Mechanical Properties of Dental Materials

1. Mechanical Properties of Dental Materials

2. 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.

3. Bulk Properties

4. 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

5. Types of stresses • Axial Compressive Tensile

6. Types of stresses • Non Axial Shear Torsion Bending

7. 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

8. D C B A Toughness Stress (Pa) Resilience Strain Stress-Strain curve

9. A A Stress (Pa) Stress (Pa) Resilience Strain Strain Resilience: the resistance of a material to permanent deformation A: Proportional limit Elastic limit

10. 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.

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

12. B B A A Stress (Pa) Stress (Pa) Resilience Strain Strain

13. 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!!!

14. B B A A Stress (Pa) Stress (Pa) Resilience Strain Strain 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

15. Poisson’s ratio • Ratio of lateral to axial strain within the elastic range

16. 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.

17. D C B A Toughness Stress (Pa) Strain Plastic deformation

18. 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.

19. D: fracture strength • The stress at which the material fractures.

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

21. Fracture toughness • The amount of energy required for fracture.

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

23. Bending and torsion • Endodontic files and reamers

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

25. Stress Stress (Pa) Strain Cycles Fatigue strength • Fatigue: Progressive fracture under repeated loading • The importance of endurance limit?

26. Fluid behaviour and Viscosity • Viscosity: the resistance of a fluid to flow • Viscosity= Shear stress/shear strain rate.

27. Pseduplastic Newtonian Dilatant Shear stress Shear stress Stress (Pa) Shear rate Strain Viscous fluids The importance of thixotropic impression materials

28. 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

29. Surface mechanical properties

30. 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

31. Stress analysis • Lab based studies. • Photoelasticity • Finite Element Analysis.

32. Wear • Loss of material due to contact between two surfaces

33. Oxide layer Surface phenomena • Atoms or molecules at surface different to bulk Stainless steel Vs steel

34. Colloidal systems • Two or more phases with one highly dispersed on the other. Types: * Sols and Gels * Emulsions

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

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

38. Θ Θ Surface tension and wetting High contact angle= less wetting Low contact angle= better wetting

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

40. Optical properties • Basic colours:Red, Green and Blue. Why only three?? Munsell colour system Hue Chroma Value

42. Hue • Basic colour

43. Chroma • Colourfulness OR saturation

44. Value • lightness

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

46. 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.

47. 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