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Mechanical Properties of Dental Materials

Mechanical Properties of Dental Materials

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Mechanical Properties of Dental Materials

  1. 1. 1
  2. 2. MECHANICAL PROPERTIES OF DENTAL MATERIALS Dr. Nithin Mathew
  3. 3. Mechanical Properties of Dental Materials - Dr. Nithin Mathew CONTENTS • Introduction • Force • Stress • Tensile • Compressive • Shear • Flexural • Strain • Elastic & Plastic Deformation • Stress – Strain Curve 3 • Mechanical Properties based on Elastic Deformation • Young’s Modulus / Modulus of Elasticity • Dynamic Young’s Modulus • Shear Modulus • Flexibility • Resilience • Poisson’s Ratio
  4. 4. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Strength Properties • Proportional Limit • Elastic Limit • Yield Strength • Ultimate tensile strength, shear strength, compressive strength & flexural strength 4 • Mechanical Properties based on Plastic Deformation • Flexural Strength • Impact Strength • Toughness • Fracture toughness • Brittleness • Ductility • Malleability • Hardness
  5. 5. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Hardness tests • Brinell • Rockwell • Vickers • Knoops • Stress Concentration Effects • Methods To Minimize Stress Concentration • Conclusion • References 5
  6. 6. Mechanical Properties of Dental Materials - Dr. Nithin Mathew INTRODUCTION • In the oral environment restorative materials and dental appliances are exposed to chemical, thermal and mechanical challenges. • These challenges can cause deformation of materials. • The mechanical properties of a material define how materials respond to mechanical challenges. • Mechanical properties are defined by the laws of mechanics. i.e It is the physical science that deals with energy, forces and their effects on the bodies. 6
  7. 7. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • So it is necessary to understand the principles involved in a variety of mechanical properties to optimise the clinical service of a material. • Mechanical properties are measured responses, both elastic (reversible on force removal) and plastic (irreversible on force removal), of materials under an applied force or distribution of forces. • They are expressed most often in units of stress and strain. 7
  8. 8. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • They can represent measurements of • Elastic deformation (reversible) • Proportional limit • Resilience • Modulus of elasticity • Plastic deformation (irreversible) • Percentage of elongation • Combination of both • Toughness • Yield strength 8
  9. 9. Mechanical Properties of Dental Materials - Dr. Nithin Mathew FORCE • In physics, a force is any influence that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction. • A force is defined by 3 characteristics: • Point of application • Magnitude • Direction of application • The SI unit of force is Newton (N). 9
  10. 10. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Occlusal Forces • Max. occlusal forces: 200 – 3500N • Occlusal forces between adult teeth are highest in the posterior region closest to the mandibular hinge axis and decrease from the molar to the incisors. • Forces on first and second molars vary from 400 to 800N. • Averageon bicuspids, cuspids and incisors is about 300, 200 and 150N. • Increase in force from 235 – 494N in growing children with an average yearly increase of about 22N. 10
  11. 11. Mechanical Properties of Dental Materials - Dr. Nithin Mathew STRESS & STRAIN Stress • The force per unit area acting on millions of atoms or molecules in a given plane of a material. • Force acting per unit area. • Unit of measurement is Megapascal (Mpa). • Stress is the internal resistance of a material to an external load applied on that material. Stress (σ)= 𝐹𝐹 (𝑁𝑁) 𝐴𝐴 (𝑚𝑚2 ) 11
  12. 12. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Type Of Stress Produced By Examples Residual Stress Stress caused within the material during the manufacturing process During welding Structural Stress Stresses produced in the structure during function. Weights they support provide the loadings In abutments of fixed partial denture Pressure Stress Induced in vessels containing pressurized materials In dentures during processing under pressure and heat Flow Stress Force of liquid striking against the wall acts as the load Molten metal alloy striking the walls of the mould during casting Thermal Stress Material is subjected to internal stress due to different temperatures causing varying expansions in the material Materials that undergo thermal stress such as inlay wax, soldering and welding alloys Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue 12
  13. 13. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • By means of the direction of force, stresses can be classified as: • Tensile stress • Compressive stress • Shear stress • Flexural stress 13
  14. 14. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Tensile Stress • Tensile stress occurs when 2 sets of forces are directed away from each other in the same straight line. • Also when one end is constrained and the other end is subjected to a force away from the constraint. 14 • It is caused by a load that tends to stretch or elongate a body.
