Physical and mechanical properties of dental material


Published on

Indian Dental Academy: will be one of the most relevant and exciting training center with best faculty and flexible training programs for dental professionals who wish to advance in their dental practice,Offers certified courses in Dental implants,Orthodontics,Endodontics,Cosmetic Dentistry, Prosthetic Dentistry, Periodontics and General Dentistry.

Published in: Technology, Business

Physical and mechanical properties of dental material

  1. 1. CONTENTS • INTRODUCTION • PHYSICAL PROPERTIES o Hardness o Viscosity o Creep and flow o Color and color perceptions o Thermo physical properties o Tarnish o Corrosion o Galvanic currents • MECHANICAL PROPERTIES o Stress o Strain o Elastic modulus o Strength properties o Toughness o Brittleness o Ductility and malleability • CONCLUSION • REFERENCES
  2. 2. INTRODUCTION The principal goal of dentistry is to maintain to improve the quality of life of the dental patient. This requires the replacement or alteration of existing tooth structure; the main challenges for centuries have been the selection and development of good prosthetic materials that can withstand the adverse conditions of the oral environment. Physical properties are the measures of a material. These properties have great significance in dental research because they provide the information needed to assess the characteristics of and improvement in materials under development. The physical properties of a tooth set the standard for materials attached to a tooth. Theory suggests that if a restorative material can be made to hold properties similar to those of natural tooth structure, it should perform as well as original tooth.
  3. 3. HARDNESS : The property of hardness is one of the major properties in the comparison of restorative materials. Hardness may be defined as “the resistance to permanent surface indentation or penetration”. The most common concept of hard and soft substances is their relative resistance to indentation. Hardness is a measure of resistance to plastic deformation and is measured as a force per unit area of indentation. - Based on this definition of hardness, it is clear why this property is so important to dentistry. Hardness is indicative of the case of finishing of a structure and its resistance to in service scratching. - There are many ways to areas use hardness depending on the shape of the object used to deform the surface being tested. Some of the most common methods of testing the hardness of restorative materials are: Brinell Knoop Vickers Rockwell Barcol and Shore a hardness tests. Each of these tests differs slightly from the others, and each presents certain advantage and disadvantages. They have a common quality, however, in that each depends on the penetration of some
  4. 4. small, symmetrically shaped indenter into the surface of the material being tested. - The various hardness tests differ in the indenter material, geometry and load. - The indenter may be made of steel, tungsten carbide or diamond and be shaped as a sphere cone, pyramid or needle. - Loads typically range from 1-3000 kg. - The choice of a hardness test depends on the material of interest, the expected hardness range, and the desired degree of localization. The general procedure for testing hardness, in dependent of the specific test and is as follows. KNOOP HARDNESS TEST: - The Knoop hardness test was developed to fulfill the needs of a micro indentation test method. - A load is applied to a carefully prepared diamond indenting tool with a pyramid shape and the lengths of the diagonals of the resulting indentation in the material are measured. - This is the shape of the shape of the indenter and the resulting indentation. - KHN is the ratio of the load applied to the area of the indentation. - The units of KHN are also kg/mm2 . - Higher values for KHN represent hardness materials.
  5. 5. - The Knoop method is designed so varying loads may be applied to the indenting instrument. The resulting indentation area, therefore, varies according to the load applied and the nature of material tested. Advantage : Materials with a great range of values. • Silicon carbide abrasive 2480 • Feldspathic porcelain 460 • Enamel 343 • Gold foil 69 • Dentin 68 • Cementum 40 • Zinc phosphate cement 38 • Denture acrylic 21
  6. 6. BRINELL HARDNESS TEST : - This is among the oldest methods used to test metals and alloys used in dentistry. - Method depends on resistance to the penetration of a small steel or tungsten carbide ball, typically 1.6 mm in diameter, when subjected to a weight of 123 N . - In testing the brinell hardness of a material the penetrates remains in contact with the specimen tested for a fixed time of 30 seconds, after which it is removed and the indentation diameter is carefully measured. - The resulting brinell hardness member (BHN) is computed as a ratio of the load applied to the area of the indentation produced. - Units of BHN are kg/mm2 .
