chapter4 mechanical properties of biomaterials 4.1 introduction molecular mechanisms behind the...

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

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]

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)

(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+)

(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

Elastic deformation Yield strength Plastic deformation Ultimate tensile strength

[tensile strength] Necking Plastic deformation

with decreased stress Fracture

Yield strength key design parameter

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

Engineering stress and strain --- True stress and strain

(5) Molecular causes of plastic deformation

ceramics metals polymers

Elastomers (rubbers)

(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

(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

(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)

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

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