Chapter 6 - Mechanical Properties and Behavior.pdf
- 1. 1 The Science and Engineering of Materials, 4th ed Donald R. Askeland – Pradeep P. Phulé Chapter 6 – Mechanical Properties and Behavior
- 2. 2 Chapter 6 Outline 6.1 Technological Significance 6.2 The Tensile Test: Use of the Stress-Strain Diagram 6.3 Properties Obtained from the Tensile Test 6.4 Hardness of Materials 6.5 Impact Behavior 6.6 Fatigue 6.7 Creep
- 3. 3 Section 6.1 Technological Significance The materials used in sports equipment must be lightweight, stiff, tough, and impact resistant. Aircraft, such as the one shown here, makes use of aluminum alloys and carbon-fiber-reinforced composites.
- 4. 4 A unidirectional force is applied to a specimen in the tensile test by means of the moveable crosshead. The cross-head movement can be performed using screws or a hydraulic mechanism. Section 6.2 The Tensile Test: Use of the Stress-Strain Diagram
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- 7. 7 Section 6.3 Properties Obtained from the Tensile Test Engineering stress - The applied load, or force, divided by the original cross-sectional area of the material. Engineering strain - The amount that a material deforms per unit length in a tensile test. Yield strength - The level of stress above which a material begins to show permanent deformation. Tensile strength - The stress that corresponds to the maximum load in a tensile test.
- 8. 8 Stiffness - A measure of a material’s resistance to elastic deformation. Stiffness is the slope of a load-displacement curve and is proportional to the elastic modulus. It depends on the geometry of the component under consideration, whereas the elastic modulus is a materials property. The inverse of stiffness is known as compliance. Shear modulus (G) - The slope of the linear part of the shear stress-shear strain curve in the elastic region. Hooke’s law - The linear-relationship between stress and strain in the elastic portion of the stress-strain curve.
- 9. 9 Poisson’s ratio - The negative of the ratio between the lateral and longitudinal strains in the elastic region. Necking - Local deformation causing a reduction in the cross-sectional area of a tensile specimen. Many ductile materials show this behavior. The engineering stress begins to decrease at the onset of necking.
- 10. 10 Modulus of resilience (Er) - The maximum elastic energy absorbed by a material when a load is applied. Tensile toughness - The area under the true stress–true strain tensile test curve. It is a measure of the energy required to cause fracture under tensile test conditions. Ductility - The ability of a material to be permanently deformed without breaking when a force is applied. Glass temperature (Tg) is a temperature above which many polymers and inorganic glasses no longer behave as brittle materials.
- 11. 11 Ductile Materials Brittle Materials
- 12. 12 (a) Determining the 0.2% offset yield strength in gray cast iron, and (b) upper and lower yield point behavior in a low-carbon steel.
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- 14. 14 Comparison of the elastic behavior of steel and aluminum. For a given stress, aluminum deforms elastically three times as much as does steel.
- 15. 15 The effect of temperance (a) on the stress-strain curve and (b) on the tensile properties of an aluminum alloy.
- 16. 16 Section 6.4 Hardness of Materials Hardness test - Measures the resistance of a material to penetration by a sharp object.
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- 18. 18 Section 6.5 Impact Behavior Impact test - Measures the ability of a material to absorb the sudden application of a load without breaking.
- 19. 19 Section 6.6 Fatigue Fatigue - The lowering of strength or failure of a material due to repetitive stress which may be above or below the yield strength. Rotating cantilever beam test
- 20. 20 The stress-number of cycles to failure (S-N) curves for a tool steel and an aluminum alloy. S-N curve (also known as the Wöhler curve) - A graph showing stress as a function of number of cycles in fatigue.
- 21. 21 Section 6.7 Creep Creep - A time dependent, permanent deformation at high temperatures, occurring at constant load or constant stress. Creep test - Measures the resistance of a material to deformation and failure when subjected to a static load below the yield strength at an elevated temperature.