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  • Add as Friendphysical and mechanical properties and its application in orthodontics

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    2 : Physical and Mechanical Properties and its application in orthodontics
    3 : Prepared by Dr.Hardik Lalakiya Guided by Dr.Ajay Kubavat Dr.Chintan Agrawal Dr.Ketan Mashru Dr.Bhavik Patel Dr.Manish Desai Dr.Vishal Patel Department of Orthodontics and Dentofacial Orthopaedics
    4 : OUTLINE Introduction Crystal structure and its arrangement Principal metal structures and its arrangement Classification Stress and its types Strain True Stress strain curve Poisson’s ratio Mechanical properties based on elastic deformation Toughness Impact strength Proportional limit Elastic limit Yield strength
    5 : Permanent Plastic deformation Strain hardening Strength and its types Fatigue Static fatigue Brittleness Ductility Malleability Physical Properties Abrasion and abrasion resistance. Hardness Viscosity Creep and flow Color and color perception Bezold brucke effect
    6 : Mechanical properties are defined by the laws of mechanics that is the physical science that deals with the energy and forces and their effects on bodies the discussion centers primarily on the static bodies –those at rest-rather than on dynamic bodies. Many factors must be taken into account when considering which properties are relevant to the successful performance of the material used in dentistry
    8 : Atomic arrangements in crystalline solids can be described with respect to a network of lines in three dimensions. The intersections of the lines are called “lattice sites” (or lattice points). Each lattice site has the same environment in the same direction.
    9 : A particular arrangement of atoms in a crystal structure can be described by specifying the atom positions in a repeating “unit cell”.
    10 : 14 Bravais lattices
    11 : Principal metal crystal structures There are three principle crystal structures for metals: –(a) Body-centered cubic (BCC) –(b) Face-centered cubic (FCC) –(c) Hexagonal close-packed (HCP)
    12 : Principal structures
    13 : Body centered cubic (BCC)
    14 : (BCC)
    15 : Face centered cubic (FCC)
    16 : (FCC)
    17 : Hexagonal closed packed (HCP)
    18 : (HCP)
    19 : Classification
    20 : Definition: When a force acts on a body tending to produce deformation . A resistance is developed to this external force application. The INTERNAL reaction is equal in intensity and opposite in direction to the applied external force and is called stress. Stress = Force/Area STRESS
    21 : Commonly expressed as Pascal 1Pa = 1N/m2. It is common to report stress in units of Megapascals (MPa) where 1 MPa = 106 Pa. TYPES OF STRESS :- Tensile Compressive Shear In english system of measurement ,the stress is usually expressed in pounds per square inch.
    22 : 3 Types of stress Tensile Compressive Stress Shear stress
    23 : Tensile Stress A tensile Stress is caused by a load that tends to stretch or elongate a body . for eg stress developed on the gingival side of 3 unit bridge bridge
    24 : 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 compressive stress.
    25 : Shear stress A stress that tends to resist a twisting motion or sliding of one portion of a body over another is shearing stress 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 may debond by shear stress failure of the resin luting cement
    26 : Complex stress Complex stress those produced by applied forces that cause flexural or torsional deformation are called flexural stress More than two They are also called as bending stress.
    27 : STRAIN A force is applied to a body it undergoes deformation. Strain is described as the change in length (? L = L – LO) per unit length of the body when it is subjected to a stress. Strain (? ) = Change in length = L – Lo = ? L Original length Lo Lo
    28 : Strain has no units of measurement.·   It is a Dimensionless quantity.· Reported as an absolute value or as a percentage.
    29 : Facts The Average max sustainable biting force is 756N (170 pounds) or (77kgs) The Guiness Book Of World records (1994) lists the highest biting force as 4337N (975 pounds).
    30 : Each type of stress is capable of producing a corresponding deformation in a body. Tensile stress produces tensile strain. Compressive stress produces compressive strain. Shear stress produces shear strain.
    31 : Stress strain curve Represents energy storage capacity of the wire so determines amount of work expected from a particular spring in moving a tooth.
    32 : True stress strain curve A stress strain curve based on stresses calculated from a Non Constant Cross sectional area is called a true stress strain Curve. A true-stress strain curve may be quite different from an engineering stress-strain curve at high loads because significant changes in the area of specimen may occur.
    34 : Mechanical Properties Based On Elastic deformation Elastic Modulus Shear Modulus Flexibility Resilience Poisson’s ratio.
