stress vs strain

[Rikuson]

Hello everyone

I'm doing a sensitivity study on 2 models. One of the parts has been defined as plastic material. After running thermal cycling tests on these 2 models, I found that plastic strain in model 1 is greater than that in model 2, however the von mises stress is just opposite (higher stress in model 2).

I'm wondering if this is a typical behavior in plastic material or something wrong with my model?

Thanks very much for any points!

Riku

 

[curlyq]

What does the material model look like for your plastic material (#1) and how does it compare to material #2? Is it possible, that #1 has yielded, and has some kind of stress-softening? I am working with biological tissue models that resemble a bell curve. After I pass the top of the bell curve my "plastic" strains go up, while the stresses go down. BTW, how do you plot plastic strains?

 

[Rikuson]

Hi Curlyq

Thanks for your ideas!

These two models share exactly same material property. I'm modeling 2 IC(integrated circuit) packages under temperature cycling test, the difference between them is: the pitch distance value of their solder joints. I expected that the model with smaller pitch will have higher stress and strain, but the stress is not the case (like I described in last post)

Yes, you are right, some material called softening material may have this kind of characteristic.

Yeah, the plastic strain in model 1 is 10 times higher than that in modle 2, so the material probably is yielding (stress decreasing)...

To plot the plastic strain: just go to toolbox area find 'create XY data', choose ODB field output, pick PE and select your element, then plot

Thanks a lot!

Riku

 

[mrgoldthorpe]

You say you are modelling thermal cycling.

Could it be that, at the point in your analysis when you are examining stresses, elastic unloading is occurring in Model 1 in the region you are studying (solder joint?).

In Model 2, that particular region might be experiencing continuing plastic deformation at the same point in the analysis, or might be undergoing elastic unloading too; but that elastic unloading might be smaller than in Model 1, thus giving higher stresses.

Also, don't forget von Mises stress is an positive scalar, telling you little about the sign and magnitude of particular components of stress.

I suggest you look at the time history at particular positions in your region of interest (solder joint?) of different components of stress and strain. If you plot these results for your two models side-by-side, it may give you more insight into their relative behaviour/performance.

 

[Rikuson]

Thank you very much for your reply, mrgoldthorpe!

Yes, I agree with you, thats what I'm thinking, I just dont know how to technically explain it.
So elastic unloading really means softening, right?

I'll look at all the components of stress and strain.

Thanks again.

Riku

 

[mrgoldthorpe]

Elasic unloading and softening are different. Elastic unloading is simply elastic deformation caused by, say, a reduction in applied load. Here continued yielding is suspended and the deformation is elastic - no plastic strain is accumulated during this phase of loading.

Softening is a reduction of yield stress with increasing equivalent plastic strain. Normally this will only occur if you supply material *PLASTIC data that incorporates such a softening response.