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Elasticity by Ibrhim AlMohimeed

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1 Elasticity by Ibrhim AlMohimeed
Chapter 4 Elasticity by Ibrhim AlMohimeed 19/11/2013 BMTS 353

2 Video 19/11/2013 BMTS 353

3 Deformation When a object crash a car, the car may not move but it will noticeably change shape. A change in the shape due to the application of a force is a deformation. Even very small forces are known to cause some deformation. 19/11/2013 BMTS 353

4 where F is force, ∆L is the change in length
Cont. Deformation For deformations, two important characteristics are observed: The object may returns to its original shape when the force is removed. The size of the deformation is proportional to the force. 𝐹∝ ∆𝐿 where F is force, ∆L is the change in length 19/11/2013 BMTS 353

5 Hooke’s law where ΔL is the amount of deformation (the change in length) produced by the force F, and k is a proportionality constant that depends on the shape and composition of the object and the direction of the force. 𝐹=𝑘 ∆𝐿 19/11/2013 BMTS 353

6 Cont. Hooke’s law The proportionality constant k depends upon a number of factors for the material. For example, a guitar string made of nylon stretches when it is tightened, and the elongation ΔL is proportional to the force applied. Thicker nylon strings and ones made of different material (steel) stretch less for the same applied force, implying they have a larger k. 19/11/2013 BMTS 353

7 Hooke’s law Examples Example 4.1: A force of 600 N will compress a spring 0.5 meters. What is the spring constant of the spring? Example 4.2: A spring has spring constant 0.1 N/m. What force is necessary to stretch the spring by 2 meters? 19/11/2013 BMTS 353

8 Elastic Modulus ΔL depends on the material of the subject.
ΔL is proportional to the force F. ΔL is proportional to the original length L0. ΔL is inversely proportional to the cross-sectional area of the subject 19/11/2013 BMTS 353

9 Cont. Elastic Modulus All these factors can be combined into one equation for ΔL : ∆𝐿= 1 𝑌 𝐹 𝐴 𝐿 0 ΔL the change in length. F the applied force. Y is Young’s modulus, or elastic modulus (that depends on the subject material). A is the cross-sectional area, L0 is the original length 19/11/2013 BMTS 353

10 Stress and Strain The ratio of force to area, 𝐹 𝐴 , is defined as stress. So the stress is the force per unit area (pressure!!!) 𝜎= 𝐹 𝐴 𝑁/ 𝑚 2 or Pa The ratio of the change in length to length, ∆𝐿 𝐿 0 , is defined as strain (a unitless quantity) 𝜀= ∆𝐿 𝐿 0 19/11/2013 BMTS 353

11 𝝈 𝒔𝒕𝒓𝒆𝒔𝒔 =𝒀 𝒀𝒐𝒖𝒏 𝒈 ′ 𝒔 𝒎𝒐𝒅𝒖𝒍𝒖𝒔 𝜺 (𝒔𝒕𝒓𝒂𝒊𝒏)
Cont. Stress and Strain So, the equation of the elastic modulus can be rearranged in term of stress and strain: 𝝈 𝒔𝒕𝒓𝒆𝒔𝒔 =𝒀 𝒀𝒐𝒖𝒏 𝒈 ′ 𝒔 𝒎𝒐𝒅𝒖𝒍𝒖𝒔 𝜺 (𝒔𝒕𝒓𝒂𝒊𝒏) 19/11/2013 BMTS 353

12 Stress and Strain Example
Example 4.3: A steel wire 10 m long and 2 mm in diameter is attached to the ceiling and a 200-N weight is attached to the end. What is the applied stress? If the wire stretches 3.08 mm. What is the longitudinal strain? L DL A = π × r2 19/11/2013 BMTS 353

13 Stress and Strain Curve
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14 Cont. Stress and Strain Curve
Flash 19/11/2013 BMTS 353

15 Cont. Stress and Strain Curve
Elastic behavior: the object will returns to its original length (or shape) when the stress acting on it is removed. Plastic behavior: the object will NOT returns to its original length (or shape) when the stress acting on it is removed. Proportionality limit: the stress above is not longer proportional to strain. 19/11/2013 BMTS 353

