mechanical properties of dental material.pptx

mechanical properties of dental materials

1. MECHANICAL PROPERTIES OF DENTAL MATERIALS Presented by : Guided by: Dr Ritu Khichar Dr Bobbin Gill Dr Nirmala Bishnoi Dr vaishak Augustine

2. CONTENTS Introduction Stress and its types Strain Mechanical properties based on elastic deformation Strength properties Mechanical properties based on plastic deformation conclusion

3. Introduction Mechanical properties are defined by the laws of mechanics, that is, the physical science that deals with energy and forces and their effects on bodies. Mechanical properties are the measured responses, both elastic (reversible on force removal) and plastic (irreversible or nonelastic ), of materials under an applied force.

4. Mechanical properties are expressed most often in units of stress and/or strain. They can represent measurements of: (1) elastic or reversible deformation (i.e., proportional limit, resilience, and modulus of elasticity); (2) plastic or irreversible deformation (e.g., percent elongation and hardness); or (3) a combination of elastic and plastic deformation such as toughness and yield strength,

5. Stresses can be classified as Tensile stress Compressive stress Shear stress Flexural stress

6. In the oral environment shear failure is unlikely to occur for at least four reasons : (1) many of the brittle materials in restored tooth surfaces generally have rough, curved surfaces . (2) The presence of chamfers, bevels, or changes in curvature of a bonded tooth surface would also make shear failure of a bonded material highly unlikely. ( 3) To produce shear failure the applied force must be located immediately adjacent to the interface

7. ( 4) Because the tensile strength of brittle materials is usually well below their shear strength values, tensile failure is more likely to occur.

8. Strain Strain, or the change in length per unit length, is the relative deformation of an object subjected to a stress . Strain may be either elastic or plastic or elastic and plastic. Elastic strain is reversible. The object fully recovers its original shape when the force is removed . Plastic strain represents a permanent deformation of the material that does not decrease when the force is removed.

9. Clinical Application While providing a restoration on occlusal surface, care should be taken that the occlusal contacts should entirely be placed on restoration or tooth structure but never at interface otherwise it will lead to stress generation at margins of restoration. For crown cementation cements which can transfer stress to tooth structure without undergoing permanent deformation should be used.

10. Mechanical properties based on elastic deformation Modulus of elasticity Flexibility Resilience Poissons ratio

11. Elastic modulus or Young’s modulus/ modulus of elasticity It is relative stiffness or rigidity of a material . It is the ratio of elastic stress to elastic strain. It is constant for a material . It is unaffected by the amount of elastic or plastic stress and is independent of the ductility of the material.

12. Implants: Function of implant depends upon rigidity of implant structure . This is related to dimensions and modulus of elasticity of material from which implant is manufactured. Use of high modulus materials enables implants of smaller cross-sectional bulk to be used.

13. Hooke’s Law According to law , within the limits of elasticity the strain produced by a stress is proportional to the stress. The stress at which a material ceases to obey Hooke’s Law is known as limit of proportionality. Stress/Strain= Constant Value of constant depends on material and type of stress.

14. • Dynamic Young’s modulus The velocity of the sound wave and the density of the material can be used to calculate the elastic modulus and poisson’s ratio values by using ultrasonic longitudinal and transverse wave transducers and appropriate recievers .

15. Flexibility The maximum flexibility is defined as the flexural strain that occurs when the material is stressed to its proportional limit.

16. • Resilience or “Springiness” It is the amount of energy absorbed within unit volume of a structure when it is stressed to its proportional limit, or the relative amount of elastic energy per unit volume released on unloading of a test specimen . The restorative material should exhibit a moderately high elastic modulus and relatively low resilience, thereby limiting the elastic strain that is produced.

17. A high value of resilience is one parameter often used to characterise elastomers. Such materials which maybe used to apply a cushioned lining to a hard denture base are able to absorb considerable amounts of energy without being distorted.

18. • Poisson’s ratio During axial loading in tension or compression there is a simultaneous axial and lateral strain . Within the elastic range, the ratio of the lateral to the axial strain is called Poisson’s ratio. Poisson’s ratio of some dental materials are Amalgam-0.35 Zinc phosphate cement- 0.35 Enamel- 0.30 Resin composite-0.24

19. Strength Properties Strength is the stress necessary to cause either fracture (ultimate strength) or a specified amount of plastic deformation (yield strength). Strength may be described by one or more of the following properties : Proportional limit Elastic limit Yield strength Ultimate tensile strength/Shear strength/Flexure strength

20. • Proportional limit •Maximum stress at which stress is proportional to strain and above which plastic deformation occurs. •If the material obeys Hooke’s law it behaves elastically up till PL and it springs back to its initial shape and size at the instant force is removed.

21. A practical example of situation in which high proportional limit is required in connectors of partial dentures. Materials such as Cobalt chromium alloy with high value of proportional limit is popular for this application since it can withstand high stresses without distortion.

22. • Elastic limit The elastic limit of a material is defined as the greatest stress to which a material can be subjected such that it returns to its original dimensions when the force is released.

23. • Yield strength • It is a property that represents the stress value at which a small amount (0.1 or 0.2%) of plastic strain has occurred. A value of 0.1 or 0.2% is selected and is referred to as percent offset . • The YS is the stress required to produce the particular offset strain that has been chosen.

24. • Cold working (strain hardening or work hardening) • When a material has been stressed beyond its PL , the hardness and strength of the metal increase at the area of deformation, but the ductility of the metal decreases.

25. Plastic Deformation If the material is deformed by a stress at a point above the proportional limit before fracture, the removal of the applied force will reduce the stress to zero, but the strain does not decrease to zero because plastic deformation has occurred. Thus , the object does not return to its original dimension when the force is removed. It remains bent, stretched, compressed, or otherwise plastically deformed.

