Mechanical properties 2

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MECHANICAL PROPERTIES MECHANICAL PROPERTIES OF OF

DENTAL MATERIALSDENTAL MATERIALSByBy

Dr Khawaja Rashid HassanDr Khawaja Rashid HassanAssistant Professor Assistant Professor

RAWAL INSTITUTE OF HEALTH SCIENCESRAWAL INSTITUTE OF HEALTH SCIENCESRAWAL COLLEGE OF DENTISTRYRAWAL COLLEGE OF DENTISTRY

ISLAMABADISLAMABAD

MECHANICAL PROPERTIES OF MECHANICAL PROPERTIES OF DENTAL MATERIALSDENTAL MATERIALS

Defined by the laws of mechanics.Defined by the laws of mechanics.The physical science that deals with The physical science that deals with

energy and forces and their effects on energy and forces and their effects on the bodies.the bodies.

Mechanical properties need to be Mechanical properties need to be considered collectively.considered collectively.

Intended application of a material is Intended application of a material is important.important.

MECHANICAL PROPERTIES OF MECHANICAL PROPERTIES OF DENTAL MATERIALSDENTAL MATERIALS

Failure or success potential of any Failure or success potential of any prosthesis / restoration is dependent prosthesis / restoration is dependent upon the mechanical properties of the upon the mechanical properties of the material.material.

The material response may be,The material response may be,1.1. Elastic …. reversible on force removal.Elastic …. reversible on force removal.2.2. Plastic …… Irreversible / non-elastic.Plastic …… Irreversible / non-elastic. Mechanical properties are expressed Mechanical properties are expressed

in terms of stress and/or strain.in terms of stress and/or strain.

MASTICATORY FORCESMASTICATORY FORCES

Occlusal forces Occlusal forces applied by adult applied by adult dentition is greatest dentition is greatest in posterior region.in posterior region.

In growing children In growing children there is an average there is an average annual increase in annual increase in force of 22 N.force of 22 N.

Denture wearers Denture wearers only apply 40% of only apply 40% of the forces given in the forces given in table.table.

ToothTooth AverageAverage

force (N)force (N)

Second Second molarmolar

800800

First molarFirst molar 390390

BicuspidsBicuspids 288288

CupidsCupids 208208

IncisorsIncisors 155155

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STRESSSTRESS When a force acts on the body, a resistance When a force acts on the body, a resistance

is developed to the external force applied.is developed to the external force applied. This internal reaction is equal in This internal reaction is equal in

magnitude/intensity and opposite in magnitude/intensity and opposite in direction to the applied force and is called as direction to the applied force and is called as ““STRESS”STRESS”

Denoted by “S” or “Denoted by “S” or “σσ”” Designated as force per unit area (Designated as force per unit area (σσ=N/m²)=N/m²) Pascal = 1 N / m².Pascal = 1 N / m². Commonly stress is reported in terms of Commonly stress is reported in terms of

megaPascals.megaPascals.

STRAINSTRAIN

Relative deformation of an object that Relative deformation of an object that is subjected to stress.is subjected to stress.

It is change in length per unit length.It is change in length per unit length. It may be elastic, plastic or both It may be elastic, plastic or both

elastic and plastic.elastic and plastic. It is denoted by “It is denoted by “εε””Designated as ∆L / L.Designated as ∆L / L.

TYPES OF FORCES APPLIEDTYPES OF FORCES APPLIED

Generally, the force applied may beGenerally, the force applied may be

1.1. Axial (tensile or compressive)Axial (tensile or compressive)

2.2. Shear (sliding, rubbing)Shear (sliding, rubbing)

3.3. Bending (bending movement)Bending (bending movement)

4.4. Tortional (twisting movement)Tortional (twisting movement)


Tension results when a body is Tension results when a body is subjected to two sets of forces subjected to two sets of forces directed away from each other in a directed away from each other in a straight line. Force is directed away straight line. Force is directed away from the objcet.from the objcet.

Compression results when the body is Compression results when the body is subjected to two sets of forces subjected to two sets of forces directed towards each other in a directed towards each other in a straight line.straight line.


TENSIONTENSION COMPRESSIONCOMPRESSION

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Shear is a result of two sets of forces Shear is a result of two sets of forces directed parallel to each other , but directed parallel to each other , but not along the same straight line.not along the same straight line.

Torsion results from the twisting of the Torsion results from the twisting of the body.body.

Bending results by applying bending Bending results by applying bending movement.movement.

