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Yield Stress Myths!

  1. May 2, 2008 #1
    Is Yield Stress for a fluid a real property?

    Or is it a time dependent illusion we are measuring?

    For example - some glass materials are referred to as super-cooled fluids and yet behave and appear like solids. In reality though these super cooled fluids flow - very slowly.

    Some old glass windows on churches etc, are thicker at the bottom than the top.

    One of the oldest continuous experiments is still been monitored in Australia- "The Pitch Drop Experiment"

    This experiment involves the filling of a glass funnel with a very viscous pitch material. The experiment shows that the pitch flows very slowly through the opening - about one drop per decade. Its been running sonce the early 1930's and still going.

    If you strike this material with a hammer it shatters into pieces like a block of glass.


    So what is yield stress? Techincally its the stress that must be overcome in order for a fluid to begin shearing or flowing. But what if it takes decades for the flow to be measured? An apparentlt sold material is flowing!

    Weird stuff
     
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  3. May 2, 2008 #2

    mgb_phys

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    Try belly-flopping into a swimming pool for proof.

    Most 'glass' isn't technically a glass
    Refuted many times - the reason it is thicker at the bottom is that it was made that way.
     
  4. May 2, 2008 #3

    Mapes

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    Sit down, foolosophy, I have some news for you. All materials, crystalline and amorphous; metal, ceramic, or polymer, flow to some degree in response to a load. That does not mean that it is useful to model all materials as fluids; the time scale is important too.

    In metals, the rate of flow is negligible below about 30-50% of the material's absolute melting temperature. In glass and amorphous polymers, the rate of flow is negligible below the glass transition temperature.

    The flow of ordinary glass from gravity at room temperature is far too small to account for this. The thickness variation is a result of the manufacturing process.

    By convention, the yield stress is a 0.2% offset from linearity on a stress-strain curve.
     
    Last edited: May 2, 2008
  5. May 2, 2008 #4
    Are you confusing the stress vs strain curve of a solid under load?

    This is not the yield stress that I am referring to.

    Rocks flow slowly over geological time scales under he weak force of gravity.

    Does that make them fluid like?

    Yield stress is a time dependent illusion - it implies that there can be NO flow (or shear) when the shear stress applied is below this value. BUT if one waits, you will get flow.

    So how are people measuring this fluid property?

    It's an important parameter is fluid flow, food industries, mining etc. BUT this is becasue of the time scale involved.

    Just to give another example - a non-Newtonian fluid that has a yield stress component and is being pumped though a pipe will have a central PLUG FLOW region or shearless region. What I am suggesting is that if the pipe was infinite in length. that plug flow region will vanish and you will only have an ideal shearless region at the centre of the pipe where the velocity is maximum.

    It's a philosophical approach to what yield stress is, rather than a practical one.

    Nobody is denying the effect of yield stress in practice under normal time scales.
     
  6. May 2, 2008 #5

    Gokul43201

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    The problem is easily overcome by specifying a strain rate as a parameter of the yield strength. Most fluids (including non-Newtonian ones) have zero shear strength at zero strain rate. That is not surprising. There are only a small number of materials (e.g., viscoplastics) that retain non-zero strength at zero strain rate.
     
  7. May 3, 2008 #6

    Astronuc

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    With respect to solids, I believe one is referring to creep, which is what materials do under relatively low load, tensile or compressive, and which is a very slow process, as opposed to flow under higher loads, even at stresses less than yield. Geological formations, which are massive, will creep, but these could be rates of cm, or m, over thousands or millions of years.

    Creep is a function of temperature, as well as load, and will occur below the yield stress of a material.

    http://www.engineersedge.com/material_science/creep.htm

    http://courses.washington.edu/mengr354/jenkins/notes/chap8.pdf

    http://thayer.dartmouth.edu/defmech/
     
    Last edited: May 3, 2008
  8. May 3, 2008 #7

    Mapes

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    It's the same thing: bonds break and permanent deformation occurs. The concept is the same whether you're talking about cobalt or ketchup. Even in metals, the phenomenon is called "plastic flow."

    I think the point you're getting at is that the concept of a non-zero yield stress lies in contrast with the fact that materials permanently deform to some extent at any stress. The yield stress arises because in practice we are often interested in permanent deformation occurring in time scales on the order of a second. If this is what you're saying, I agree.
     
  9. May 3, 2008 #8
    i always thought it was due to creep. :uhh:
    It really isn't so?? And why would the bottom be made thicker?? that doesn't make any sense at all:grumpy:. at least till now!!
     
  10. May 4, 2008 #9
    Most practical fluids in industrial applications possess a measureble "yield Stress" component.

    Foods, slurries, pastes, wet concrete, bitumens, thick oils, glue etc.........

    Here is an example that may help.

    If you get a piece of plasticine or paly dough and try to stretch it you will find that it behaves differently depending on how fast you pull it apart.

    If you pull the plasticine apart with both hands VERY QUICKLY it snaps and beahaves and fails similar to a conventional solid material.

    If on the other hand you stretch it slowly, the plasticine elengates and flows like a fluid.

    What may seem to a solid may well be behaving like a fluid but over very long time scales - so the solid-like appearance is an illusion!
     
  11. May 4, 2008 #10

    Gokul43201

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    I don't get why you call this (positive strain rate hardening) an illusion. The overwhelming majority of materials exhibit it (from metals and alloys, to Newtonian fluids to many non-Newtonian fluids).

    On a small number of materials, like thermoplastics (hot Teflon will snap if you stretch it slowly, but will admit strains much bigger than 1 when stretched rapidly), you have negative strain rate hardening which results from processes too fast to allow cross links between polymer strands to "catch up".

    Either way, any illusion comes only from approximating quasi equilibrium behavior as true equilibrium behavior. Creep, for instance, is not an equilibrium process.
     
  12. May 5, 2008 #11
    Well i don't buy that.
    simply imagine it as a ball suspended from a spring. If the load(weight of the ball) exceeds the spring force, the ball accelerates downwards, and the moment spring force balances the load, the ball stops , acceleration goes to 0 and a moment later, is reversed.
    Now same is the case for any solid, if the self weight produces some stress, there are bonds which act like spring and doesn't let the solid flow(as you are saying).
    Obviously there are materials which do flow under self weight, but they are not exactly solid.

    Why it is not?? There is no other factor than time.
     
  13. May 6, 2008 #12
    Hey what am I supposed to say to this. I only gave a reason for my point, you just changed the topic
    (no offense)but is this a serious discussion or some science kicking thread
     
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