seconds year PROPERTIES OF DENTAL MATERIAL 2.pptx

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1. MECHANICAL PROPERTIES BY DR. NUHA ELKADIKI BDS , MDS

2. Mechanical properties Restorative materials used in dentistry are subjected to different types of force during fabrication or mastication. Mechanical properties : are properties of materials deals with application of force to a body and reaction of this body to these force. The range of masticatory force varies from on area to another , from one individual to another, from male to female and from adult to child.

3. Mechanical properties Biting force Force teeth 665 N Molar 45O N bicuspids 220 N incisors

4. Force I.1. Stress. I.1.1. Types of stress. I.1.1.1 Normal or Axial . Tensile stress. Compressive stress. I.1.1.2 Tangential Stresses. Shear stress. I.1.1.3 Complex Stresses .

5. Force I.1 . Stress ( The internal reaction to the external force which is equal in intensity and opposite in direction ). stress = Force Area UNITS: Pa =N/m 2 MPa, MPa, MN/m 2 =N/mm 2, Kg/cm 2 , Ib /in 2

6. Force I.1.1. types of stress. I.1.1.1 Normal or Axial . Tensile Stress .( σ t) Results when the body is subjected to two sets of force acting away from each other in the same plane and in the same straight line . Tensile stresses tend to elongate the body.

7. Force I.1.1. types of stress. I.1.1.1 Normal or Axial . b. Compressive Stress .( σ c) Results when the body subjected to two sets of force acting toward each other in the same straight line and the same plane . They tend to shorten the body.

8. Force I.1.1. types of stress. I.1.1.2 Tangential Stresses. Shear stress . Results when two sets of force are directed parallel to each other no in the same line or plane . Shear tend to slide the atoms against each other and cause tearing .

9. Force I.1.1. types of stress. I.1.1.3 Complex Stresses .( τ ) Results when all types of stresses develop in a structure as a result of application of load .( Dental bridge)

10. Force I.1. Stress. I.2.Deformation and strain. I.2.1. Types of strain. 1.2.2. Poissons ration .(µ)

11. I.2.Deformation and strain. A body undergoes deformation when a force is applied to it . If we considered the change (deformation ) in the length ( L )the deformation equals Lf-L0. Strain ( € ) is the change in the length per unit length. Strain has no unit. Strain= deformation original length

12. I.2.Deformation and strain. I.2.1. Types of strain. Each type of stress is capable of producing a corresponding type of strain in the body . Tensile , compressive and shear strains can be either elastic or plastic. Elastic strain ( temporary )( elastic deformation ) when the body returns to its original dimension with the removal of stress. Plastic strain ( permanent ) when the body demonstrated permanent amount of deformation on removal of stress.

13. I.2.Deformation and strain. 1.2.2. Poissons ration .(µ) During axial loading . There is a simultaneous axial and lateral strain .e.g. under tensile loading ,the material elongated in the axial direction and compressed in lateral direction .within elastic range . Poisson ratio . Indicated that reduction in cross section is proportional to the elongation during the elastic deformation.

14. I.2.Deformation and strain. 1.2.2. Poissons ration .(µ) µ= Lateral strain Axial strain Brittle material show less poissons ratios. Ductile materials show higher poissons ratios.

15. II. Relation between stress and strain (stress-strain curve) The stress-strain relationship of dental material is studied by measuring the load and deformation and the calculating the corresponding stress and atrain . After testing material a plot of stress versus strain can be obtained. The resulting stress-strain curve is called an engineering stress-strain curve.

16. Until a certain stress value at P.L. we find that the strain is proportional to the stress induced . This part of the curve obeys hooks law which states that ˝ strain is directly proportional to the stress until a stress value known as the proportional limit ̋

17. Definitions II. 1 Proportional limit ( value of stress at P.L.) It’s the greater stress the material can withstand without deviation. II.2 Elastic limit (value of stress at E.L.) Its greater stress the materials can withstand without permanent deformation resulting .it is Therefore , describing the elastic behavior of the material.

18. Definitions GIVE REASON : for most of the materials the proportional limit and elastic limit represent the same value , they differ however in the fundamental concepts? Elastic behavior describe the behavior of the material meanwhile ; proportional limit describe the proportionality between stress and strain.