TYPES OF STRESSESTYPES OF STRESSES 3 simple types.3 simple types.1.1. TENSILE STRESS:TENSILE STRESS:

causes the body to stretch or elongate. causes the body to stretch or elongate. Tensile stress is always accompanied Tensile stress is always accompanied by tensile strain.by tensile strain.

2.2. COMPRESSIVE STRESS:COMPRESSIVE STRESS:causes the body to shorten or causes the body to shorten or compress. Compressive compress. Compressive

3.3. SHEAR STRESS:SHEAR STRESS:resist the sliding or twisting of one resist the sliding or twisting of one portion of the body over another.portion of the body over another.


Complex stressesComplex stresses

FLEXURAL STRESS:FLEXURAL STRESS: Also called as bending stress.Also called as bending stress. Produced by bending forces over the Produced by bending forces over the

dental appliance.dental appliance. Application of shear force may Application of shear force may

produce elastic shear strain or plastic produce elastic shear strain or plastic shear strain.shear strain.

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Hooke's LawHooke's Law

Hooke's LawHooke's Law states that "within the states that "within the limits of elasticity the strain limits of elasticity the strain produced by a stress (of any one produced by a stress (of any one kind) is proportional to the stress".kind) is proportional to the stress".

The stress at which a material The stress at which a material ceases to obey Hooke's Law is known ceases to obey Hooke's Law is known as the limit of proportionality. as the limit of proportionality.

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Hooke's LawHooke's Law

Hooke's law can be expressed by the Hooke's law can be expressed by the formula formula stress / strain = a constant. stress / strain = a constant.

The value of the constant depends The value of the constant depends on the material and the type of on the material and the type of stress. For tensile and compressive stress. For tensile and compressive forces it is called Young's modulus, forces it is called Young's modulus, EE; ; for shearing forces, the shear for shearing forces, the shear modulus, modulus, SS; and, for forces affecting ; and, for forces affecting the volume of the object, the the volume of the object, the bulk modulus, , KK..

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PROPORTIONAL LIMITPROPORTIONAL LIMIT

It is the maximum stress at which It is the maximum stress at which the stress is equivalent/proportional the stress is equivalent/proportional to strain and above this limit the to strain and above this limit the plastic deformation of a material plastic deformation of a material occurs.occurs.

The material may be subjected to The material may be subjected to any type of applied force.any type of applied force.

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STENGTHSTENGTH

Strength is the maximum stress that Strength is the maximum stress that a material can withstand without a material can withstand without sustaining a specific amount of sustaining a specific amount of plastic strain.plastic strain.

OROR

Stress at the point of fracture.Stress at the point of fracture.

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STRENGTH PROPERTIESSTRENGTH PROPERTIES

ULTIMATE TENSILE STENGTH :ULTIMATE TENSILE STENGTH :Simply called as TENSILE STRENGTH.Simply called as TENSILE STRENGTH.It is defined as the Tensile stress at the It is defined as the Tensile stress at the point of fracture.point of fracture.YIELD STRENGTH :YIELD STRENGTH :It is the stress at which a test specimen It is the stress at which a test specimen exhibits a specific amount of plastic exhibits a specific amount of plastic strain.strain.Used in the conditions when proportional Used in the conditions when proportional limit cannot be determined with accuracy.limit cannot be determined with accuracy.

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SHEAR STRENGTH:SHEAR STRENGTH:Maximum shear stress at the point of Maximum shear stress at the point of fracture.fracture. FLEXURAL STRENGTH:FLEXURAL STRENGTH:Defined as “force per unit area at the Defined as “force per unit area at the point of fracture of a specimen that point of fracture of a specimen that is subjected to flexural loading”is subjected to flexural loading”Also called as “BENDING STRENGTH” Also called as “BENDING STRENGTH” or “MODULUS OF RUPTURE”or “MODULUS OF RUPTURE”

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FATIGUE STRENGTH:FATIGUE STRENGTH: Determined by subjecting a material to Determined by subjecting a material to

cyclic stress of maximum known value and cyclic stress of maximum known value and determining the number of cycles required determining the number of cycles required to cause failure of the material.to cause failure of the material.

Maximum service stress (endurance limit) Maximum service stress (endurance limit) can be maintained without failure over an can be maintained without failure over an infinite number of cycles.infinite number of cycles.

Endurance limit is lower for materials with Endurance limit is lower for materials with brittle and rough surface.brittle and rough surface.

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FATIGUE STRENGTH:FATIGUE STRENGTH: Dental restorative materials may exhibit Dental restorative materials may exhibit

static fatigue failurestatic fatigue failure or or dynamic dynamic fatigue failurefatigue failure..

