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Difference beteween Strength, Hardness and Toughness

  1. Mar 19, 2012 #1
    Hi, am I right to say that:

    Hardness is a material's resistance to plastic deformation. E.g. if a spoon is hard, it's difficult to bend it.

    Toughness is how much deformation a material can undergo before fracture. E.g. how much energy a spoon can absorb while deforming before it breaks.

    Strength is the maximum amount of stress an object can take before deforming. Though I don't really see the difference between this and hardness...
     
  2. jcsd
  3. Mar 19, 2012 #2

    tiny-tim

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    hi lokifenrir96! :smile:

    wikipedia (http://en.wikipedia.org/wiki/Hardness) is very good, and clear, on this …

    "Hardness is the measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied."

    "Strength is a measure of the extent of a material's elastic range, or elastic and plastic ranges together."

    "The toughness of a material is the maximum amount of energy it can absorb before fracturing, which is different from the amount of force that can be applied."​

    So hardness is about permanent change, strength is about elastic (non-permanent) change, and toughness is about energy (rather than force).

    (See also http://en.wikipedia.org/wiki/Strength_of_materials and http://en.wikipedia.org/wiki/Toughness)
     
  4. Mar 19, 2012 #3
    The details in the above post by Tiny Tim are very important. :-)

    As one sees these are very different terms and knowing the difference is crucial.

    (Like you said)If it is very difficult to deform a body, it is called hard.

    Strength is something else
    and infact strength is defined in two ways.

    1) Based on fracture point


    Imagine a body being stretched and stretched.
    There will be a point after which it will break(into two or more pieces).This point is called the fracture point.

    If the amount of streching (and thus force )required to break it is large its called a strong material.
    And in laymen' terms is the definition of strength.

    2) based on ultimate tensile strength

    There is another way which is by measuring its ultimate tensile strength.
    (this strength is used in technical aspects like engineering)

    You can read more about it on


    http://en.m.wikipedia.org/wiki/Ultimate_tensile_strength
     
    Last edited: Mar 19, 2012
  5. Mar 19, 2012 #4
    My text book states:
    STRENGTH ...how great a force a material can withstand before BREAKING
    TOUGH material is not BRITTLE, ie does not crack readily
    STIFFNESS is opposition to being DISTORTED

    People very often use the word 'strong' when they means 'stiff'
     
  6. Dec 3, 2012 #5
    The text I have highlighted in bold is incorrect. Strength is not about elastic change. The correct definition for strength, from wikipedia, is:
    So strength is about the ability to withstand forces without PERMANENT change e.g breakage. So hardness is one type of strength, just like tensile and compressive strength.

    What Tiny Tim was talking about is stiffness, not strength. Stiffness is
    The opposite of stiffness is elasticity - how far an object will stretch when pulled with a given amount of force.

    Strength and stiffness are not at all the same thing and should never be confused for each other.

    The above quote sums it up quite nicely.
     
    Last edited: Dec 3, 2012
  7. Dec 5, 2012 #6
    it is important to define exactly what "failure" means. In some applications, exceeding the yield point of a material can be considered a failure. In other applications, the part can function quite well until it atually breaks. Therefore, as has been said, there are different types of "strength".
     
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