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

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

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

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

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

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

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

ENGR-45_Lec-15_Metal_MechProp-2.ppt 8 Bruce Mayer, PE Engineering-45: Materials of Engineering Plastic Deformation  -   Simple Tension Test (Temperature <T melt /3) Tensile Stress,  engineering strain,  Elastic+Plastic at larger stress Elastic initially permanent (plastic) after load is removed PP plastic strain Elastic Recovery

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 = (0.2%) tensile stress,  engineering strain,  PP = yy  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)

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

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 yy TS

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

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%)

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

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

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

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

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

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)

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”

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

ENGR-45_Lec-15_Metal_MechProp-2.ppt 22 Bruce Mayer, PE Engineering-45: Materials of Engineering Plastic Behavior →  -  Typical Metal

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 K n necking fracture

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)

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

ENGR-45_Lec-15_Metal_MechProp-2.ppt 26 Bruce Mayer, PE Engineering-45: Materials of Engineering Elastic Recovery cont. Elastic Energy, U r  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

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.

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

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

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