Lecture15-15_16938_Lecture 15 to 17-converted.pptx

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1. Course Code: MEC208 Engineering Materials MEC208 LPU Most of the images are courtesy of the book “Material Science and Engg. an Introduction” by William D. Callister, Jr. & David G. Rethwisch. Few were adopted from “ The Science and Engineering of Materials” by Donald. R. Askeland. Images collected from Wikipedia and google images were also used. 1

2. Unit 3 2 Fundamentals Unit 3: Mechanical Properties of the Materials

3. Unit 3 Fundamentals Mechanical Properties: o What are these mechanical Properties? 3

4. Unit 3 Fundamentals Mechanical Properties: o What are mechanical Properties? 4

5. Mechanical Properties: Strength Elasticity Plasticity Hardness Toughness Ductility Malleability Brittleness Fatigue

6. Mechanical Properties: Strength: The ability of a material to resist the externally applied forces without breaking or yielding. Elasticity: T he property of a material to regain its original shape after deformation, when the external forces are removed.  Plasticity: I t is the property of a metal that gives the ability to deform non-elastically; without fracture, they do not regain their original shape and size Hardness: is defined by a material's ability to resist various forms of deformation, indentation and penetration Toughness: It is the ability to absorb energy up to failure or fracture, or  toughness  is the ability of a material to resist any deformations due to bending Ductility: A material is ductile when it can be drawn into the wire under the action of tensile force.  Malleability: T he term  malleability  may be defined as the property of a metal by virtue of which it can be deformed into thin sheets by rolling  Brittleness: It is the property of a material and indicates fracture without appreciable deformation, and is opposite to toughness and ductility Fatigue: The  fatigue  is the prolonged effect of repeated straining due to application and removal of stress, by which material breaks or fractures.

7. Unit 3 Fundamentals Mechanical Properties: o Why are mechanical Properties important? Graphene Sheet: It takes a adult elephant standing on a pencil tip, to puncture a Graphene sheet which is just 1 atom thick. 7

8. Unit 3 Fundamentals 6 Mechanical Properties: o Why are mechanical Properties important? Graphene Aero-gel: ρ = 0.16 milligrams/cc Density slightly higher than Hydrogen, but lower than Helium Flexible Touch Screens

9. Unit 3 Fundamentals 7 Human Thigh Bone ~ Load of ~8600 Kg/ inch 3 ~ Stronger than steel and half its weight, ~ 4 times stronger than concrete Mechanical Properties: Bullet Proof Glass

10. Unit 3 Fundamentals Mechanical Properties: Spray on Fabric: ~ A fabric made by Aerosol based material can be sprayed directly on the body. ~ The fabric can be simply removed, washed and worn again. Developed by: Imperial College, London.

11. Unit 3 11 Fundamentals Basic Mechanical Properties: Isotropy Anisotropy Elasticity Plasticity Ductility Brittleness Hardness Toughness Stiffness 10. Resilience Fatigue Creep e.t.c 13. Strength: Ultimate Strength Elastic Strength Plastic Strength Tensile Strength Compressive Strength Shear Strength Bending Strength Torsional Strength

12. Unit 3 Mechanical Properties Isotropy: Body is said to be isotropic if its physical properties are not dependent upon the direction in the body along which they are measured, Ex: A polycrystalline material in which the grains or crystals are oriented behaves isotropically, i.e. its properties are independent of direction. 12

13. Unit 3 Mechanical Properties Anisotropy: The quality of variation of a physical property with the direction in a body along which the property is measured. It is the state of having different properties in different directions. Ex. Single crystals, Multi Laminar Composites e.t.c 13

14. Unit 3 Mechanical Properties Elasticity : It is the property of a material which enables it to regain its original shape and size after deformation within the elastic limit. However, in nature no material is perfectly elastic, i.e., a certain limit exists for every material beyond which it will not be able to regain its original shape and size. This limit is termed as elastic limit. 14

15. Unit 3 Mechanical Properties Plasticity : It is the ability of material to be permanently deformed (without fracture) even after the load is removed. To some extent all materials are plastic. Metals possess more plasticity at high temperatures. Usually, plasticity of a material increases with increase in temperature and this is important in deciding the further operations. 15

16. Unit 3 Mechanical Properties o Stress and Strain Stress: A measure of the average amount of force exerted per unit area . When the force is applied on the body, it deforms. If the force is compressive, the body compresses. If the force is tensile, the body expands. Both the compressive and tensile stresses are called direct stresses. (σ) stress = stress measured in Nm -2 or Pascals (Pa) F = force in newtons (N) A = cross-sectional area in m 2 16

17. Unit 3 Mechanical Properties o Stress and Strain (Types of Stresses) There are five basic types of stresses possibly applied on materials. T ensional 17 C o mp r ess i ve Be n ding

18. Unit 3 Mechanical Properties o Stress and Strain (Types of Stresses) There are five basic types of stresses possibly applied on materials. S h ear 18 T ors i onal

