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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 1 Bruce Mayer, PE Engineering-45: Materials of Engineering Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu Engineering 45 Mechanical Properties of Metals (2)

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 2 Bruce Mayer, PE Engineering-45: Materials of Engineering Learning Goals.1 – Mech Props STRESS and STRAIN: What they are and why they are they used instead of LOAD and DEFORMATION ELASTIC Behavior How much deformation occurs when Loads are SMALL? Which materials deform least

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 3 Bruce Mayer, PE Engineering-45: Materials of Engineering Learning Goals.2 – Mech Props PLASTIC Behavior Determine the point at which Dislocations cause PERMANENT deformation Which materials are most resistant to Permanent Deformation TOUGHNESS and DUCTILITY What they are How to Measure them

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 4 Bruce Mayer, PE Engineering-45: Materials of Engineering Properties of Solid Materials Mechanical: Characteristics of materials displayed when forces and/or torques are applied to them. Physical: Characteristics of materials that relate to the interaction of materials with various forms of energy. Chemical: Material characteristics that relate to the structure of a material. Dimensional: Size, shape, and finish

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 5 Bruce Mayer, PE Engineering-45: Materials of Engineering Material Properties Chemical Physical Mechanical Dimensional Composition Melting Point Tensile propertiesStandard Shapes Microstructure Thermal Toughness Standard Sizes Phases Magnetic DuctilitySurface Texture Grain Size Electrical FatigueStability Corrosion Optical HardnessMfg. Tolerances Crystallinity Acoustic Creep Molecular Weight Gravimetric Compression Flammability

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 6 Bruce Mayer, PE Engineering-45: Materials of Engineering Recall ELASTIC Deformation Apply/Remove a SMALL Force-Load to a Specimen 1. Initial3. Unload return to initial 2. SMALL load bonds stretch F F Force Load (lb or N) Deformation in Response to the Load (in or m) F Linear- elastic Non-Linear- elastic ELASTIC means REVERSIBLE

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 7 Bruce Mayer, PE Engineering-45: Materials of Engineering Recall PLASTIC Deformation Apply/Remove a LARGE Force Load to a Specimen PLASTIC means PERMANENT 1. Initial 3. Unload Planes Still Sheared & planes 2. LARGE load bonds stretch shear F elastic+plastic plastic F linear elastic linear elastic plastic

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 8 Bruce Mayer, PE Engineering-45: Materials of Engineering Plastic Deformation - Simple Tension Test (Temperature

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 9 Bruce Mayer, PE Engineering-45: Materials of Engineering YIELD Strength, y Define YIELD Strength as the Stress at Which NOTICEABLE Plastic Deformation Occurs Define NOTICEABLE as 0.2% → P = 0.002 (0.2%) tensile stress, engineering strain, PP = 0.002 yy σ y ≡ Yield Strength For Matl’s WithOUt a well Defined Yield Pt σ y = σ(ε = 0.2%) For a 2” gage-length ΔL = 2”0.002 = 0.004” (0.1 mm)

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 10 Bruce Mayer, PE Engineering-45: Materials of Engineering Room T values Based on data in Table B4, Callister 6e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered Yield Strength: Comparison y, ceramics >> y,metals >> y,polymers

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 11 Bruce Mayer, PE Engineering-45: Materials of Engineering TENSILE/ULTIMATE Strength Define TENSILE/ULTIMATE Strength (TS/σ u ) as the MAX-σ Point on the σ-ε Curve Metals: occurs when noticeable NECKING starts Ceramics: occurs when CRACK PROPAGATION starts Polymers: occurs when POLYMER BACKBONES are aligned and about to break engineering strain engineering stress yy TS

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 12 Bruce Mayer, PE Engineering-45: Materials of Engineering Room T values Based on data in Table B4, Callister 6e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered AFRE, GFRE, & CFRE = aramid, glass, & carbon fiber-reinforced epoxy composites, with 60 vol% fibers. Tensile Strength: Comparison TS ceramics TS metals TS comp >> TS polymers

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 13 Bruce Mayer, PE Engineering-45: Materials of Engineering Ductility → Strain at Fracture At Tensile Fracture Define Ductility in Terms of ELONGATION L o L f A o A f Plastic Strain At Tensile Failure Engineering tensile strain, Engineering tensile stress, smaller %EL (brittle if %EL<5%) Larger %EL (ductile if %EL>5%)

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 14 Bruce Mayer, PE Engineering-45: Materials of Engineering Ductility → Strain at Fracture Alternative Definition is Reduction of Area L o L f A o A f RA Ductility Note: %RA and %EL Tend to Be Quite Comparable Reason: crystal slip does not change material VOLUME. %RA < %EL possible if internal voids form in neck. %EL is More Common Than %RA

