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OT: Tensile strength of steel

Discussion in 'Electronic Design' started by linnix, Aug 29, 2008.

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  1. linnix

    linnix Guest

    This is not really electronics, but most of us are engineers here.
    Right?

    I need to estimate the tensile strength of a steel bar of 25mm width
    and 1mm thick.

    Do I have the numbers correct?

    Using low end estimate of 400MPa steel.

    400 MPa = 400,000,000 Newton / sq. meter

    25mm x 1mm = 0.025 * 0.001 = 0.000025 sq. meter

    400,000,000 * 0.000025
    = 10,000 Newtons or 2250 pounds (4.4 pounds per Newton?)
     
  2. JeffM

    JeffM Guest

    The book I consulted says there's a grade even lower than that:
    http://www.google.com/search?q=cach...s.Handbook+Tensile.strength+steel+324&strip=1
    The math looks kosher to me.
     
  3. linnix

    linnix Guest

    It's stainless steel screwed on a piece of wood to increase the
    tensile strength of the structure. It could be subject to a few
    hundred pounds of vibrations, but no where near 2000 pounds. I just
    need to figure out if it should be 1mm or 2mm thick.
     
  4. Any failure is likely around the screws

    --
    Dirk

    http://www.transcendence.me.uk/ - Transcendence UK
    http://www.theconsensus.org/ - A UK political party
    http://www.onetribe.me.uk/wordpress/?cat=5 - Our podcasts on weird stuff
     
  5. neon

    neon

    1,325
    0
    Oct 21, 2006
    I guess he means stainless steel as opposed to cold roll stell. WELL ANYHOW POUNDS/1000 feet /circular mil is 0.002671 now you do your conversation you may get a A. NOW THAT IS AT 20 DEGREES
     
  6. Rich Grise

    Rich Grise Guest

    How do you intend to fasten it? In any case, if it's attached to wood,
    then the part that fails won't be the steel.

    Good Luck!
    Rich
     
  7. JosephKK

    JosephKK Guest

    Typical SS runs about 35000 PSI at yield (3% elongation). Useful
    capacity is not over 15% of that. Your dimensions are about 0.038
    square inches for 1 mm thickness. 35000 * 0.15 * 0.038 = 199.5 lbf. I
    recommend at least 2 mm.
     
  8. linnix

    linnix Guest

    I would expect yield strength of 50%, not 15%. But as someone already
    mentioned, wood tissue around the screws could be a problem anyway.
     
  9. linnix

    linnix Guest

    Sorry, we are both wrong. I read your message wrong. SS is 70PSI
    tensile and 35PSI yield. You got my size wrong. It should be 35000 *
    0.15 * 1 (25mm is approx. 1") = 2000 pounds.
     
  10. Tim Williams

    Tim Williams Guest

    I should hope not; you meant KSI right?

    (Hmm, are there even any materials with single-digits PSI yield? Jello
    perhaps?)

    Tim
     
  11. linnix

    linnix Guest

    Yes, you are right. It's KSI.
     
  12. Tim Shoppa

    Tim Shoppa Guest

    Maybe it's just english, but I'd describe a 25mm x 1mm shape not as a
    bar but as a 1-inch wide strip of 20-gauge sheet metal.

    Now you're drilling or punching holes in that sheet metal, and then
    you're going to subject it to vibrations of hundreds of pounds.

    Maybe what you describe (wood, sheet metal strips, etc.) are more
    appropriately described as "banding", like what they put outside a
    wooden crate for a little extra strength?

    My words of advice are:

    1: Whatever fastener system (screws? glue?) you're planning on using
    will probably fail long before the metal does.

    2: Hundreds of pounds of vibrations are not things to ignore. With
    many metals, if you know the rig is going to be seeing hundreds of
    pounds of vibrations, you have to design for a static strength in the
    thousands of pounds.

    3. Sheet metal... bent around corners? I can guarantee that when it
    does fail it'll fail at a sharp corner!

    Tim.
     
