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					                          Engineering 45


     Material
    Failure (2)
                                            Bruce Mayer, PE
                     Registered Electrical & Mechanical Engineer
                                           BMayer@ChabotCollege.edu

Engineering-45: Materials of Engineering                                                   Bruce Mayer, PE
1                                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
Learning Goals.1 – Failure
      How Flaws In A Material Initiate Failure
      How Fracture Resistance is Quantified
         • How Different Material Classes Compare
      How to Estimate The Stress To Fracture
      Factors that Change the Failure Stress
         • Loading Rate
         • Loading History
         • Temperature

 Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
 2                                          BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
Learning Goals.2 – Failure
      FATIGUE Failure
         • Fatigue Limit
         • Fatigue Strength
         • Fatigue Life
      CREEP at Elevated Temperatures
         • Incremental Yielding at <y Over a Long
           Time Period at High Temperatures



 Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
 3                                          BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
    Fatigue Defined
     ASTM E206-72 Definition
         The Process of PROGRESSIVE
      LOCALIZED PERMANENT Structural
          Change Occurring in a Material
       Subjected to Conditions Which Produce
      FLUCTUATING Stresses and Strains at
          Some Point or Points Which May
        Culminate in CRACKS or Complete
       FRACTURE After a Sufficient Number
                   of Fluctuations
Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
4                                          BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
    Fatigue Failure
        Caused by Load-
         Cycling at <y
        Brittle-Like Fracture
         with Little Warning
         by Plastic
         Deformation
        •       May take Millions of
                Cycles to Failure          1. Crack Initiation Site(s)
                                           2. “Beach Marks” Indicate of
     Fatigue Failure                         Crack Growth
      Time-Stages                          3. Distinct Final
                                              Fracture Region
Engineering-45: Materials of Engineering                                       Bruce Mayer, PE
5                                             BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
    Fatigue Parameters
     Recall Fatigue Testing (RR Moore Tester)
      specimen                  compression on top

                                                    motor            counter
                                                 flex coupling
                                                                                            m   max   min  2
                                     tension on bottom
                                                                                          S   max   min  2
     Stress Varies with Time;                                 max
      Key Parameters                                                   m                                        S
        • m  Mean Stress (MPa)                                                                                     time
                                                                min
        • S  Stress Amplitude (MPa)
     Failure Even though                            Cause of ~90% of
      max < c                                       Mech Failures
Engineering-45: Materials of Engineering                                                     Bruce Mayer, PE
6                                                           BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
    More Fatigue Parameters

     σmax = maximum
      stress in the cycle
     σmin = minimum
      stress in the cycle
     σm = mean stress in
      the cycle = (σmax + σmin)/2
     σa = stress amplitude = (σmax - σmin)/2
     Δσ = stress range = σmax - σmin = 2σa
     R = stress ratio = σmax/σmin
Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
7                                          BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
    Fatigue Design Parameter
                                           S = stress amplitude
     Fatigue (Endurance)                                                                                   case for
                                                                            unsafe                          steel (typ.)
      Limit, Sfat in MPa
        • Unlimited Cycles if              Sfat
                                                                safe
          S < Sfat
                                              103 105      107     109
                                                 N = Cycles to failure

     Some Materials will                  S = stress amplitude
                                                                                                            case for
      NOT permit                                                           unsafe                            Al (typ.)
      Limitless Cycling
        • i.e.; Sfat = ZERO                                    safe

                                             103 105      107     109
                                                N = Cycles to failure
Engineering-45: Materials of Engineering                                           Bruce Mayer, PE
8                                                 BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
    Factigue Crack Growth
     Fatigue Cracks Grow INCREMENTALLY
      during the TENSION part of the Cycle
     Math Model for Incremental Crack Extension

                                                typ. 1 to 6
                 da
                     K
                         m
                                               K I ~   a
                 dN        Opening-Mode (Mode-I) Stress Intensity Factor

                                           increase in crack length per loading cycle

     Example: Austenitic Stainless Steel
    da
    dN
       m / cyc  5.6 10  K MPa m
                          12
                                                                                                             3.25


Engineering-45: Materials of Engineering                                                       Bruce Mayer, PE
9                                                             BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
Improving Fatigue Performance
                                                   S = stress amplitude