  15. 15. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 15 • In fixed prosthodontics, a sticky candy (Jujube) can be used to remove crowns by means of a tensile force when the patient tries to open the mouth after the candy has mechanically bonded to opposing teeth or gums.
  16. 16. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Compressive Stress • Compressive stress occurs when 2 sets of forces are directed towards each other in the same straight line. • Also when one end is constrained and the other end is subjected to a force towards the constraint. • It is caused by a load that tends to compress or shorten a body. 16
  17. 17. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Shear Stress • Shear stress occurs when 2 sets of forces are directed parallel to each other but not along the same straight line. • A shear stress tends to resist the sliding of one portion of a body over another. • Shear stress can also be produced by a twisting or torsional action on a material. 17
  18. 18. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 18 • Eg: If a force is applied along the surface of tooth enamel by a sharp-edged instrument parallel to the interface between the enamel and the orthodontic bracket, the bracket may debond by shear stress failure of the resin luting agent.
  19. 19. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 19 In the oral cavity, shear failure is unlikely to occur due to: 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. 3. To produce shear failure, the applied force must be located immediately adjacent to the interface. The farther away from the interface the load is applied, more likely that it is a tensile failure.
  20. 20. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 20 5. Since the tensile strength of brittle materials is usually well below their shear strength values, tensile failure is more likely to occur.
  21. 21. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Flexural Stress (bending) • Force per unit area of a material that is subjected to flexural loading (bending). • A shear stress tends to resist the sliding of one portion of a body over another. • A flexural force can produce all the three types of stresses in a structure, but in most cases fracture occurs due to the tensile component. 21
  22. 22. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 22 • Flexural stresses produced in a three-unit fixed dental prosthesis. • Flexural stresses produced in a two-unit cantilever bridge.
  23. 23. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Strength: Strength of a material is defined as the average level of stress at which a material exhibits a certain amount of plastic deformation or at which fracture occurs in several test specimens of the same shape and size. 23
  24. 24. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 24 • Clinical strength of brittle material (ceramics, amalgams, composites) may appear to be low when large flaws are present or if stress concentration areas exist because of improper design of a prosthetic component. • So under these conditions, such appliances may fracture at a much lower applied force because the localized stress exceeds the strength of the material at the critical location of the flaw or stress concentration.
  25. 25. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Elastic stress in ductile material such as gold alloys do not cause any permanent damage. • Plastic stresses does cause permanent deformation and sometimes it may be high enough to produce a fracture. 25
  26. 26. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Since the stress in structure varies directly with the force ad inversely with area, the area over which the force acts is an important consideration. • This is true when considering dental restorative materials where the area over which the occlusal forces acts are extremely small such as the cuspal areas of contact. 26
  27. 27. Mechanical Properties of Dental Materials - Dr. Nithin Mathew STRAIN • Defined as the change in length per unit original length. • Strain of a material is reported as percentage(%). • Strain may be either elastic, plastic or a combination of both elastic and plastic. • Elastic strain is reversible. ie it disappears when force is removed. • Plastic strain represents permanent deformation of the material which never recovers when the force is removed. Strain (ε)= 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑖𝑖 𝑖𝑖 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 (𝛥𝛥𝑙𝑙) 𝑈𝑈𝑈𝑈𝑈𝑈𝑈𝑈 𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 (𝑙𝑙0) 27
