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Mechanical Properties of Metals
 

Mechanical Properties of Metals

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Mechanical Properties of Metals

Mechanical Properties of Metals

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    Mechanical Properties of Metals Mechanical Properties of Metals Presentation Transcript

    • Mechanical Properties of Metals
    • Mechanical Properties
      • Stiffness - Elastic Modulus or Young’s Modulus (MPa)
      • Strength - Yield, Ultimate, Fracture, Proof, Offset Yield. Measured as stress (MPa)
      • Ductility - Measure of ability to deform plastically without fracture - Elongation, Area Reduction, Fracture Strain - (no units or mm/mm)
      • Toughness, Resilience - Measure of ability to absorb energy (J/m 3 ).
      • Hardness - Resistance to indentation/abrasion (Various scales, e.g.; Rockwell, Brinell, Vickers.)
    • Stress and Strain
      • In a simplistic sense, stress may be thought of as Load/Area .
      • Similarly, strain is the deformation of the component/original length.
      • A stress may be direct , shear , or torsional - leading to corresponding deformations.
      • Stress cannot be measured directly, but deformation can be.
    • Direct Stress Examples Direct Stress - Tension Direct Stress - Compression Engineering Stress Engineering Strain
    • Tension Test Typical Universal Testing Machine Extensometer Measures  L Measures P
    • Modern Materials Testing System Hydraulic Wedge Grips Specimen Extensometer
    • ASTM Tension Test Specimen 2” Gauge Length A o =0.20 in 2 L o
    • Raw Data Obtained
      • Elongation,  L (mm)
      Load, P (kN) Uniform Deformation Total Elongation Elastic Deformation X Maximum Load, P max Load, P f
    • Engineering Stress-Strain Curve Elongation 0.2% offset yield stress Proportional Limit E E (Ultimate) Engineering Strain, e =  L/Lo) Engineering Stress, S=P/Ao S y S u
    • Duke’s Quick Tip!
      • Express Load in Newtons (N) and Area in mm 2 to get Stress in MPa.
      • Mechanical properties of metals are almost always given in MPa or ksi.
      • Imperial units: Load in kips (1000 lbf) & Area as in 2 gives Stress in ksi (kips/in 2 )
      • 1000 psi = 1 ksi = 6.89 MPa
    • Hooke’s Law Elastic Deformation
      • Elastic deformation is not permanent; it means that when the load is removed, the part returns to its original shape and dimensions.
      • For most metals, the elastic region is linear. For some materials, including metals such as cast iron, polymers, and concrete, the elastic region is non-linear.
      • If the behavior is linear elastic, or nearly linear-elastic, Hooke’s Law may be applied:
      • Where E is the modulus of elasticity (MPa)
    • Modulus of Elasticity - Stiffness
    • Atomic Origin of Stiffness
    • Shear Stress and Strain shear stress,  = Shear Load / Area shear strain,  = angle of deformation (radians) shear modulus, G =  /  (elastic region) Shear Stress Shear Strain
    • Elastic Properties of Materials
      • Poisson’s ratio: When a metal is strained in one direction, there are corresponding strains in all other directions.
      • For a uniaxial tension strain, the lateral strains are constrictive.
      • Conversely, for a uniaxial compressive strain, the lateral strains are expansive.
      • i.e.; the lateral strains are opposite in sign to the axial strain.
      • The ratio of lateral to axial strains is known as Poisson’s ratio,  .
    • Poisson’s Ratio,  For most metals, 0.25 <  < 0.35 in the elastic range Furthermore :
    • Plastic Deformation Stress Strain 0.002 0.002 0.002 S y S y S y Most Metals - Al, Cu Clad Al-Alloys Low carbon Steel Elastic Plastic Elastic Plastic Elastic Plastic
    • Microstructural Origins of Plasticity
      • Slip, Climb and Slide of atoms in the crystal structure.
      • Slip and Climb occur at Dislocations and Slide occurs at Grain Boundaries.
       
    • Elastic and Plastic Strain Stress Strain Plastic Elastic e e e p P Total Strain (e,S) The 0.2% offset yield stress is the stress that gives a plastic (permanent) strain of 0.002.
    • Elastic Recovery Strain Stress Loading Unloading Loading Unloading Reloading elastic strain Strain
    • Ductility - EL% & AR%
      • El ongation
      • A rea R eduction
      L o A o L f A f
    • Ductile Vs Brittle Materials
      • Only Ductile materials will exhibit necking .
      • Ductile if EL%>8% (approximately)
      • Brittle if EL% < 5% (approximately)
      Engineering Stress Engineering Strain
    • Toughness & Resilience
      • Toughness: A measure of the ability of a material to absorb energy without fracture. (J/m 3 or N.mm/mm 3 = MPa)
      • Resilience: A measure of the ability of a material to absorb energy without plastic or permanent deformation.
      • (J/m 3 or N.mm/mm 3 = MPa)
      • Note: Both are determined as
      • energy/unit volume
    • Toughness, U t Engineering Strain, e =  L/Lo) Engineering Stress, S=P/Ao S u S y
    • Resilience, U r Engineering Strain, e =  L/Lo) Engineering Stress, S=P/Ao S u S y E e y
    • Typical Mechanical Properties Metals in annealed (soft) condition