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Determining Young's Modulus

  1. Aug 21, 2012 #1
    Hello everyone,

    I'm currently undertaking a work placement at the medical physics department of Leicester Royal Infirmary, investigating the potential application of a novel ultrasound technique in assessing carotid plaque.

    As part of my research, I'm evaluating the accuracy of the technique by determining the Young's Modulus of several cylinders of tissue mimicking material (PVA-cryogel, between 4 and 6 freeze/thaw cycles) by experiment before evaluating the YM using the ultrasound scanner, so I can compare the results.

    At the moment I'm using the classic method of loading masses and measuring the extension of the cylinders etc, however I'm convinced that this is an ineffective method for this material. The typical stiffness is between around 50 - 180 kPa and the material is fairly slipply and fragile, so it's difficult to attach masses and the cylinders often break and I'm not sure that I'm getting very accurate results.

    I've suggested that instead we load the masses on top of the cylinders and measure the compression, but again, that's not ideal.

    I was wondering if anyone knew any better ways of determining the YM?

    (We don't have any tensometers etc)


    Thanks,

    Sam
     
  2. jcsd
  3. Aug 21, 2012 #2
    Does it have to be using ultrasound? You can't just shove a strain gauge on it? You know the force you're applying to it and therefore the stress. You also get the strain from the strain gauge. That gives you the YM.

    I've only used ultrasound to find the location of internal cracks of a structure. Using it to find YM seems pretty annoying.
     
  4. Aug 21, 2012 #3

    AlephZero

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    I don't know anything abouit the properties of your matieral, but you might be able to find the modulus by dynamic testing. Either find the resonant frequency of a suitable sized sample, or attach a mass to one end of the sample, excite it by tapping it, and measure its vibration frequency with an acclerometer on the mass.
     
  5. Aug 21, 2012 #4
    If you are able to make another shape such as a plate or disk you could treat it as membrane or diaphragm or thin plate, support it around the edges, centre load the plate with a weighted plunger and measure the deflection with a dial gauge. The plumger foot will need a load spreader to prevent it punching into the material rather than deflecting the whole membrane.
    Alternatively you could set the membrane as the only flexible wall of a pressure cell and measure the deflection (with a dial gauge) as you pump up the cell air or water pressure.
     
  6. Aug 24, 2012 #5
    First, be aware that what determines ultrasound propagation is not the usual Young's modulus but the modulus whan the material can't move laterally, which makes it stiffer by 1/(1-2*µ^2) where µ is the transverse coefficient. This makes a huge difference on weak materials like elastomers.

    Then, if you want the acoustic properties of materials, measure them acoustically! And near the frequency (or frequencies) they will be used at! Properties change radically (I mean, Young*100) depending on the frequency, especially for weak materials like elastomers or tissues.

    You can for instance look for resonant frequencies for longitudinal waves, putting your material between good reflectors, which means a dense metal, at best tungsten, molybdenum... Then, the material's density will tell you the modulus and the impedance.
     
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