  7. 7. - The smaller the area of indentation, the harder the material and the larger the BHN value. - Because BH test yields relatively large indentation area, this test is good for determining average hardness values and poor determining very localized values. Condensed gold foil 69 powdered 46 Gold alloys type I 45 type II 95 type III 120 type IV 220 Stainless steel 350 Co-Cr alloys 270-370 Amalgam 90 Dentin 60-70 Composite 25 -35 Aluminium 18-35 Pure gold 18-30
  8. 8. ROCKWELL HARDNESS : Rockwell hardness is a rapid testing method in which an instrument applies a load to a material and a dial quickly calculates a hardness number. This method is commonly used with plastics, since the device can be kept on the material for varying amounts of time to measure percent of recovery. - Depth of indentation is measured with a sensitive micrometer. - 60-150 kg - Good foe testing visco elastic materials. - Readings are directly read. - Indentation rapidly disappears.
  9. 9. BARCOL HARDNESS : - Used to study the depth of cure of resin composites. - Has a spring loaded needle with a diameter of 1 mm that is pressed against the surface. - If no penetration occurs, needle reads 0. Reading decreases as indentation increases.
  10. 10. VICKERS HARDNESS: This hardness test uses a 136 diamond pyramid, it is used in applied loads, it is used in applied loads. It is commonly used in dentistry and measure very hard materials and if small areas all to be tested. - A squarish indentation is produced. - Diagonals are measured. - Kg/mm2 . - Application varies from 1-120 kg. NANOINDENTATION : - Traditional tests used high loads and indentation areas were large. - But many materials have microstructural constituents and to accurately measure these microphases, it is necessary to be able to create indentations of a smaller size scale and also to be able to control the location of indentations.
  11. 11. Therefore nanoindentation has recently been introduced and are able to apply loads in the range of 0.1-5000 mg. - Indentations are of 1µm in size. - Studies compared the efficacy by comparing values obtained earlier. N.H. KHN Dentin 71 kg/mm2 68 kg/mm2 Enamel 457 kg/mm2 343 kg/mm2
  12. 12. Hardness can be tested simply by varying the test load. because very light load applications produce extremely delicate micro indentations, this method of testing can be employed to examine materials that vary in hardness over an area of interest. Disadvantage : The used for a highly polished and feat test specimen and the time required to complete the test operation. VISCOSITY : Materials that have mechanical properties independent or loading rate are termed – Elastic. Materials that have mechanical properties dependent on loading rate are termed – Viscoelastic. In other words, these materials have characteristics of an elastic solid and a viscous fluid. Most liquids when placed in motion resist imposed forces that cause them to move. This resistance to motion is called viscosity and is controlled by the interval frictional forces within the liquid. Viscosity is the measure of the consistency of a fluid and its inability to frontier. So, a highly viscous fluid flows slower because of its high viscosity. The units of viscosity are poise P. (1P = 0.1 Pas = 0.1 NS/m2 ), or is also reported in centipoises CP (100 CP = 1P) to put this concept on a quantitative basis ….. A liquid occupies the space between 2 metal plates. The lower plate is fixed and upper plate is moved to the right with a certain velocity.
  13. 13. A force is required to overcome the drag produced by the friction of the liquid. Stress is the force per unit area that develops within a structure when an external force is applied. The stress produced causes a deformation or strain to develop and can be circulated. If the plates haven area (A), a shear. Stress (T) can be defined as T = F/A. the shear strain rate or rate of change of deformation is ε = v/d where d is the distance between the 2 plates and v is the velocity of the liquid. Similarly, a shear stress versus stain rate curve can be plotted. An ‘ideal’ fluid demonstrates a shear stress that is proportional to the strain rate and thus the plot is a straight line. Such behavior is called Newtonian. A Newtonian fluid has a constant viscosity and exhibits a constant scope of shear stress plotted against the strains rate (a straight line), many dental materials exhibit pseudoplastic behavior – their viscosity deceases with increasing shear rate until it reaches a nearly constant value. E.g. Rubber impression materials.
