grinding welds

Discussion in 'Metal Boat Building' started by alanrockwood, Feb 18, 2011.

  1. gonzo
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    gonzo Senior Member

    Steel is also plastic, which is one of the properties that make it dent without cracking.
     
  2. Poida
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    Poida Senior Member

    But plastic is not steel.
     
  3. MikeJohns
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    MikeJohns Senior Member


    There's nothing pedantic about this. And there's nothing to take offense over. Clarification is for the benefit of all readers it's not a personal attack.

    Remember this is a boat design forum and engineering terms like Elasticity are not subjective, there is no room for opinion here. Definitions are absolute there’s no give or take. Concepts like stiffness and Elastic Instability are fundamental to understanding structures.

    What you call flexibility is actually Strain and its not good structurally, the less strain you have the better.

    A lot of people seem to struggle with basic material properties and it's worth trying to reduce that confusion when it occurs..

    Elasticity doesn't make steel a good boatbuilding material. Its stiffness, toughness, ductility, resistance to fatigue and ease of welding make it a good material. Stiffness is the opposite of elasticity. In other words, less strain for a given stress.

    Toughness and Elasticity and are quite different concepts.

    And as for plasticity, mentioned by Gonzo, that's really ductility. Metals as we use them deform plastically between yield and rupture.
     
  4. gonzo
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    gonzo Senior Member

    I did mean plastic deformation with impact or other damage.
     
  5. Frosty

    Frosty Previous Member

    I have always thought the word plastic to be a petroleum derivative. Ive not heard it been used to describe its ma liable properties.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Elastic:
    A material that is capable of sustaining stress (a load) without permenent deformation (ie change shape). Apply a load, it bends, release, it goes back to its orginal shape.

    Platsic:
    As above, except it can sustain appreciable deformation, which is permenent, without rupture/fracture. Apply a load, it bends, release, it stays at that shape it acquired when loaded, no change back as above.
     
  7. Frosty

    Frosty Previous Member

    Then steel is elastic, it can bend -distort until it reaches its point of elasticity from which when passed will then not return to its original.

    A1 metallurgy.

    The only plastic metal that comes to mind would be such as lead.
     
  8. Poida
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    Poida Senior Member

    Thank you Ad Hoc, that is why when you design in steel, you design within the allowable deflection. This is the range in which steel is elastic and as defined by Ad Hoc it will return back to it's original length after being stretched. Within this range steel is very elastic otherwise bridges would get longer as thay flex.

    There is a saying, from whence it came I know not. "If you are standing on a bridge and it stops moving, get off quick, it is about to collapse."
     
  9. MikeJohns
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    MikeJohns Senior Member

    Poida
    To illustrate how intuition can lead to poor comprehension I'll ask you the following:

    Do you think both Glass and Concrete are more or less 'elastic' than steel ?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Incorrect.

    When designing with steel, it is stress driven.

    When designing with aluminium or composite, it is deflection driven ...why?..because they both have a low Youngs modulus of elasticity!

    Steel is not very elastic!
     
  11. Frosty

    Frosty Previous Member

    The springs in my car are really springy and bouncy, infact so bouncy that have to have hydraulic shock absorbers to stop them from bouncing so much. The springs on the early mini was rubber balls, but now they use steel springs for suspension.

    Point-- steel has different qualities.
     
  12. MikeJohns
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    MikeJohns Senior Member


    Exactly ! The spring steel is so stiff the system requires damping. There's a direct relationship between the materials stiffness and the rate at which it absorbs and returns strain energy.

    A more elastic material such as Nylon or rubber would not require as much damping.

    Polyurethane springs for example are more effective by weight size and volume than steel springs because they are more elastic.
     
  13. Poida
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    Poida Senior Member

    Ad Hoc - mate we are saying the same thing. Steel has to work within it's allowable deflection because after that it will stress. ie the point at which it will not return to it's original length.

    If steel wasn't very elastic then every time it flexes, it didn't return to it's original length the Golden Gate Bridge would be in New York By now.

    Assuming that New York is in line with the Golden Gate Bridge if not, it would have reached New York if you moved New York in line with the Golden Gate Bridge.

    Before anyone dissagrees with this, OK I did exagerate it would only have got to Chicago, assuming, oh whatever!

    Mike - wouldn't have a clue buddy.
     
  14. RayThackeray
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    RayThackeray Senior Member


    This still doesn't make sense. My understanding is that its steel's combination of elasicity, strength and other properties make it a very good boatbuilding material (the point that Poida has tried to make).

    Do you dispute that the property of elasticicy (as relatively small as it is in steel) helps in this respect?

    If not, then steel would be even better if it had all the existing properties but one change - as elastic as ceramics. We all know where that would lead.

    Again, I charge the offence of pedantry.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Poida/Ray

    You’re getting mixed up, because you are incorrectly attributing “elastic” with “elasticity”.

    Any material that returns to its original shape once unloaded i.e. consider: “elastic”, Steel, aluminium, wood or rubber.

    The measure of how much is deforms while under a given load is a measure of its elasticity. We call this the Young’s Modulus.

    So, here to explain.
    Take an I-beam that has a web of 200x5, with a 100x10 rider bar. What would the deflections be in steel, ally, wood and rubber, if 2.0m long, simply supported and with a 10 tonne point load?

    Steel = 3.1mm
    Ally = 9.3mm
    Wood = 64.4mm
    Rubber = 160925 km!

    So, all these materials are elastic, but which is VERY elastic…oh look, rubber!

    Which is NOT very elastic, oh look, steel!

    It is all about structural stiffness the EI…..E (Youngs modulus) x I (Inertia).

    So, what would the stress be, in the same 4 examples? The value in (..MPa) is the elastic limit.

    Steel = 212MPa….pass (235 MPa)
    Ally = fail by 170% (125MPa)
    Wood = fail by 2120% (10MPa)
    Rubber = fail by 75,900,000,000% (0.004MPa)

    So, the rubber is VERY elastic, but cannot take the load. Whereas steel is not very elastic, but can take the load.

    The reason why steel is used in shipbuilding is clear to see. It can take load, be within its “elastic” limit, ie returns to its original shape after loading and does not deflect much.

    To make Ally do the same, much more material is required to pass, approximately twice as much….how is this achieved, we increase the “I” in the EI term. The second moment of inertia.

    To make wood pass, we require even more…..and for rubber, forget it.
     
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