lecture6 - Chapter Chapter 6 Mechanical Properties 1...

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Chapter Chapter 6 Mechanical Properties 1
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echanical Properties SUES TO ADDRESS Mechanical Properties 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?
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Elastic Deformation 1. Initial 2. Small load 3. Unload bonds stretch tt d return to initial F F Linear- astic Elastic means reversible ! elastic Nonlinear- elastic d
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Plastic Deformation (Metals) 1. Initial 2. Small load 3. Unload nes bonds planes still sheared stretch & planes shear astic + plastic d plastic F d elastic + plastic Plastic means permanent ! linear astic linear astic F d elastic elastic d plastic
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Engineering Stres Engineering Stress Shear stress, : F t F Tensile stress, : F t Area, A F s Area, A F t F F s = F s F t F N lb tress has units: A o original area = t A o 2 f 2 m or in = Stress has units: N/m 2 or lb f /in 2 J/m 3 before loading
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Common States of Stress Simple tension: cable A o = cross sectional area (when unloaded) F F o = F A M F s Torsion (a form of shear): drive shaft Ski lift (photo courtesy P.M. Anderson) = F s A A o A c Note: t = M / A c R here. o M 2 R
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Other Common Stress States (1 Other Common Stress States (1) Simple compression: A o (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o = F A Note: compressive structure member ( < 0 here). (photo courtesy P.M. Anderson) Balanced Rock, Arches National Park
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Other Common Stress States (2 Other Common Stress States (2) Bi-axial tension: Hydrostatic compression: Pressurized tank (photo courtesy Fish under water 0 (photo courtesy P.M. Anderson) P.M. Anderson) 0 > 0 < 0 h r > 0
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Engineering Strain Tensile strain: Lateral strain: d /2 e= d L o - d e L = L w o L o w o Shear strain: d L /2 x = = n Strain is always 90º - y x / y tan y dimensionless. 90º
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Stress Stress-Strain Testing Strain Testing • Typical tensile test machine • Typical tensile specimen Adapted specimen extensometer Callister 8 th Ed. gauge length g Adapted from Callister (Ref: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.)
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Linear Elastic Properties Modulus of Elasticity, E : (also known as Young's modulus) Hooke's Law : = E e s E F Linear- e elastic F simple tension st test
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Poisson's ratio, Poisson's ratio, : e L e - e =- L e Units: E : [GPa] or [psi] imensionless metals : ~ 0.33 ceramics : ~ 0.25 polymers : ~ 0.40 : dimensionless
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Mechanical Properties Slope of stress strain plot (which is proportional to the elastic modulus) depends on bond strength of metal (or any material…) Adapted from Callister, 8 th Ed.
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Temperature Dependence of Young’s odulus Modulus Structure-Property elation: Relation: 1 E µ m T T<T
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lecture6 - Chapter Chapter 6 Mechanical Properties 1...

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