    35 : Elastic modulus(young’s modulus or Elasticity) The term elastic modules describes the relative STIFFNESS or RIGIDITY of a material which is measured by the elastic region of stress – strain diagram. It is denoted by letter E Determined from stress stain curve by calculating ratio of stress to strain or slope of linear portion of curve. Stress 6 Elastic Modulus = = Strain ?
    36 : Stress strain curve
    37 : Modulus of elasticity is independent of the ductility of a material and it is not a measure of its strength. It is an inherent property of a material and cannot be altered appreciably by heat treatment, work hardening or any other kind of conditioning. This property is called STRUCTURAL INSENSITIVITY.
    38 : The Elastic modulus of a tensile test specimen can be calculated as follows where E is elastic modulus P is the applied force or load A is the cross sectional area of material under stress ^l is the increase in length Lo is the original length
    39 : Flexibility The maximum flexibility is defined as the strain that occurs when the material is stressed to its proportional limit. For example in an orthodontic appliance, a spring is often bent a considerable distance with a small stress resulting in such a case structure is said to be flexible.
    40 : Resilience Popularly the term Resilience is associated with “springiness”. Definition: It is defined as the amount of energy absorbed by a structure when it is stressed to its proportional limit. Area bounded by the elastic region is measure of Resilience.
    41 : Poisson’s ratio Any material when subjected to a tensile or compressive stress, there is simultaneous axial and lateral strain. Within elastic range the ratio of lateral to axial strain is known as poisson’s ratio. Dental materials have poisson’s ratio in the range of 0.3 to 0.5.
    42 : TOUGHNESS It is defined as energy required to fracture a material. It is measured as a total area under stress strain curve. Toughness of the material is dependent on the ductility and malleability of the material than upon the flexibility or elastic modulus.
    43 : Conventional Tensile Stress Strain Curve
    44 : IMPACT STRENGTH IMPACT: It is the reaction of a stationary object to a collision with a moving object. Depending on the resilience of the object , energy is stored in the body without causing deformation or with deformation. Impact resistance decreases with increase in stiffness. Resilient material have high impact strength. Increase in volume leads to increase in impact resistance.
    45 : Impact Strength (continue).. It is the energy required to fracture a material under force. A charpey type tester is used. It has a heavy pendulum which swings down to fracture the specimen. Another instrument called Izod impact tester can also be used.
    46 : Strength properties Strength is the stress necessary to cause either fracture(ultimate strength) or a specified amount of plastic deformation(yields strength). The strength of a material can be described by Proportional limit Elastic strain Yield strength Ultimate tensile strength, shear ,compressive and flexural strength.
    47 : Proportional limit (PL) It is defined as the greatest stress that a material will sustain without a deviation from the linear proportionality of stress to strain.
    48 :
    49 : Hooke’s Law :- States that stress – strain ratio is constant upto the proportional limit, the constant in this linear stress-strain relationship is Modulus of Elasticity. Below PL no permanent deformation occurs in a structure. Region of stress stain Curve. Below PL – ELASTIC REGION Above PL – PLASTIC REGION
    50 : Elastic limit (EL) Definition: It is defined as maximum stress that a material can withstand before it undergoes permanent deformation. For all practical purposes PL and EL represent same stress. But they differ in fundamental concept :-
    51 : PL deals with proportionality of strain to stress in structure. EL describe elastic behavior of the material. EL PL limits are usually assumed to be identical although their experimental values may differ slightly.
    52 : Yield Strength(yield stress or proof stress) It is defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain. Amount of permanent strain is arbitrarily selected for material being examined and may be indicated as 0.1%, 0.2% or 0.5% (0.001, 0.002, 0.005) permanent strain
    53 : Amount of permanent strain may be referred to as PERCENT OFFSET. Many specifications use 0.2% as convention.
    54 : Permanent (Plastic) deformation If the material is deformed by a stress at a point above the proportional limit before fracture,the removal of applied force will reduce the stress to zero,but the strain does not decrease to zero because the plastic deformation has occurred . Thus the object does not return to its original dimension when the force is removed.It remains bent,streched,compressed or otherwise plastically deformed.
    55 : Strain hardening Strengthening by increase of dislocation density (Strain Hardening = Work Hardening = Cold Working) Ductile metals become stronger when they are deformed plastically at temperatures well below the melting point. The reason for strain hardening is the increase of dislocation density with plastic deformation.
