emphasis on stress and strain

1. Properies of dental materials
Dr Mumtaz ul Islam
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2. Why properties
 To evaluate performance of a material
 Many factors were considered
 Situation where a material used
 Manipulation
 In situ (in its original place)
 In vitro
 In vivo
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3. Properties in different situations
 Unmixed materials (shelf life)
 During mixing manipulation and setting (thorough
mixing, standard manipulation and setting process)
 Set material (physical &chemical)
 Working time: time available for mixing and
manipulating a material
 Setting time: the time taken by a material to attain a
certain level of rigidity or elasticity
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4. Physical properties
 Mechanical properties
 Stress
 Tensile
 Compressive
 Fracture (flexural)
 A stress resisting a compressive force is referred to as a
compressive stress and that resisting a tensile force a
tensile stress
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5. Strain
 Measure of the fractional change in length caused by
an applied force
 When strain becomes large, the dimensions of test
specimens may change in a direction at 90º to that of
the applied force
 Poissons ratio
 The ratio of strain occurring at 90º to the direction of
the applied force to that occurring in the direction of
the force
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6. Stress-strain relationship
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7. Stress-strain relationship
 Tensile strength
 Compressive strength
 The value of stress which corresponds to the limit of
proportionality, P, is referred to as the proportional
limit
 Point E is the yield stress. This corresponds to the
stress beyond which strains are not fully recovered
 High value of proportional limit indicates that a
sample of the material is more likely to withstand
applied stress without permanent deformation
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8. Proof stress
 This indicates the value of stress which will result in a
certain degree of permanent deformation upon removal
of the stress. For example, the 0.1% proof stress
(commonly used for alloys) is the level of stress which
would result in a 0.1% permanent deformation
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9. Modulus of elasticity
 Modulus of elasticity
 Stress / Strain
 A steep slope giving a high modulus value, indicates a
rigid material
 shallow slope, giving a low modulus value, indicates
flexible material
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10. Ductility and malleability
 The value of strain recorded between points E and T
indicates the degree of permanent deformation which
can be imparted to a material up to the point of fracture
 For a tensile test this gives an indication of ductility
 For a compressive test it indicates malleability
 Elongation at fracture is the property, ductile materials
shows
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11. Swagging machine
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12. Resilience and Toughness
 Resilience may be defined as the energy absorbed by a
material in undergoing elastic deformation up to the
elastic limit
 Total amount of energy which a material can absorb up
to the point of fracture
 Brittleness is opposite of toughness
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13. Fracture toughness and impact strength
 For brittle materials fracture may occur suddenly at a
stress which is apparently well below the ideal fracture
stress
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14. single-edge-notched specimen (SEN)
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15. Impact test &impact strength
 When the stress is increased very rapidly it may be
termed an impact test
 The important practical property obtained is the impact
strength
 When the presence of a small notch or crack in the
surface of a material has a marked effect on impact
strength the material is said to be notch sensitive
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16. Charpy impact tester
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17. Different stress strain graphs
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18. Fatigue life and fatigue limit
 Application of a cyclic stress at a given magnitude and
frequency and to observe the number of cycles
required for failure. The result is often referred to as
the fatigue life
 Applying number of stress cycles, say 10 000, and
determine the value of the cyclic stress required to
cause fracture within this number of cycles
 The result in this case is referred to as the fatigue limit
 Fatigue crack always occurs in the surface of material
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19. Fatigue life and fatigue limit
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20. Wear
 Indenting and scratching of the surface by abrasive
toothpastes or food is termed abrasive wear
 Fatigue wear minimal degree of scratches due to
intermittent stresses
 Erosion loss of material by chemical action
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21. Abrasion
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22. Attrition
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23. Attrition
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24. Erosion
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