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 15 Bruce Mayer, PE Engineering-45: Materials of Engineering Desirable Mechanical Properties Without Considering Such Factors Cost, Weight, Weldability, etc., The Typically Desired Combination of Strength and Ductility HIGH σ y HIGH %EL σ y, is the Mechanical DESIGN PARAMETER, not The Ultimate Strength YIELDING permanently deforms (bends) Structures; typically rendering them NON-functional

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 16 Bruce Mayer, PE Engineering-45: Materials of Engineering Resilience → Energy Storage Consider the σ·ε Product Now FδL has Units of ENERGY (J) AL has Units of Volume (cu-m) Let U → J/m 3 Next Consider the σ-ε Curve in the Elastic Range dεdε

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 17 Bruce Mayer, PE Engineering-45: Materials of Engineering Resilience cont. In The Elastic Range the Material Stretches and then Returns to the Original Size Thus Define Resilience, U r, as the REVERSIBLE Energy Storage U r → Area under σ·ε curve in elastic Rng In the Elastic Range Then the U r Integral

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 18 Bruce Mayer, PE Engineering-45: Materials of Engineering Toughness smaller toughness- unreinforced polymers Engineering tensile strain, , Engineering Tensile Stress smaller toughness (ceramics) larger toughness (metals, some composites) An Extension of RESILIENCE Beyond the Elastic Range to Plastic-Flow & Fracture A Measure of the TOTAL Energy-per-Vol Absorbance Capability of a Material to the Total Plastic-Def. Area under the σ-ε curve

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 19 Bruce Mayer, PE Engineering-45: Materials of Engineering TRUE Stress & Strain Engineering Stress F Applied Pull A o Original Area But the Specimen NECKS-DOWN, Reducing the Area So the TRUE Stress A i Instantaneous Area = f(σ) or f(ε)

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 20 Bruce Mayer, PE Engineering-45: Materials of Engineering TRUE Stress & Strain cont Engineering Strain In the Instantaneous Case (see Rt) Integrating L0L0 LiLi Thus Original (UnLoaded) Load at Instant “i”

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 21 Bruce Mayer, PE Engineering-45: Materials of Engineering Engineering/True Stress/Strain Engineering/True Stress/Strain For Strain Now Assume Constant Material VOLUME

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 22 Bruce Mayer, PE Engineering-45: Materials of Engineering Plastic Behavior → - Typical Metal

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 23 Bruce Mayer, PE Engineering-45: Materials of Engineering Typ. Work-Hardening Parameters For Most Metals, True Stress Increases in the Plastic Range (not ElastoPlastic) The Material “Hardens” as it is WORKED Log (true plastic strain, ) Log(true stress, ) MPa 1.00.100.0100.0010 K n necking fracture

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 24 Bruce Mayer, PE Engineering-45: Materials of Engineering Strain-Hardening K Work-Hardening Prefactor in MPa or Ksi n Work-Hardening Exponent (unitless)

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 25 Bruce Mayer, PE Engineering-45: Materials of Engineering Elastic Recovery When a material is released prior to fracture: Some of the total energy is stored elastically Some is absorbed by the plastic deformation The plastic deformation energy represents the lattice strains. The elastic energy will be recovered once the material is released i.e., the material will unstretch

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 26 Bruce Mayer, PE Engineering-45: Materials of Engineering Elastic Recovery cont. Elastic Energy, U r 1 3 2 To determine the amount that the material recovers: 1.draw a line PARALLEL to the elastic modulus line that goes back to the strain axis 2.The difference in strains provides the recovered length 3.The area under this line is the recovered energy

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 27 Bruce Mayer, PE Engineering-45: Materials of Engineering Hardness Short Definition = Resistance to Penetration Metals HandBook "Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation.

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 28 Bruce Mayer, PE Engineering-45: Materials of Engineering Hardness, cont. Hardness Resistance to Plastic Indentation LARGE Hardness Indicates Properties: Resistance to plastic deformation or cracking when loaded in COMPRESSION Better Wear Resistance e.g., 10mm sphere apply known force (1 to 1000 kg) measure size of indent after removing load d D Smaller indents mean larger hardness increasing hardness most plastics brasses Al alloys easy to machine steelsfile hard cutting tools nitrided steelsdiamond

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 29 Bruce Mayer, PE Engineering-45: Materials of Engineering WhiteBoard Work

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BMayer@ChabotCollege.edu ENGR-45_Lec-15_Metal_MechProp-2.ppt 30 Bruce Mayer, PE Engineering-45: Materials of Engineering Elastic Strain RECOVERY UrUr Parallel Lines When a Post- Yield Load is Removed the Material Recovers along a Line PARALLEL to the initial ELASTIC extension Line

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