Fractures and Failures

1
Fractures and Failures
2
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

K ?8???a ?Y(a/W)
3
Failure Modes
Proximal Zone
Central Zone

• Cement Fracture

• Stem Fracture

• Bone-cement interface

Distal Zone

• Cement Fracture

• Bone Resorption

• Bone Fracture

4
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 lt 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.

5
Stress corrosion in implants

• Source of relative motion--fretting
• Bending in the cone
• Bending of the long neck extension (skirt) with
  proximal-distal slipping
• Bore angle too large
• Bore angle too small

6
Impingement fracture in a ceramic hip replacement
7
Different damage modes occur in different devices
8
Post fracture
9
Through-fracture in a tibial plateau