19. II.3 Yield strength (stress value at Y.P.) The stress at which the material begins to function in a plastic manner . At this stress a limited permanent strain has occurred . Percent offset : it’s the amount of permanent strain equivalent to yield strength .( 0.1 %) E.L Y.P STRESS STRAIN

20. Definitions Importance of yield strength yield strength is an important property of dental material as the material should withstand high stresses while in function without permanent deformation . During adjustment of restoration the stress applied must be greater than yield strength to produce permanent deformation e.g. burnishing . If the restoration is permanently deformed while in function even without breakage ,it is considered as functional failure .

21. Definitions II.4 Ultimate strength (the stress value at U.S.) The maximum stress that material can withstand before fracture . GIVE REASON : The yield strength is of greater importance than ultimate strength? Because at the yield strength the material will start to deform. II.5 Fracture strength( fracture toughness) The stress at which a material fracture .

22. Definitions II.6 Modulus of elasticity (young's Modulus) ̋ E ̋ The stiffness of the material within the elastic range(resistance for elastic deformation) . It represent the slop of elastic portion of the stress and strain curve. E= σ /ɛ ( same unit of stress ). The stronger the basic attraction force inside the materials the greeter the value of modulus.

23. represent the steepness of the stress strain curve and so the rigidity of the materials. Material( b) demonstrated more elastic deformation , which mean ( a) is rigid . Rubber has low modulus while the metal & alloy has high modulus as they deformed at high stress. stress strain a b

24. Definitions Importance Modulus of elasticity : Materials with high modulus of elasticity shows an even distribution over the are to which the load is applied . Bridge particularly long span bridge . Denture base materials : denture materials with high modulus of elasticity can be used in thinner section without fear of uneven stress distribution (cobalt chromium) Base under restoration should be rigid to increase fracture resistance of the filling.

25. Definitions II.7 Flexibility Is the strain that occur when the material is stressed to its proportional limits . Clinical importance : Orthodontic wires & endodontic files and reamers as considerable amount of elastic bending is needed with little stress Elastic impression materials to allow ease removal of impression material after impression making .

26. STRESS STRAIN

27. Definitions II.8 Ductility and Malleability Is ability of the material to be permanently deformed without rupture or fracture. Ductility: is ability of material to be drawn into wire under tensile force. Malleability : ability to be hammered or rolled into thin sheet without fracture. Importance : Ductility is property that has been related to the workability of a material in the mouth. Malleability ability of the material to be burnished .

28. Importance are properties of metal or alloy. They indicated the workability of the alloy e.g. the burnish -ability of alloy .

29. Definitions II.9 Brittleness Material which demonstrate little or no plastic deformation on load application. Brittle fracture occur by crack propagation. Importance : As brittle material are weak in tension , dentist should design brittle restoration to receive compressive load during function and minimize tensile load. Amalgam is highly brittle material.

30. Ductile material Brittle material

31. Definitions II.10 Elongation Is the deformation that result from application of tensile force. The represented the maximum amount of permanent deformation . %Elongation = ( increase in length / original length )x100 Clinical importance It is a measure of the ductility of the material Clasps ,crown , and inlay should have high elongation % so they can be burnished . They higher the yield strength of the material the less the elongation.

32. Definitions II.12 Resilience Amount of energy needed to deform the material to its proportional limit . This is called the stored energy because when the load is removed it is released ʻ causing complete recovery of the deformed material ʼ It is represented by the area under the straight portion of the stress-strain curve i.e. the area of the triangular . Clinical importance : It is an important requirement of Orthodontic wires because their stored energy can be released over the required time to move teeth. Denture base materials

33. Definitions II.13 Toughness It is the energy required to stress the material to the point of fracture. it is represented by the area under the elastic and plastic portion of the stress-strain curve. The toughest materials are those with High proportional limit and Good ductility.

34. Definitions II.14 Fracture toughness The amount of energy required to fracture the material with crack propagation. Brittle material have low fracture toughness , while ductile material have high fracture toughness. Clinical importance: Give me indication about the materials ability to resist crack propagation , so we can make modification in the material composition to increase fracture toughness.

35. Other mechanical properties and tests Cantilever bending bending properties are usually measured by clamping a sample at one end and applying a force at fixed distance from the face of the clamp . As the force is increased and the sample is bend , corresponding value for the angle of the bending and the bending moment are recorded . bending moment =force x distance Bending moment Angular deflection

36. Other mechanical properties and tests Cantilever bending Clinical importance bending properties of many materials (endodontic files )are important in ortho and endodontic

37. Other mechanical properties and tests Fatigue strength test Fatigue is a progressive fracture under repeated cyclic loading below proportional limit. Fatigue strength is stress at which a material fails under cyclic loading . Fatigue test determines the magnitude of stress at or below which the material can withstand an infinite no. of cyclic loading without fracture.

