Mechanical properties of dental biomaterials 2

To Compare the properties of two materials:

Elastic Modulus of material (A) is higher than that of material (B)Elastic Modulus of material (A) is higher than that of material (B)

#Strength = Height of the curve

#Stiffness (Rigidity) = Slope of the curve

A

A

B

B

#Ductility & Flexibility

A B

X Y Z X Y Z

X Y Z X Y Z

# Resilience & Toughness = Area under the curve

#Ductility & Flexibility

A B

X Y Z X Y Z

X Y Z X Y Z

# Resilience & Toughness = Area under the curve

Question?

• Is rubber a rigid or a flexible material?

• Which is stronger? Glasswaxstainless steel

Testing compressive strength of a material:

• Specimen shape: Cylindrical

• Type of force used: compression

•Tester: Universal Testing Machine

Tensile strength testing:

• Specimen shape: Dumble-shape

• Type of force: Tension

• Tester: Universal Testing Machine

Brittle materials: They are materials which

undergo fracture or rupture with little or no prior permanent deformation.

They are stronger under compression rather than tension.

Their tensile strength can be calculated using:

Diametral compression test. (Indirect tensile test)

Fig 15: A drawing to illustrate how compression force develops tensile stress in brittle material

Fig 15: A drawing to illustrate how compression force develops tensile stress in brittle material

Diametral compression test of brittle materials:

• Specimen shape: Disk-shaped

• Type of force applied: Compression

• Resultant force: Tension

• Tester: Universal Testing Machine

Stress = 2P Load π D T Diameter X Thickness

Compression

Tension

Shear

Transverse strength - Modulus of Rupture:(3 point bending test)

It is the ability of a material to bend before it breaks.

Stress= 3 Load X Length

2 X Width X Thickness2

Strain= Load X Length3

4 Elastic Modulus X Width X Thickness3

Fig 16-a: Diagrammatic representation of a 3-point bending test or transverse test bending of the beam introduces both

tensile and compressive stresses.

Fig 16-a: Diagrammatic representation of a 3-point bending test or transverse test bending of the beam introduces both

tensile and compressive stresses.

Impact strength

Materials such as glasses,

ceramics, cements and amalgam

have low resistance to breakage

when a load is applied by

impact. (dynamic load).

Denture mid-line fracture Porcelain bridge fracture

The impact strength is defined

as the energy required to

fracture a material under an

impact force.

Impact strength (Charpy tester)

Hardness:

It is defined as the resistance of a material to surface penetration or indentation.

There are four common standard test methods for expressing the hardness of a material: Brinell, Rockwell, Vickers, and Knoop.

HARDNESS

The property of hardness is of major

importance in the comparison of

restorative materials.

Hardness is defined as to resistance

to permanent surface indentation or

penetration.

1. Brinell hardness test• Shape of indentor: Sphere• Indentor material: Tungestun or carbide• Size of indentor: 1.6 mm in diameter•Type of indentor: Macro-indentor• Measurement: Depth of indentation

2. Rockwell hardness test• Shape of indentor: Cone shaped• Indentor material: Tungestun or carbide• Size of indentor: 1.6 mm in diameter•Type of indentor: Macro-indentor• Measurement: Depth of indentation

Limitation:Can not be used to test the hardness of brittle materials.

3. Vickers Hardness Test

• Shape of indentor: Pyramid-shaped• Indentor material: 136 degree Pyramid• Type of indentor: Micro-indentor• Measurement: Diagonal of indentation

4. Knoop hardness test

• Shape of indentor: Pyramid-shaped• Indentor material: Diamond• Type of indentor: Micro-indentor• Measurement: The longer diagonal of indentation.

Shore A test• Shape of indentor: Blunt pointed• Indentor material: Steel• Type of indentor: For rubber materials.• Measurement: The depth of indentation.

Why do we need to know the mechanical properties of the materials? Human Dentin

Density 2.1 ~ 2.2E-9 Mg/mm3Modulus of Elasticity 12 ~ 14000 MPaYield Stress 240 MPa

Cementing Agents – Zinc PhosphateDensity 3.94E- 09 Mg/mm3Modulus of Elasticity 13400 MPaYield Stress  6.9 MPa

Aluminum OxideDensity 3.72E- 09 Mg/mm3Modulus of Elasticity 14000 MPaYield Stress   352 MPa