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CHAPTER 6: MECHANICAL PROPERTIES

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Presentation on theme: "CHAPTER 6: MECHANICAL PROPERTIES"— Presentation transcript:

1 CHAPTER 6: MECHANICAL PROPERTIES
ISSUES TO ADDRESS... • Stress and strain: What are they and why are they used instead of load and deformation? • Elastic behavior: When loads are small, how much deformation occurs? What materials deform least? • Plastic behavior: At what point do dislocations cause permanent deformation? What materials are most resistant to permanent deformation? • Toughness and ductility: What are they and how do we measure them? 1

2 ELASTIC DEFORMATION 1. Initial 2. Small load 3. Unload Example: metals
Elastic means reversible! Example: rubber 2

3 PLASTIC DEFORMATION (METALS)
1. Initial 2. Small load 3. Unload Plastic means permanent! 3

4 ENGINEERING STRESS • Tensile stress, s: • Shear stress, t:
Stress has units: N/m2 or lb/in2 4

5 ENGINEERING STRESS 4

6 COMMON STATES OF STRESS
• Simple tension: cable Ski lift (photo courtesy P.M. Anderson) • Simple shear: drive shaft Note: t = M/AcR here. 5

7 OTHER COMMON STRESS STATES (1)
• Simple compression: (photo courtesy P.M. Anderson) Note: compressive structure member (s < 0 here). (photo courtesy P.M. Anderson) 6

8 OTHER COMMON STRESS STATES (2)
• Bi-axial tension: • Hydrostatic compression: Pressurized tank (photo courtesy P.M. Anderson) (photo courtesy P.M. Anderson) s < 0 h 7

9 ENGINEERING STRAIN • Tensile strain: • Lateral strain: • Shear strain:
Strain is always dimensionless. 8

10 STRESS-STRAIN TESTING
• Typical tensile specimen • Typical tensile test machine Adapted from Fig. 6.2, Callister 6e. • Other types of tests: --compression: brittle materials (e.g., concrete) --torsion: cylindrical tubes, shafts. Adapted from Fig. 6.3, Callister 6e. (Fig. 6.3 is taken from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, p. 2, John Wiley and Sons, New York, 1965.) 9

11 LINEAR ELASTIC PROPERTIES
• Modulus of Elasticity, E: (also known as Young's modulus) • Hooke's Law: s = E e • Poisson's ratio, n: metals: n ~ 0.33 ceramics: ~0.25 polymers: ~0.40 Units: E: [GPa] or [psi] n: dimensionless 10

12 LINEAR ELASTIC PROPERTIES
• Modulus of Elasticity, E: (also known as Young's modulus) • Hooke's Law: Units: E: [GPa] or [psi] s = E e E  (dF/dr)ro Note: E is a function of temperature in general 10

13 Other Definitions of E in case of Non-linear Elastic behavior
10

14 OTHER ELASTIC PROPERTIES
• Elastic Shear modulus, G: simple torsion test t = G g • Elastic Bulk modulus, K: pressure test: Init. vol =Vo. Vol chg. = DV • Special relations for isotropic materials: 11

15 YOUNG’S MODULI: COMPARISON
Graphite Ceramics Semicond Metals Alloys Composites /fibers Polymers E(GPa) Based on data in Table B2, Callister 6e. Composite data based on reinforced epoxy with 60 vol% of aligned carbon (CFRE), aramid (AFRE), or glass (GFRE) fibers. 12

16 USEFUL LINEAR ELASTIC RELATIONS
• Simple tension: • Simple torsion: • Material, geometric, and loading parameters all contribute to deflection. • Larger elastic moduli minimize elastic deflection. 13


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