Lecture18-18_16938_Lecture 18 and 19 (1)-converted.pptx

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1. Unit 3 Mechanical Properties 1 Is Thor’s hammer “Mjölnir ” Ductile or Brittle or Malleable ?

2. Unit 3 Mechanical Properties o Stress and Strain (example problem) 2

3. Unit 3 Mechanical Properties o Stress and Strain (example problem) 2

4. Unit 3 Mechanical Properties o Stress and Strain (example problem) 2

5. Unit 3 Mechanical Properties o Stress and Strain (example problem) 2

6. Unit 3 Mechanical Properties 6 Tensile Strength It is the stress at the maximum on the engineering stress – strain curve. This corresponds to the maximum stress that can be sustained by a structure in tension, i.e. if this stress is applied and maintained, fracture will result. Str e ss St r ain TS Till the point M, there will be uniform deformation and later, there will be a small constriction or neck begins and this behaviour is called necking. In the structural engg. applications, Yield strength is used rather than Tensile strength.

7. Unit 3 Mechanical Properties o Tensile Strength B y the t i me struct u re, rea c hes p o int M, it has alre a dy we nt through significant plastic deformation that, the structure is useless. Tensile strength can be anywhere from 50 MPa for Al to ~ 3000 MPa for HSS. 11

8. Unit 3 Mechanical Properties Toughness: It may be defined as the property of a metal by virtue of which it can absorb maximum energy before fracture takes place. It is the m e asur e m ent o f ulti m a t e en e r gy stren g th o f m aterial a nd is expressed as work units/unit volume, i.e. kg fm/m 3 . Toughness is also calculated in terms of area under stress-strain curve. 11

9. Unit 3 Mechanical Properties Toughness: Toughness is the property of materials which enables a material to be twisted, bent or stretched under a high stress before rupture. The value of toughness falls with the rise in temperature. T o u g h ness is hig h l y desi r able pro p erty for struct u r al and m e chanical parts which have to withstand shock and vibration. 11

10. Unit 3 Mechanical Properties Stiffness: This may be defined as the property of a metal by virtue of which it resists deformation. Modulus of rigidity is the measure of stiffness. The term flexibility is quite opposite of stiffness. su f fer have l e s s high The materials which deformation under load degree of stiffness. 11

11. Unit 3 Mechanical Properties Resilience: Resilience is the ability of a material to absorb energy when it is deformed elastically , and release that energy upon unloading. Proof resilience is defined as the maximum energy that can be absorbed within the elastic limit, without creating a permanent distortion. The modulus of resilience is defined as the maximum energy that can be absorbed per unit volume without creating a permanent distortion. 11

12. Unit 3 Mechanical Properties Resilience: Modulus of Resilience can also be defines as the strain energy per unit volume required t stress a material from an unloaded state up to the point of yielding. Assuming linear elastic region: ═► 11

13. Unit 3 Mechanical Properties o True Stress and True Strain: True Stress: True Strain: If A i l i = A l 11

14. Unit 3 Mechanical Properties Hardness: This is the property of a material (metal) by virtue of it is able to resist abration, indentation (or penetration) and scratching by harder bodies. It is the resistance of a material to permanent deformation of the surface. In other words, one can define it as the resistance of the metal to penetration by an indenter. 11

15. Unit 3 11 Mechanical Properties Hardness: Hardness tests are performed more frequently than any other mechanical test for several reasons: They are simple & Inexpensive, The test is non-destructive, Other mechanical properties can be estimated from hardness data. Common Hardness Measuring Apparatus: Rockwell Hardness Tester, Knoop and Vickers micro-indentation Hardness Tester, Brinell Hardness Tester, Mohs Hardness.