19. Unit 3 Mechanical Properties 17 Poisson’s Ratio When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect . Poisson's ratio ( nu ) is a measure of this effect. T ens i on

20. Unit 3 Mechanical Properties Poisson’s Ratio The Poisson ratio is the fraction (or percent) of expansion divided by the fraction (or percent) of compression. C o mp r ess i on 20

21. Unit 3 Mechanical Properties o Stress and Strain Strain: It is the deformation per the unit of the original length. When ever force is applied on a body, there will be deformation i.e. there will be variation in the size of the object (either decreases or increases). This deformation is called Strain. ( ε ) Δ l 21 l Δ l is the variation in size (or extension) l i s the original length Strain does not have any units.

22. Unit 3 Mechanical Properties o Stress and Strain (example problem) 22

23. Unit 3 Mechanical Properties o Stress and Strain Hooks Law: It states that Stress and Strain in a material are proportional to each other within the elastic limits. σ ∝ ε The ratio between Stress and strain is always a constant. σ = E ε E = σ / ε Where the constant E is called the Young’s modulus or Modulus of Elasticity. 23

24. Significance of E Elastic modulus may be thought of as stiffness, or a material’s resistance to elastic deformation. The greater the modulus, the stiffer the material, or the smaller the elastic strain that results from the application of a given stress. The modulus is an important design parameter used for computing elastic deflections.

25. Significance of E On an atomic scale, macroscopic elastic strain is manifested as small changes in the interatomic spacing and the stretching of interatomic bonds. As a consequence, the magnitude of the modulus of elasticity is a measure of the resistance to separation of adjacent atoms, that is, the interatomic bonding forces.

26. Factor of Safety What is Factor of Safety? Which applications require high factor of safety? How factor of safety can be increased?

27. Unit 3 Mechanical Properties o Stress and Strain Hooks Law: It states that Stress and Strain in a material are proportional to each other within the elastic limits. E = σ / ε 27

28. Unit 3 Mechanical Properties o Stress and Strain Hooks Law: It states that Stress and Strain in a material are proportional to each other within the elastic limits. E = σ / ε 28

29. Unit 3 Mechanical Properties Ductility : It is defined as the property of a metal by virtue of which it can be drawn into wires or elongated before rupture takes place. It is the deformation produced in a material at the breaking point and measured by the percentage of elongation and the percentage of reduction in area before rupture of test piece. 29

30. Unit 3 Mechanical Properties Ductility: Its value is expressed as elongation, i.e., percentage elongation is most widely used to measure ductility. Percentage of Reduction 30

31. Unit 3 Mechanical Properties Ductility & Malleability: Ductility is an important property of a material which governs its ability to be deformed in processes, e.g. drawing, rolling and forging. Adequate ductility ensures that the material during these processes will not fracture. There is an associated property by virtue of which sheets can be rolled from material is called ‘ malleability’. Both Ductility & Malleability of material are inversely proportional to strength of the material. And both the properties are effected by temperature. 31

32. Unit 3 Mechanical Properties o Stress – Strain Curve of Ductile Materials: L = the limit of proportionality, Hooke’s law applies up to this point. E = elastic limit, beyond this point the material is permanently stretch and it will not go back to its original length. Elastic behaviour is when a material returns to its original length, plastic behaviour is when the stretched material does not return to its original length. 32

33. Unit 3 Mechanical Properties o Stress – Strain Curve of Ductile Materials: Y = yield point, beyond this point small increases in force give much big increases in length. B = breaking point / breaking stress, the material breaks at this point. 33

34. Location of yield point in various heat treated steels and other ductile alloys? True Stress Strain Failure criteria of ductile and brittle materials

35. Properties identified from Stress Strain curve E YS UTS %age elong %age reduction area Modulus of Resilience Modulus of toughness The modulus of resilience is defined as the maximum energy that can be absorbed per unit volume without creating a permanent distortion.

36. Unit 3 Mechanical Properties o Elastic Properties of the materials: Above point Y, material tends to show plastic behavior, i.e., any deformation occurring is permanent. The Point Y is called Yield Point. Yield Strength is the lowest stress that produces a permanent deformation in a material. 36

37. Unit 3 Mechanical Properties o Elastic Properties of the materials: a t e nsi o nal stre s s is to a m a ter i al, it When applied elo n gates. T ill the P o i nt Y , Stress is equal to Strain of the material, i. e . m aterial retains its ela s tic property till point Y. 37

38. Unit 3 Mechanical Properties Brittleness: A m aterial is brittle if, when su b j e cted to stres s , it breaks witho u t significant deformation ( strain ). Brittle materials absorb relatively little energy prior to fracture, even those of high strength . 38

39. Unit 3 Mechanical Properties Brittleness: Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics , glasses and concrete (which do not deform plastically) Many steels become brittle at low temperatures, depending on their composition and processing. . 39