7.Mechanical%20Properties - 1 MSE 280: Engineering...

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Unformatted text preview: 1 MSE 280: Engineering Materials Reading: Chapter 7 Why mechanical properties? Need to design materials that can withstand applied load Mechanical Properties e.g. materials used in building bridges that can hold up automobiles, pedestrians materials for skyscrapers in the Windy City materials for NASA MSE280 2007-2009 Moonsub Shim, University of Illinois 1 materials for and designing MEMs and NEMs Space elevators? materials for space exploration Issues to address Stress and strain Elastic behavio Elastic behavior Plastic behavior Strength, ductility, resilience, toughness, hardness MSE280 2007-2009 Moonsub Shim, University of Illinois 2 Mechanical behavior of different classes of materials Design/safety factors 2 Stress and Strain Stress: Pressure due to applied load. area force stress = = tension, compression, shear, torsion, and combination. MSE280 2007-2009 Moonsub Shim, University of Illinois 3 Strain : response of the material to stress (i.e. physical deformation such as elongation due to tension). Tension Compression MSE280 2007-2009 Moonsub Shim, University of Illinois 4 Shear Torsion 3 COMMON STATES OF STRESS Simple tension: cable = F A o = cross sectional Area (when unloaded) F F MSE280 2007-2009 Moonsub Shim, University of Illinois 5 o A Ski lift (photo courtesy P.M. Anderson) From Cal ister 6e resource CD. COMMON STATES OF STRESS Simple compression: Canyon Bridge, Los Alamos, NM A o MSE280 2007-2009 Moonsub Shim, University of Illinois 6 Balanced Rock, Arches National Park o = F A Note: compressive structure member ( < 0 here). (photo courtesy P.M. Anderson) (photo courtesy P.M. Anderson) From Cal ister 6e resource CD. 4 COMMON STATES OF STRESS Bi-axial tension: Hydrostatic compression: MSE280 2007-2009 Moonsub Shim, University of Illinois 7 Pressurized tank < 0 h (photo courtesy P.M. Anderson) (photo courtesy P.M. Anderson) Fish under water z > 0 > 0 Tension and Compression Tension Engineering stress = o A F = Engineering strain = o o o i l l l l l = = A o = original cross sectional area l i = instantaneous length l o = original length Note: strain is unitless MSE280 2007-2009 Moonsub Shim, University of Illinois 8 Note: strain is unitless. Compression Same as tension but in the opposite direction (stress and strain defined in the same manner). By convention, stress and strain are negative for compression. 5 Shear Pure shear stress = o A F = Pure shear strain = tan = MSE280 2007-2009 Moonsub Shim, University of Illinois 9 Elastic Deformation 1. Initial 2. Small load 3. Unload bonds stretch return to initial MSE280 2007-2009 Moonsub Shim, University of Illinois 10 F Elastic means reversible !...
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7.Mechanical%20Properties - 1 MSE 280: Engineering...

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