16. Unit 3 Mechanical Properties o Hardness: 11

17. Unit 3 Mechanical Properties 17 o Hardness: (conversions) Hardness No. calculated using one apparatus can be used to estimate the possible Hardness No. outcome from the other equipment. Tensile Strength of a material of a material can also be estimated using (~ approximately) the below formulae:

18. Unit 3 18 Mechanisms of Strengthening of Metals

19. Unit 3 Mechanisms of Strengthening materials Strengthening of materials by grain size reduction: Grain Boundaries Grain boundary is a narrow region between two grains of about two to few atomic diameters in width, and is the region of mismatch between adjacent grains. 18

20. Strengthening of materials by grain size reduction: The size of the grains, or average grain diameter, in a polycrystalline metal influences the mechanical properties. Adjacent grains normally have di f ferent cr y s tallograp h ic orientations and, of course, a common grain boundary, as indicated in Figure. During plastic deformation, slip or dislocation motion must take place across this common boundary—say, from grain A to grain B in Figure Mechanisms of Strengthening materials Unit 3 18

21. 21 Strengthening of materials by grain size reduction: The grain boundary acts as a barrier to dislocation motion for two reasons: Because the two grains are of different orientations, a dislocation passing into grain B will have to change its direction of motion; this becomes more difficult as the crystallographic mis-orientation increases. The atomic disorder within a grain boundary region will result in a discontinuity of slip planes from one grain into the other. It should be mentioned that, for high-angle grain boundaries, it may not be the case that dislocations traverse grain boundaries during deformation; rather, dislocations tend to “pile up” (or back u p) at grain boundaries. Mechanisms of Strengthening materials Unit 3

22. d is the average grain size; σ y is the yield stress; σ o and k y are constants. 22 Strengthening of materials by grain size reduction: T h ese pile- up s introd u ce stress co n c entrati o ns a h ead o f their sl i p planes, which generate new dislocations in adjacent grains. A fine-grained material (one that has small grains) is harder and stronger than one that is coarse grained, because the former has a greater total grain to i m p ede boundary area dislocation motion. Hall–Petch equation: Mechanisms of Strengthening materials Unit 3

23. 23 Strengthening of materials by grain size reduction: Grain size may be regulated by the rate of solidification from the liquid phase, and also by plastic deformation followed by an appropriate heat treatment It sh o u l d a l s o b e m ent i o n ed t hat gra i n si z e r e d uct i on i m p r oves not only strength, but also the toughness of many alloys. Boundaries between two different phases are also impediments to movements of dislocations; this is important in the strengthening of more complex alloys The si z es a nd s h a pes o f the c onst i t u e nt phas e s s i g n ifi c a ntly a f fe c t the mechanical properties of multiphase alloys. Twin boundaries will effectively block the slip and increase the strength of the material. Mechanisms of Strengthening materials Unit 3

24. Plastic Deformation : The process of permanent deformation of metal beyond elastic state. Such state is called as plastic state. Various aspects of plastic deformation i.e. strain hardening, recovery, re-crystallization are discussed below… Strain Hardening : Also called as work hardening. Most of the metals are polycrystalline, having grains in different orientation. During plastic deformation, due to obstacles such as surrounding grain boundaries, foreign atoms and other dislocations, the movement of the dislocations is considerably restricted, i.e., the shear strain in the metal increases. This increase in the shear stress is called strain hardening/ work hardening and metal is said to have been strain hardened or work hardened . Mechanisms of Strengthening materials Unit 3

25. Strain Hardening : Ductile materials exhibit a marked increase in their hardness & strength when subjected to plastic deformation at temperatures below crystallization temperature. If plastic deformation is continued beyond the elastic limit, even a small deformation further causes a significant increase in strain hardening, but if continued, strain hardening remains constant until fracture takes place. Strain hardening is considered as strengthening mechanism and is used to process metals to provide greater strength and hardness through cold working. Ductility, plasticity & electrical conduction is decreased as the result of strain hardening. Mechanisms of Strengthening materials Unit 3

