2 MECHANICAL PROPERTIES of dental material

Dental material notes

1. MECHANICAL PROPERTIES BASIC SCIENCES FOR DENTAL MATERIALS LECTURE # 2 DR MUHAMMAD KALEEM

2. Force Force, in physics, any action or influence that accelerates an object Characteristics of force Point of application of force Direction of application Magnitude Unit of force Newton (N ) Occlusal Force Ranges from 200 to 3500 N Molars :- 400-800 N Bicuspid Cuspid & incisor :- 300,200,150 N These forces keep on changing with growth

3. Stress & Strain Stress Stress is the force per unit cross-sectional area that is acting on a material . Stress , is given by  = F/A Units of stress are N/m2 or Pa Strain Is the fractional change in the dimensions caused by the force Strain ,e is given by e =(L1-L0)/L0 L0 L1 Area=A Force =F

4. Principal types of stress

5. Principal types of stress

6. Principal types of stress (Tension) Universal testing machine

7. Principal types of stress (Tension) Bending of the beam resulting in both compressive  and tensile stress  Flexural Strength of Selected Dental Materials

8. Principal types of stress (Tension) Clinical Application A, Stresses induced in a three-unit bridge by a flexural force (P). B, Stresses induced in a two-unit cantilever bridge. Note that the tensile stress develops on the gingival side of the three-unit bridge and on the occlusal side of the cantilever bridge. Phillips page 77 11 th edit

9. Principal types of stress (Shear force) Atomic model illustrating Elastic shear deformation Atomic model illustrating Plastic shear deformation Phillips page 79 11 th edit

10. Compressive Strength Ratio of compressive force to cross sectional area perpendicular to axis of applied force. Compressive Strength: compressive stress within a compression test specimen at the point of fracture Complex stress pattern developed in cylinder subjected to compressive stress. Compressive stress (S C ), Shear stress (S S ), Tensile stress (S T )

11. Compressive Strength Compressive Strength of Selected Dental Materials

12. Strength of a material Is defined as the average level of stress at which a material exhibits a certain amount of initial plastic deformation or at which fracture occurs in test specimens of the same shape and size. The strength is dependent on several factors : strain rate. The shape of the test specimen. The surface finish. The environment in which a material is tested.

13. Elastic deformation Vs plastic deformation Elastic deformation It can be defined as a deformation which can be recovered after the stress is removed Plastic deformation It can be defined as a deformation which can not be recovered after the stress is removed

14. Stress & Strain Curve

15. Stress –Strain Curves

16. Practical Utilization Of The Stress Strain Curve (Tensile force) Plotting stress-strain curves. A, Stress-strain curve for a material subjected to tensile stress. Specimens illustrate amount of deformation at each point (A-D). B, Elastic deformation is exhibited up to the proportional limit (PL) and plastic deformation is exhibited from PL to the failure point (FP).

17. Young’s Modulus It is a constant that relates the stress and the strain in the linear elastic region, and is a measure of the stiffness or resistance of the material. young’s Modulus = stress/ strain Unit = Giga pascal ( GPa ) Also known as elastic modulus, or modulus of elasticity stress strain Young’s modulus

18. Elastic Modulus ( GPa ) of Selected Dental Materials

19. Proportional limit Is defined as the stress above which stress is no longer proportional to strain. Can also be defined as a limit to which a metal can bear a load without deforming permanently. Below this point there is no permanent deformation. Elastic limit The maximum stress a material can withstand before it become plastically deformed.

20. Yield strength or proof stress The stress required to produce a given amount of plastic strain Is a point beyond which strain are not fully recovered. It is different for different materials Brittle material :- high yield strength Ductile material :-low yield strength

21. Brittle and Ductile Materials Brittle materials Ductile Materials

22. Practical Utilization Of The Stress Strain Curve Resilience May be defined as the energy absorbed by a material in undergoing elastic deformation up to the elastic limit. Toughness Can be defined as the total amount of energy which a material can absorb up to the point of fracture.