38. Other mechanical properties and tests Fatigue strength test Endurance limit : amount of stress at or below which the material can withstand an infinite no. of cyclic loading without fracture. Fatigue limit no. of cycles required to fracture the material . Clinical importance: Dental material are subjected to alternating force during mastication rather than static force .therefore , fatigue properties can help the dentist designing the restoration (clasp of partial denture)

39. Other mechanical properties and tests Impact strength is the energy required to fracture a material under sudden force. Clinical importance: impact test simulates a sudden blow from accident or dropping of a restoration e.g. dropping of the denture on the floor .material like fused cement amalgam and some plastic have low impact strength values

40. Other mechanical properties and tests Impact strength Dental significance : Denture has low impact strength so it must be cleaned in a beaker contain full of water. Dropping of denture is an impact so must be designed to withstand dropping and sudden blow.

41. Other mechanical properties and tests Transverse strength (modulus of rupture or flexure strength) The test is per formed by subjecting a simple beam supported at end to a static load at the middle. S=3PL/2bd 2 S=flexure strength P=load L=distance between supports b=breath of the specimen. d= depth of the specimen

42. p ½ p 1/2p

43. tension compression

44. Surface mechanical property Surface hardness(surface property not bulk property) Is the resistance to permanent indentation or penetration, is the surface properties that cannot be obtained from stress-strain curve . Principles of hardness test Each test depends on the penetration of small symmetrically shaped object into the surface of the material being tested The dimension of the indentation will vary inversely with the hardness of the material being tested ( small indentation refers to hard material while large indentation is obtained from soft one )

45. Surface mechanical property Surface hardness(surface property not bulk property) Dental significant of hardness Avoid scratching structure like teeth or restoration natural teeth should not be opposed by harder like porcelain. Avoid scratching of soft material (model &die materials)because it decrease its accuracy. Restoration made from hard material like cobalt chromium Are very difficult to finish and polish.

46. Surface mechanical property Wear: Is loss of the material resulting from mechanical action ( wear is usually undesirable but during finishing and polishing is highly desirable ) Cause of wear physiological (attrition) Pathological Mechanical.(abrasion)

47. Viscoelasticity and creep Many material exhibit a combination of elastic , an elastic and viscous behavior. combination of viscoelastic strain is time dependant . Elastic and an elastic portion are recovered , but viscous Are not.

48. Viscoelasticity and creep Ideal elastic material When load is applied strain occur immediately . No increase in strain with time . When load is removed strain disappear immediately and completely to 0 . Strain is time independent .

49. Viscoelasticity and creep Ideal viscous material when load is applied strain increase as the time of load application is increased. When load removed no strain recovery occur . Strain is time dependent.

50. Viscoelasticity and creep An elastic material (delayed elasticity) When load is applied strain increase with time When load is removed there is complete delayed strain recovery.

51. Viscoelasticity and creep viscoelastic material ( sensitive to rate of loading ) When load is applied : Strain of elastic part occur immediately while strain of viscous and an elastic part is time dependent. When load is removed the elastic strain is immediately recovered and the an elastic strain is gradually recovered .however, viscous strain not recovered . Unrecovered viscous strain reduce the accuracy of the impression .

52. Viscoelasticity and creep Clinical importance Elastic impression material should be removed sharply from mouth (snap removal)to obtain . Highest recovery. Highest tear strength.

53. Viscoelasticity and creep Creep Is time dependent permanent deformation . Permanent deformation of material held for long period of time at stress below yield strength. This mechanism occur only at temperature near the softening point of materials. Since the softening temperature of metal and ceramic is above room or mouth temperature they don’t creep in dental application, many polymers such waxes are near their softening point at room temperature creep occur.

54. Viscoelasticity and creep Creep Importance: All metals tend to creep when stressed near its melting temperature(amalgam) they contain component with melting temperature slightly above room temperature , they undergo plastic deformation while in function which should be kept minimum. The term flow ,rather than creep has generally been used in dentistry in describe ,time dependant plastic deformation for amorphous material ,such as waxes