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Mechanical Properties of Metals

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Mechanical Properties of Metals Powered By Docstoc
					Mechanical Properties
     of Metals
               500

                                         CONTINUED
               400
Stress (MPa)



               300

               200

               100

                0
                 0.000 0.002 0.004 0.006 0.008 0.010
                               Strain
          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/m3).
• 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
             Load, P      Engineering Stress           Load, P
   L/2

                                P                           L/2

                             S
    Lo           Area
                                Ao
                                               Lo           Area

                                L
                 Ao
                                                            Ao

                             e
                                Lo
   L/2                                                      L/2
             P
                          Engineering Strain
                                                        P

Direct Stress - Tension                    Direct Stress - Compression
               Tension Test
                  Measures P




Extensometer
Measures L




                               Typical Universal
                               Testing Machine
Modern Materials Testing System

                                Hydraulic
                                Wedge
                                Grips


                     Specimen
   Extensometer
ASTM Tension Test Specimen
                    Ao=0.20 in2
                    0.505" Dia



         2” Gauge Length

              Lo
                     Raw Data Obtained
                       Total Elongation
               Uniform Deformation
Load, P (kN)




                                                     X

                                    Maximum
                                    Load, Pmax
                  Elastic
                                                   Load,
                  Deformation                      Pf




                             Elongation, L (mm)
                                  Engineering Stress-Strain Curve
                                                 Elongation

                             Sy
Engineering Stress, S=P/Ao



                             0.2% offset
                             yield stress




                                       E                  (Ultimate)

                                                                       Su
                                E

                                    Proportional Limit




                                              Engineering Strain, e = L/Lo)
Duke’s Quick Tip!

• Express Load in Newtons (N) and Area in
  mm2 to get Stress in MPa.
                N
                  2  MPa
               mm
• Mechanical properties of metals are almost
  always given in MPa or ksi.
• Imperial units: Load in kips (1000 lbf) &
  Area as in2 gives Stress in ksi (kips/in2)
• 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:
                         S  Ee
• Where E is the modulus of elasticity (MPa)
               Modulus of Elasticity - Stiffness
               500

                                                       CONTINUED
               400
Stress (MPa)



               300

               200             S (300  0)MPa
                          E                    2x10 5 MPa
                               e (0.015  0.0)

               100

                0
                 0.000   0.002    0.004      0.006     0.008   0.010
                                      Strain
Atomic Origin of Stiffness
    dF 
E   
     dr ro
Net Interatomic Force
                        Strongly Bonded




                                 Weakly Bonded




                            Interatomic Distance
             Shear Stress and Strain
             Shear Stress,
  Strain,
  Shear




                               Shear Stress
                                              Shear Strain

shear stress,  = Shear Load / Area
shear strain,  = angle of deformation (radians)
shear modulus, G =  /(elastic region)
   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, n.
Poisson’s Ratio, n

                ex  ey
            n  
                ez  ez
              For most metals,
               0.25 < n< 0.35
              in the elastic range
               Furthermore:
             E  2G(1  n )
                        Plastic Deformation
 Elastic Plastic            Elastic Plastic
                                                   Elastic Plastic
                                              Sy
                   Sy
                                                                     Sy
Stress




         0.002               0.002    Strain            0.002

     Most Metals - Al, Cu       Clad Al-Alloys           Low carbon Steel
   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
                         P (e,S)         e  ee  e p
                                               S
                                         ee 
                                               E
Stress




                                         e p  e  ee
         Total Strain

                                       The 0.2% offset yield stress
                             Strain    is the stress that gives a plastic
         Plastic                       (permanent) strain of 0.002.
                             Elastic
            ep          ee
                    Elastic Recovery
         Loading               Loading
                                                   Reloading
Stress




                               Unloading


                   Unloading


                   Strain                                 Strain
                                         elastic strain
     Ductility - EL% & AR%
• Elongation
         L f  Lo
EL%                x 100
               Lo           Lo
                                 Lf
• Area Reduction
         Ao  A f           Ao   Af
AR%                x 100
               Ao
                     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/m3 or N.mm/mm3= MPa)
• Resilience: A measure of the ability of a
  material to absorb energy without plastic or
  permanent deformation.
   (J/m3 or N.mm/mm3= MPa)
• Note: Both are determined as
      energy/unit volume
                                   Toughness, Ut

                                                  Su
                              Sy
Engineering Stress, S=P/Ao




                                       ef
                                Ut   S de
                                       o
                                      (S y  Su ) EL%
                                                      
                                           2       100 


                             Engineering Strain, e = L/Lo)
                                       Resilience, Ur

                                  Sy                    Su
Engineering Stress, S=P/Ao



                                                    ey            X
                                             U r   S de
                                                    o
                                                    Sy e y
                                                
                             E                           2
                                                    Sy 2
                                                
                                                    2E
                                        ey
                                 Engineering Strain, e = L/Lo)
           Typical Mechanical Properties
                      Metals in annealed (soft) condition

Material              Yield Stress     Ultimate     Ductility   Elastic Modulus   Poisson’s
                        (MPa)        Stress (MPa)    EL%             (MPa)          Ratio
1040 Steel                350             520            30         207000            0.30
1080 Steel                380             615            25         207000            0.30
2024 Al Alloy             100             200            18           72000           0.33
316 Stainless Steel       210             550            60         195000            0.30
70/30 Brass                75             300            70         110000            0.35
6-4 Ti Alloy              942            1000            14         107000            0.36
AZ80 Mg Alloy             285             340            11           45000           0.29