Presentation on theme: "Mechanical Properties of Dental Materials. Occlusal forces Average occlusal forces for fully dentate patients : 150 Newton in the anterior region to 500N."— Presentation transcript:
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.
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 Resilience A B Stress (Pa) Toughness C D
Resilience: the resistance of a material to permanent deformation A: Proportional limit Elastic limit A Stress (Pa) Strain Resilience A Stress (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.
A B Stress (Pa) Strain Resilience A B Stress (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 A B Stress (Pa) Strain Resilience A B Stress (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 A B Stress (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? Stress (Pa) StrainCycles Stress
Fluid behaviour and Viscosity Viscosity: the resistance of a fluid to flow Viscosity= Shear stress/shear strain 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 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?? Hue Chroma Value Munsell colour system
Hue Basic colour
Chroma Colourfulness OR saturation
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