23. Practical Utilization Of The Stress Strain Curve Schematic of different types of deformation in brittle (glass, steel file) and ductile (copper) materials of the same diameter and having a notch of the same dimensions. Craig ,13 th , Page 43

24. Ultimate tensile strength, shear strength, compressive strength Is a measure of stress required to fracture a material. Is the maximum stress that the specimen can withstand Is often different from the point of fracture stress strain UTS

25. Practical Utilization Of The Stress Strain Curve Ductility : the ability of a material to sustain a large permanent deformation under a tensile load before it fractures. Example, a metal that can be drawn readily into a long, thin wire is considered to be ductile. Malleability: The ability of a material to sustain considerable permanent deformation without rupture under compression, as in hammering or rolling into a sheet . Gold is the most ductile and malleable pure metal, and silver is second of the metals of interest to the dentist, platinum ranked third in ductility, and copper ranks third in malleability.

26. Significance of stress strain curve for the characterisation of the materials Rigid, strong, tough and ductile material Flexible, tough Rigid, strong, brittle;

27. Rigid, weak, brittle Flexible, weak, brittle Flexible, resilient. Significance of stress strain curve for the characterisation of the materials

28. Mechanical properties

29. Fatigue Properties Fatigue is defined as a progressive fracture under repeated loading. Fatigue tests are performed by subjecting a specimen to alternating stress applications below the yield strength until fracture occurs. Tensile, compressive, shear, bending, and torsional fatigue tests can all be performed. Page 12, 2 Flexural fatigue curve for a cobalt-chromium-nickel alloy used for partial dentures.

30. Fatigue Properties… Fatigue life If a force of a given magnitude and frequency is applied on a sample, then the number of the cycles required to cause the fracture are called as fatigue life. Fatigue limit it is the number of cyclic stress required to cause a fracture . Page 12, 2

31. Fatigue Properties …

32. Hardness Testing Hardness testing is done by applying a standardized force or weight to an indenter. This produces a symmetrically shaped indentation. Vickers test scheme

33. Hardness Testing That can be measured under a microscope for depth, area, or width of the indentation produced. The indentation dimensions are then related to tabulated hardness values. An indentation left after a Vickers hardness test A Vickers hardness tester

34. Hardness … With a fixed load applied to a standardized indenter, the dimensions of the indentation vary inversely with the resistance to penetration of the material tested. Brinell Hardness Test Knoop Hardness Test Vickers Hardness Test Rockwell Hardness Test Barcol Hardness Test Shore A Hardness Test B. Pyramid (Vickers) indentation test. A, Knoop hardness measurement.

35. Hardness … A, Indentation in soft material. B, Indentation in harder material. C, Microscopic view of indentations.

36. Hardness … Shapes of hardness indenter points (upper row) and the indentation depressions left in material surfaces (lower row). The measured dimension M that is shown for each test is used to calculate hardness. Phillips page 97 11 th edit

37. Hardness … Material VHS Enamel 350 Dentine 60 Acrylic resin 20 Dental amalgam 100 Porcelian 450 Co/Cr alloy 420 Vickers hardness numbers of some selected dental materials and tooth structure

38. Hardness …

39. Poisson Ratio If a test piece is loaded in tension or compression, then, in directions perpendicular to the load axis, corresponding lateral strains will appear. Tensile force  contraction Compressive force expansion Tensile force Compressive force

40. Rheology

41. Rheology Rheological properties are related to the time dependent deformation of a material under an applied stress, which in turn also depends on the magnitude and rate of applied stresses. Deformation may be elastic, which would recover after the removal of the force. It may be a permanent deformation.

42. Hooke’s law A perfectly elastic material obeys Hooke’s law, according to which when an applied stress is removed the material returns to its original state. For example, if shear stress τ is applied to a spring for some time t 1 and then removed the spring will return to its original position. Instant Recovery t 1 t Strain Instant Deformation Time

43. Hooke’s law … If it is a shear stress then Hooke’s law can be given by the following equation: τ γ (t) = G

44. Newton’s law of viscosity In the case of a viscous medium, Newton’s law of viscosity explains its behaviour and such a medium is called a Newtonian fluid/medium.

