Autodesk Inventor FEA Von Mises Stress Question/Explination

[Anonymous]

I am curious why a 6061 aluminum frame has a different Von Mises Stress when the frame is assigned 6061 Al and the welds are assigned 6061 Al (Welded) compared to the frame and welds both being assigned 6061 Al. 

When I assign the frame material to mild steel and welds to welded mild steel the Von Mises Stress remains the same as the frame and welds both assigned to mild steel. 

I was under the impression that stress is dependent on load applied and geometry, and the material type makes no difference. Could someone please help me understand why this is happening?

Thanks.

[swalton]

I just ran a quick FEA of two cantilevered bars.  The bars were the same size, with the same loads and fixed constraints. The only difference was the material, one was steel, the other aluminum.  As expected the stresses were the same, but the deflections were different.

Without seeing your model, I suspect the issue is due to the Modulus of Elasticity difference between aluminum and steel.

If you are modeling something more complex than a single beam, the stiffness of the components may change the load path and stress results.

Does the location of the peak stress change with the different materials?  How about the deformed shape of your model?

Are you able to post your model, or a non-proprietary one that displays the same issue?

[Anonymous]

Here is a simple welded cantilever to demonstrate. In the first picture, the base material is "6061 Al" and the welds are "6061 Al"

In the second picture, the base material is "6061 Al" and the welds are "Welded 6061 Al". 

I can see that the stress is distributed differently, but I am not sure what specifically causes this/ what material property causes this?

If you run the model with the same material for the welds and base material, the Von Mises Stress will be the same and the specific material makes no difference. The change comes when the welds are material with different properties than the base material.

kelly.young has embedded your images for clarity.

[arron.craig]

I don't know much about aluminium alloys so am unsure of the reason for the different recipies in the material properties but the reason you are getting these results is obviously because they are drastically different within inventor. Editing the yield stress value, tensile stress value and shear modulus will give the same values.