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Stress, strain, pressure

  1. Sep 18, 2013 #1
    I studied that Stress is defined as Force per unit area. Force here referred to internal force between the particles in the materials per unit area. am i right?

    Is pressure a kind of stress? (Internal force per unit volume) for fluids but i have studied that pressure is external force per unit volume

    Give me some example what is the use of finding Bulk and Shear modulus
  2. jcsd
  3. Sep 18, 2013 #2
    Pressure is an isotropic part of the stress tensor.
    Look up the general tensorial equation for a Newtonian fluid in a fluid mechanics book. This equation describes the rheological behavior of gases and most liquids.
  4. Sep 19, 2013 #3
    Is that internal force is restoring force?If yes, Does that mean all internal forces are restoring forces?
  5. Sep 19, 2013 #4


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    This is just a matter of definition, surely. A force would only be described as a 'restoring force' if you were dealing with an equilibrium situation i.e. when there are no external applied forces or when your system consists of balancing stress and applied forces.
  6. Sep 19, 2013 #5
    Pressure is not force per unit volume. It is force per unit AREA.
    Last edited: Sep 19, 2013
  7. Sep 19, 2013 #6


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    yeah. and (I think) the stress IS the entire internal force per area at some given point. (i.e. not including 'body forces' like gravity). So it is quite general, and in a general case, it will not be a restoring force. But for an example, if you have a (linear) sound wave moving through some fluid, then I guess this is an example of when stress can be a restoring force.

    edit: an example of when an internal force is not a restoring force: uh... let's say we have some fluid inside a closed cylinder, if we push one wall inward, there will be stresses inside the fluid, which propagate through the fluid (starting from the moving wall). These stresses act to change the system, so that it goes from lower density to higher density throughout the fluid.
    Last edited: Sep 19, 2013
  8. Sep 19, 2013 #7


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    In general (as chestermiller implied) stress is a tensor, described by 6 components (3 for direct stress and 3 for shear stress). These 6 components describe the "force per unit area" on an "area" that is in any orientation through a point in the body, and in general the force will be different depending on the orientation of the area. (I'm not sure if that's what you meant by the "entire internal force").

    The idea of "stress = force per unit area" is often used as an explanation of the concept of stress for beginners, in simple situations like the tension in a string or rod, when 5 of the 6 stress components are zero, and the "area" is perpendicular to the non-zero stress component.

    "Internal pressure" is the special case where the 3 direct stress components are equal, and the 3 shear components are all zero. In that case, the "force per unit area" is independent of the orientation of the "area" in the body.
  9. Sep 19, 2013 #8


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    yeah, I didn't mention the tensor aspect, since I'm guessing the OP is not on to that stuff yet.
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