  28. 28. Eg: • A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of 0.000002 m2. • If the wire is 0.1m long and if it stretches 0.001m under the load, • ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of 0.01% Strain (ε)= (𝛥𝛥𝑙𝑙) (𝑙𝑙0) = 0.001 𝑚𝑚 0.1 𝑚𝑚 = 0.0001 = 0.01 % Stress (σ)= 200 𝑁𝑁 0.000002 𝑚𝑚2 = 200 2 x 106 = 100 MPa 28
  29. 29. Mechanical Properties of Dental Materials - Dr. Nithin Mathew ELASTIC AND PLASTIC DEFORMATION 29 Elastic Shear Deformation Plastic Shear Deformation
  30. 30. PL = 1020 YS = 1536 UTS = 1625 0 200 400 600 800 1000 1200 1400 1600 1800 0 0.5 1 1.5 STRESS – STRAIN CURVE Stress(Mpa) Strain (%) YS (0.2%) = 1536 Mpa UTS = 1625 Mpa E = 1020/0.053 = 192 GPa • PL : Proportional Limit • YS : Yield Strength • UTS : Ultimate Tensile Strength • E : Elastic Modulus Mechanical Properties of Dental Materials - Dr. Nithin Mathew
  31. 31. Mechanical Properties of Dental Materials - Dr. Nithin Mathew MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION • Mechanical properties that are measures of the elastic strain of dental materials includes: • Young’sModulus / Modulus of Elasticity • Dynamic Young’s Modulus • Shear Modulus • Flexibility • Resilience • Poisson’s Ratio 31
  32. 32. Mechanical Properties of Dental Materials - Dr. Nithin Mathew YOUNG’S MODULUS • Elastic modulus describes the relative stiffness of a material which is measured by the slope of the elastic region of the stress strain graph. • It is the stiffness of a material that is calculated as the ratio of the elastic stress to elastic strain. • ie. a stiff material will have a high modulus of elasticity while a flexible material will have a low modulus of elasticity. 32 E = 1020/0.053 = 192 GPa
  33. 33. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Elastic modulus of a tensile test specimen can be calculated as: • By definition, Stress (σ)= 𝐹𝐹 𝐴𝐴 Strain (ε)= 𝛥𝛥𝛥𝛥 𝑙𝑙0 33 F : Applied Force A : Crossectional Area 𝛥𝛥𝛥𝛥 : Change In Length 𝑙𝑙0 : Original Length E : Elastic Modulus E = 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 = σ ε = ⁄𝐹𝐹 𝐴𝐴 ⁄𝛥𝛥𝛥𝛥 𝑙𝑙0
  34. 34. 34 E = 1020/0.053 = 192 GPa Steep Line Higher modulus and more rigidity Flat Line Lower modulus and less rigidity Mechanical Properties of Dental Materials - Dr. Nithin Mathew
  35. 35. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Eg: Principle of Elastic Recovery • Burnishing of an open metal margin, where a dental abrasive stone is rotated against the metal margin to close the marginal gap as a result of elastic and plastic strain. • After the force is removed, the metal springs back to an amount equal to the total elastic strain. • A final clearance of 25µm is available for the cement. 35
  36. 36. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Eg: Impression Material • The impression materials should have a low modulus of elasticity to enable it to be removed from the undercut areas in the mouth. • Modulus of elasticity should not be very low that the material cannot withstand tearing. 36
  37. 37. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Hooke’s Law • According to this law, within the limits of elasticity the strain produced by a stress (of any one kind) is proportional to the stress. • The stress at which a material ceases to obey Hooke's Law is known as the limit of proportionality. • Hooke's law can be expressed by the formula • The value of the constant depends on the material and the type of stress. 37 𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺 𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺𝑺 𝑺𝑺 = 𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪
  38. 38. STRESS – STRAIN CURVE : Enamel & Dentin PL = 235 CS = 262 PL = 176 CS = 234 0 50 100 150 200 250 300 CompressiveSress(Mpa) Strain Enamel Dentin • Dentin is capable of sustaining significant plastic deformation under a compressive load before it fractures. • Enamel - more stiffer and brittle than dentin. • But dentin more flexible and tougher. EEnamel = 11.7 GPa EDentin = 33.6 GPa Mechanical Properties of Dental Materials - Dr. Nithin Mathew
