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Difference in strength between extrusions and sheet metal

  1. Aug 1, 2015 #1
    Hello,

    How do I take in to account the difference in stiffness when fabricating a part (for example, an L-bracket) from sheet metal or using an extrusion?

    In particular, where I work, it is "common knowledge" that extrusions are stiffer. How do i factor this in the stress and strain calculations?

    I thought of using the theory of curved beams for sheet metal parts, and using two perpendicularly connected straight beams for the extrusion.

    Does anyone have a standard procedure for this?

    Thank you.
     
  2. jcsd
  3. Aug 1, 2015 #2

    OldEngr63

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    Is it "common knowledge" that extrusions are stiffer when all of the dimensions of each are exactly the same?

    I would imagine that in most cases, the extrusion has a bit of a filet at the vertex of the bend that is not found on the bent sheet metal. This could make a lot of difference for some loading types.

    If the two cross sections are exactly the same in every detail, then I cannot see any reason for a difference in stiffness.
     
  4. Aug 1, 2015 #3
    Yes, it is common knowledge among some of the more experienced engineers, although there is no concrete theory to back it up. I am just fact checking.

    I understand that identical cross sections should have the same stiffness, but allegedly, the fabrication process itself (bending or extrusion) makes some sort of difference.

    So far, I have not managed to find a study or paper which deals with this.
     
  5. Aug 1, 2015 #4

    billy_joule

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    Yes, the fabrication process can have a large effect on the properties. Introductory textbooks on materials science and/or manufacturing processes are a good start.
    Any of these may be relevant;
    work hardening can occur during plastic deformation (ie folding operations)
    https://en.wikipedia.org/wiki/Work_hardening

    But it can also occur during extrusion (and rolling of sheet for that matter) so it depends on the details of the process.
    https://en.wikipedia.org/wiki/Extrusion

    Many extrusions undergo subsequent heat treatments to alter their properties:

    https://en.wikipedia.org/wiki/Heat_treating

    https://en.wikipedia.org/wiki/Hardenability
     
  6. Aug 1, 2015 #5

    SteamKing

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    It would help if we knew what material you are dealing with here. Is it aluminum, steel, what?

    From a design standpoint, the properties presented for various sections (area, moment of inertia, etc.) are based on nominal dimensions. For rolled or extruded shapes, there will likely be some tolerances associated with the fabrication process (rolling or extrusion), just like extrusion or rolling may "work-harden" the material.

    Because it is difficult to pin down these differences in dimensions and material properties from their nominal values, designers use the standard strength values for the grade of material and the nominal dimensions of the section.

    If your design doesn't work unless you take into account these variations, then perhaps you need to re-evaluate the design from the ground up. :wink:
     
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