EGN3365-6

EGN3365-6 - Chapter 6 - 1 Why mechanical properties? Why...

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Unformatted text preview: Chapter 6 - 1 Why mechanical properties? Why mechanical properties? Why mechanical properties? Why mechanical properties? Need to design materials that can withstand applied load e.g. materials used in building bridges that can hold up automobiles, pedestrians materials for skyscrapers in the Windy City materials for and designing MEMs and NEMs Space elevators? materials for space exploration NASA Chapter 6:Mechanical Properties Chapter 6 - 2 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 does permanent deformation occur? What materials are most resistant to permanent deformation? Toughness and ductility : What are they and how do we measure them? Chapter 6 - 3 Stress and Strain Stress: Pressure due to applied load. area force stress = = tension, compression, shear, torsion, and their combination. Strain : response of the material to stress (i.e. physical deformation such as elongation due to tension). Chapter 6 - 4 Tension Compression Shear Torsion Chapter 6 - 5 COMMON STATES OF STRESS Simple tension: cable o = F A A o = cross sectional Area (when unloaded) F F Ski lift (photo courtesy P.M. Anderson) From Callister 6e resource CD. Chapter 6 - 6 COMMON STATES OF STRESS Canyon Bridge, Los Alamos, NM Simple compression: A o 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 Callister 6e resource CD. Chapter 6 - 7 COMMON STATES OF STRESS Hydrostatic compression: Fish under water < 0 h (photo courtesy P.M. Anderson) From Callister 6e resource CD. Chapter 6 - 8 Tension and Compression 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. Tension Engineering stress = o A F = 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. Chapter 6 - 9 Shear Pure shear stress = o A F = Pure shear strain = tan = Strain is always dimensionless. Chapter 6 - 10 Elastic means reversible ! Elastic Deformation 1. Initial 2. Small load 3. Unload F bonds stretch return to initial F Linear- elastic Non-Linear- elastic-a non-permanent deformation where the material completely recovers to its original state upon release of the applied stress. Chapter 6 - 11 Plastic means permanent ! Plastic Deformation (Metals) F linear elastic linear elastic plastic 1. Initial 2. Small load 3. Unload planes still sheared F elastic + plastic bonds stretch & planes shear plastic Chapter 6 - 12 Stress-Strain Testing Typical tensile test machine Adapted from Fig. 6.3, Callister 7e. (Fig. 6.3 is taken from H.W. (Fig....
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EGN3365-6 - Chapter 6 - 1 Why mechanical properties? Why...

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