Stress & Strain Properies of dental materials

• 1. Properies of dental materials Dr Mumtaz ul Islam110/30/2013

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 vivo210/30/2013 3. Properties in different situations Unmixed materials (shelf life) During mixing manipulation and setting (thoroughmixing, 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 elasticity310/30/2013 4. Physical properties Mechanical properties Stress Tensile Compressive Fracture (flexural) A stress resisting a compressive force is referred to as acompressive stress and that resisting a tensile force a tensile stress410/30/2013 5. Strain Measure of the fractional change in length caused byan applied force When strain becomes large, the dimensions of test specimens may change in a direction at 90 to that of the applied force Poissons ratio The ratio of strain occurring at 90 to the direction of the applied force to that occurring in the direction of the force510/30/2013 6. Stress-strain relationship610/30/2013 7. Stress-strain relationship Tensile strength Compressive strength The value of stress which corresponds to the limit ofproportionality, 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 710/30/2013 8. Proof stress This indicates the value of stress which will result in acertain 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 deformation810/30/2013 9. Modulus of elasticity Modulus of elasticity Stress / Strain A steep slope giving a high modulus value, indicates arigid material shallow slope, giving a low modulus value, indicates flexible material910/30/2013 10. Ductility and malleability The value of strain recorded between points E and Tindicates 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 shows1010/30/2013 11. Swagging machine1110/30/2013 12. Resilience and Toughness Resilience may be defined as the energy absorbed by amaterial 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 toughness1210/30/2013 13. Fracture toughness and impact strength For brittle materials fracture may occur suddenly at astress which is apparently well below the ideal fracture stress1310/30/2013 14. single-edge-notched specimen (SEN)1410/30/2013 15. Impact test &impact strength When the stress is increased very rapidly it may betermed 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 sensitive1510/30/2013 16. Charpy impact tester1610/30/2013 17. Different stress strain graphs1710/30/2013 18. Fatigue life and fatigue limit Application of a cyclic stress at a given magnitude andfrequency 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 material1810/30/2013 19. Fatigue life and fatigue limit1910/30/2013 20. Wear Indenting and scratching of the surface by abrasivetoothpastes or food is termed abrasive wear Fatigue wear minimal degree of scratches due to intermittent stresses Erosion loss of material by chemical action2010/30/2013 21. Abrasion2110/30/2013 22. Attrition2210/30/2013 23. Attrition2310/30/2013 24. Erosion2410/30/2013 25. 2510/30/2013