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Chapter 3 - σ ε true engineering TS failure failure...

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1 1 Chapter 3 : Plasticity Plastic deformation elastic plastic unloading permanent deformation loading elastic recovery 2 Chapter 3 : Plasticity 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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2 3 Chapter 3 : Plasticity Engineering stress-strain curve σ f 4 Chapter 3 : Plasticity Tensile strength strain Typical response of a metal F = fracture Neck – acts as stress concentrator, region of high local deformation engineering TS stress engineering strain σ y
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3 5 Chapter 3 : Plasticity Elastic - plastic behavior TS 6 Chapter 3 : Plasticity True stress-strain Change in Poisson’s ratio from elastic to plastic deformation
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4 7 Chapter 3 : Plasticity True stress-strain curves The maximum tensile strength occurs at failure Not true for engineering stress-strain curves 8 Chapter 3 : Plasticity 1.0 0.10 0.010 0.0010 n = slope start of yielding fracture K Log (true plastic strain, ε PT ) Log σ PT - σ y ) Log-log plot to determine K, n
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5 9 Chapter 3 : Plasticity
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Unformatted text preview: σ ε true engineering TS failure failure corrected Bridgeman correction σ y 10 Chapter 3 : Plasticity 0.002 σ y ε y ! y 2 2 E Engineering tensile strain, ε Engineering tensile stress, σ smaller % EL larger % EL L f A o A f L o Ductility Resilience 6 11 Chapter 3 : Plasticity Toughness small toughness (polymers) (low strength, high ductility) Engineering tensile strain, ε Engineering tensile stress, σ small toughness (ceramics) (high strength, low ductility) large toughness (metals) (optimal combination of strength and toughness 12 Chapter 3 : Plasticity Plastic deformation of polymers the neck region keeps stretching, unlike metals 7 13 Chapter 3 : Plasticity Hardness measurement 14 Chapter 3 : Plasticity Comparison between Knoop, Brinell, Rockwell and Moh’s hardness scales Hardness scales Hardness numbers can be used to give the approximate strength For most steels: TS = 3.45 HB HV ~ 3 σ y...
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