FEA safety factors? 2

[paulkel]

Hi, I work for a company that makes heavy duty mechanical machines.

I am wondering what safety factors you guys use to comply with local standards?

I am in Australia and haven’t noticed any standards that mention FEA.

Some help would be great!

Thanks!

 

[johnhors]

haven't noticed any standards that mention FEA[\quote]

- Ask yourself why should there be any?

FEA is just a tool, nothing more nothing less (it is not an "expert system" as some vendors might lead you to believe !). As with any tool the quality of work is only as good as the craftsman using it. (Remember the old saying a bad workman blames his tools). For FEA for as long as I can remember there has always been the saying "garbage in equals garbage out", which is as true today as ever (and probably even more so).

Any standards or codes of practice apply, and are specific to the industry you are working in, and may or may not include FEA in their procedures.

Seeing your other post, I would strongly recommend that you attend a course on FEA and purchase some reading material. Avoid vendors training courses since all they teach you is how to use their software and a lot of the time induce attendees into using bad practices. For a start check out NAFEMS as their courses are vendor neutral.

http://www.Roshaz.com

 

[rb1957]

the basic question is how confident are you in your FEA ?
beyond a reasonable doubt ? ... what's reasonable ??
test data ? similar experience ??
what are the consequences of a structural failure ? dead people ?? $m of costs ??

 

[corus]

Design standards usually refer to hand calculations. For your use, FEA is probably more valuable in assessing stresses for fatigue damage. In a previous job I used a crane code for heavy machinery as that was most apt for the application and it provided impact factors, load factors etc., that could be applied to static loads. The nominal stresses could be worked out by hand but the FEA gave detailed results at welds etc. that you'll need for fatigue.

corus

 

[gwolf2]

Agree with corus, had exactly the same experience recently with crane codes (I am usually aerospace).

ME, Cranes! Times are definitely tough

 

[kellnerp]

FEA is a tool used to calculate stresses and displacements (deformations) of a structure. It is typically used when a hand calculation cannot capture the behavior of the structure. Generally the answers given by FEA or hand calcs are considered to be true to the assumptions made in formulating the problem. Factors of safety are then applied to those answers regardless of the method chosen.

Some codes/standards place requirements on how FEA is to be done to capture the behavior of the structure, but again the FOS is a separate subject.

TOP
CSWP, BSSE

http://www.engtran.com

http://www.niswug.org

"Node news is good news."

 

[btrueblood]

I had an engineering professor who stated that the uncertainties in analysis are generally in the area of boundary conditions (loads and restraints, joints) and in knowledge of the material properties. The analysis, whether by hand or FEA can be made more precise by a plethora of methods. But that is just semantics.

 

[GregLocock]

"safety factor" is the polite term for "factor of ignorance".

You need to decide on your level of ignorance in the following areas

-wrong loads
-wrong material properties
-wrong lifecycle expectations
-wrong analysis
-wrong customer usage profile
-wrong load
-wrong assembly methods
-environmental effects
-system interactions

You may observe a fair bit of redundancy in that list. There may be a good reason for that. And I may have forgotten something. There is no good reason for that.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[KnacKeN]

Is the NAFEMS site down? I cant access.

 

[jenDL]

Factor of safety:
Typically in analyzing structures the service loads would be factored up and the member capacities are factored down. Then the two are compared against each other. It makes no difference how you do the analysis. The aggregate "factor of safety" is built into the two means of factoring and is hard to quantify with a single number.

Standard building and material codes will have your load factors (ASCE7, IBC etc). Material codes will have the capacity reduction factors (AISC, ACI, etc).

 

[EngAddict]

You want AS 1210 - Supplement 1. This is for pressure vessels but can be used for most FEA analysis. Basic limits for bending and membrane stress do not account for peak stresses, however AS 1210 - Sup.1 (taken from PD5500 Annex A & ASME) has factors to increase the allowable stress for peak stress, provided the stress is localised.

These are not safety factors though. The safety factor is in the allowable stress for the material, which for vessels is taken from AS 1210 or supplement 1 depending on your method used. This is generally UTS/3.5 or Yield/1.5 but for structural work you should refer to the relevant code which will probably allow around 90% of yield.

You should read the codes and check first.