201%20Week%208%202008%20Posted

201%20Week%208%202008%20Posted - Chapter 6 - 1 ISSUES TO...

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Unformatted text preview: Chapter 6 - 1 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? Toughness and ductility : What are they and how do we measure them? Chapters 6, 7, & 8: Mechanical Properties/Strengthening/Failure Why are dislocations observed primarily in metals and alloys? How are strength and dislocation motion related? How can we increase strength? How do flaws in a material initiate failure ? How do loading rate, loading history, and temperature affect the failure stress? Chapter 6 - Elastic means reversible ! Elastic Deformation 1. Initial 2. Small load 3. Unload F bonds stretch return to initial F Linear- elastic Non-Linear- elastic Chapter 6 - 3 Plastic means permanent ! Plastic Deformation (Metals) F linear elastic linear elastic plastic 1. Initial 2. Small load 3. Unload p lanes still sheared F elastic + plastic bonds stretch & planes shear plastic Chapter 6 - 4 Stress has units: N/m 2 or lb f /in 2 Engineering Stress Shear stress, : Area, A F t F t F s F F F s = F s A o Tensile stress, : original area before loading Area, A F t F t = F t A o 2 f 2 m N or in lb = Chapter 6 - 5 Simple tension: cable Note: = M / A c R here. Common States of Stress A o = cross sectional area (when unloaded) F F o = F A o = F s A M M A o 2 R F s A c Torsion (a form of shear): drive shaft Ski lift (photo courtesy P.M. Anderson) Chapter 6 - 6 (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o = F A Simple compression: Note: compressive structure member ( < 0 here). (photo courtesy P.M. Anderson) OTHER COMMON STRESS STATES (1) A o Balanced Rock, Arches National Park Chapter 6 - 7 Bi-axial tension: Hydrostatic compression: Pressurized tank < 0 h (photo courtesy P.M. Anderson) (photo courtesy P.M. Anderson) OTHER COMMON STRESS STATES (2) Fish under water z > 0 > 0 Chapter 6 - 8 Tensile strain: Lateral strain: Shear strain: Strain is always dimensionless. Engineering Strain 90 90 - y x = x / y = tan = L o- L = L w o Adapted from Fig. 6.1 (a) and (c), Callister 7e. /2 L /2 L o w o Chapter 6 - 9 Stress-Strain Testing Typical tensile test machine Adapted from Fig. 6.3, Callister 7e. (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.) specimen extensometer Typical tensile specimen Adapted from Fig. 6.2, Callister 7e....
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201%20Week%208%202008%20Posted - Chapter 6 - 1 ISSUES TO...

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