45. Newton’s law of viscosity .. For example, if shear stress τ is applied on a piston for some time t in a dashpot, filled with a Newtonian fluid with a viscosity of η : on removal of the applied stress the piston will not move back to its original position, resulting in permanent or plastic deformation. Linear Strain t 1 t Strain Permanent Deformation Time

46. Newton’s law of viscosity .. .t η γ(t) τ =

47. Viscoelasticity Materials usually can be classified as either viscous or elastic, but this can only be truly applied to substances such as water and steel, respectively; whereas all other materials lie in-between. Most biomaterials used in dentistry lie in-between elastic and viscous materials and are called viscoelastic materials, i.e: whenever stress is applied, some part of it is stored within the material (elastic response) while the other part is dissipated as heat (viscous response).

48. Viscoelasticity Such a material which shows a combination of viscous and elastic response can be explained with the help of : Maxwell model (close to the behaviour of a real liquid) Kelvin-Voigt Model (close to a real solid)

49. Maxwell Model A Maxwell model is a simple combination of a spring and a dashpot in series.

50. Maxwell Model .. When shear stress τ is applied to the system at time to, the system shows an immediate elastic response due to extension of the spring, followed by a slow time dependent viscous response by the movement of the piston in the dashpot. t 1 t Strain Elastic Response (spring) Viscous response (Dash pot) Time

51. Kelvin-Voigt Model Spring and dashpot are arranged in parallel and the extension of the spring depends on the movement of the piston in the cylinder, which in turn depends on the viscosity of the liquid. So any applied stress is shared between the two components of the model. When stress is applied on this mechanical model, the extension of the spring corresponds with the movement of the piston.

52. Kelvin-Voigt Model .. In other words, no strain occurs instantly, it needs time for the movement of the piston against the viscous fluid in the cylinder, and similarly on the removal of the stress the recovery is slow. However, in this model recovery is complete as there is a stored energy in the spring which causes the returning back of the piston in the cylinder Time Strain t 1 t

53. Creep & Flow In creep relatively small deformation produced by a relatively large stress over a long period of time In flow a greater deformation produced more rapidly with a smaller applied force. Creep recovery curve , showing, A , elastic, B , anelastic , and, C, viscous strain.

54. Rheological Properties .. It is applicable to both solids and liquids. For solids → creep and viscoelasticity For liquids → viscosity Viscosity ( η ) is given by η = shear stress/ shear rate Shear stress = pressure required to press it Shear rate = flow rate viscosity =K P/Q Pressure=P Plunger speed= Q

55. Viscosity … Low viscosity = low pressure  high flow rate High viscosity = high pressure  low flow rate Shear stress = K (shear rate ) n Where K = constant n = flow index

56. Newtonian fluids n=1 Shear stress α shear rate Viscosity is constant at all shear rate E.g. water, mineral oils Pseudoplastic fluids n < unity No measureable yield point, so flow can occur at any stress Increase in shear rate without increase in shear stress E.g. Polymers, adhesives and suspensions. Dilatant fluids n > unity Increase in viscosity with an increase in strain rate Shear diagrams of Newtonian, pseudo plastic, and dilatants liquids. (The viscosity is show n by the slope of the curve at a given shear rate.)

57. Dilatant fluids .. E.g. silicone products, pastes with higher filler loadings. Resin composites. In these materials increase in shear rate results in inter locking of the particles and they will increase the viscosity of the medium.

58. Time-dependence of viscosity Many materials used in dentistry involve the mixing of two components, thus initiating a chemical reaction which causes the material to change from a fluid to a rigid solid or elastomer.

59. Time-dependence of viscosity (Working time) Working time refers to the time after which the material cannot be manipulated without creating distortion in the final product. OR Manipulation becomes impossible when viscosity has increased beyond a certain point. The time taken to reach that point is the working time of the material

60. Time-dependence of viscosity (working time) For curve A; The material may become unmanageable when it reaches a viscosity value of V 1 , thus the material has a working time of T 1 . For curve B; viscosity does not begin to increase until the time T 2 and the viscosity has not reached V 1 until the time T 3 .

61. Time-dependence of viscosity (Setting time) It can be defined as the time taken for the material to reach its final set state or to develop properties which are considered adequate for that application. For alginate , it is the time when can be withdrawn without distortion or tearing. For gypsum , it is the time when it can be separated from the impression without fracture.

62. Assessment of setting time A : Material is unset B: Material is set

63. The End