Fatigue Crack Propagation and Stable Tearing in Friction-Stir-Welded by rdf19918

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									Fatigue Crack Propagation
and Stable Tearing in
Friction-Stir-Welded
Aluminum Sheet


                 Presented by:   Eui I. Lim
                                 The Boeing Company

                 At:             ASIP 2006
                                 San Antonio, Texas
                                 November 28, 2006
                    Collaborative Effort
                                             Univ of So Carolina




  Raphael Muzzolini                          Nathalie Fuzier
  Jean-Christophe Ehrstrom, PhD              Anthony Reynolds, PhD




                           Kenneth K. Chan
                           Eui I. Lim

ASIP 2006                         2
                           Initiation
 • Seemingly Consistent Crack Turnings
      – FCGR test on 7xxx-T7 Sheet
      – 3 Samples each, from 3 Lots




            FSW sample                  Base metal
ASIP 2006                       3
                         Motivation


 • