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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