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)

# Strength = Height of the curve A B # Stiffness (Rigidity) = Slope of the curve A B

# Ductility & Flexibility A X Y Z X Y Z # Resilience & Toughness = Area under the curve X Y Z X Y Z

# Ductility & Flexibility A X Y Z X Y Z # Resilience & Toughness = Area under the curve X Y Z X Y Z

Question? Is rubber a rigid or a flexible material? Which is stronger? Glass wax stainless 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

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 Shear Tension

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.

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

Shore A test Indentor material: Steel 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/mm3 Modulus of Elasticity 12 ~ 14000 MPa Yield Stress 240 MPa Cementing Agents – Zinc Phosphate Density 3.94E- 09 Mg/mm3 Modulus of Elasticity 13400 MPa Yield Stress  6.9 MPa Aluminum Oxide Density 3.72E- 09 Mg/mm3 Modulus of Elasticity 14000 MPa Yield Stress   352 MPa

Questions: 1) If material (A) has a lower elastic modulus than material (B), then this means that: Material (A) is more flexible than material (B) Material (A) is less flexible than material (B) Material (A) is more elastic than material (B) Material (A) is less elastic than material (B)

Questions: 2) ____________ measures the resistance of a material to indentation Tensile testing Compressive testing Hardness testing Diametric compression

Thank you