  39. 39. Mechanical Properties of Dental Materials - Dr. Nithin Mathew POISSON’S RATIO • During axial loading in tension or compression, there is a simultaneous strain in the axial and transverse or lateral directions. • Under tensile loading, as a material elongates in the direction of the load, there is a reduction in cross- section. • Under compressive loading, there is an increase in the cross-section. 39
  40. 40. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Within the elastic range, the ratio of the lateral to the axial strain is called the Poisson’s Ratio. • Poisson’s ratio is a unit-less value since it is a ratio of 2 strains. • Most rigid materials such as enamel, dentin, amalgam, composite, etc. exhibit a poisson’s ratio of about 0.3 • More ductile materials such as soft gold alloys show a higher degree of reduction in cross-sectional area and higher poisson’s ratio. 40
  41. 41. Mechanical Properties of Dental Materials - Dr. Nithin Mathew DYNAMIC YOUNG’S MODULUS • Elastic modulus can be measured by a dynamic method as well as a static method. • The velocity at which the sound travels through a solid can be readily measured by ultrasonic transducers and receivers. 41 • The velocity of the sound wave and the density of the material can be used to calculate the elastic modulus and poisson’s ratio.
  42. 42. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • If a shear stress was induced instead of a uniaxial tensile or compressive stress, the resulting shear strain could be used to define a shear modulus for the material. • The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and Poisson’s Ratio (v) : • The value of 0.25 to 0.30 for Poisson’s ratio is typical. • Therefore the shear modulus is usually 38% of the elastic modulus value. 42 G = 𝐸𝐸 2(1+𝑣𝑣) = 𝐸𝐸 2(1+0.3) = 0.38E
  43. 43. Mechanical Properties of Dental Materials - Dr. Nithin Mathew FLEXIBILITY • Defined as the flexural strain that occurs when the material is stressed to its proportional limit. • Materials used to fabricate dental appliances and restorations, a high value for the elastic limit is a necessary requirement. 43 • This is because the structure is expected to return to its original shape after it has been stressed and the force removed. • Maximum flexibility – flexural strain that occurs when the material is stressed to its proportional limit
  44. 44. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • There are instances where a large strain or deformation may be needed with a moderate or slight stress such as in an orthodontic appliance. • Here a spring is often bent a considerable distance under the influence of a small stress. • In this case, the structure is said to possess the property of flexibility. 44
  45. 45. Mechanical Properties of Dental Materials - Dr. Nithin Mathew RESILIENCE • It is the amount of energy per unit volume that is sustained on loading and released upon unloading of a test specimen. 45 • Term resilience is associated with springiness of a material but it means precisely the amount of energy absorbed within a unit volume of a structure when it is stressed to its proportional limit.
  46. 46. • Resilience of 2 or more materials can be compared by observing the areas under the elastic region of their stress-strain graph. • ie The material with the longer elastic area has the higher resilience. 46
  47. 47. • When a dental restoration is deformed during mastication, it absorbs energy. • If induced stress is not greater than proportional limit, the restoration is not permanently deformed. • ie only elastic energy is stored in it. • So restorative material should exhibit a moderately high elastic modulus and relatively low resilience – limiting the elastic strain produced. 47
  48. 48. • Chewing force on dental restoration causes Deformation (determined by the magnitude of the induced stress) • Large deformation do not occur due to the proprioceptive receptors in the periodontal ligament. • The pain stimulus causes the force to be decreased and induced stress to be reduced. • This prevent damage to teeth or restoration. 48
  49. 49. Mechanical Properties of Dental Materials - Dr. Nithin Mathew STRENGTH PROPERTIES • Strength can be defined as the • Maximum stress that a structure can withstand without sustaining a specific amount of plastic strain (yield strength). OR • Stress at the point of fracture (ultimate strength). 49 • When we describe the strength of a material, we are most often referring to the maximum stress that is required to cause fracture of the material.