Depends upon the nature of loading or Depends upon the nature of loading or residual stress situations.residual stress situations.

Failure begins as a flaw that propagates Failure begins as a flaw that propagates till the catastrophic fracture occurs.till the catastrophic fracture occurs.

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IMPACT STRENGTH:IMPACT STRENGTH: Impact is the reaction of a stationary Impact is the reaction of a stationary

object to a collusion with a moving object to a collusion with a moving body.body.

Impact strength is defined as energy Impact strength is defined as energy required to fracture a material under an required to fracture a material under an impact force.impact force.

The energy units are joules.The energy units are joules.

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ELASTIC MODULUSELASTIC MODULUS

Also called as modulus of elasticity or Also called as modulus of elasticity or Young’s modulus.Young’s modulus.

It is the relative stiffness or rigidity of a It is the relative stiffness or rigidity of a material.material.

Measured by the slope of the elastic Measured by the slope of the elastic region of the stress strain curve.region of the stress strain curve.

If a tensile or compressive stress (below If a tensile or compressive stress (below the proportional limit) is divided by the proportional limit) is divided by corresponding strain value, a constant of corresponding strain value, a constant of proportionality will be obtained.proportionality will be obtained.

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ELASTIC MODULUSELASTIC MODULUS

Unaffected by the amount of elastic or Unaffected by the amount of elastic or plastic stress induced in the material.plastic stress induced in the material.

Independent of ductility of a material.Independent of ductility of a material.The lower the strain for a given stress, The lower the strain for a given stress,

greater will be the elastic modulus.greater will be the elastic modulus.E.g. two wires of same shape and size.E.g. two wires of same shape and size.Polyether impression materials.Polyether impression materials.Unit is Giganewtons/m² (GPa).Unit is Giganewtons/m² (GPa).

FIRST MONTHLY CLASS TESTFIRST MONTHLY CLASS TEST

THEORY PAPERTHEORY PAPER 33RDRD MAY 2012 MAY 2012

(THURSDAY)(THURSDAY)

LECTURE TIMINGLECTURE TIMING

VIVAVIVA 44THTH MAY 2012 MAY 2012

(FRIDAY)(FRIDAY)

TUTORIAL TIMINGSTUTORIAL TIMINGS

TOPICS:TOPICS:1)1) INTRODUCTION TO INTRODUCTION TO

DENTAL MATERIALSDENTAL MATERIALS

2)2) SELECTION & EVALUATION SELECTION & EVALUATION OF DENTAL MATERIALS.OF DENTAL MATERIALS.

3)3) BIOCOMPATIBILITY OF BIOCOMPATIBILITY OF DENTAL MATERIALS.DENTAL MATERIALS.

4)4) PHYSICAL PROPERTIES OF PHYSICAL PROPERTIES OF DENTAL MATERIALS.DENTAL MATERIALS.

5)5) MACHANICAL PROPERTIES MACHANICAL PROPERTIES OF DENTAL MATERIALSOF DENTAL MATERIALS

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STRESS-STRAIN CURVESTRESS-STRAIN CURVE

For materials in which strain is For materials in which strain is independent of the length of time independent of the length of time that a load is applied “ STRESS that a load is applied “ STRESS STRAIN CURVES“ are important.STRAIN CURVES“ are important.

ANALYSIS FOR A STRESS ANALYSIS FOR A STRESS STRAIN CURVESTRAIN CURVE

STIFFNESS & FLEXIBILITYSTIFFNESS & FLEXIBILITY

1)1) If longitudinal portion of the curve is If longitudinal portion of the curve is closer to the long axis the material closer to the long axis the material is stiff & not flexible.is stiff & not flexible.

2)2) If it is away from the long axis the If it is away from the long axis the material is flexible.material is flexible.

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TOUGHNESS & BRITTLENESSTOUGHNESS & BRITTLENESS

1)1) If material fractures after a long If material fractures after a long concave portion of the curve, it concave portion of the curve, it donates that the material is tough & donates that the material is tough & ductile.ductile.

2)2) If elastic portion of the curve is If elastic portion of the curve is minimal, it shows the brittleness of minimal, it shows the brittleness of the material.the material.

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STRNGTH & WEAKNESSSTRNGTH & WEAKNESS If longitudinal portion of curve is longer, If longitudinal portion of curve is longer,

means that the material is strong.means that the material is strong. If longitudinal portion is short the If longitudinal portion is short the

material is weak.material is weak.