  14. 14. The viscosity of a dilatant liquid increases with increasing shear rate. E.g. Fluid denture base resins. These liquids become more liquid as the rate of deformation increases. - There are also some materials that behave like a rigid body until some minimum value to shear stress is reached. E.g. Ketchup is a familiar A sharp blow to the bottle is usually required to produce an initial flow. - A liquid that becomes less viscosity and more fluid under repeated applications of pressure is referred to as thyrotrophic. Eg. Dental prophylaxis pastes, plaster of Paris
  15. 15. CREEP AND FLOW If a metal is held at a temperature near its melting point and is subjected to a constant applied stress, the resulting strain will increase over time. Creep is defined as the time dependent plastic strain of a material under a static load or constant stress. The related phenomenon if sag occurs in the permanent deformation of long – span metal bridge structures at porcelain – rising temperatures under the influence of the mass of the prosthesis. Dental amalgams contain from 42-52 wt% of Hg and begin melting at low temperatures. Because of its low melting range, dental amalgam can slowly creep from a restored tooth site under periodic sustained stress, such as those imposed by patients who clench their teeth. Because creep produces continuing plastic deformation, the process can be destructive to a dental prosthesis. The term ‘flow’ rather than creep has generally been used in dentistry to describe the rheology of amorphous materials such as waxes. The flow of wax is a measure of its potential to deform under a small static load even that associated with its own mass. - Although creep or flow may be measured under any type of stress, comparison is usually employed in the testing of dental materials. - A cylinder of prescribed dimensions is subjected to a given compressive stress for a specified time and temperature. Creep or flow is measured as the % decrease in length that occurs under these testing conditions. - Creep may cause unacceptable deformation of dental restorations (such as low Cu amalgam) made from a material that is used clinically at a temperature near its melting point for an extended period.
  16. 16. COLOR An important goal of dentistry is to restore the color and appearance of natural dentition. Color is all about light. The perception of the color of an object is the result of a physiological response to a physical stimulus, light is an electromagnetic radiation that can be detected by the human eye. The eye is sensitive to wavelengths from approximately 400 nm to 500 nm. Light is reflected from on object and stimulates the neural sensors in the eye’s retina to send a signal i.e. interpreted in the visual cortex of the brain. The reflected light intensities and the combined intensities of wave lengths present in incidence and reflected light determine the appearance properties (hue, value and chroma). For an object to be visible, it must reflect or transmit light incident on it from an external source. The incident light is usually polychromatic, that is, a mixture of the various wavelengths. Incident light is selectively absorbed or scattered (or both) at retain wavelengths. The spectral distribution of the transmitted or reflected light resembles that of incident light, although certain wavelengths are reduced in magnitude. Verbal descriptions of color are not precise enough to describe the appearance of teeth. Hence, 3 variables must be measured to accurately describe once perception of light reflected from a tooth’s restoration surface : Hue Value
  17. 17. Chroma Hue : Describes the dominant color of an object. E.g.: Red, green or blue. This refers to the dominant wavelength in the spectral distribution. The continuous of these were creates a color. Value : Increases towards the whiter and decreases towards black. Teeth can be separated into lighter shades (increases value) and darker shades (lower value) value identifies the lightness or a darkness of a color, which can be measured independently of here. Chroma : The yellow of a lemon is more ‘vivid’ than a yellow of a banana – which is a dull yellow. This is a difference in color intensity. Chroma represents this degree of saturation of a particular here. The higher the chroma, the more intense the color. Chroma is not considered separately in dentistry. It is considered along with here and value of dental tissues. Because the spectral distribution of the light reflected from or transmitted through an object is dependent on the spectral content of the incident light, the appearance of an object is dependent in the nature of the light in which the object is viewed. Day light, and fluorescent lamps are common sources of light in dental operators and each of there has a different spectral distribution. Objects that appear to be color matched under one type of light may appear different under another light source. This phenomenon is called “Metamerism”. Measurement of color: The color of dental restorative materials is most commonly measured in reflected light by instrumental or visual techniques. Instrumental techniques : Curves of spectral reflectance versus wavelength can be obtained over the visible range (405-407 nm) with a
