chapter4 mechanical properties of biomaterials 4.1 introduction molecular mechanisms behind the...
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CHAPTER
44Mechanical Properties of Biomaterials
4.1 Introduction
Molecular mechanisms behind the mechanical properties Nature of chemical bonds and subunit structures
Mechanical properties1. tensile/compressive properties 2. shear/torsion properties3. bending properties 4. viscoelastic properties 5. hardness
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4.2 Mechanical Testing Methods, Results, and Calculations Forces: tensile, compressive, shear, torsion
4.2.1 Tensile and shear properties(1) Calculation for tension and shear tests 1) tension testing
dog-bone geometryload and specimen elongation [stress () & strain () relationship]
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4.2.1 Tensile and shear properties
(1) Calculation for tension and shear tests 1) tension testing
engineering stress & engineering strain and relationship [Hooke’s law] geometry of specimen (shapes)
2) compression testing < 0, <0
3) shear testing shear stress ()
and shear strain () 4) torsion forces
torsion stress () and torsion strain ()
torque force (T)
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(2) Stress-strain curve and elastic deformation
stress-strain curve = E x
modulus of elasticity or Young’s modulus[stiffness of materials]
= G x shear modulus [slope of the stress-strain curve in the elastic region]
Elastic elongation & contraction (transverse strain) Poisson’s ratio () isotropic material = 0.25 relationship between the shear and elastic moduli E = 2G (1+)
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(3) Molecular causes of elastic deformation
resistance (interatomic bonding force)E & (dF/dr)ro
high E (very stiff materials) force separation curveceramics > metals > polymers
(4) Stress-strain curves and plastic deformation
permanent deformation (metals and polymers)
linear and non-linear regions elastic and plastic deformation
Yield strength (y) Yield point strain (yp) 0.2% strain offset
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Elastic deformation Yield strength Plastic deformation Ultimate tensile strength
[tensile strength] Necking Plastic deformation
with decreased stress Fracture
Yield strength key design parameter
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Ductility: ability of a material to deformplastically before breaking
Low ductility --- brittle
% elongation vs. % area reduction
Semicrystalline polymers
Yield point -- neck -- polymer chain orientation-- resistance -- growth of the necked region-- stress increase to deform the polymer -- fracture
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Engineering stress and strain --- True stress and strain
(5) Molecular causes of plastic deformation
ceramics metals polymers
Elastomers (rubbers)
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(6) Causes of plastic deformation- metals and crystalline ceramics –
Slip: Force vs. slip plane
1) single crystal material in tension shear forces --- dislocation glide resolved shear stress (r)
r > crss [slip]
2) polycrystalline materials more complex multigrain structure macroscopic deformation
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(7) Causes of plastic deformation - amorphous polymers and ceramics (glasses) –
deformation via viscous flow Newton’s law [rate of deformation & applied stress]
= x
amorphous materials as cooled liquid shear force ---- continuous deformation with time
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(8) Causes of plastic deformation - polymers (general) - testing temperature 증가 , strain rate 감소
----- E 감소 , tensile strength 감소 , ductility 증가 1) temperature
ductility vs brittleness 2) strain rate
necking phenomenon
(9) Causes of plastic deformation - semi-crystalline polymers and elastomers –
1) semicrystalline polymers tensile forcechain orientation change in spherulite shape (necking phenomenon)
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Synthesis and processing parameters ---- deformation behavior 영향
Chain mobility 감소 ---- strength 증가 , ductility 감소 a) polymer crystallinity b) mol. wt. c) X-linking
2) Elastomers
Amorphous with coiled chains w/ free bond rotations
--- X-linking (prevention of plastic deformation)
T>Tg tensile strength
chain alignment
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4.2.2. Bending Properties
Ceramics: inherent brittleness of the materials
Bending test compressive forcetensile force
Modulus of rupture
Stress-strain curves with little plastic deformation