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ch07 - Chapter 7 Mechanical Properties Elastic Deformation...

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2/11/2011 1 Chapter 7 - 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? What materials are most resistant to permanent deformation? Toughness and ductility : What are they and how do we measure them? Chapter 7: Mechanical Properties Chapter 7 - 2 Elastic means reversible ! Elastic Deformation 2. Small load F d bonds stretch 1. Initial 3. Unload return to initial F d Linear- elastic Non-Linear- elastic Chapter 7 - 3 Plastic means permanent ! Plastic Deformation (Metals) F d linear elastic linear elastic d plastic 1. Initial 2. Small load 3. Unload planes still sheared F d elastic + plastic bonds stretch & planes shear d plastic Chapter 7 - 4 Stress has units: N/m 2 or lb f /in 2 Engineering Stress Shear stress, t : Area, A o F t F t F s F F F s t = F s A o Tensile stress, s : original area before loading s = F t A o 2 f 2 m N or in lb = Area, A o F t F t Chapter 7 - 5 Simple tension: cable Note: t = M / A c R here. Common States of Stress o s = F A o t = F s A s s M M A o 2 R F s A c Torsion (a form of shear): drive shaft Ski lift (photo courtesy P.M. Anderson) A o = cross sectional area (when unloaded) F F Chapter 7 - 6 (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o s = F A Simple compression: Note: compressive structure member ( s < 0 here). (photo courtesy P.M. Anderson) OTHER COMMON STRESS STATES (i) A o Balanced Rock, Arches National Park
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