Part passing FEA but failing lab test 2

[windsoreng]

I am designing brake pedal.

Its made of plastic, I am using CATIA/ANSYS for FEA analysis.

Part is within stress limits and sress counters are under yield stress, but during lab test, it fails.

I think that I am not modeling plastic correctly in FEA, I am using non linear model.

Any Ideas?

 

[IRstuff]

Your part is failing despite being subjected to stresses that are less than "stress limits." Doesn't that suggest that your stress limits are incorrect? Shouldn't you then get an objective test of the material? Are you doing something in the fabrication process that weakens the material?

TTFN

FAQ731-376

 

[mgage]

Well a few things that come to mind are:

1) your model does not capture the test correctly
2) there is something wrong with your material properties

there are many more possibilities but without more specific information that's where I would start looking.

Marcus

 

[windsoreng]

We are using injection molding to manufacture parts.

I will ask for ASTM dog-bone test done on material, I am looking for its stress-strain curve. That might tell me something.

I can refine my mesh further.

should I use linear model or non linear model?

Thanks

 

[IRstuff]

I think the question was more pointed at whether your FEA model accurately reflects the test conditions, as opposed to anything related to the FEA process itself. FEA programs are often faced with physical conditions for which there is no direct analog for in the program. Or, the stimulus used for testing may deviate from what you put into your analysis.

TTFN

FAQ731-376

 

[johnhors]

Please post an image of your model showing the boundary conditions.

http://www.Roshaz.com

 

[mrgoldthorpe]

Is the test just one proof loading, or is is repeated loading?

How does the part fail: does it deform excessively, or does it break? Where does it fail - near a change in section for example?

 

[DanStro]

In your model how close is the stress getting to the materials yield stress?

 

[Twoballcane]

Did you do any hand calcs to see if it correlates to the test or FEA? This may give you some more insight of what may be going on physically.

Tobalcane
"If you avoid failure, you also avoid success."

 

[Elabbasi]

Good idea to start with the ASTM material tests. Since plastics have high strain rate dependence you should ideally do multiple strain rates and loading unloading as well. Most manufacturers do not provide sufficient material data for plastics. Using only initial Young's modulus and ultimate stress and strain is not enough. Do not use a linear material model because plastics don't behave linearly.

There is of course the possibility of something wrong in the FEA model. That can be wrong settings (unsuitable element selection, coarse mesh), or the FEA model not matching the physics of your problem (wrong boundary conditions, wrong loads, material model, material properties, etc). In that case you're getting the right solution to the wrong problem!

Nagi Elabbasi
Veryst Engineering

 

[321GO]

Model stiffness and loading are influenced by large deformtions, so i agree with the non-linear solution.

Did you measrure the actual force on the pedal during the test? Without this feedback, how can you be sure your loading is correct?

Google "fea plastics". There is a link called "Why is FEA of plastic parts so often wrong?" that seems informative.

The material model is highly non-linear, but personaly i think this has more effect on the displacements than on the stress.



"If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack."
Winston Churchill

 

[ESPcomposites]

If its a FEM problem: More times than not, the problem will be with poor choice of boundary conditions and not mesh density.

What about producibility issues? I remember having a great variance in strength between injection molded parts. Have you tested enough samples to determine what type of statistical distribution you might have? If there is a lot of scatter, you won't be able to capture that in your FEM.

Brian

http://www.espcomposites.com

 

[rb1957]

the lab test is right, and there's something wrong with the FEA ! (bleeding obvious i know) ... FEA constraints or material data would be likely suspects, maybe detailed loading. maybe strain gauges will help clarify the stresses ... this is a plastic part, why not a photo-elastic model ?? else reinforce where the part breaks in th elab !

as for linear vs non-linear ... how much is the part deflecting (geometric non-linearity) and how close to yield (material non-linearity) ?

 

[GregLocock]

I'm guessing the lab test is a mixture of high cycle low force, and low cycle high force tests. In which is it failing? All of the big 3 (1.7?) use plastic brake pedals already, your customer should be able to suggest whether the material properties you have used are reasonable.



Cheers

Greg Locock


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