26. Amalgam undergoes certain amount of plastic deformation or creep when subjected to dynamic intra-oral stresses. Creep causes amalgam to flow such that unsupported amalgam protrudes from margin of cavity. These unsupported edges are weak and may be further weekend by corrosion. Fracture causes formation of ditch around margins of amalgam.

27. • Diametral tensile strength/Diametral compression strength • This test is used only for materials that exhibit elastic deformation and little or no plastic deformation . • Tensile stress= 2P/ π Dt

28. • Flexure strength/transverse strength/ modulus of rupture A strength test of a bar supported at each end, or a thin a disk supported along a lower support circle, under a static load . This test is a collective measurement of tensile, compressive and shear stresses simultaneously.

29. • Fatigue strength Stress values well below the UTS of a material can produce premature fracture of a dental prostheses because microscopic flaws grow slowly over many cycles of stress. This phenomenon is called as fatigue failure . Fatigue behavior is determined by subjecting a material to cyclic stress of a maximum known value and determining the number of cycles that are required to produce failure.

30. •Impact strength Defined as the energy required to fracture a material under an impact force. The term impact is used to describe the reaction of a stationary object to a collision with a moving object. A Charpy -type impact tester/ izod impact tester is used to measure impact strength

31. Toughness The amount of elastic and plastic deformation energy required to fracture a material . It is indicated by total area under the stress-strain graph . Toughness increases with increase in strength and ductility. The greater the strength and the higher the ductility, the greater the toughness.

32. • Fracture toughness Fracture toughness is a mechanical property that describes the resistance of brittle materials to the catastrophic propagation of flaws under an applied stress. Unit - MPa-m1/2

33. Low copper amalgam is tougher than high copper amalgam. Modelling waxes are tough enough to resist fracture when withdraw from shallow under cuts.

34. Brittleness It is the relative inability of a material to sustain plastic deformation before fracture of a material occurs . Or a brittle material fractures at or near its proportional limit.

35. Enamel is brittle when compared to dentin. So during cavity preparation care should be taken that enamel has sound support which is more capable of bearing stress. Porcelain is brittle and hard so teeth constructed from this material are more likely to chip and fracture than acrylic teeth.

36. Ductility and malleability Ductility represents the ability of a material to sustain a large permanent deformation under a tensile load before it fractures. Material which sustains tensile stress and considerable permanent deformation without rupture, it is ductile. The ability of a material to sustain considerable permanent deformation without rupture under compression, is termed as malleability

37. There are three common methods for the measurement of ductility 1 . the percent elongation after fracture . 2. the reduction in area of tensile test specimens . 3. the maximum number of bends performed in a cold bend tests

38. Gold is most malleable metal. Because of this property burnish ability of margins of restoration is possible. Clasps or wires of dentures constructed from ductile alloys Can be altered by bending during alteration of appliances.

39. • Hardness The relative hardness of a substance is based on its ability to resist scratching . “Resistance to indentation ”. The properties that are related to the hardness of a material are compressive strength, proportional limit and ductility. The different tests of to measure hardness are: Barcol , Shore, Brinell , Vickers, Rockwell, and Knoop .

40. • Brinell hardness tests • A hardened steel ball is pressed under a specified load into polished surface of a material. The load is divided by the area of the projected surface of the indentation, and the quotient is referred to as the Brinell Hardness Number. BHN

41. • Rockwell hardness tests • Steel ball or a conical diamond point is used. instead of measuring the diameter of penetration the depth is measured directly by a dial gauge on the instrument (RHN)

42. • Vickers hardness tests • Square based pyramid is used. • VHN is computed by dividing the load by the projected area of indentation . • This test is suitable for determining the hardness of brittle materials, for example tooth structure.

43. • Knoop hardness tests • Diamond tipped tool is used • The projected area is divided into the load to give KHN . • The KHN is independent of the ductility of the tested material.

44. Micro-hardness tests • Knoop and Vickers • Less than 9.8 N loads are used and the resulting indentations are small and are limited to depth of less than 19 micron . • Hence they are capable of measuring the hardness in small regions of thin objects .

45. • Macro-hardness tests • Brinell and Rockwell tests • Give average hardness values over much larger areas.

46. Shore and barcol tests • Measure the hardness of rubber and plastic type of dental materials. • The principle of these tests is based on resistance to indentation.

47. Hardness of amalgam is lower than enamel,a factor that may be responsible for amalgam restorations developing surface facets when they make contact with cusps of opposing teeth Gold wears at a rate similar to enamal therefore does not cost excessive loss of enamal of opposing teeth. Harder materials are difficult to polish by mechanical means .

48. Hardness is also used to give indication of abrasion resistance of a material. There is a vast difference in hardness between acrylic resin and porcelain. Acrylic teeth are more likely to suffer abrasion than porcelain teeth. Whereas porcelain teeth cause more abrasion of natural enamel opposing to the restoration.

49. Stress concentration factors Areas of high stress concentration are caused by one or more of the following factors : 1. Surface flaws, such as porosity, roughness or machining damage. 2 . Interior flaws such as voids or inclusions. 3 . Marked changes in contour 4 . A large difference in elastic moduli or thermal expansion coefficient across bonded interface. 5 . A Hertzian load (or point contact)

50. Conclusion Three inter-related factors are important in long term function of restorative materials Material choice Component geometry Component design

51. The failure potentials of prosthesis under applied forces is mainly related to these properties of prosthetic material . Goal should be to ensure that properties of oral restorations must adequately withstand stresses of mastication for e.g. In areas of high stress materials having high elastic moduli and strength should be used. Restorations should be designed that resulting forces of mastication are distributed as uniformly as possible.