  18. 18. recording spectrophotometer and integrating sphere. Typical curves for a composite resin before and after 300 hours of accelerated aging in a weathering chamber SPECTROPHOTOMETER From the reflectance values and tabulated color matching functions, the tristimulus values (X,Y, Z) can be computed relative to a particular light source. These tristimulus values are related to the amounts of the three primary colors required to give by additive mixture, a match with the color being considered. Typically, the tristimulus values are considered relative to the commission international de I’Eclairage (CIE) a diagram of the CIE. L*a*b* color space is this. The L*a*b* color space is characterized by uniform chromacities. Value (black to white) is denoted as L*, whereas chroma (a*b*) is denoted as red (+a*), green (-a*), yellow (+b*) and blue (- b*). Visual technique : A popular system of visual determination is the Munsell color system, the parameters of which are represented in 3 dimensions.
  19. 19. THERMOPHYSICAL PROPERTIES : Thermal Conductivity : Heat transfer through solid substances most commonly occurs by means of conduction. The conduction of heat through metals occurs through the interactions of crystal lattice vibrations and by the motions of electrons and their interaction with atoms. Thermal conductivity is a thermo physical measure of low well heat is transferred through a material by conductive flow. The measurement of thermal C is preformed under ‘steady state conditions’. Under these conditions, temperatures in the system (i.e. the temperature gradient) do not change over time. The rate of heat flow through a structure is
  20. 20. proportional both to the area (perpendicular to heat is conducted and to the temperature gradient across the structure. thus, if significant porosity exists in the structure, the area available for conduction is reduced and the rate of heat flow is reduced. The thermal conductivity or coefficient of thermal conductivity is the quantity of heat in calories per second that passes through a specimen 1 cm thick having a cross- sectional area of 1 cm2 when the temperature difference between the surfaces perpendicular to heat flow of the specimen is 10 K. According to the 2nd law of thermodynamics, heat flow from points of higher temperature to points of lower temperature. - Materials that have a high thermal and are called conductors. - Materials of low thermal conductivity are called insulators. - ISI unit or measure for thermal conductivity is watt per meter per second per degree Kelvin (w x m-1 x s-1 x k-1 ). Thermal diffusivity: The value of thermal diffusivity of a material control the time rate of temperature change as heat passes through a material. - It is a measure of the rate at which a body with a non-uniform temperature reaches a state of thermal equilibrium. - The square root of thermal diffusivity is indirectly proportional to the thermal insulation ability. - In the oral environment, temperatures are not constant during the ingestion of foods and liquids. For these unsteady state conditions, heat transfer through the material deceases the thermal gradient under such conditions, the thermal diffusivity is
  21. 21. - The effectiveness of a material in preventing heat transfer is directly proportional to the thickness of the liner and inversely proportional to the square root of the thermal diffusivity. Thus, the thicknesses of the remaining dentin and the base are as important as, if not more important than, the thermal properties of the materials. The S.I. unit of thermal diffusivity is typical of diffusion processes, that is, square meter per second values. The COTE refers to the amount of expansion and contraction a material undergoes in relation to temperature. a tooth expands and contracts with thermal changes. COEFFICIENT OF THERMAL EXPANSION: This is an important thermal property. Coefficient of thermal expansion is defined as the change in length per unit of the original length of a material when its temperature is raised 10 K. A tooth expands and contracts with thermal changes. A high COTE indicates a relatively high degree of dimensional change is expansion to temperature. Values of coefficients of thermal expansion of some materials of interest are
  22. 22. The units of α are typically expressed in units of µm/m0 K or ppm / 0 K. Studies show that there is a direct revolution behavior marginal leakage and thermal changes. The greater the difference in flow between tooth structure and the restorative, the greater the leakage. A tooth restoration may expand or contract more than the tooth during a change in temperature; thus there may be marginal microleakage adjacent to the restoration, or the restoration may debond from the tooth. - Restorative materials may change in dimension upto 4.4 times more than the tooth enamel for every degree of temperature change. - The high thermal expansion coefficient of inlay wax is important because it is highly susceptible to temperature changes. - Although these thermal stress is cannot be eliminated completely, they can be reduced appreciably by selection of materials whose expansion or contraction coefficients are matched fairly closely (within 4%).
