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Homework Help: Poissons ratio close-packed spheres

  1. Oct 7, 2013 #1

    See question 7.4 from the link.

    http://books.google.com/books?id=0N0...ow&w=685&w=800 [Broken]

    "Assuming that atoms are hard elastic spheres, show that Poisson's ratio for a close-packed array of spheres is 1/3"

    I am having trouble explaining the proof for this.

    I know the that the volume modulus, K, = E(elastic modulus) / ((3(1-2υ)) where υ is Poisson's ratio

    K = E / (3(1-2υ))

    When υ = 1/3, K=E.

    I'm thinking that since for a hexagonal close packed structure, HCP, the angles between lattice sites is 120°, or 1/3 of the plane of a full crystal structure.

    Refer to:
    http://www.science.uwaterloo.ca/~cch...ct/fig/hcp.gif [Broken]

    Therefore the elastic properties for a given volume is split in thirds? It seems like a misleading argument, but I can't find a way to explain it with math!
    1. The problem statement, all variables and given/known data

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    Last edited by a moderator: May 6, 2017
  2. jcsd
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