Stress & Strain Properies of dental materials

emphasis on stress and strain

1. Dr Mumtaz ul Islam Properies of dental materials 10/30/2013 1

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 10/30/2013 2

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 10/30/2013 3

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 10/30/2013 4

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 10/30/2013 5

6. Stress-strain relationship 10/30/2013 6

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 10/30/2013 7

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 10/30/2013 8

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 10/30/2013 9

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 10/30/2013 10

11. Swagging machine 10/30/2013 11

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 10/30/2013 12

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 10/30/2013 13

14. single-edge-notched specimen (SEN) 10/30/2013 14

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 10/30/2013 15

16. Charpy impact tester 10/30/2013 16

17. Different stress strain graphs 10/30/2013 17

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 10/30/2013 18

19. Fatigue life and fatigue limit 10/30/2013 19

20. Wear 10/30/2013 20 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

21. Abrasion 10/30/2013 21

22. Attrition 10/30/2013 22

23. Attrition 10/30/2013 23

24. Erosion 10/30/2013 24

25. 10/30/2013 25