  23. 23. MECHANICAL PROPERTIES Defined by the laws of mechanics, that is, the physical science that deals in the energy and forces and their efforts on bodies. All mechanical properties are measures of the resistance of a material to deformation or fracture under an applied force. For distal applications stress forces are usually expressed as Mpa. A free scale named after French scientist Blasé Das et al in 1667. Newton is a free scale named after British mathematician sie Issac Newton. STRESSES AND STRAINS : Defined as force per unit area within a structure subjected to an external force or pressure. Expressed in Newton’s per sq. mm (N/mm2 ) or pounds per sq. inch (psi). The unit N/mm2 is properly known as the Pascal and abbreviated pa. The pascal is a small unit. For dental applications, there are several types of stress that develop according to the nature of the applied forces and the object shape. These include tensile stress, shear stress and compressive stress.
  24. 24. STRAIN : Defined as the change in the length of materials on the applications of stress. Calculated by dividing by its original length – a unit with no dimensions. A material capable of high strain, such a rubber or latex, can tolerate a strain value of 0.5 – 50.0% before failure. For most solids, stain is expressed as micro strain is parts per millions (PPM) or 10-6 strain.
  25. 25. TENSILE STRESS: A tensile stress is caused by a load that tends to stretch or elongate a body. A tensile stress is always accompanied by tensile strain. There are very few pure tensile stress situations in dentistry. However, a tensile stress can be generated when structures are flexed. (The deformation of a bridge and the diametral compression of a cylinder) Because most dental materials are quite brittle, they are highly susceptible to crack initiation in the presence of surface flaws when subjected to tensile stress, such as when they are subjected to flexural loading. Although some brittle materials are strong, they fracture with little warning, because little or no plastic deformation occurs to indicate high levels of stress. COMPRESSIVE STRESS : If a body is placed under a load that tends to compress or shorten it, the internal resistance to such a load is called a compressive stress. A compressive stress is associated with a compressive strain. To calculate either tensile stress or compressive stress, the applied force is divided by the cross-sectional area perpendicular to the force direction.
  26. 26. SHEAR STRESS: A shear stress tends to resist the sliding or twisting of one portion of a body over another. Shear stress can also be produced by twisting or toesional action on a material. For 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 an orthodontic bracket, the bracket may debond by shear stress failure of the resin luting agent. Shear stress is calculated by dividing the force by the area parallel to the force direction. In the oral environment, shear failure is unlikely to occur because: Presence of chamfers, bevels or changes in curvatures of a bonded tooth surface would also make shear failure of a bonded material highly unlikely. The further away from the interface the load is applied, the more likely that tensile failure rather than shear failure will occur because the potential for bending stresses would increase Because the tensile strength of brittle materials is usually well below their shear strength values, tensile failure is more likely to occur.
