1. 4 Mechanical Properties of Biomaterials CHAPTER 4.1 Introduction
Molecular mechanisms behind the mechanical properties
Nature of chemical bonds and subunit structures
Mechanical properties
1. tensile/compressive properties
2. shear/torsion properties
3. bending properties
4. viscoelastic properties
5. hardness

2. 4.2 Mechanical Testing Methods, Results, and Calculations
Forces: tensile, compressive, shear, torsion
Tensile and shear properties
Calculation for tension and shear tests
1) tension testing
dog-bone geometry
load and specimen elongation
[stress (s) & strain (e) relationship]

3. 4.2.1 Tensile and shear properties
Calculation for tension and shear tests
1) tension testing
engineering stress & engineering strain
s and e relationship [Hooke’s law]
geometry of specimen (shapes)
2) compression testing
s < 0, e<0
3) shear testing
shear stress (t)
and shear strain (g)
4) torsion forces
torsion stress (t)
and torsion strain (g)
torque force (T)

4. (2) Stress-strain curve and elastic deformation
s = E x e
modulus of elasticity or Young’s modulus
[stiffness of materials]
t = G x g
shear modulus
[slope of the stress-strain curve in the elastic region]
Elastic elongation & contraction (transverse strain)
Poisson’s ratio (n)
isotropic material n = 0.25
relationship between the shear and elastic moduli
E = 2G (1+n)

5. (3) Molecular causes of elastic deformation
resistance (interatomic bonding force)
E & (dF/dr)ro
high E (very stiff materials)
force separation curve
ceramics > 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 (sy)
Yield point strain (eyp)
0.2% strain offset

6. Elastic deformation
Yield strength
Plastic deformation
Ultimate tensile strength
[tensile strength]
Necking
with decreased stress
Fracture
key design parameter

7. Ductility: ability of a material to deform
plastically 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

8. Engineering stress and strain
--- True stress and strain
(5) Molecular causes
of plastic deformation
ceramics
metals
polymers
Elastomers (rubbers)

9. (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 (tr)
tr > tcrss [slip]
2) polycrystalline materials
more complex multigrain structure
macroscopic deformation

10. (7) Causes of plastic deformation
- amorphous polymers and ceramics (glasses) –
deformation via viscous flow
Newton’s law [rate of deformation & applied stress]
t = h x g
amorphous materials as cooled liquid
shear force ---- continuous deformation with time

11. (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 force
chain orientation
change in spherulite shape
(necking phenomenon)

12. 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

13. Bending Properties
Ceramics: inherent brittleness of the materials
Bending test
compressive force
tensile force
Modulus of rupture
Stress-strain curves with little plastic deformation