HENCE FROM THE ANALYSIS OF THE HENCE FROM THE ANALYSIS OF THE STRESS STRAIN CURVE IT IS STRESS STRAIN CURVE IT IS

POSSIBLE TO HAVE AN IDEA ABOUT POSSIBLE TO HAVE AN IDEA ABOUT THE PROPERTIES OF A MATERIAL.THE PROPERTIES OF A MATERIAL.

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STRAIN TIME CURVESSTRAIN TIME CURVES

For materials in which the strain is For materials in which the strain is dependent upon the time for which the dependent upon the time for which the load is being applied “STRAIN TIME load is being applied “STRAIN TIME CURVES” are mor useful in explaining the CURVES” are mor useful in explaining the properties of a material than stress strain properties of a material than stress strain curves.curves.

Examples:Examples:

Alginate & rubber base impression Alginate & rubber base impression materials, dental amalgam & human materials, dental amalgam & human dentin.dentin.

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STRESS STRAIN CURVESSTRESS STRAIN CURVES

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STRESS STRAIN CURVESSTRESS STRAIN CURVES

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Dynamic Young’s ModulusDynamic Young’s Modulus

Can be measured by dynamic Can be measured by dynamic method.method.

Ultrasonic longitudinal and Ultrasonic longitudinal and transverse wave transducers and transverse wave transducers and appropriate receivers are used.appropriate receivers are used.

The velocity of sound wave and The velocity of sound wave and density of material are used to density of material are used to calculate elastic modulus.calculate elastic modulus.

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RESILIENCERESILIENCE

The amount of elastic energy per unit The amount of elastic energy per unit volume released when the stress is volume released when the stress is removed.removed.

With increase in interatomic spacing With increase in interatomic spacing the internal energy increases.the internal energy increases.

Until the stress is lower than Until the stress is lower than proportional limit, the energy is proportional limit, the energy is called as RELILIENCE.called as RELILIENCE.

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TOUGHNESSTOUGHNESS

Amount of elastic and plastic deformation Amount of elastic and plastic deformation energy required to fracture a material.energy required to fracture a material.

Measured by the area under the elastic Measured by the area under the elastic region of the stress strain curve.region of the stress strain curve.

Toughness increases with increase in Toughness increases with increase in strength and ductility.strength and ductility.

Tough materials are generally strong.Tough materials are generally strong. Resistance of a brittle material to Resistance of a brittle material to

propagation of flaws under an applied propagation of flaws under an applied stress (FRACTURE TOUGHNESS)stress (FRACTURE TOUGHNESS)

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DUCTILITY and MALLEABILITYDUCTILITY and MALLEABILITY

DUCTILITY:DUCTILITY:

Ability of a material to deform Ability of a material to deform plastically under a tensile stress plastically under a tensile stress before fracture. e.g. metal drawn before fracture. e.g. metal drawn readily into long thin wires.readily into long thin wires.

MALLEABILITY:MALLEABILITY:

The ability of a material to sustain The ability of a material to sustain plastic deformation, without fracture plastic deformation, without fracture under compression.under compression.

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DUCTILITY and MALLEABILITYDUCTILITY and MALLEABILITY

Gold is the most ductile and Gold is the most ductile and malleable pure metal, followed by malleable pure metal, followed by silver.silver.

Platinum is ranked third in ductility.Platinum is ranked third in ductility.Copper ranks third in malleability.Copper ranks third in malleability.

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HARDNESSHARDNESS

In mineralogy, relative hardness of a In mineralogy, relative hardness of a substance is based upon its ability to substance is based upon its ability to resist scratching.resist scratching.

In metallurgy and mostly in all other In metallurgy and mostly in all other disciplines, hardness is defined as disciplines, hardness is defined as resistance to indentation.resistance to indentation.

Designated as Designated as KNOOP HARDNESS NUMBER.KNOOP HARDNESS NUMBER. BRINELL HARDNESS NUMBER.BRINELL HARDNESS NUMBER. VICKERS HARDNESS NUMBER.VICKERS HARDNESS NUMBER. ROCKWELL HARDNESS NUMBER.ROCKWELL HARDNESS NUMBER.

TERMS TO REMEMBERTERMS TO REMEMBER

Shapes produced by indentorsShapes produced by indentors

On materialsOn materials

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KNOOP HARDNESS TEST

BRINELL & ROCKWELL

HARDNESS TEST

VICKERS HARDNESS

TEST

QUESTIONS???QUESTIONS???

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Mechanical properties 2

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