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bc.inter.edufacultadeherreraMaterials_20Science

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									       ELASTIC DEFORMATION
     1. Initial       2. Small load    3. Unload

                            bonds
                            stretch

                                      return to
                                      initial
                  
                      F

Elastic means reversible!
PLASTIC DEFORMATION (METALS)
    1. Initial   2. Small load              3. Unload




                                 F

Plastic means permanent!   linear               linear
                           elastic              elastic
                                                          
                                     plastic
          ENGINEERING STRESS
• Tensile stress, s:               • Shear stress, t:




       Ft
    s
       Ao
   original area
   before loading      Stress has units:
                       N/m2 or lb/in2
            ENGINEERING STRAIN
• Tensile strain:
                                       /2
    l l f  lo
     
    lo     lo                            Lo
                                 wo
                                        /2
• Shear strain:
                          L/2        L/2
    /2

                     = tan           Strain is always
                                       dimensionless.
      /2 - 

    /2             /2
    LINEAR ELASTIC PROPERTIES
• Modulus of Elasticity, E:
 (also known as Young's modulus)
• Hooke's Law:
       s=E
• Poisson's ratio, n:
      x  y
  n  
      z  z
Units:
E: [GPa] or [psi]
n: dimensionless
       STRESS-STRAIN TESTING
• Typical tensile specimen   • Typical tensile
                                test machine
         TENSILE STRENGTH, TS
• Maximum possible engineering stress in tension.




• Metals: occurs when noticeable necking starts.
• Ceramics: occurs when crack propagation starts.
• Polymers: occurs when polymer backbones are
 aligned and about to break.
           PLASTIC (PERMANENT)
              DEFORMATION
                         (at lower temperatures, T < Tmelt/3)
• Simple tension test:
            YIELD STRENGTH, sy
• Stress at which noticeable plastic deformation has
   occurred.
                           when p = 0.002
          tensile stress, s
     sy




                  engineering strain, 
     p = 0.002
              DUCTILITY and %EL
                                           L f  Lo
• Plastic tensile strain at failure: %EL           x100
                                              Lo




                                       Ao  A f
• Another ductility measure:     %AR           x100
                                         Ao
                RESILIENCE, Ur
• Resilience is the capacity of a material to absorb
energy when it is deformed elastically and then, upon
unloading, to have this energy recovered.




                                 1        sy
                                           2

                            U r  s y y 
                                 2         2E
                 TOUGHNESS
• Energy to break a unit volume of material
• Approximate by the area under the stress-strain
   curve.
Engineering     smaller toughness (ceramics)
tensile                           larger toughness
stress, s                         (metals, PMCs)

                                               smaller toughness-
                                               unreinforced
                                               polymers


                Engineering tensile strain, 

								
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