  50. 50. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Strength of a material can be described by one or more of the following properties: • Proportional Limit • Elastic Limit • Yield Strength • Ultimate tensile strength, shear strength, compressive strength & flexural strength 50
  51. 51. PROPORTIONAL LIMIT • Defined as the magnitude of elastic stress above which plastic deformation occurs. 51 A B C D 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 14 Stress(Mpa) Strain Elastic Plastic • As stress is increased, the strain is also increased. • Initial 0 – A portion shows that stress is linearly proportional to strain. • As strain is doubled, stress is also doubled.
  52. 52. • After the point A, stress is no longer linearly proportional to strain. • Here, the value of stress at A is known as the proportional limit. 52 A B C D 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 14 Stress(Mpa) Strain Elastic Plastic • So, it can also be defined as the highest stress at which the stress-strain curve is a straight line. • ie stress is linearly proportional to strain • Below the proportional limit, there is no permanent deformation in a structure, ie the object will return to its original dimension when force is removed.
  53. 53. • The material is elastic in nature below the proportional limit. 53 A B C D 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 14 Stress(Mpa) Strain Elastic Plastic • The region of the stress – strain curve before the proportional limit is called the elastic region and the region beyond is called the plastic region.
  54. 54. • When a material is said to have high value of proportional limit, it indicates that the material is more likely to withstand applied stress without permanent deformation. 54 • The connectors of partial dentures should have high proportional limit. • Materials like Cobalt/Chromium (alloy) which has high proportional limit is widely used for the fabrication of connectors because they can withstand high stresses without permanent deformation.
  55. 55. Mechanical Properties of Dental Materials - Dr. Nithin Mathew ELASTIC LIMIT • Defined as the maximum stress that a material will withstand without permanent deformation. • For linearly elastic materials, the proportional limit and the elastic limit represents the same stress within the structure. 55
  56. 56. Mechanical Properties of Dental Materials - Dr. Nithin Mathew YIELD STRENGTH • Defined as the stress at which a test specimen exhibits a specific amount of plastic strain. • It is a property that represents the stress value at which a small amount of (0.1% - 0.2%) plastic strain has occurred. • It is a property often used to describe the stress at which the material begins to function in a plastic manner. • The amount of permanent strain is referred to as the percent offset. 56
  57. 57. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • A value of either 0.1% or 0.2% of the plastic strain is often selected and is referred to as the percent offset. 57 0.2 0.2% offset • The point at which at the parallel line intersect the stress-strain curve is the yield strength. • Elastic limit, proportional limit & yield strength are defined differently but their values are fairly close to each other in many cases.
  58. 58. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • These values are important in the evaluation of dental materials because they represent the stress at which permanent deformation begins. • If these values are exceeded by the masticatory stresses, the restoration or appliance may no longer function as originally designed. 58 • Also, A fixed partial denture becomes permanently deformed through the application of excessive occlusal forces since a stress equal to or greater than yield strength is developed.
  59. 59. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • In the process of shaping an orthodontic appliance or adjusting the clasp of on a removable partial denture it is necessary to apply a stress into the structure in excess of yield strength if the material is to be permanently bent or adapted. 59
  60. 60. Mechanical Properties of Dental Materials - Dr. Nithin Mathew ULTIMATE STRENGTH • Ultimate tensile strength/stress (UTS) is defined as the maximum stress that a material can withstand before failure in tension. • Ultimate compressive strength/stress (UCS) is the maximum stress that a material can withstand in compression. • The ultimate strength / stress is determined by dividing the maximum load in tension (or compression) by the original cross-sectional area of the test specimen. 60
  61. 61. Mechanical Properties of Dental Materials - Dr. Nithin Mathew MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION • If a material is deformed by the stress at a point above the proportional limit before fracture, and upon removal of the applied force, the stress will reduce to 0, but the plastic strain (deformation) remains. • Thus the object will not return to its original shape when the force is removed. • It remains bent, stretched or compressed, ie it becomes plastically deformed. 61
  62. 62. Mechanical Properties of Dental Materials - Dr. Nithin Mathew COLD WORKING (Strain Hardening/Work Hardening) • When metals are stretched beyond the proportional limit, hardness and strength increases at the area of deformation but their ductility decreases. • Repeated plastic deformation of the metal during bending of orthodontic wire can lead to brittleness of the deformed area of the wire which may fracture on further adjustment. 62 • To minimize the risk of brittleness, is to deform the metal in small increments so as not to plastically deform the metal excessively.
