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Thread: Welded Uprights

  1. #1
    Hi,

    For those of you who construct your uprights by welding sheet metal or tubes, how do you FEA your uprights?
    Horace Lai

  2. #2
    Hi,

    For those of you who construct your uprights by welding sheet metal or tubes, how do you FEA your uprights?
    Horace Lai

  3. #3
    Same way you'd FEA anything else? Shell mesh may work well depending on the construction.

    The problem is the material properties. They will be hiiighly dependent on your fitup, welder quality, filler rod, and post-weld procedure.

    This is why I stayed away from welded uprights and stuck with billet. Plus, I could make billet ones much faster.

    Best way with a weldment like that is to do a one-off prototype and proof test it to load. Thats just my opinion though.

  4. #4
    Weldments are very easy to analyze. just connect the shell elements together. The problem comes when you have to correlate the FEA with the live part. The software's assumptions (isotropic etc.) can make it very challenging.
    "Gute Fahrer haben die Fliegenreste auf den Seitenscheiben."
    --Walter Röhrl

  5. #5
    Model it as a single part instead of a sheet metal weldment assembly. Then run it in cosmos. Make sure you have a decent FOS, it will be lighter, and cheaper than a billet one, and will not break. Yeah the welds change the material properties, but you can take care of that with a little heat treating.
    Mike D
    UTSA 04-

  6. #6
    One nice part about welding them is that you can close out those sections and reap your good torsional stiffness. We have welded them (4130 steel) in an aluminum jig and then had a material science student heat treat them for the past two years with no problems.

    I would argue that the difficult part of the FE analysis is knowing what forces to apply and applying them correctly. We start with some lateral, bump, braking and accel assumptions, applied at the contact patch or wheel center as appropriate. We treat all supsension members as pin-ended members (force only along member, no bending moment) so a simple truss analysis can be constructed. We consider all possible combinations of these forces and pick some "worst case" loadings for each part. The proper application of these forces to the upright takes some thought. LU Bolton... care to weigh in???

  7. #7
    Since we use CosmosWorks to do all our fea (learning ansys too ), i'll talk in terms of it.

    For an upright you'll have two zones to pay attention, the upright body that's made with sheet metal, the hub bearing supports and balljoint/rodend fixtures. Sheet metal will be modelled with shell elements and the bearing supports for sure will need to be modelled with solid elements (since those always have section changes and such), so a mixed mesh will do the job if you define properly all the bonded contacts between the sheet metal edges and the solid parts. The main challenge is to define properly you load cases and be SURE to don't overconstrain your model (ie if you do a free body diagram of your wishbones and then fix the upright from the spindle you'll overconstrain it). So the approach that Composites Guy said, using poin joints and the loads applied to the bearing races (and brake caliper jigs) is the way to go.

    See the CosmosWorks tutorial on mixed mesh to learn how to do the whole thing. You'll need to do an assembly with all the solids as one part and all the shells as another, the fast way to do that is to pick the original model and copy it twice on a folder, then rename one of the two copies as name_of_the_model_here shells.sldprt Then you can open it and cut extrude all the solid features to left the shells alone. Do the same with the solids on the another copy.

    Then you can assemble the shells and the solids, and run your analysis.

    Post weld normalization help with stress relieving reduce brittleness on the welds
    -----------------------------------
    Drivetrain Leader 07-08
    Technical Director 08-09
    Team FSAE USB - Caracas - Venezuela.



  8. #8

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    I like your approach AFX, very simple and effective.

    I'm gonna try that tonight!
    Kind Regards,

    Chris Lane
    Design Team Leader - ECR 2008
    http://engineering.ecu.edu.au/ecr
    ------------------------------------------------------
    "Race Tape is like 'The Force'. It has a dark side, a light side, and it holds the Unive

  9. #9
    Material properties are still going to throw you for a loop I think, even if you post-weld normalize.

    I could be wrong, but from what I recall stuff like 70S-2 filler is designed to have 70ksi strength in the as-welded condition. Likewise 80S-2 is in the 80ksi range.

    The idea is for something like 4130 you have a low carbon filler rod which 'dilutes' the weld pool of 4130 so when it quickly cools when you remove the heat, you have a more ductile weld zone. HAZ of the base metal is still brittle, obviously.

    Depending on how much filler you use, and how good your welder is, you can have varying amounts of dilution in the weld area. And if you go ahead and normalize the whole weldment I'd think that while you make the base metal back into a more ductile microstructure, your low carbon weld area likewise is going to lose strength. How much? Good question.

    That's the way its always been in my mind. I could be wrong. But that's why I stayed away from weldments and FEA on weldments. Plus.. the shape of the bead your welder puts down is going to change the geometry and stress concentration in the area.

    Build it to a high stress-based FOS is only thing I can say. Will help keep the thing stiff at least, which is just as critical.

  10. #10
    ...But if you're going to heat treat a 4130 weldment, why would you use carbon steel filler? Yes, more ductile joint area, but weaker. With 4130 filler, yes more brittle, but the same strength, and closer to the same microstructure. Nice fillets at the joint should help alleviate a lot of the concern too, since the joint section will be greater thickness.

    That was how I understood it all anyway.

    Anything we made from 4130 and annealed was welded with ER70-S2 specifically to help keep the joints from being too brittle, even after the annealing process.

    Best,
    Drew
    _______________________________________

    Northwestern Formula Racing Alum
    Head Engineer, Frame/Suspension 2006-2009

    My '73 Saab 99 Road Race Build

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