Ist Year - Dental Materials

1. TIME DEPENDENTTIME DEPENDENT
MECHANICALMECHANICAL
PROPERTIESPROPERTIES

2. Time dependent mechanical propertiesTime dependent mechanical properties
I. ViscosityI. Viscosity
II. ViscoelasticityII. Viscoelasticity
1. Creep1. Creep
2. Stress relaxation2. Stress relaxation
III. FatigueIII. Fatigue

3. I. ViscosityI. Viscosity
 It a property of the liquid or paste state.It a property of the liquid or paste state.
 It is the resistance of a fluid to flow.It is the resistance of a fluid to flow.
 It is caused by the internal friction of a fluid.It is caused by the internal friction of a fluid.
 Viscosity = Shear stress / Shear rateViscosity = Shear stress / Shear rate
 It is measured with Viscometers .It is measured with Viscometers .
 Its unit is dyne-sec/cmIts unit is dyne-sec/cm22
or poiseor poise

4. Measurements of viscosityMeasurements of viscosity

5. Measurements of viscosityMeasurements of viscosity

6. Pouring a melted gold alloyPouring a melted gold alloy
to cast a crownto cast a crown

7. Model to discuss viscosityModel to discuss viscosity

8. Importance of viscosity in dentistryImportance of viscosity in dentistry
 It is important to have a low viscosity moltenIt is important to have a low viscosity molten
metal during casting in order to completely fillmetal during casting in order to completely fill
the mold.the mold.
 Impression materials should have a sufficientImpression materials should have a sufficient
degree of fluidity to get the details and also thedegree of fluidity to get the details and also the
fairly complex areas of tooth structure.fairly complex areas of tooth structure.
 Changes of viscosity of impression materialsChanges of viscosity of impression materials
with time is important.with time is important.

9. Changes of viscosity with time inChanges of viscosity with time in
impression materialsimpression materials

10. Factors affecting viscosityFactors affecting viscosity
 The nature of substanceThe nature of substance
 TemperatureTemperature
 PressurePressure
 Shear rateShear rate
 Shear historyShear history
The liquid that becomes less viscous underThe liquid that becomes less viscous under
pressure is referred to aspressure is referred to as thixotropicthixotropic..

11. RheologyRheology
 Is the science of the study of flow andIs the science of the study of flow and
deformation of matter.deformation of matter.
 Deformation of solids may be:Deformation of solids may be:
1. Elastic1. Elastic
2. Plastic2. Plastic
3. Viscoelastic3. Viscoelastic

12. Flow behavior of matterFlow behavior of matter
 Newtonian flowNewtonian flow
The shear stress and shear rate are proportional to eachThe shear stress and shear rate are proportional to each
other. The coefficient of viscosity is constant.other. The coefficient of viscosity is constant.
Coefficient of viscosity = Shear stress / Shear rate.Coefficient of viscosity = Shear stress / Shear rate.
 Non-Newtonian flowNon-Newtonian flow
1. Bingham flow:1. Bingham flow: Composite filling materialsComposite filling materials
2. Plastic flow2. Plastic flow
3. Pesudoplastic flow:3. Pesudoplastic flow: Rubber impression materials.Rubber impression materials.
4. Dilatant flow:4. Dilatant flow: Fluid of denture base resin.Fluid of denture base resin.

13. Types of flow behaviorTypes of flow behavior

14. Effect of shear rate on apparent viscosityEffect of shear rate on apparent viscosity

15. II. ViscoelasticityII. Viscoelasticity
 It is a combination of both elastic and viscousIt is a combination of both elastic and viscous
properties.properties.
 It can be represented by using modelsIt can be represented by using models
consisting of springs and dashpots (Maxwell-consisting of springs and dashpots (Maxwell-
type and Voigt type).type and Voigt type).
 Creep and stress relaxation are viscoelasticCreep and stress relaxation are viscoelastic
properties.properties.

16. Spring and dashpotSpring and dashpot

17. Maxwell and Voigt modelsMaxwell and Voigt models

18. Models discussing behavior ofModels discussing behavior of
materialsmaterials

19. Strain-time curve for springStrain-time curve for spring
(Elastic behavior)(Elastic behavior)

20. Strain-time curve for dashpotStrain-time curve for dashpot
(Viscous behavior)(Viscous behavior)

21. Strain-time curve for vsicoelasticStrain-time curve for vsicoelastic
materials (Maxwell model)materials (Maxwell model)

22. Strain-time curve for vsicoelasticStrain-time curve for vsicoelastic
materials (Voigt model)materials (Voigt model)

23. Strain-time curve for vsicoelasticStrain-time curve for vsicoelastic
materials (universal model)materials (universal model)

25. CreepCreep
 Is the time dependent (slow) deformationIs the time dependent (slow) deformation
(strain) that occurs in a material under constant(strain) that occurs in a material under constant
stress.stress.
 In most metals, It occurs in three stagesIn most metals, It occurs in three stages
1. Primary or transient creep1. Primary or transient creep
2. Secondary or steady-state creep2. Secondary or steady-state creep
3. Tertiary creep (it may followed by creep3. Tertiary creep (it may followed by creep
rupture).rupture).

26. Creep curve of metalsCreep curve of metals

27. Definition of creepDefinition of creep

28. Factors affecting creepFactors affecting creep
 The materialThe material
 The temperatureThe temperature
 The loadThe load
 Time of application of the loadTime of application of the load
Creep may cause loss of marginal adaptationCreep may cause loss of marginal adaptation
and ditching of the filling material at theand ditching of the filling material at the
margin.margin.

29. Creep of amalgam fillingCreep of amalgam filling

30. Creep of gold coping at highCreep of gold coping at high
temperaturetemperature

31. Stress relaxationStress relaxation
 It is time dependent decrease in stress thatIt is time dependent decrease in stress that
occurs in a solid under given loadingoccurs in a solid under given loading
conditions.conditions.
 When a viscoelastic material is strained to aWhen a viscoelastic material is strained to a
given extent and this strain is maintained thengiven extent and this strain is maintained then
the stress within the material decreases withthe stress within the material decreases with
time.time.
 It is determined by constant strain test.It is determined by constant strain test.

32. Stress relaxation curveStress relaxation curve

33. FatigueFatigue
 It is the behavior or failure of materials, which areIt is the behavior or failure of materials, which are
subjected to repeated loads (stresses) below theirsubjected to repeated loads (stresses) below their
yield strength.yield strength.
 Fatigue strength is the stress at which a material failsFatigue strength is the stress at which a material fails
under repeated loading.under repeated loading.
 Fatigue failure always starts at crack or stressFatigue failure always starts at crack or stress
concentration site.concentration site.
 Fatigue is always represented by stress-life or numberFatigue is always represented by stress-life or number
of cycles (S-N) curve.of cycles (S-N) curve.

34. Fatigue failure depends on:Fatigue failure depends on:
 The magnitude of load (stress)The magnitude of load (stress)
 The number of loading cycles (load repetition)The number of loading cycles (load repetition)
 Environmental factors (temperature, humidity)Environmental factors (temperature, humidity)
Endurance limit is the stress at which theEndurance limit is the stress at which the
material can be loaded (stressed) an infinitematerial can be loaded (stressed) an infinite
number of times without failingnumber of times without failing..

35. Repeated application of loadRepeated application of load

36. Fatigue curveFatigue curve

37. Endurance limit of steelEndurance limit of steel

38. Stress concentration siteStress concentration site