  63. 63. Mechanical Properties of Dental Materials - Dr. Nithin Mathew FLEXURAL STRENGTH • Defined as the force per unit area at the instant of fracture in a test specimen subjected to flexural loading. • Also known as modulus of rupture. • It is a strength test of a bar supported at each end under a static load. 63
  64. 64. Mechanical Properties of Dental Materials - Dr. Nithin Mathew FLEXURAL STRENGTH • For a bar with a rectangular cross-section subjected to a 3 point flexure, the flexural strength can be calculated as 64 σ = 3PL 2 wt2 σ : Max. flexural stress (Mpa) P : Load at fracture (N) 𝐿𝐿 : Distance btw 2 supports (mm) 𝑤𝑤 : Width of specimen (mm) t : Thickness of specimen (mm)
  65. 65. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Most prosthesis & restoration fractures develop progressively over many stress cycles after initiation of a crack from a critical flaw, and subsequently by propagation of the crack until a sudden, unexpected fracture occurs. • This phenomenon is called fatigue failure. 65
  66. 66. Mechanical Properties of Dental Materials - Dr. Nithin Mathew IMPACT STRENGTH • Defined as the energy required to fracture a material under an impact force. 66 • Measured using a Charpy Impact tester, where a pendulum is released that swings down to fracture the center of a specimen supported at both ends.
  67. 67. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • A moving object possesses a known kinetic energy. • If the struck object is not permanently deformed, it stores the energy of collision in an elastic manner. • This ability is due to the resiliency of the material and is measured by the area under the elastic region of the stress- strain curve. 67 • Thus a material with low elastic modulus and high tensile strength is more resistant to impact forces. • A material with low elastic modulus and low tensile strength has low impact resistance.
  68. 68. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Material Elastic Modulus (Gpa) Tensile Strength (Mpa) Composite 17 30 – 90 Porcelain 40 50 – 100 Amalgam 21 27 – 55 Alumina ceramic 350 – 418 120 Acrylic 3.5 60 68
  69. 69. Mechanical Properties of Dental Materials - Dr. Nithin Mathew TOUGHNESS • It is the ability of a material to absorb elastic energy and to deform plastically before fracturing. • Measured as the total area under a plot of the tensile stress v/s strain. • It can be defined as the amount of elastic and plastic deformation energy required to fracture a material. • Toughnessincreases with increase in strength and ductility. • ie. Greater the strength and higher the ductility, the greater is the toughness. 69 TOUGHNESS
  70. 70. Mechanical Properties of Dental Materials - Dr. Nithin Mathew FRACTURE TOUGHNESS • It is the mechanical property that describes the resistance of brittle materials to the propagation of flaws under an applied stress. 70 • The longer the flaw, the lower is the stress needed to cause fracture. This is because the stress which would normally be supported by the material are now concentrated at the tip of the flaw. • The ability of a flaw to cause fracture depends on the fracture toughness of the material. • Fracture toughness is a material property and is proportional to the energy consumed in plastic deformation.