16 Cont. Stress and Strain Curve
Elastic behavior: the object will returns to its original length (or shape) when the stress acting on it is removed. Elastic Limit: The maximum stress that can be applied without resulting permanent deformation. Yield Stress: at which there are large increases in strain with little or no increase in stress. steel exhibits this type of response. 19/11/2013 BMTS 353

17 Cont. Stress and Strain Curve
Strain Hardening: when yielding has ended, a further load can be applied to the object. Ultimate Strength: the maximum stress the object can withstand. Necking: after the ultimate stress, the cross-sectional area begins to decrease in a localized region of the object. 19/11/2013 BMTS 353

18 Stress & Strain Curve Example
Example 4.4: The elastic limit for steel of 3.14 x 10-6 m2 is 2.48 x 108 Pa and The ultimate strength is 4.89 x 108 Pa. a)What is the maximum weight that can be supported without exceeding the elastic limit? b) What is the maximum weight that can be supported without breaking the wire? 19/11/2013 BMTS 353

19 Stress & Strain Curve Example
a stress applied to an object was 6.37 x 107 Pa and its strain was 3.08 x Find the modulus of elasticity for object? 19/11/2013 BMTS 353

20 Stress & Strain Curve Example
Young’s modulus for brass is 8.96 x 1011Pa. A 120-N weight is attached to an 8-m length of brass wire; find the increase in length. The diameter is 1.5 mm. 8 m DL 120 N 19/11/2013 BMTS 353

21 Strain Hardening 19/11/2013 BMTS 353

22 Cont. Strain Hardening Strain Hardening
- If the material is loaded again from the beginning stage of strain hardening, the curve will be the same Elastic Modulus (slope). - The material will has a higher yield strength. 19/11/2013 BMTS 353

23 Types of Material Isotropic materials:
have elastic properties that are independent of direction. Anisotropic materials: whose properties depend upon direction. 19/11/2013 BMTS 353

24 Material Behavior Behavior of materials can be broadly classified into two categories: Brittle (Example: glass, ceramics) Ductile (Example: Metals; Gold, silver, copper, iron ) 19/11/2013 BMTS 353

25 Brittle Behavior Brittle \fracture at much lower strains.
yielding region is nearly nonexistent. often have relatively large Young's moduli and ultimate stresses. 19/11/2013 BMTS 353

26 Ductile Behavior Ductile \withstand large strains before failure.
yielding region often takes up the majority of the stress-strain curve capable of absorbing much larger quantities of energy before failure. 19/11/2013 BMTS 353

27 Resilience & Toughness
\Amount of energy stored in material up to elastic limit up to elastic limit per unit volume. Toughness: Amount of energy stored in material up to fracture per unit volume 19/11/2013 BMTS 353

28 Cont. Resilience & Toughness
19/11/2013 BMTS 353

29 Shear modulus The shear modulus (S) is the elastic modulus we use for the deformation which takes place when a force is applied parallel to one face of the object while the opposite face is held fixed by another equal force. 19/11/2013 BMTS 353

30 Cont. Shear modulus Shear deformation behaves similarly to tension and compression and can be described with similar equations. 𝜏 𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 =𝑆 𝑠ℎ𝑒𝑎𝑟 𝑚𝑜𝑑𝑢𝑙𝑒 ×𝜙(𝑠ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑖𝑛) 19/11/2013 BMTS 353

31 Bulk Modulus An object will be compressed in all directions if inward forces are applied evenly on all its surfaces. It is relatively easy to compress gases and extremely difficult to compress liquids and solids. 19/11/2013 BMTS 353

32 Bulk Modulus 𝐵= 𝑉𝑜𝑙𝑢𝑚𝑒 𝑠𝑡𝑟𝑒𝑠𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 𝑠𝑡𝑟𝑎𝑖𝑛 =− 𝐹/𝐴 Δ𝑉/𝑉 19/11/2013
BMTS 353

33 Bulk Modulus Example Example 4.6:
A hydrostatic press contains 5 liters of oil. Find the decrease in volume of the oil if it is subjected to a pressure of 3000 kPa. (Assume that B = 1700 MPa.) 19/11/2013 BMTS 353

34 End of the Chapter 19/11/2013 BMTS 353


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