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Learning Objectives × After the Material Properties lecture and assignment, you
should be able to: × Describe the similarities/differences between metallic,
covalent, ionic, and secondary forces × Differentiate between bulk and surface properties × Correctly categorize several properties as being bulk or
surface properties × Draw and identify the important aspects of a stress-strain
curve × Describe stress shielding
Chemical Bonds and Forces
Metallic Ionic
Covalent
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Secondary Forces
× Electromagnetic Forces control atomic interactions × Van der Waals interactions
× Hydrogen Bonding
http://iverson.cm.utexas.edu/courses/310M/POTD%20Fl12/POTD8-31-12.html
Consequences of Chemistry × Atoms bond together using these attractive forces, creating
molecules with different properties
× A material’s properties are intimately connected to its chemical structure and atomic forces
× Consequently, a material’s biocompatibility is also intimately linked to its chemical composition
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Material Properties
× Bulk Properties vs. Surface Properties × A continuous material interacts with itself within the bulk but
structural units at the surface are asymmetrically oriented (including electrons), leading to surface energy.
× Surface energy: excess energy at the surface compared to the bulk, i.e. surface tension (force per unit length)
× Unique properties at the surface compared to bulk (i.e. reactivity)
× Surface properties largely dictate biocompatibility…more on this later
Material Properties
× Depend not only on composition but also on how molecules are arranged (microstructure, down to 10-9 m)
× Intrinsic properties (depend on composition only) × Density × Heat capacity
http://www.g-polymer.com/eng/kaihatukonseputo/images/110417153403430109549.gif
× Extrinsic properties (depend on microstructure and composition)
Stress
× Apply a load to a material: rotation, translation, deformation
× Nominal Stress: force (F) applied over initial cross-sectional area (A1)
σ n =FA1
× Tension: pulling force, elongating sample × Compression: pushing force, shortening
sample
× Units: N/m2 = Pa
× True stress difficult to calculate due to changing area A
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Strain
× Strain: Length change per unit length, dimensionless
× l = length, λis extension ratio
× True strain incorporates the changing length of the sample with applied force:
ε =l2 − l1l1
=Δll1= λ1→2 −1 λ1→2 =
l2l1
dεt =dll
εt =dlll1
l2∫ = ln l2l1
Shear Stress and Strain
× Shear stress—applied forces parallel to a pair of opposite faces
× Shear Strain—shape change caused by shear stress
Shear stress and the endothelium, Barbara J Ballermann, Alan Dardik, Eudora Eng and Ailian Liu
JAMA 1999; 282:2035 - 2042
τ =FA1
γ = tanθ
Stress-Strain Curves
Nominal Strain
Nom
inal
Str
ess
(GPa
)
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Stress-Strain: Elastic Deformation
× Initially there is a linear relationship between stress and strain
× Slope of this line is the Young’s Modulus, E.
× In the case of shear stress, the proportionality constant is the shear modulus, G.
× Poisson’s ratio Nominal Strain
Nom
inal
Str
ess
(GPa
) σ = Eε
τ =Gγ
v = − εtransverseεlongitudinal
Stress-Strain: Plastic Deformation
× Plastic deformation is irreversible (rearrangement of molecules occurs) × Non-linear response to
stress
× Ductility: plastic tensile strain required to break the material
× Note: malleability is plastic compressive strain required to break material Nominal Strain
Nom
inal
Str
ess
(GPa
)
Ductility
Stress-Strain Curves
× Proportional Limit: departure from linearity
× Yield Stress: stress at which noticeable (0.2% for metals) plastic strain occurs
× Ultimate Tensile Strength: the maximum on nominal strength-strain plot
× Breaking strength: actual material break point
Nominal Strain
Nom
inal
Str
ess
(GPa
)
Proportional Limit Yield Stress
Ultimate Tensile Strength (UTS)
Breaking Strength
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Stress-Strain Curves
× Hardness is typically used as an estimate of yield strength and UTS; measure by loading a small indenter
× Resilience is the elastic energy of a volume before yield
× Toughness is the energy required to deform a volume to break
Nominal Strain
Nom
inal
Str
ess
(GPa
)
Ur = σ dε0
εy∫
Ubreak = σ dε0
εbreak∫
Yield Drop
× Why is the polymer different? × Yield Drop—chains align during first phase, then once aligned
they are easier to elongate so there’s a yield drop, but then stress increases again × Try pulling apart a six pack ring—gets much harder just before
breaking
Metal Ceramic Polymer
Stress Shielding
× Reduction in bone density as a result of removal of normal stress from the bone by an implant × Wolff's law: bone in a healthy person will remodel in response
to the loads it is placed under.
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Material Properties Define the suitability of a material for an application
Primary Bonds Metallic, Ionic, Covalent
Secondary Bonds Van der Waals, hydrogen bonds,
A material’s properties Are caused by the underlying chemistry
Bulk vs. Surface Surfaces have unique properties because of surface energy caused by atoms/molecules having to interact with non-bulk molecules
Intrinsic vs. Extrinsic properties
Intrinsic (density, heat capacity): dictated by composition only; Extrinsic (yield strength): sensitive to microstructure (like crystal size and polymer chain arrangement) too!
Stress/Strain Stress: Force applied over area; Strain: length change due to stress
Deformation Elastic: reversible and described by Young’s modulus E; Plastic: irreversible and nonlinear response
Ductility Plastic tensile strain required to break the material
Yield Stress Stress at which noticeable (0.2% for metals) plastic strain occurs
Ultimate Tensile Strength Maximum on nominal strength-strain plot
Breaking strength Actual material break point
Stress shielding Reduction in bone density as a result of removal of normal stress from the bone by an implant
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