Materials in Mechanical Design

Materials in Mechanical Design - Materials in Mechanical...

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1 Materials in Mechanical Design Materials taken from Chapter 2 of Mott, Machine Elements in Mechanical Design, 2003 Materials in Mechanical Design  The designer must specify the required materials for each part of a mechanical device. Physical properties Mechanical properties Match the properties to the expectations placed on them Classes of Material  Metals and their alloys  Elastomers  Plastics  Woods  Composites  Ceramics and glasses
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2 Properties of Metals  Tensile tests determine strength, elastic, and ductility properties for metals, plastics, and other types of materials.  A round or flat bar of the material is clamped between jaws and pulled slowly until it breaks in tension.  The data from these tensile tests are shown on stress-strain diagrams. Tensile strength  The peak of the stress-strain curve is considered the ultimate tensile strength (su). (AKA: ultimate strength, tensile strength)  Measures the highest apparent stress on a test bar of the material Stress-Strain Diagram for Steel Mott, Machine Elements in Mechanical Design, 2003
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3 Stress-Strain Diagram for Aluminum Mott, Machine Elements in Mechanical Design, 2003 Yield Strength  Yield Strength (s y ): The portion of the stress-strain diagram where there is a large increase in strain with little or no increase in stress.  Proportional Limit : The point on the stress-strain curve where the straight line deviates.  Elastic Limit : The point at which a material experiences some amount of plastic strain and thus will not return to its original shape after release of the load. Modulus of Elasticity in Tension  For part of the stress-strain diagram that is straight, stress is proportional to strain.  There, the value of E, the modulus of elasticity, is the constant of proportionality.  E = stress = σ strain ε
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4 Ductility and Percent Elongation  Ductility : The degree to which a material will deform before ultimate fracture. Antonym = brittleness  When ductile materials are used in machine members, impending failure is detected easily, so sudden failure is unlikely.  Percent Elongation : The usual measure of ductility. Percent Elongation = L f –L o * 100% L o Shear Strength  Both the yield strength and the ultimate strength in shear are important properties of materials.  s ys = s y / 2 = 0.50 s y = yield strength in shear  s us = 0.75 s u = ultimate strength in shear Poisson’s Ratio  During a tensile strain, the cross-sectional dimensions perpendicular to the direction of the tensile strain shorten.  Poisson’s ratio (v): The ratio of the shortening strain to the tensile strain.
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5 Illustration of Poisson’s Ratio Mott, Machine Elements in Mechanical Design, 2003 Modulus of Elasticity in Shear  The Modulus of Elasticity in Shear (G): The ratio of shearing stress to shearing strain. Â
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Materials in Mechanical Design - Materials in Mechanical...

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