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Chapter6_yy - Homework is due today Chapter 6 1 Why do we...

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Chapter 6 - 1 Homework is due today!
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Chapter 6 - 2 Why do we care about mechanical properties of materials? An oil barge fractured in a brittle manner by crack propagation around its girth
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Chapter 6 - 3 Why care? Structural failure of a ladder Failure can result from poor design – choice of material versus load Structural failure - is driven by repeat loading, which is a necessary requirement of a ladder, a drive shaft on an automobile, and the wing on an airplane. Heverill Fire Department aerial ladder failure.
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Chapter 6 - 4 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: Mechanical Properties
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Chapter 6 - 5 Elastic means reversible ! Elastic Deformation 1. Initial 2. Small load 3. Unload F δ bonds stretch return to initial F δ Linear- elastic Non-Linear- elastic
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Chapter 6 - 6 Plastic means permanent ! Plastic Deformation (Metals) F δ linear elastic linear elastic δ plastic 1. Initial 2. Small load 3. Unload planes still sheared F δ elastic + plastic bonds stretch & planes shear δ plastic
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Chapter 6 - 7 Simple tension: cable τ Common States of Stress A o = cross sectional area (when unloaded) F F o σ = F A o τ = F s A σ σ M M A o 2 R F s A c Torsion (a form of shear): drive shaft Ski lift (photo courtesy P.M. Anderson)
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Chapter 6 - 8 (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o σ = F A Simple compression: Note: compressive structure member ( σ < 0 here). (photo courtesy P.M. Anderson) OTHER COMMON STRESS STATES (1) A o Balanced Rock, Arches National Park
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Chapter 6 - 9 Bi-axial tension: Hydrostatic compression: Pressurized tank σ < 0 h (photo courtesy P.M. Anderson) (photo courtesy P.M. Anderson) OTHER COMMON STRESS STATES (2) Fish under water σ z > 0 σ θ > 0
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Chapter 6 - 10 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. gauge length
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Chapter 6 - 11
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Chapter 6 - 12 Hooke’s law Simplest linear stress-strain law σ ( ε ) Law of the spring Ut tensio, sic vis As the stretch, so the force σ = constant ε Robert Hooke 1635-1703
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Chapter 6 - 13 Linear Elastic Properties Modulus of Elasticity, E : (also known as Young's modulus) Hooke's Law : σ = E ε σ Linear- elastic E ε F F simple tension test
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Chapter 6 - 14 Mechanical Properties Slope of stress strain plot (which is proportional to the elastic modulus) depends on bond strength of metal Adapted from Fig. 6.7, Callister 7e.
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Chapter 6 - 15 Properties from Bonding: E 16 Elastic modulus , E cross sectional area A o
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