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Relation between compression and strength

  1. Jan 31, 2016 #1
    Whats the relation between compression and strength? When we build a structure, the ground or the land underneath the structure gets compressed and becomes rock solid. So how does compression make a substance strong? Is a compressed substance having more tensile strength?
     
  2. jcsd
  3. Jan 31, 2016 #2
    Compression measures how much you can push something into a smaller space, such as a snowball, wheres tensile strength measures how much you can stretch something before it loses cohesion and breaks, such as a rope pulling a sled.

    As a Canadian, it is wintertime here, thus I used snow in my examples. :D
     
  4. Jan 31, 2016 #3

    billy_joule

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    There is no relationship at all. Compression is the application of balanced inward (generally coaxial) forces. It has nothing to do with strength.

    https://en.wikipedia.org/wiki/Compression_(physics)

    It seems you may be thinking of compressive strength?

    https://en.wikipedia.org/wiki/Compressive_strength

    That is not true, the leaning tower of Pisa is one of many counterexamples. Poor foundation design leads to subsidence.
    https://en.wikipedia.org/wiki/Foundation_(engineering)
    https://en.wikipedia.org/wiki/Geotechnical_engineering
    https://en.wiktionary.org/wiki/build_on_sand


    It doesn't.
    If stress is applied to a material, whether its compressive, tensile or shear, and that applied stress exceeds a certain level the material will fail. Applying any compressive force will bring the stress level closer to failure - not make it stronger.
    I'd recommend you study the basics of mechanics of materials before attempting soil mechanics.
    I assume what you're thinking of is compaction:
    https://en.wikipedia.org/wiki/Soil_compaction

    If by stretch you mean extension then that is not tensile strength, it is ductility.

    https://en.wikipedia.org/wiki/Ductility
    https://en.wikipedia.org/wiki/Ultimate_tensile_strength

    Tensile strength does not go hand in hand with ductility, in fact quite the opposite. High strength materials generally have low ductility.
     
  5. Jan 31, 2016 #4

    Ductility measures how how thin a material can become and still maintain cohesion, such as forming a strand of wire. This can be accomplished through stretching, but also through extrusion and other methods. Ductility is more akin to Malleability in that it doesn't directly indicate a breaking point nor does require a limit on the amount of stress that "can be" applied.

    Tensile Strength measures the amount of pulling stress that can be exerted on that strand of wire and is referenced to the breaking point of the material.

    I somewhat agree, though I think they are not opposing properties.
    Tensile Strength indicates the maximum stress level at the moment immediately before fracture, whereas Ductility indicates the maximum change that can occur prior to the Tensile Strength limit being exceeded.
     
  6. Feb 3, 2016 #5
    Contrary to what has been said it is not true that compression does not increase strength in certain substances, and that adding stress will only bring a substance closer failure. Pre stressed concrete is a perfect example of a substance that gains strength from compression. If you are unfamiliar with the process, it is where concrete is poured in a mold with cables running the length, these cables are stretched and when the concrete is dry they are released, thus putting the concrete under stress. Cocnrete has very high compression strength and very low tensile strength. So yes, in a way you are bringing the concrete closer to its compression fail level but taking it further from its tensile fail level, which is a good thing since concrete will in all probability never even come close to its compression fail level. For all intents and purposes, compression makes concrete stronger.

    Another example of compression increasing strength is in the forging process, where a piece of metal is heated and then a pressure is applied forcing it into shape. This results in the metal being stronger than through the cast process(molten metal poured into a form) because the pressure aligns the grains of the metal, which adds strength to the piece.

    As to how compression might help a buildings stability, if the ground is compressed, it will increase the force needed to compress it even more, thus making that plot more stable and less likely to settle. But as far as I know, and my knowledge in this area is limited, most homes are built on solid ground, and if the ground isn't solid, the loose material is removed and replaced with something more solid, ie replacing sand with dirt/clay. and there is not much compacting. Not sure what they do for bigger structures.

    to answer your question, the relationship between compression and strength depends on the material. some materials will lose strength from being compressed, others will gain. in some Materials there is an increase in strength from an added stress.
     
  7. Feb 9, 2016 #6
    As what was said about the subject in soil mechanics problems are not generally about tensile stresses. Failures generally happen because of the soil couldnt bear the sheer stress applied by the structure. We cannot generalize compression enhancing soil's sheer strenght, it should be examined in lab enviorment. Also, we do not want the soil to settle more than the safety limits. And in soil samples which have water in them, especially clay/silt like soils, consolidation settlement is a very important consept to study. These typr of soil settles over time due to pressure applied to the water in the soil that leads the water to escape and therefore the soil gets compressed and settles even further.
     
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