26. When a polycrystalline metal with uniform equi-axed grains (grains having equal dimensions in all directions) is subjected to plastic deformation at room temperature (a process known as cold working), the grains become deformed and elongated, During plastic deformation, the grain boundaries remain intact and mass continuity is maintained. The deformed metal exhibits higher strength, because of the entanglement of dislocations with grain boundaries and with each other. Mechanisms of Strengthening materials Unit 3

27. The increase in strength depends on the degree of deformation (strain) to which the metal is subjected; the higher the deformation, the stronger the metal becomes. The strength is higher for metals with smaller grains, because they have a larger grain- boundary surface area per unit volume of metal and hence more entanglement of dislocations. Anisotropy (Texture): In the above image, as a result of plastic deformation, the grains have elongated in one direction and contracted in the other. Consequently, this piece of metal has become anisotropic, and thus its properties in the vertical direction are different from those in the horizontal direction. The degree of anisotropy depends on the temperature at which deformation takes places and on how uniformly the metal is deformed. Anisotropy influences both mechanical and physical properties of metals Mechanisms of Strengthening materials Unit 3

28. 28 Strengthening of materials by grain size reduction: Percent of cold work (% CW) : A o is the original area. A d is the area after deformation. Mechanisms of Strengthening materials Unit 3

29. We have seen that plastic deformation at room temperature causes distortion of the grains and grain boundaries (leading to anisotropic behavior), a general increase in strength, and a decrease in ductility. These effects can be reversed, and the properties of the metal can be brought back to their original levels, by heating the metal to a specific temperature range for a given period of time-a process called annealing. Three events take place consecutively during the heating process: Recovery Re-crystallization Grain Growth Mechanisms of Strengthening materials Unit 3

30. Recovery: During recovery, which occurs at a certain temperature range below the re-crystallization temperature of the metal (described next), the stresses in the highly deformed regions of the metal piece are relieved. Sub-grain boundaries begin to form (a process called polygonization), with no significant change in mechanical properties such as hardness and strength. Mechanisms of Strengthening materials Unit 3

31. Recrystallization: When the deformed metal is heated to a temperature above the recovery range, nucleation and growth of the new grain takes place. The formation of new equiaxed grain in the heating process, instead of the oriented fibrous structure of the deformed metal is called recrystallization. The temperature at which recrystallization takes place, i.e., new grains are formed is called recrystallization. The recrystallization temperature depends on the degree of prior cold work (work hardening): The more the cold work, the lower the temperature required for recrystallization. The reason is that, as the amount of cold work increases, the number of dislocations and the amount of energy stored in dislocations (stored energy) also increase. This energy supplies some of the work required for recrystallization. Mechanisms of Strengthening materials Unit 3

32. Recrystallization: Recrystallization is also a function of time, because it involves diffusion-the movement and exchange of atoms across grain boundaries. The effects on recrystallization of temperature, time, and plastic deformation by cold working are as follows: For a constant amount of deformation by cold working, the time required for recrystallization decreases with increasing temperature; The more the prior cold work, the lower the temperature required for recrystallization; The higher the amount of deformation, the smaller the grain size becomes during recrystallization; this effect is a commonly used method of converting a coarse-grained structure to one having a finer grain, and thus one with improved properties; Some anisotropy due to preferred orientation usually persists after recrystallization; to restore isotropy, a temperature higher than that required for recrystallization may be necessary. Mechanisms of Strengthening materials Unit 3

33. Annealing Time, Annealing Temperature, Amount of initial Deformation, Grain Growth: If the temperature of the metal is raised further, the grains begin to grow, and their size may eventually exceed the original grain size; called grain growth, this phenomenon adversely affects mechanical properties. During this process, first small grains become even smaller and are finally absorbed by the larger ones. Thus, the smaller ones disappear and the larger ones become still larger. During this process there is a loss in strength & hardness, and there is improvement in ductility. The parameters which effect the grain growth: Rate of Heating, Alloying Elements, Insoluble Impurities. Mechanisms of Strengthening materials Unit 3

34. End of UNIT - 3 MEC 2 08 LPU Please read the book.