Fractures and Failures

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					Fractures and Failures
       Fracture mechanics concepts

• The stress intensity factor, K, is derived from
  linear elastic fracture mechanics
• This parameter is used to describe the
  magnitude of the stresses, strains and
  displacements ahead of the crack tip

               KI                  3 
      yy           cos   sin sin
                            1             
               2 r      2      2     2 
              K = ∞a Y(a/W)
                   Failure Modes
               Central Zone         Proximal Zone
              •Stem Fracture        •Cement Fracture

                                  •Bone-cement interface


Distal Zone
•Cement Fracture

          •Bone Fracture       •Bone Resorption
              Stress Corrosion Cracking

• Stress corrosion cracking (SCC) is the formation of cracks in a normally
  sound material through the synergistic action of a stress and a corrosive
  environment, where the differences in stress concentrations results in
  different chemical potentials. Usually, most of the surface remains
  unattacked, but there fine cracks penetrating into the material. SCC fractures
  have a brittle appearance. SCC is especially in important when studying the
  fracture behavior of implants presence of the corrosive environment since it
  lowers the critical stress intensity factor, KIC so that KISCC < KIC..
• Materials are susceptible to SCC in aqueous, ionic and high temperature
  environments where stress gradients are also present. For example, a flexed
  component will experience tensile stresses on one side and compressive
  stresses on the other and this can result in a chemical potential difference
  that renders the tensile portion subject to chemical attack resulting in local
  crack development.
          Stress corrosion in implants

•    Source of relative
     motion--fretting
i. Bending in the cone
ii. Bending of the long
     neck extension (skirt)
     with proximal-distal
     slipping
iii. Bore angle too large
iv. Bore angle too small
 Impingement fracture in
a ceramic hip replacement
Different damage modes occur
      in different devices
Post fracture
Through-fracture in a
    tibial plateau

				
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posted:3/9/2012
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