Safety factor applied to structural design

Discussion in 'Boat Design' started by makobuilders, Jan 18, 2017.

  1. rxcomposite
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    rxcomposite Senior Member

    The working stress/allowable stress is the maximum safe stress the material can carry. In design, the working stress is designed around the proportional limit (the straightest portion of the stress/strain curve) so as not to invalidate the stress/strain relationship of the Hooke's law.

    In most cases, the yield point is 60% of the material to be used and the proportional limit is below the yield point or 1/2 (50%) of the ultimate strength.

    A working stress set at the 1/2 of the ultimate strength has an FoS of 2. Setting it at 1/2 of the proportional limit, the FoS is 4. If the FoS is 1 (the ultimate strength), the load has exceeded the yield point and the material has deformed permanently.

    While some engineering practice uses the ultimate strength as a guide, the designer is well aware of the material property. They set the working stress either below the yield point, or at best, stay within the proportional limit to be accurate in computation.

    Note that in using ultimate strength, the material has deformed only at the ultimate strength. It will break/separate from each other at break/rupture at the far end. If the material is ductile, the break will occur long after the ultimate strength. Never design at ultimate strength or worse at breaking stress.
     

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  2. upchurchmr
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    upchurchmr Senior Member

    Stresses can be and are used into the plastic bending region in aircraft.

    Occasionally and with a great deal of care.
     
  3. rxcomposite
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    rxcomposite Senior Member

    Agree, and it is complicated. Here is one from aerospace.

    >rb1957 (Aerospace)

    FoS is equivalent to MS (=FoS-1).

    One interpretation for FoS might be the ultimate factor, re FAR25.303, but then i wouldn't tie this definition to MS.

    1) first we define a limit load on the structure, say a maneouvre load. the allowable for limit load is yield, ie no plasic deformation.

    2) then ultimate load = 1.5*limit; 1.5 is defined in FAR25.303 as FoS. the allowable for ultimate load is ultimate strength, plastic deformation is acceptable.

    3) then we calc RF = allowable/applied or MS = RF-1

    4) then we apply other factors as required, fitting factor, casting factor, ... RF = (allowable/applied)/(FF*...) or MS = RF-1.<
     
  4. idkfa
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    idkfa Senior Member

    Some keels on race boats can be seen to deflect visibly, if the boat is held horizontally, like when to doing a roll over test. Are they designed at the limit of the elastic region, with little or no SF and the assumption is made that they will operate/spend most of its life boat at 30deg heel or less. How much is dynamic loading more than the 90deg static loading? And what allowance is/should be made for metal fatigue?

    https://www.youtube.com/watch?v=LIplsOf_DBQ
     
  5. Barry
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    Barry Senior Member





    Is this comment at odds with the graph that you provided?
    Ie the graph shows the elastic zone stopping at the elastic limit, which is below yield and the plastic area beginning before yield.

    I would have thought that the elastic area would be up to the Yield point, ie the definition of yield, and the plastic area from yield to the right

    Some clarification please
     
  6. Barry
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    Barry Senior Member

    Allowing for the factor of safety, correct? Ie not for normal working loads
     
  7. rxcomposite
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    rxcomposite Senior Member

    The graph is mine but the explanation on the MS or MoS referring to aircraft is not mine as duly noted. Please check.
     
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  8. upchurchmr
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    upchurchmr Senior Member

    Barry,

    What usually happens in my experience, is that you do the first design to yield, with the safety factor.

    You find out later there is an error, ground test shows loads are higher or there is an error on the individual part design, or flight test shows the loads are higher.

    Now you need to show the design is still good or redesign/ rebuild any aircraft already built.

    Since the initial design has as many conservative assumptions as you could stand, you now can take away as much conservatism as you can stand. One of those things is using the parts in the elastic region.
    When you do that, any increased load in the local area will get picked up in the surrounding structure. Assuming that structure around can take the increased load, and the problem structure will not fail for fatigue or life consideration, you are set to keep on flying with the original design.
    Just not the original analysis.
     
  9. Barry
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    Barry Senior Member

    Your original comment lead me to believe that that you were saying that the allowable stresses could be in the plastic range, ie below the factor of safety limit.
    The concern would be higher fatigue failures (at less cycles) at higher stresses closer to yield.

    Stresses into the plastic range will cause permanent deformation and a geometry change as well as work hardening will be accelerated.

    With enough cycling, even below the plastic limit can cause issues as evidenced by the Hawaiian Air event that part of the fuselage fell off and the issue that they had with the engine pylon work hardening, causing cracks, with at least one crash due to the engine breaking off the wing. I could not find the link. It was several years ago. Update, DC10 and Chicago

    My first thought would be to determine the allowable stresses then apply the factor of safety, but this upper design stress would still have to be below the yield plastic limit
     
  10. upchurchmr
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    upchurchmr Senior Member

    Barry,

    The overall point is that you can work the stress level much higher than normally acceptable if you have enough physical testing of the material, specific analysis methods (which go beyond the normally use methods) and some historical evidence.

    All that takes a lots of money, time and expertise not normally shared openly.

    This is a subject which will not have any applicability to a boat forum. I'm sorry I stirred this up.

    I assure you. Normal process always results in the "upper design stress" below the yield plastic limit. Believe me or not, each aircraft I've worked on has had some point at which the aircraft could exceed that point in acceptable operation.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Correct

    Any relaxation of said FoS, as upchurchmr notes, comes with actual in-service experience to counter the original assumptions.
     

  12. Wayne Grabow
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    Wayne Grabow Senior Member

    Fascinating discussion to me. I am a dentist, and all this applies to dental restorations as well. Think of the endless cycles of chewing a person does; not counting possible bruxing habits. My undergraduate degree was mech. engineering. With a hobby of designing and building small boats (< 20'), I am usually depending on following scantlings for similar existing designs when building. Nothing has broken so far. I worry most about the weight and power of an OB engine hanging from the transom.
     
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