Elasto-plastic

[4782]

Hi all,

I am doing elasto-plastic analysis using bi-linear material curve. I am new to this type of analysis and for the plasticity application. I have problem in interpreting with my results.

In elastic analysis I got the von Mises stress of about 257MPa which is more than the yield limit of the material which is 240 MPa and UTS is about 400MPa for the material. To study further I did elasto-plastic analysis using BISO model in Ansys (Bilinear isotropic hardening)

From elasto-plastic analysis using bilinear material curve, the von Mises stress got reduced to 240.543 MPa and the von Mises plastic strain was found to be 0.000353 and the von Mises elastic strain found to be 0.001423.

What this plastic strain indicates and how we can access the component using the plastic strain?

And the stress value in the elasto-plastic analysis was found to be very close to the yield limit of the material.

Can anyone help how to interpret the results.

Thanks in Advance.

Kan

 

[gurmeet2003]

4782,

It appears that your part has barely started to yield. When yielding occurs the stress is redistributed and reduced. With a non linear analysis, the stresses will be lower, because the modulus value past yielding is much lower. The linear analysis does not account for change in modulus which occurs at yield point. Therefore it gives higher stresses.

Your results appear to make sense.

Gurmeet

 

[4782]

Thank you Gurmeet,

Your explanation helped me to understand much better.

As you said the material barely started to yield, is that indicates that component will fail. Or i need to study in detail whether this plastic region will grow futher, but this plastic strain is for my extreme load case.

I am bit confused. Is that i need to go far still more detailed study?

Thanks.

Kan

 

[cbrn]

Hi,
well, no, local yielding is not an indication of failure "per se". Of course, now you've got to ensure that the fatigue life (loading / unloading) is not affected by that:
you have to make sure that the yielded zone won't progress indefinitely with the cycles (this is called ratchetting); several Norms (e.g. EN13445) explain very well what you have to do to check for ratchetting risk or, on the other hand, for the existence of "shakedown" (the phenomenon by which a locally plasticized component will execute all the load/unload cycles within the elastic range, after a certain number of "settle-down" cycles).
The reason why you want to design within yield limit is to avoid such headaches !!!
OK, this is not the only reason of course.
Anyway, if you don't have any cyclic load, and you verify that all the functionality requirements of your part are fulfilled (caution: sometimes deformations are the real limit, not stresses...), you may accept a LOCALIZED yielding, provided that it is ---TRUELY--- localized; but that's a complicated matter and, sincerely, I don't know any customer of ours (here where I work) who would accept a beyond-yield operation, even if we can demonstrate that it is safe...

Regards

 

[Stringmaker]

I think you need to consult your design standards here and get an idea of what condition you're analyzing for/against and know what the underlying criteria is for that. Hopefully if you're yielding material this is only under a limit or ultimate loading condition.

Also, be certain that you're not seeing some sort of stress singularity.

 

[4782]

Thank you all for your more valuable information.

Load for which the plastic strain has been developed is not a cyclic loading. This high stress is mainly due to the shrink fit pressure which is simulated by means of contact elements.

The stress mainly get concentrated at the holes and at the outer fiber of the larger cylinder where we have a small protusion interacts with the cylinder with fillets.

Apart from this we have only inertia load which is cyclic.
So my fatigue assessment will not include the shrink fit pressure and also the impact of ineria load in terms of stresses was found to be very less.

In this case will the plastic region grows further asuuming applied load as the ultimate.

As stingmaker said we will also check our standard.Also i would like to thank cbrn for giving me the valuable information.

Thanks.