  71. 71. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Material Fracture Toughness Enamel 0.7 – 1.3 Dentin 3.1 Amalgam 1.3 – 1.6 Ceramic 1.2 – 3.0 Composite 1.4 – 2.3 Porcelain 0.9 – 1.0 71
  72. 72. Mechanical Properties of Dental Materials - Dr. Nithin Mathew BRITTLENESS • It is the relative inability of a material to sustain plastic deformation before fracture of a material occurs. • Eg: amalgam, ceramics, composites are brittle at oral temperatures. • They sustain no/little plastic strain before they fracture. • ie a brittle material fractures at or near its proportional limit. • Dental materials with low or 0% elongation such as amalgam, composite, ceramics, etc will have little or no burnishability because they have no plastic deformation potential. 72
  73. 73. Mechanical Properties of Dental Materials - Dr. Nithin Mathew DUCTILITY • It is the ability of a material to sustain a large permanent deformation under a tensile load upto the point of fracture. • Eg: a metal can be drawn readily into long thin wire is said to be ductile. • Ductility is the relative ability of a material to be stretched plastically at room temperature without fracturing. • Its magnitude can be assessed by the amount of permanent deformation indicated by the stress-strain curve. 73
  74. 74. Mechanical Properties of Dental Materials - Dr. Nithin Mathew Methods to determine ductility are: • Percent elongation after fracture • Reduction in area of tensile test specimens • Maximum number of bends performed in a cold bend test. 74
  75. 75. Mechanical Properties of Dental Materials - Dr. Nithin Mathew MALLEABILITY • It is 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, followed by silver. 75
  76. 76. Mechanical Properties of Dental Materials - Dr. Nithin Mathew HARDNESS • It is the resistance of a material to plastic deformation which is typically produced by an indentation force. • In mineralogy, the relative hardness of a material is based on its ability to resist scratching. 76
  77. 77. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 77 CLASSIFICATION OF HARDNESS TEST Method of Application Size of the indenter Amount of load applied to the indenter Static Loading – slowly applied Macro-indentation - Large indenter tip Macrohardness - ˃ 1kg load Dynamic Loading – rapidly applied Micro-indentation - Small indenter tip Microhardness - < 1kg load
  78. 78. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Varioushardness tests include: • Brinell Test • Rockwell Test • Vicker’s Test • Knoop’s Test • Selection of the test should be done on the basis of the material being tested. 78
  79. 79. Mechanical Properties of Dental Materials - Dr. Nithin Mathew BRINELL TEST • Used extensively for determining the hardness of metals and metallic materials used in dentistry. • Related to the proportional limit and ultimate tensile strength. • The methods depends on the resistance to the penetration of a small steel ball, typically 1.6mm diameter when subjected to a specified load. 79
  80. 80. Mechanical Properties of Dental Materials - Dr. Nithin Mathew BRINELL TEST • Method: • A hardened steel ball is pressed under a specified load into a polished surface of the material to be tested. • Load remains in contact with the material for a fixed time of 30s. • After 30s it is then removed and the indentation diameter is measured. 80 • Load value is then divided by area of projected surface of indentation and the quotient is referred to as the Brinell Hardness Number (BHN).
  81. 81. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Since the brinell test yields a relatively large indentation area, this test is good for determining average hardness value and poor for determining very localized values. • Thus for a given load, the smaller the indentation, the larger is the number and harder is the material. 81 ADVANTAGES DISADVANTAGES Best suited for testing ductile materials Hardness of cold-worked materials are difficult to measure Not suitable for brittle materials
  82. 82. Mechanical Properties of Dental Materials - Dr. Nithin Mathew ROCKWELL HARDNESS TEST • This test was developed as a rapid method for hardness determinations. • Here, instead of a steel ball, a conical diamond point is used. • The depth of the penetration is directly measured by a dial gauge on the instrument. • This test is not suitable for testing brittle materials. 82
  83. 83. Mechanical Properties of Dental Materials - Dr. Nithin Mathew The value is the Rockwell Hardness Number (RHN) 83 ADVANTAGES DISADVANTAGES Direct reading of the depth of indentation Not suitable for brittle materials Rapid testing time
  84. 84. Mechanical Properties of Dental Materials - Dr. Nithin Mathew VICKER’S HARDNESS TEST • This test uses a square based pyramidal indenter. • The impression obtained on the material is a square. • The method is similar to Knoop’s and Brinell tests. 84 • The load value divided by the projected area of indentation gives the Vicker’s Hardness Number (VHN). • The lengths of the diagonals of the indentations are measured and averaged.
