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Mechanical Properties of Dental Materials. Occlusal forces. Average occlusal forces for fully dentate patients : 150 Newton in the anterior region to 500N in posterior region Maximum occlusal forces: different reports in the literature up to 3500N. - PowerPoint PPT Presentation
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Mechanical Properties of
Dental Materials
Resilience
AB
Str
ess
(Pa)
Strain
Resilience
AB
Str
ess
(Pa)
Strain
Occlusal forces
• Average occlusal forces for fully dentate patients :
150 Newton in the anterior region to 500N in posterior region
• Maximum occlusal forces: different reports in the literature up to 3500N.
• The occlusal forces for edentulous patients 15% of dentate patients.
Bulk Properties
Stress
Force per unit area; a force exerted on one body
that presses on, pulls on, pushes against, or tends to invest,
compress another body; the deformation caused in a body by such a force;
an internal force that resists an externally applied load or
force. It is normally defined in terms of mechanical stress, which
is the force divided by the perpendicular cross sectional area over
which the force is applied. GPT 2005, J Prosthetic Dentistry
Stress: Internal resistance to applied external force.
Stress= Force/Area
Types of stresses
• Axial
Compressive
Tensile
Types of stresses
• Non Axial Shear
Torsion
Bending
Strain
• Strain: change in length per unit length when stress is applied; the change in length/original length
GPT 2005, J Prosthetic Dentistry
• Strain(ε)= Deformation/Original length
Stress-Strain curve
Strain
Res
ilien
ceA
B
Str
ess
(Pa) Toughness
C D
Resilience: the resistance of a material to permanent deformation
A: Proportional limit
Elastic limit
A
Str
ess
(Pa)
Strain R
esili
ence
A
Str
ess
(Pa)
Strain
A: Proportional limit
• The greatest stress that a material will sustain without a deviation from the proportionality of stress to strain, below which no permanent deformation happens.
Elastic limit
• The maximum stress that a material will withstand without permanent deformation.
AB
Str
ess
(Pa)
Strain
Res
ilien
ce
AB
Str
ess
(Pa)
Strain
B:Yield strength(YS)
• The stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain
* YS indicates a degree of permanent deformation (usually 0.2%)
YS indicates a functional failure!!!
Elastic modulus
• Is a measure of elasticity of the material: how stiff the material is in the elastic range
• Elastic modulus= Stress/Strain
• The slope of the curve
AB
Str
ess
(Pa)
Strain R
esili
ence
AB
Str
ess
(Pa)
Strain
Poisson’s ratio
• Ratio of lateral to axial strain within the elastic range
Ductility and malleability
• Ductility: The ability of a material to be plastically deformed.
• Malleability: The ability of a material to be hammered or rolled into thin sheets without fractureing.
Plastic deformation
Strain
AB
Str
ess
(Pa) Toughness
C D
C: Ultimate strength
• Tensile or Compressive.
• The Ultimate strength: The maximum that a material can withstand before failure (tension or compressive).
it does give an indication of the needed thickness (cross section) of the restorations before failure.
D: fracture strength
• The stress at which the material fractures.
Toughness
• The resistance of a material to fracture
So what does yellow area under curve represent?
Fracture toughness
• The amount of energy required for fracture.
• Bond strength:
the bond strength between two dental materials.
Either tensile or shear
Fatigue bond strength?
Bending and torsion
• Endodontic files and reamers
Transverse strength
• Modulus of rupture or flexural strength
3- point bending test
Fatigue strength
• Fatigue: Progressive fracture under repeated loading
• The importance of endurance limit?
Str
ess
(Pa)
Strain Cycles
Str
ess
Fluid behaviour and Viscosity
• Viscosity: the resistance of a fluid to flow
• Viscosity= Shear stress/shear strain rate.
Viscous fluids
She
ar s
tres
s
Newto
nian
Str
ess
(Pa)
Strain Shear rate
Pseduplasti
c
Dilata
nt
She
ar s
tres
s
The importance of thixotropic impression materials
Creep and stress relaxation
• Creep is the increase in strain in a material under constant pressure.
Creep test is used for study of new amalgam materials
Surface mechanical properties
Indentation hardness
• Brinell hardness test. Ball,(steel or T carbide),
• Knoop hardness: Microindentation, pyramid shape.
• Vickers: 136 degrees diamond pyramid.
• Rockwell:metal cone.
• Shore A hardness for rubber
Stress analysis
• Lab based studies.
• Photoelasticity
• Finite Element Analysis.
Wear
• Loss of material due to contact between two surfaces
Surface phenomena
• Atoms or molecules at surface different to bulk
Stainless steel Vs steel
Oxide layer
Colloidal systems
• Two or more phases with one highly dispersed on the other.
Types:
* Sols and Gels
* Emulsions
Gels
• Entangled framework of solid colloidal praticles in which liquid is tapped in the intestices in which liquid is trapped
Emulsions
• A uniform dispersion of minute droplets of one liquid into another with the aid of emulsifier.
Surface tension and wetting
Θ Θ
High contact angle= less wetting
Low contact angle= better wetting
Adhesion
• The bonding of dissimilar materials by either:
^ Chemical bonding (True) OR
^ Mechanical bonding (retention).
Optical properties
• Basic colours:Red, Green and Blue.
Why only three??
HueChromaValue
Munsell colour system
Hue
• Basic colour
Chroma
• Colourfulness OR saturation
Value
• lightness
Metamerism
When two colour samples match when viewed under one light source but not another.
Any significance in dentistry?
Flouresence
• The emission of luminous energy by a material when a beam of light is shown on it.
What impact does this have in anterior restorations.
Thermal properties
• Heat of fusion: melting or freezing heat.
• Coefficient of thermal expansion: of paramount importance in clinical dentistry, why???
• Glass transition temperature??? For non metallic structures; glasses and polymers