 1. Impose a
    Compressive
                                                                               moderate compressive, m
                                                                              near zero ortensile,
    Surface Stress (to
                                                                               larger tensile, m
                                                                                        m
    Suppress Surface
                                                   N = Cycles to failure
    cracks from growing)
            • Method 1: shot peening                     • Method 2: carburizing (interstitial)
                       shot
                                                                      C-rich gas
                                               put
                                             surface
                                               into
                                           compression



 2. Remove                                                      bad                              better
    Stress-Concentrating
    sharp corners                                               bad                              better
Engineering-45: Materials of Engineering                                                  Bruce Mayer, PE
10                                                       BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
 Creep Deformation
  Creep Defined
  HIGH TEMPERATURE PROGRESSIVE
       DEFORMATION of a material at
    constant stress. High temperature is a
     relative term that is dependent on the
          material(s) being evaluated.
  For Metals, Creep Becomes important
   at Temperatures of About 40% of the
   Absolute Melting Temperature (0.4Tm)
Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
11                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
 Creep: ε vs t Behavior
  In a creep test a
   constant load is
   applied to a tensile
   specimen
   maintained at a
   constant temp.
   Strain is then                           Stage-1 → Primary
   measured over a                           • a period of primarily
   period of time                              transient creep. During
                                               this period deformation
        • Typical Metallic                     takes place, and Strain
          Dynamic Strain at                    Hardening Occurs
          Upper-Right
Engineering-45: Materials of Engineering                                      Bruce Mayer, PE
12                                           BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
 Creep: ε vs t Behavior cont.1
  Stage-II → Steady
   State Creep
        • a.k.a. Secondary
          Creep
        • Creep Rate, dε/dt is
          approximately
          Constant                         • a reduction in cross
        • Strain-Hardening                   sectional area due to
          and RECOVERY                       necking, or effective
          Roughly Balance                    reduction in area due to
                                             internal void formation
  Stage-III →                             • Creep Fracture is often
   Tertiary Creep                            called “Rupture”
Engineering-45: Materials of Engineering               Bruce Mayer, PE
13                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
 Secondary Creep
  Most of Material Life Occurs in this Stage
  Strain-Rate is about Constant for Given T & σ
        • Work-Hardening Balanced by Recovery
  The
                                           d                     Qc 
   Math Model                                     s  K 2 exp  
                                                           n
                                                                      
        • Where
                                           dt s                   RT 
                – K2  A Material-                           – Qc  The Activation
                  Dependent Constant                           Energy for Creep
                – σ  The Applied Stress                     – R  The Gas Constant
                – n  A Material                             – T  The Absolute
                  Dependent Constant                           Temperature

Engineering-45: Materials of Engineering                                             Bruce Mayer, PE
14                                                  BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
 Creep Failure                              Estimate Rupture Time
                                               • S590 Iron, T = 800 °C,
  Occurs Along Grain                            σ = 20 Ksi
   Boundaries
                                                                               100
       g.b. cavities




                                                                                    Stress, ksi
                                                                                20
                                                                               10
                            applied
                                           data for
                            stress         S-590 Iron
                                                              1
  The Time-to-Rupture                     12 16 20 24 28
                                                L(10 3K-log hr)                                   24x103 K-log hr
   Power-Law Model
   T(20  log t r )  L                        T(20  log t r )  L
temperature          function of            1073K
                     applied stress
time to failure (rupture)                             Ans: tr = 233hr
Engineering-45: Materials of Engineering                                        Bruce Mayer, PE
15                                             BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
                                                                                      P
 WhiteBoard Work
  Problem 8.17
        • Ø 0.60” 2014-T6 Al Round bar
        • Cyclic Axial Loading in                                                    Al              σm =
                                                                                 2014-T6             5 ksi
          Tension-Compression
        • Design Life, N = 108 Cycles                                         0.60”
        • σmean = 5 ksi
        • S-N per Fig 8.34
  Find Loads: Pmax, Pmin
        • See NEXT Slide                                                             P
Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
16                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
 S-N Data for 2014-T6 Al




                                                                                               19.5 ksi




Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
17                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
18                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
19                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt
                                                                                      Creep Test Instrument
Engineering-45: Materials of Engineering                                    Bruce Mayer, PE
20                                         BMayer@ChabotCollege.edu • ENGR-45_Lec-20_Failure-2.ppt

				
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posted:11/24/2011
language:English
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