  85. 85. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • Used for testing dental casting gold alloys as well as tooth structure. • Suitable for determining the hardness of brittle materials. 85 ADVANTAGES DISADVANTAGES Useful for measuring the hardness of small areas and hard materials Material requires highly polished surface Longer time required to complete the test
  86. 86. Mechanical Properties of Dental Materials - Dr. Nithin Mathew KNOOP’S HARDNESS TEST • This test was developed mainly to fulfill the needs of a micro-indentation test method. • Suitable for testing thin plastic or metal sheets or brittle materials where the applied force does not exceed 35N. • This test is designed so that varying loads may be applied to the indenting instrument. • Therefore the resulting indentation varies according to the load applied and the nature of the material tested. 86
  87. 87. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • The impression is rhomboid in outline and the length of the largest diagonal is measured. • The load value is then divided by the projected area to get the Knoop’s Hardness Number (KHN) 87 ADVANTAGES DISADVANTAGES Useful for very brittle materials or thin sheets Material requires highly polished surface Longer time required to complete the test
  88. 88. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 88 MATERIAL KHN Enamel 343 Dentin 68 Cementum 40 Denture Acrylic 21 Zinc Phosphate Cement 38 Porcelain 460
  89. 89. Mechanical Properties of Dental Materials - Dr. Nithin Mathew STRESS CONCENTRATION EFFECTS • The cause of strength reduction is the presence of small microscopic flaws or microstructural defects on the surface or within the internal structure. • These flaws are especially critical in brittle materials in areas of tensile stress because tensile stress tends to open cracks. 89 • Stress at the tips of these flaws is greatly increased which leads to crack inititation and broken bonds.
  90. 90. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • When a brittle or a ductile material is subjected to compressive stress, it tends to close the crack and this stress distribution is more uniform. 90 • When a ductile material is subjected to tensile force, it tends to opening of the flaw and only plastic deformation has occurred.
  91. 91. Mechanical Properties of Dental Materials - Dr. Nithin Mathew • 2 important aspects of the flaws : 1. Stress intensity increases with the length of the flaw. 2. Flaws on the surface are associated with higher stresses than flaws of same size in the interior region. Therefore, the surface of brittle materials such as ceramics, amalgams, etc. are extremely important in areas subjected to tensile stress. 91
  92. 92. Mechanical Properties of Dental Materials - Dr. Nithin Mathew CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM 1. Surface defects such as porosity, grinding roughness. • Polish surface to reduce the depth of the defects. 2. Interior flaws such as voids. • Little can be done about the interior flaws but to ensure highest quality of the structure or to increase the size of the object . 92
  93. 93. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 3. Marked changes in contour – sharp internal line angle at axio-pulpal line angle. • Design of the prosthesis should vary gradually than abruptly. • Notches should be avoided • All internal line angles should be rounded 4. A large difference in elastic moduli or thermal expansion coefficient across bonded surface. • The elastic moduli of the 2 materials should be closely matched • Materials must be closely matched in their coefficients of expansion or contraction. 93
  94. 94. Mechanical Properties of Dental Materials - Dr. Nithin Mathew 5. A Hertzian load (load applied to a point on the surface of a brittle material). • Cusp tip of an opposing crown or tooth should be well rounded such that the occlusal contact areas in the brittle material is large. 94
  95. 95. Mechanical Properties of Dental Materials - Dr. Nithin Mathew CONCLUSION • While designing a dental appliance or a restorative material, it should have adequate mechanical properties to withstand the stress and strain caused by the forces of mastication. • All the methods must be employed to minimize stress concentration so that the restorative material or the appliance is in harmony with the different types of forces occurring in the oral cavity. 95
  96. 96. Mechanical Properties of Dental Materials - Dr. Nithin Mathew REFERENCES • Phillip’s Science of Dental Materials – 10th & 12th Edition • Craig’s Restorative Dental Materials – 13th Edition • Dental materials and their selection: William J’O Brien – 3rd Edition • Materials Used in Dentistry: S. Mahalaxmi – 1st Edition 96
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