    56 : Average distance between dislocations decreases and dislocations start blocking the motion of each other. The percent cold work (% CW) is often used to express the degree of plastic deformation: %CW is just another measure of the degree of plastic deformation, in addition to strain.
    57 :
    58 :
    59 :
    60 : Strength It is the maximal stress required to fracture a structure. Strength is not a measure of individual atom to atom attraction or repulsion , but rather it is a measure of the interatomic forces collectively over the material which is stressed. STRENGTH IS BASICALLY OF FOUR TYPES: Tensile Compressive Shear Flexure
    61 : Tensile strength Tensile Strength is determined by subjecting a rod , wire or a dumbbell shaped specimen to a tensile loading. It is defined as the maximal stress the structure will withstand before rupture.
    62 : Diametral Tensile Strength Brittle material an indirect tensile test called Diametral compression test or Brazillian test is used . A compressive load is placed on the diameter of a short cylindrical material .
    63 : Compressive strength Crushing strength is determined by subjecting a cylindrical specimen to a compressive load. The strength is obtained from the cross sectional area and force applied. Complex failure
    64 : SHEAR SRENGTH Maximum stress a material can withstand before failure in a shear mode of loading. It is tested using punch or pushout method. Shear strength = Force/ ? punch dia * thickness
    65 : FLEXURE STRENGTH Transverse strength or modulus of rupture or flexure strength Obtained using a beam supported at each end and load applied in the middle. Also called three point bending test. Used in long span bridges. Neutral Axis
    66 : Fatigue A Structure subjected to repeated or cyclic stress below its proportional limit can produce abrupt failure of these structure. Fatigue behavior is determined by subjecting a material to a cyclic stress of known value and determining the number of cycles that are required to produce failure.
    67 :
    68 : Static fatigue Some material support a static load for a long period of time and fail abruptly. This type of failure may occur in wet environment. Eg ceramic materials.
    69 : Brittleness A brittle material fractures at or near its proportional limit. It is opposite of toughness. Brittle material will not bend appreciably without breaking. Though a brittle material may have a very high compressive strength. E.g. glass.
    70 : Ductility Ability of a material to withstand permanent deformation under a tensile load without rupture. It is the ability of the metal to be drawn into wires. Ductility depends on tensile strength. It decreases with increase in temperature.
    71 : MEASUREMENT OF DUCTILITY 1.Percentage elongation after fracture Gauge length = 51 mm( STANDARD GAUGE LENGTH FOR DENTAL MATERIALS) 2.Measuring reduction in cross sectional areas of fractured ends in comparison to the original area of the wire. This is also called as reduction in area method. 3. cold bend test
    72 : Malleability It is the ability of a material to withstand rupture under compression. It is seen in hammering or rolling of a material into sheets. It is not dependent on the strength of the material It increases with temperature. Gold is most ductile and malleable and silver stands the second. Platinum is third most ductile and copper ranks third in malleability.
    73 : Stress concentration factors THESE INCLUDES Surface flaws Internal voids air bubbles. Inclusions of other materials Hertzian load Sharp angles Notches Thermal mismatch
    74 : Some clinical relations with orthodontic wire Tension Test Results; UTS and E for stainless steel and titanium material. Material Type UTS (MPa) E (GPa) Stainless steel 1300 193 titanium 1615 179
    75 : Stress-Strain curve of stainless steel specimen the x-axis the strain in the specimenand the y-axis stress (MP/mm2). By wp 300 tensile testing machine
    76 :
    77 : Physical Properties
    78 :
    79 :
    80 : Abrasion and abrasion resistance Phenomenon of wearing/ removal process that occurs whenever surfaces slide against each other The material which causes wearing is called abrasive The material which is worn is called substrate.
    81 : Hardness is one of the common index of a material to resist abrasion or wear but not the only index. Other factor which cause and influence abrasion / abrasion resistance are Biting force Frequency of chewing, Abrasiveness of diet, Intra oral liquid, temperature changes, Surface roughness, Impurities and irregularities (Pits and grooves)
    82 : hardness Resistance to surface penetration / surface scratching /ability to resist indentation. Indentation is produced on the surface of the material from a applied force of a sharp point or an abrasive particle. Most hardness test are based on ability of a surface of a material to resist penetration by diamond point or a steel ball under a specified
    83 : Common tests are Barcol Brinell (BH) Rockwell (RH) Shore Vickers (HV) Knoop (KH) Microhardness test Macrohardness test
    84 : Brinell hardness number (BHN) Oldest, simplest , convenient extensively used Hardened steel ball pressed with standard load on polished surface of material . Load is divided by the area of projected surface of indentation . Thus for a given load smaller the indentation, larger is the number and the harder is the material
    85 : Rockwell hardness number (RHN) Conical diamond point is used. Depth of penetration is measured directly by the dial gauge on the Instrument RHN and BHN are used for measuring hardness of metal and alloys and they are not suitable for brittle materials.