  27. 27. FLEXURAL (BENDING) STRESS : Taking an e.g. of a 3 unit bridge or FPD and a 2 unit cantilever FPD. These stresses are produced by bending forces in dental appliances in one of the 2 ways : 1. By subjecting a structure such as an FPD to 3 point loading, where by the end points are fixed and a force is applied between these end points. 2. By subjecting a cantilevered structure that is supported at only one end to a load along any part of the unsupported section. Also, when a patient bites into an object, the anterior teeth receive forces that are at an angle to their long axes thereby creating flexural stresses within the teeth. - A tensile stress develops on the tissue side of the FPD and compressive stress develops on the occlusal side. between these 2 areas is a neutral axis that represents a state with no tensile stress and no compressive stress. For a cantilevered FPD (as show in fig), the maximum tensile stress develops with the occlusal surface or the surface that is becoming more convex (indicating a stretching action). If you can visualize this unit bending downward toward the tissue, the upper
  28. 28. surface becomes more convex or stretched and the opposite surface becomes compressed. ELASTIC DEFORMATION : There are several important mechanical properties and parameters that are measures of the elastic strain or plastic strain behaviour of dental materials. These are elastic modulus, dynamic Young’s modulus, shear modulus, flexibility, resilience and Poisson’s ratio. Elastic modulus (young’s modulus of elasticity) : Determines resistance to flexes and deformation, of the anterior of bending when loaded. Elastic modulus describes the relative stiffness or rigidity of a material, which is measured by the slope of the elastic region of the stress-strain graph. OR The measure of elasticity of a material is described by the term elastic modulus denoted by the variable E. It represents the stiffness of a material within the elastic range. 1. Can be measured by placing a force on a material and measuring the deformation, can be calculated in anon-destructive way by measuring the harmonics if a material when vibrated.
  29. 29. 2. Stress and strain are related in that the elastic modulus is the ratio of stress over stain. 3. The elastic modulus can be determines by a stress-stain curve by calculating the ratio of stress to strain or the slope of the linear region of the curve. The modulus is calculated from the equation : Elastic modulus = stress / stain Because strain in dimensionless, the modulus has the same units as stress and is usually reported in MPa or GPa (1GPA = 1000 MPa). - The elastic qualities of a material represent a fundamental property of the material. The inter atomic or intermolecular forces of the material are responsible for the property of elasticity. - The stronger the basic attraction forces, the greater the values of the elastic modulus and the more rigid or stiff the material. Because this property is related to the attraction forces within the material, it is usually the same when the material is subjected to either tension or compression. This property is generally independent of any heat treatment or mechanical treatment that a metal or alloy has received, but is quite dependent on the composition of the elastic modulus indicates the material amount of stress that needs to be applied to achieve a certain strain, or if the strain is knows, what level of stress is in effect. DYNAMIC YOUNG’S MODULUS: Elastic modulus can be measured by a dynamic method using Poisson’s ratio. During axial loading in tension or compression, there is a simultaneous axial and lateral strain. Under tensile loading as a material elongates in the direction of load, there is a reduction in cross
  30. 30. section. Under compressive loading, there is an increase in cross section. Within the elastic range, the ratio of the lateral to the axial strain is called Poisson’s ratio (v). In tensile loading, the Poisson’s ratio indicates that the reduction in cross section is proportional to the elongation during the elastic deformation. The reduction in cross-section continues until the material is fractured. RESILIENCE : Resilience is the resistance of a material to permanent deformation. It indicates the amount of energy necessary to deform the material to the proportional limit. Resilience is therefore measured by the area under the elastic portion of the stress-strain curve. Resilience can be measured by idealizing area of interest as a triangle and calculating the area of the triangle. The units are MN / m3 (meter x mega newtons per cubic meter), which represents energy per unit volume of material. resilience has particular importance in the evaluation of orthodontic wires because the amount of work expected from a particular spring in moving a tooth is of interest. STRENGTH The strength of a material is defined as the average level of stress at which a material exhibits a certain amount of initial plastic
