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