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 dbonds stretch 1. Initial 3. Unload return to initial F dLinear- elastic Non-Linear- elastic Chapter 7 - 3 Plastic means permanent! Plastic Deformation (Metals) F dlinear elastic linear elastic dplastic 1. Initial 2. Small load 3. Unload planes still sheared F delastic + plastic bonds stretch & planes shear dplastic Chapter 7 - 4 Stress has units: N/m2or lbf/in2 Engineering Stress • Shear stress, t: Area, AoF 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, Ao F t F t Chapter 7 - 5 • Simple tension: cable Note: t= M/AcRhere. Common States of Stress o s=F A o t=F s A ssM M A o 2R 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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