  31. 31. deformation or at which fracture occurs in test specimens of the same shape and size. The strength is dependent on several factors including: 1. Strain rate 2. Shape of test specimen 3. The surface finish 4. The environment in which a material is tested. However, the strength of brittle materials may appear to be low when large flows are present or if stress concentration areas exist because of improper design of a prosthetic component (such as a notch on the clasp arm on a partial denture) under these circumstances, a clinical prosthesis may fracture at a much lower applied force because the localized stress exceeds the strength of the material at the critical location of the flow (stress concentration). PROPORTIONAL LIMIT : As a wire is stretched steadily in tension, the wire eventually fractures. However, in dentistry, we are also interested in the stress at which plastic deformation begins to develop. A stress – strain curve for a hypothetical material that was subjected to increasing tensile stress until fracture. The stress is plotted vertically and the strain is plotted horizontally. As the stress is increased, the strain is increased. In fact, the initial portion of the curve, from O to A, the strain is linearly proportional to the stress, and as the stress is doubled, the amount of strain is also doubled. When a stress that is higher than the value registered at A is achieved, the strain changes are no longer linearly proportional to the stress changes. Hence, the value of the stress at A is known as proportional limit and is defined as the greatest stress that a
  32. 32. material will sustain without a deviation from the linear proportionality of stress to strain. - Below the proportional limit, no permanent deformation occurs in a structure. When the stress is removed, the structure will return to its original dimensions. The region of stress-strain curve before proportional limit is called elastic region. - The application of stress grater than the proportional limit results in a permanent or irreversible strain in the specimen, the region of the stress-strain curve beyond the proportional limit is called plastic region. - The elastic limit is defined as the maximum stress that a material will withstand without permanent deformation. There fore, for all practical
  33. 33. purposes, the proportional limit and elastic limit represent the same stresses within the structure and terms are used inter changeably. YIELD STRENGTH : The conditions needed for the definitions of elastic limit and proportional limit are not always realized under practical conditions. The yield strength or yield stress (YS) of a material is a property that can be determined readily and is often used to describe the stress at which the material begins to function in a plastic manner. - At this stress, a limited permanent strain has occurred in a material. - The yield strength is defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain. - A value of either 0.1% or 0.2% of the plastic strain is often selected and is referred to as “percent offset”. COMPRESSIVE STRENGTH : Compressive strength is important in many restorative materials used in dental technique and operations. - Compressive strength in measure of the amount of force a material can support in a single impact before breaking. - This property is particularly important in the process of mastication because many of the forces of mastication are compressive. - One of the easiest to measure and is often cited in advertisements for materials. - There is no direct correlation between compressive strength and clinical perform.
  34. 34. - Compressive strength is most useful for comparing materials that are brittle and generally weak in tension and that are therefore not employed in regions of oral cavity where tensile forces predominate. - Certain characteristics of a material subjected to tension are also observed when a material is in compression. - When a structure is subjected to compression, note that the failure of the body may occur as a result of complex stress formations in the body. This is illustrated by a cross-sectional view of a right cylinder subjected to compression. It is apparent that forces of compression are resolved into forces of shear along a cone shaped area at each end and as a result of the action of the 2 cones on the cylinder, into tensile forces in the center of the cylinder.
  35. 35. - Because of this resolution of the forces in the body, it has become necessary to adopt standard sizes and dimensions to obtain reproducible test results. - This fig shows that if a material too short, the force distributions becomes more complicated as a result of the cone formations over lapping in the ends of the cylinder. - If the specimen is too long, buckling may occur. Therefore the cylinder should have a length twice that of diameter for the most satisfactory results. - Unfortunately, the punch test has no direct correlation to the clinical performance of a material. Further, there is little agreement in the research community on how to conduct this test, although standards are being developed.
  36. 36. SHEAR STRENGTH : Shear stress is the maximum stress that a material can withstand before failure in a shear mode of loading. - It is particularly important in the study of interfaces between 2 materials and has been used to measure bond strength between different materials. - One method of testing shear strength of dental materials is the punch or pushout method – in which an axial load is applied where ‘F’ is the compression force applied to the specimen, ‘d’ is the diameter of the punch and ‘h’ is thickness of specimen, then shear strength = S.S. = F / π d h - In this test, shear strength is calculated from the compressive force applied, the diameter of punch and the thickness of material tested.