    86 : Vickers hardness test HV test employs square based pyramid of 136 Degrees Method of computation is the load divided by the projected area of Indentation. The length of the diagonals are measured and averaged. Can be used for brittle materials. also called 136 degree diamond pyramid test.
    87 : Knoop hardness number (KHN) Uses diamond tip tool. Rhombohedral pyramid diamond tip is used of dimension 130 degree and 172.30 degree The length of the largest diagonal is measured . The projected area is divided in to the load to give KHN Can be used for extremely hard and soft materials.
    88 : KHN and HV are called as micro hardness test. BHN and RHN are macro hardness test. Shore and Barcol test are sometimes employed to measure hardness of rubber and plastic type of dental materials. These have spring loaded metal indenter point.
    89 : Viscosity Resistance of a liquid to flow Study of flow character of a material is the basis for Rheology Importance of knowing flow: impressions, Gypsum products, cements, waxes. Resistance to flow is controlled by internal frictional forces. Thus viscosity is the measure of consistency of a medium and its inability to flow.
    90 : Change in Viscosity Whenever a force is applied to a material it will deform. The force / area is called stress. The calculation of deformation is the strain. Strain = change in length / initial length. Unit of viscosity is MPa / second or CETIPOISE
    91 : Viscosity of most liquids decreases with increase in temperature i.e. its flow increases To explain viscous nature of some materials , shear stress / shear strain rate curve is plotted .
    92 : Based on Rheologic behavior fluids are classified in to four types Newtonian fluid Pseudoplastic Dilatant fluid Plastics
    93 : Newtonian fluid Ideal fluid which demonstrates a shear strain proportional to the shear stress The plot on the graph is a straight line Newtonian fluids has a constant viscosity and is independent of the shear strain rate.
    94 : Pseudoplastic fluid When the viscosity of a material decreases with increasing strain rate until it reaches the constant value such a material is called Pseudoplastic materials or fluid.
    95 : Dilatant fluid These are the liquids that becomes more rigid as the rate of deformation increases. These liquids show opposite tendency as described for pseudoplastic
    96 : Plastic Some classes of material behave like a rigid body until some minimum value of shear stress is reached (off set value) These fluids which exhibits rigid behavior initially and then attend constant viscosity are referred to as plastic. Ketchup is a familiar example .
    97 : Thixotrophic material Viscosity of liquid also depends on previous deformation of liquid A liquid of this type that becomes less viscous and more fluid under more repeated application of pressure is called as Thixotrophic materials Examples: Dental polishing paste, plaster of paris, impression materials, resins and cements
    98 : Importance of Viscosity Properties Teaches us the best way to manipulate the materials Guides as on the best use of the materials Measure of working time Thixotropic materials stays on tray but on applying pressure in the mouth the material flows
    99 : Creep and flow If the metal is held at the temperature near its melting point and subjected to constant applied stress, the resulting strain will increases over time. Creep is defined as the time dependant plastic strain of a materials under static / constant load. Sag is same as creep but the load is the mass of the same material .
    100 : Creep and flow (continue…) A filling material called “Amalgam” has low melting range. So when in mouth it is close to the melting point and is subjected to constant biting forces. It gets get deformed. Here the biting forces keep changing and continuous Dyanamic creep. For waxes term flow rather than creep is used as it is amorphous. The flow of wax is its potential to deform under small static load / or its own mass.
    101 : Creep and flow (continue…) Flow is measured using compressive forces mostly. Testing flow: A cylinder prescribed dimension is subjected to a given compressive stress for a specified time and temperature. The creep or flow is measured as percentage decrease in length. Significance of creep / sag.
    102 : Thermophysical properties Heat transfer through solid substances most commonly occur by means of conduction. The conduction of heat through metals occurs through the interaction with atoms. Thermal conductivity (k) is the thermophysical measure of how well heat is transferred through a material by conductive flow. The measurement of thermal conductivity is performed under steady state conditions.