  13. JosephKK

    JosephKK Guest

    Umm, 1 mm is a little less than 0.040 inch. 25 mm is about 2 % less
    than one inch, i stand by my very conservative cross section estimate.
    It is highly desirable to stay well below yield. (say 15% of yield.)
    Ultimate tensile strength is at the breaking point, not a real
    operating load. At least that is the way i read the "Metals
    Handbook, (1963) Volume VI properties and material selection", IIRC.
    Took the book to work to clarify some discussions about various
    materials and typical strengths.
     
  14. Tim Shoppa

    Tim Shoppa Guest

    Nothing wrong with going to the books to look up material
    properties, but it's amazing that anyone here thinks that
    this is the only concern when someone wants to support
    something that vibrates with hundreds of pounds of force
    with a piece of sheet metal an inch wide and as skinny as
    20 gauge all held to a wood support with a screw.

    In other words, it's not just material handbooks and multiplication,
    there's some handyman's common sense that ought to come in too :).

    Tim.
     
  15. linnix

    linnix Guest

    I find it hard to believe that the usable range is so low. Yield is
    where it deform permanently, but temporary deformation is OK (for
    example: a spring).
    Actually, it's more like 18 guage (1.27mm) with six screws of various
    length and sizes (long thin one and short thick one). There are also
    plywood on top and other supports around. Worst case is a 200 to 300
    pounds person jumping directly on top of it, which would create
    tensile forces parallel to the plate, but perpendicular to the
    screws. But I am still open to 10 to 12 gauge metal plates.
     
  16. Tim Shoppa

    Tim Shoppa Guest

    You're now talking about human safety! It sounds worse every time you
    talk about it, you should not support a human with a one-inch wide
    piece
    of skinny sheet metal attached to wood with a bunch of screws.

    For non-human loads, it is common to derate ropes and chains (under
    tensile load) by a factor of ten to fifty from their breaking load.
    The derating factor
    for a strip of sheet metal and a non-human-safety load has gotta be
    far far more severe. And I don't personally get to deal with anything
    involving
    human safety.

    And finally, a 300 pound guy jumping a foot and coming back down,
    stopping in a fraction of a second, is a force far far greater than
    300
    pounds.

    You should probably be asking someone familiar with OSHA safety
    requirements etc. for railings and platforms and fire code
    requirements
    for fire escapes. There is far far more to engineering a safe
    structure
    than looking up some numbers related to a material in a handbook!

    Tim.
     
  17. linnix

    linnix Guest

    OK, I'll go with 2mm steel bar. The structure is mostly jointed with
    1mm steel plates. I am just trying to reenforce the weak points. The
    25mm x 2mm steel bar should give me at least 5000 pounds yield
    strength.
     
  18. linnix

    linnix Guest

    Douglas Fir, or similar construction wood has a tensile
    strength of 87MPa (12.6KSI) in the X direction (lenght) but only
    2MPa (290PSI) in the Y direction (width) Stainless steel has
    a tensile strength of 860MPa (125KSI) in either direction.

    For a typical 2x4 wood (cross sectional area of 8 sq. in)
    and stainless steel plate of 4x0.04 (18 guage steel plate),
    Wood has a cross sectional area of 12 times of steel.

    So, steel has 80 percent of tensile strength of wood
    in the X direction, but 30 times in the Y direction.

    My purpose is to reenforce the weak Y direction tensile strength.
    Shouldn't 1mm steel plate be more than enough? What am I missing?
     
  19. Rich Grise

    Rich Grise Guest

    Use Southern Yellow Pine. To see how a support structure is made,
    find a wood roller coaster, buy the attendant a 6-pack (or bottle),
    and have him show you the details. :)

    Good Luck!
    Rich
     
  20. JosephKK

    JosephKK Guest

    Three things:

    1.) No sane engineer designs anything at more than 10 % of failing
    strength. That is the meaning of the upper number of tensile
    strength, that is when it breaks over 95% of the time. There are also
    differences between fairly elastic materials like most metals and
    fairly inelastic materials like wood, and brittle materials like most
    ceramics.

    2.) Stated dimensions are not always the physical dimensions. For
    example a finished 2 by 4 is actually about 1-3/4 by 3-1/2 inches; go
    ahead put a measuring stick to it.

    3.) I will not even touch this any more without dimensioned drawings
    with fully declared materials. I have many civil engineering friends
    that will help just for the asking, but they will not do it
    blindfolded and hogtied by inadequate information.
     
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