  37. 37. - It is important to note that the stress distribution caused by this method is not ‘pure’ shear and that results often differ because of differences in specimen distribution, surface geometry, composition and preparation and mechanical testing procedure. - However, it is a simple test to perform and has been used extensively. Alternatively, shear properties may be determined by subjecting a specimen to tensional loading as well. TENSILE STRENGTH : - Is the amount of force that can be used to stretch a material in a single impact prior to breaking. - This physical property is more difficult to measure than CST. - The tolerance of measuring device is critical – materials must be pulled at an exact 1800 angle from each other to eliminate the influence of shear forces. The clinical relevance of tensile strength is limited.
  38. 38. Diametral tensile strength : Theoretical tensile strength measurement that is calculated by measuring the CS of a disc of material. This test is easier to perform and more consistent than the clinical tensile strength test. TOUGHNESS : - Toughness is defined as the amount of elastic and plastic deformation energy required to fracture a material. It is a measure of the energy required to propogate critical flows in the structure. - Toughness is indicated as the total area under the stress stain graph, from zero stress to fracture stress. - Toughness increases with increase in strength and ductility. - Fracture toughness is the critical stress intensity factor at the beginning of the rapid crack propagation in a solid containing a crack of known shape and size.
  39. 39. - It describes the resistance of brittle materials to the catastrophic propagation of flows under an applied stress. - Fracture toughness is given in units of stress times the square root of crack length that is MPa M ½ or the equivalent form, MN. M– ½ .
  40. 40. BRITTLENESS : Shown in this figure are 3 materials, their stress-stain curves with variable properties. - Material A is stronger, stiffer and more ductile than material B and C. - Material B has less ductility than material A and is thus more brittle. - Material C has no ductility and is perfectly brittle, it is also the weakest of the 3 materials. - Brittleness is the relative inability of material to sustain plastic deformation before fracture of a material occurs. For eg : amalgams, ceramics and composites are brittle at oral temperatures (5-550 C). They sustain little or no plastic strain before they fracture. In other words, a brittle material fractures at or near its proportional limit. This behaviour is shown by material C. - However, a brittle material is not necessarily weak. For eg : if a glass is drawn into a fiber with very smooth surface and insignificant internal flows, its tensile strength may be as high as 2800MPa, but it will have no ductility (0% elongation).
  41. 41. DUCTILITY AND MALLEABILITY : For eg : a metal that can be drawn readily into long, thin, wire is considered to be ductile. - The ability of a material to sustain considerable permanent deformation without rupture under compression, as in hammering or rolling into a sheet, is termed “MALLEABLE”. - Gold is the most ductile and malleable pure metal and silver is second, copper ranks third. - Ductility is the maximum plastic deformation a material can withstand when it is stretched at room temperature. Measurement of ductility : There are 3 common methods for measurement of ductility 1) The present elongation after fracture 2) The maximum number of bends performed in a cold bend test. - The most common and simplest used method is to compare the increase in length of a wire or rod after fracture in tension to its length before fracture. 2 marks are placed on the wire or rod after fracture in tension to its length before fracture. 2 marks are placed on the wire or rod a specified distance apart and this distance is designated as the ‘gauge length’. The wire or rod is then pulled apart under a tensile load. The guage length is again measured. The ratio of the increased length is % is called ‘percent elongation” and this represents the quantitative value of ductility. - Another method for manifestation of ductility is the COLD BEND TEST : the material is clamped in a rise and bent around a mandrel of a
  42. 42. specified radius. A number of bends to fracture is counted, and the greater the number, the greater the ductility. CONCLUSION It is very important to know the properties of the materials we use in dentistry, especially as restorative materials. This will enable us to select a material that will have properties close to that of natural tooth structure. Also we will be able to better understand and select materials from the wide range that are coming in to the market. Hence, a thorough knowledge of the properties of restorative materials is a must.
  43. 43. REFERENCES • Phillips science of dental materials-Anusavice(11 edn) • Restorative dental materials-Craig and Powers (11edn) • Materials in dentistry- Ferracane • Tooth colored restoratives-Albers