    103 : Thermoconductivity Properties 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 1cm2 ,when the temperature difference between the surfaces Thermoconductivity Properties perpendicular to the heat flow of the specimen is 10 K. Materials that have a high thermal conductivity are called conductors, whereas materials of low thermal conductivity are called insulators.
    104 : Thermoconductivity Properties(Cont..) The international system (SI) unit or measure for thermal conductivity is watt / meter / second /o Kelvin Increase in thermal conductivity , greater is the ability to transfer thermal energy. Metal restoration – increase conductivity compared to other materials.
    105 : Thermal Diffusivity The value of thermal diffusivity of a material controls 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 nonuniform temperature reaches a state of thermal equilibrium. For a given volume of material, the heat required to raise the temperature , to a given amount depends on its heat capacity or specific heat and the density.
    106 : Thermal Diffusivity (cont).. The formula that related thermal diffusivity to thermal conductivity is h = k / cp? h = Thermal diffusivity k = Thermal conductivity cp = Heat capacity ? = temperature dependent density
    107 : Thermal Diffusivity (cont).. Square root of thermal diffusivity is indirectly proportional to thermal insulation ability. SI unit is square meter per second commonly used.
    108 : Coefficient of thermal expansion Coefficient of thermal expansion, is defined as the change in length / unit of the original length of a material when its temperature is raised 1degree K. SI unit µm /m0 K or ppm / k0 A tooth restoration may contract or expand more than the tooth during the change in temp which may cause micro leakage or debond of restoration of teeth. To reduce this, selection of material whose expansion or contraction coefficient should be matched approximately within 4%. PFM
    109 : Color and color perception (cont).. Sensation induced from color of various wavelength reaching the eye. Eye is sensitive to wavelength of 400nm(violet) to 700nm(dark red). For an object to be visible, it must reflect and transmit incident light at certain wavelength. Color is measured using munsell system.
    110 : Color and color perception (cont)..
    111 : Color and color perception (cont).. Thus, Light from object Incident on eyes Focused in retina ?rods and cones Converted into nerve impulses Transmitted to brain
    112 : Color and color perception (cont).. Three dimension of color are: 1. Hue 2. Value 3. Chroma
    113 : Color and color perception (cont).. Hue: Dominant color of an object E.g. red, blue, green (dominant wavelength). The normal human teeth have hue range of 6.3 yellow red to 9.3 yellow red.
    114 : Color and color perception (cont).. Value Relative lightness or darkness of color. The human teeth have a value in the range of 0-7.
    115 : Color and color perception (cont).. CHROMA Degree of saturation of particular hue. Higher the chroma, more intense and mature the color. Chroma cannot exist itself and it is always associated with hue and value. Normal human teeth has chroma of 4 to 7.
    116 : Color and color perception (cont).. Color Solid: Central rod = value Spikes = hue Volume = chroma
    117 : Color and color perception (cont).. CIE SYSTEM: Commission International Eclairage. Based on Adam system Colour in L*a*b L = value a = measure along r-g axis b= measure along y-b axis
    118 : Color and color perception (cont).. Shade Guide : In the dental laboratory, color matching is usually performed by the shade guide. The most commonly used guide is VITA shade guide. The range is from A1 to D4 .From left to right the darkness increase.
    119 : Color and color perception (cont).. Metamerism: Object that appear to be color matched under one type of light may appear different under another light source. Day light, incandescent lamps, fluorescent lamps are most common source of light in dental operatory. Two or more sources of light should be used to prevent metamerism causing wrong selection of
    120 : Metamerism
    121 : Color and color perception (cont).. Near ultraviolet radiation: Natural tooth structure absorbs light at wave lengths too short to be visible at human eye. These wave lengths between between 300nm- 400nm are referred as near ultraviolet radiation. Sources are natural sunlight, photoflash lamps, UV light
    122 : Color and color perception (cont).. Fluorescence: Energy that the tooth absorbs is converted into light with longer wavelength in which case the tooth actually becomes a light source. The phenomenon is called Fluorescence. Ceramics, composites – fluorescent agents are added.
    123 : Fluorescence
    124 : Color and color perception (cont).. BEZOLD BRUCKE EFFECT: At low light levels, rods of human eye are dominant and color perception is lost. As the brightness becomes more intense , color appears to change.
    127 :
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