Interpreting FEM results

[Justanengineer703]

Hello all, I have been reading up on the difference between max principal stress and von mises stress. Those principles are easy to understand but I can't seems to make sense of it for a forged material.

I am using DEFORM to simulate the cold forging of a workpiece and the result shows that both the max principal stress and vM stresses are way above the material's UTS and yield strength.

Am I right to say that for a forged material, its stresses should be above the yield strength since it have to plastically deform but below its UTS?

*The said workpiece is already under production. It hasn't crack and looks fine.

 

[inline6]

you need to get true stress vs true strain data instead of using engineering stress vs engineering strain

 

[DrBwts]

How are you deforming your model (I have no experience of DEFORM) if the material is under compression then I would expect some work hardening to occur are you taking that into account or it it just a linear elastic model?

 

[rb1957]

forging causes permanent deformation so, sorry, of course cold forging stresses exceed allowables,'cause the material deforms and flows.


another day in paradise, or is paradise one day closer ?

 

[stressebookllc]

The question is a bit confusing to me. But if it is a linear static analysis, the stress strain curve is simply a straight line along the slope of the linear elastic curve and the stress keeps going up along that line with elastic 'strain'.

So your stresses will be way above UTS as you are not using a fully defined stress strain curve. Same here, not sure what DEFORM solution is, is it able to account for the nonlinear material stress strain curve? If yes, there is no way you can see any stresses (other than numerical noise) well above the defined stress strain curve values because you are dictating to the solver that at this strain level this is the stress level.

http://www.stressebook.com

Stressing Stresslessly!

 

[Justanengineer703]

Thank you everyone for your reply. There is a complete stress-strain curve of the materials for us. It is linear-elastic for the first part, followed by plastic region. What I am trying to ask is, is it right for me to say that my forged workpiece's von mises stress OR max principal stress should be below UTS? And any stresses above UTS means material crack or tearing.

I have also come across Cockcroft-Latham damage criterion (also used by DEFORM). Is there a general value which my material shouldn't excees? I have seen a paper somewhere stating that a damage value exceeding 1 means crack/tearing will occur, is it true? Some the workpiece cracked when DEFORM shows damage value of 0.8 while other workpieces came out fine even with DEFORM's damage value indicating 1.5.

I have been reading up on the above topics but many textbooks and research papers have been said that critical damage or stress values are obtained emperically and trial and error. I was wondering if there are any theorical guidelines that I can follow.

 

[rb1957]

I'm right in understanding that "DEFORM" means starting with a metal blank, and producing the desired shape out of it ? ie, modelling the forging process ? so that you're looking at ...
1) the stresses that occur during the forging, or
2) the residual stresses, after forging ?

during forging, I'd expect that the stresses would be above yield. It makes sense that there's some criteria that'd measure whether the forging process will produce 1 part or 2 (or many). But I've no idea how "they'd" model the metal flow, etc ...

another day in paradise, or is paradise one day closer ?

 

[stressebookllc]

There is something called a failure index, but it is used for composites in the Tsai-Hill failure criterion for example.

There, if the FI > 1.0, failure is predicted.

So this one sounds similar for forging pieces. And my guess is that may be there are internal algorithms inside 'DEFORM' that indicate the failure value depending on the shape, material and final product form etc.

Your best bet is to read up as much as you can on what DEFORM is doing in the background or contact their tech support for references. It seems not many people here are familiar with DEFORM.

http://www.stressebook.com

Stressing Stresslessly!