B
Stress (Pa)
A
ce
Re
ien
sil
Strain
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.
• 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
C
B
Stress (Pa)
A
Toughness
Strain
D
Resilience: the
resistance of a material
to permanent
deformation
A
A: Proportional limit
Stress (Pa)
Elastic limit
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.
Stress (Pa)
B
A
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
B
A
Stress (Pa)
• 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
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
C
B
Stress (Pa)
A
Toughness
Strain
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:
Progressive
fracture under
repeated
loading
• The
importance of
endurance
limit?
(Pa)
StressStress
Fatigue strength
Cycles
Strain
Fluid behaviour and Viscosity
• Viscosity: the resistance of a fluid to flow
• Viscosity= Shear stress/shear strain rate.
The
importance
of
thixotropic
impression
materials
stress
Shear
Shear
(Pa)
Stressstress
Viscous fluids
Strain
Shear
rate
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
High contact angle= less
wetting
Θ
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??
Munsell colour system
Hue
Chroma
Value
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