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What makes materials strong ?

  1. Feb 9, 2008 #1
    what is it that makes materials strong ?

    density ? melted ? chemistry reactions ? strong nuclear force ? all of that ?


    regards
    garfield1729
     
  2. jcsd
  3. Feb 9, 2008 #2
    force of attraction between the molecules I guess. Everything else is dependent on it.
     
  4. Feb 9, 2008 #3

    Mapes

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    What makes materials strong is any impediment to what makes them fail. Seriously. In metals, for example, failure often occurs by crystal planes slipping past each other via dislocation motion. Anything that impedes dislocation motion (like precipitate obstacles or other dislocations) will strengthen the metal. Brittle materials like ceramics, in contrast, often fail by crack propagation, which calls for different strengthening mechanisms.

    A related question is what makes materials stiff (strength is resistance to permanent deformation, stiffness is resistance to elastic, or recoverable, deformation). Stiffness is strongly coupled to the interatomic bond strength. So is density and melting temperature, and the element-to-element trend for these three material properties is similar.
     
  5. Feb 9, 2008 #4

    Astronuc

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    Basically, the atomic bonds. A material is a strong as the weakest bond.

    Density is a function of atomic mass, most of which resides in the nucleus, and the atomic (ionic) radii or interatomic distance. One can find low density but very strong materials.

    Strength decreases with temperature.


    See this thread - https://www.physicsforums.com/showthread.php?t=68286

    and http://www.isotruss.org/hsratio.htm
     
    Last edited: Feb 9, 2008
  6. Feb 11, 2008 #5

    Mapes

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    Basically, the atomic bonds. A material is a strong as the weakest bond.

    I'm going to go ahead and disagree with this. There are many examples of materials becoming stronger through the deliberate introduction of weak or unsatisfied bonds. One example is in metals, where additional dislocations and grain boundaries (1-D and 2-D defects, respectively, consisting of distorted and broken bonds) strengthen the material. Another example is ceramics, where voids (no bonds at all!) are used to interrupt crack propagation and again strengthen the material.

    The statement isn't even true in a general sense: covalent and ionic bonds are generally stronger than metallic bonds, but metals are generally stronger than covalent and ionic crystals in tension!
     
    Last edited: Feb 11, 2008
  7. Feb 11, 2008 #6
    I'll agree with your disagreement. Material strength is determined at a higher level than atomic bond strength. There are some excellent books by J. E. Gordon "Structures" and "The New Science of Strong Materials" that go into this in a lightly mathematical, very conversational way. He recounts an episode where a new graduate makes what he claims is a very strong material, because the bonds comprising it were stronger. Actual testing shows it had "... the strength of wet cheese."
     
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