Mechanical Properties of Dental Materials

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Mechanical Properties of Dental Materials
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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.

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• Bulk Properties

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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

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Types of stresses

• Axial
• Compressive
• Tensile

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Types of stresses

• Non Axial
• Shear
• Torsion
• Bending

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Strain

• Strain change in length per unit length when
  stress is applied the change in length/original
  length
• GPT 2005, J Prosthetic Dentistry
• Strain(e) Deformation/Original length

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Stress-Strain curve
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Resilience the resistance of a material to
permanent deformation A Proportional limit
Elastic limit
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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.

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Elastic limit

• The maximum stress that a material will withstand
  without permanent deformation.

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BYield 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!!!

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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

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Poissons ratio

• Ratio of lateral to axial strain within the
  elastic range

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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.

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Plastic deformation
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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.

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D fracture strength

• The stress at which the material fractures.

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Toughness

• The resistance of a material to fracture
• So what does yellow area under curve represent?

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Fracture toughness

• The amount of energy required for fracture.

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• Bond strength
• the bond strength between two dental
  materials.
• Either tensile or shear
• Fatigue bond strength?

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Bending and torsion

• Endodontic files and reamers

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Transverse strength

• Modulus of rupture or flexural strength
• 3- point bending test

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Fatigue strength

• Fatigue Progressive fracture under repeated
  loading
• The importance of endurance limit?

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Fluid behaviour and Viscosity

• Viscosity the resistance of a fluid to flow
• Viscosity Shear stress/shear strain rate.

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Viscous fluids
The importance of thixotropic impression materials
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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

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Surface mechanical properties
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Indentation hardness

• Brinell hardness test. Ball,(steel or T carbide),
• Knoop hardness Microindentation, pyramid shape.
• Vickers 136 degrees diamond pyramid.
• Rockwellmetal cone.
• Shore A hardness for rubber

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Stress analysis

• Lab based studies.
• Photoelasticity
• Finite Element Analysis.

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Wear

• Loss of material due to contact between two
  surfaces

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Surface phenomena

• Atoms or molecules at surface different to bulk
• Stainless steel Vs steel

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Colloidal systems

• Two or more phases with one highly dispersed on
  the other.
• Types
• Sols and Gels
• Emulsions

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Gels

• Entangled framework of solid colloidal praticles
  in which liquid is tapped in the intestices in
  which liquid is trapped

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Emulsions

• A uniform dispersion of minute droplets of one
  liquid into another with the aid of emulsifier.

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Surface tension and wetting
High contact angle less wetting
Low contact angle better wetting
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Adhesion

• The bonding of dissimilar materials by either
• Chemical bonding (True) OR
• Mechanical bonding (retention).

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Optical properties

• Basic coloursRed, Green and Blue.
• Why only three??

Munsell colour system
Hue Chroma Value
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Hue

• Basic colour

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Chroma

• Colourfulness OR saturation

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Value

• lightness

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Metamerism

• When two colour samples match when viewed under
  one light source but not another.
• Any significance in dentistry?

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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.

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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