results of a FEA on a plastic part

[relbeltagi]

Hi all

I am new to Cosmos so please bare with me.

I have been trying to analyse the darts on a plastic part which is made from acetal resin material. Please see attachment.

The darts are required to deflect 1mm. I always get a factor of safety (FOS) less than 1 to achieve this.

I have tried changing the shape several times.

My understanding, correct me if I am wrong, is that if the FOS is less than one then the part will break. Is this the case? am I missing anything?

Thanks for your help in advance.

 

[GregLocock]

There is something absurdly wrong with your loading or boundary conditions, or you are not plotting stress.

Cheers

Greg Locock

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

 

[rstupplebeen]

I would plot the stress and see if it is below the yield strength of the material assuming that this is an appropriate failure criteria for your application.

Rob Stupplebeen

 

[relbeltagi]

Thanks for your prompt replies.

All i have is a restrained face and a force on the darts. I do not think its the loading. What do you mean by boundary conditions?

I have plotted the stress and it shows that it will be above the yield strength, which is consistent with the FOS. Please see attachment.

I am still worried because from gut feeling (and playing around with other darts) i would assume it shouldn't break.

Any other suggestions of likely mistakes i might have made?

Thanks again.

 

[GregLocock]

That plot is far more sensible. I suggest that there is something wrong with the previous plot.

What is your yield stress? Is that really 1mm deflection?

Cheers

Greg Locock

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

 

[relbeltagi]

The previous plot is from the same analysis though. How does that make sense?

The deflection plot is attached.

Yield Strength is 6.7e7N/m^2 or 67MPa.

Does this mean the darts will break with this deflection?

 

[GregLocock]

On your FoS plot the tip of the dart is bright red, ie FoS ~.4, yet it sees no load, whereas the stress is below 2.8 e7, so your FoS plot is wrong.

Other than that the displacement and stress plots look OK.

Is this dart shorter than your usual design? Is the material stiffer? Does a simple beam calculation agree with your estimated stress?



Cheers

Greg Locock

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

 

[rstupplebeen]

Yield does not mean fail. You can bend a paperclip and deform it after the load but not break it. The analysis is simply saying that the dart is bent. I hope this helps.

Rob Stupplebeen