone know what I'm talking about? I hope so! Thanks for any light shed on this. Don

Reply to
dvanzile
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Please answer these questions:

  1. Loads - Static or repeated?

  1. Material - Ductile (most steels, aluminums); Brittle (grey iron, glass); Visoelastic (plastics)?

  2. Geometry - Constant section or stress raisers?\

  1. Temperature - Upper and lower limits, average

  2. Environmental factors - salt water, plating, corrosives, exposure to weather, sunlight

  1. Geometry - range of thickness due to manufacturing or wear and tear in critical areas?

  2. Standards - is this item covered by existing standards, for example Mil-specs?
Reply to
P.

Factor of Safety == strength of material (typically yield; sometimes ultimate) divided by the maximum anticipated stress at any point on the part. And yes, you do just list whatever the FOS reported by the software is in your report.

If you have a stress concentration, then that relatively high stress will be the one used to calculate the FOS. A large part of the reason for doing FEA analysis is that it allows you to identify and eliminate concentrations

In the optimum case, you add/remove material or change geometry so that the stress is the same everywhere in the part. This optimum condition, however, is largely a fantasy. You almost always have some variation in stresses; your job as an engineer is to minimize it

Reply to
Michael

That is a very simplified approach that may overbuild or underbuild the part. In some cases you will not use the maximum stress reported by the FEA program. In other cases you will not use the stress at all in determining the factor of safety. The questions have to be answered to know the nature of the load and the material.

Reply to
P.

Repeated Loading

Steels and Aluminums

Mostly constant

Average

Factory Atmosphere...

can vary depending on the part.

example

Reply to
dvanzile

For stress concentrations, linearize stress in the thickness... if you are not so far from a shell model (L/h>5, R/h>5) see ASME, CODAP (FR)... use average stress in elements instead of node stresses meshing has to be larger than thickness for shells for composite, around a hole, an edge, don't mesh under 4 mm (see average stress criteria) A ratio 1 to 1000 between element and max length is a thumb rule See KI, KII, GI, GII .... in fact we don't know how to well analyse the stress concentrations! a standard would have to be done......

a écrit dans le message de news: snipped-for-privacy@f14g2000cwb.googlegroups.com...

Reply to
vnhjvnj

agreed-- but it's a simplified approach that will (generally) keep you out of trouble if you're not fully grounded in the subject of FEA--which the OP clearly is not

Reply to
Michael

Well, I went off and had a look at his company's website to see what they were making. Modular tooling. I suspect that there are two types of failure mode that he has to watch out for. First will be loading of a nature that it strains the structure to the breaking point. It must withstand those loads without failing. For that purpose average stress on a section, not peak stress in a stress raiser will determine whether the structure fails. In this case Red is not necessarily bad unless it covers a large area.

In the second case the tooling may be subjected to repetive loads. Given the nature of the product the direction and magnitudes of the loads may not be easy to predict. In this case he may need to make an influence diagram to see which combination of loads causes the worst case stresses in his part. There may also be cyclic loads superimposed on high static loads in clamping hardware. For cyclic loads the stress concentrations will be the problem. But the allowable stress will not be yield, it will depend on the magnitude of the stress and the number of cycles.

Perhaps more important than the FEA at this po> agreed-- but it's a simplified approach that will (generally) keep you out

Reply to
P.

Thanks for your input! This group is always helpful.

Reply to
dvanzile

a écrit dans le message de news: snipped-for-privacy@f14g2000cwb.googlegroups.com...

One of my teachers liked to speak about "factor of ignorance".

It depends on your knowledge of the real conditions. In aerospace they use a factor close to one ...

Reply to
Jean Marc BRUN

Hey P.

I really appreciate your help on this. You wouldn't by chance have any really good examples of parts/assemblies setup with realistic restraints, loads, surface contacts etc... that you can correlate to real world testing? I would like to contact you through e-mail but I can't find your full e-mail since they block it now. I would really like to show you a couple simple pictures of what I'm trying to do. I want to be sure my setup is accurate so I don't have a garbarge-in garbage-out scenario. The Engineer who mainly works on FEA here believes that it's garbage out no matter what you put in (yes, he's gone through training too). I would really like to understand how others setup simple contraints seen all the time in machine design if at all possible. I really would like to try and change his attitude towards it. Thanks for all the help.

Reply to
dvanzile

This question has been answered elsewhere on the forum. But I will recap a bit.

Building Better Products with FEA by Vince Adams and Askenazi. Roark and Young.

The first is specific to FEA and the second is specific to determining whether a structure will fail or survive.

You can respond to me directly using Google and clicking on options.

Reply to
P.

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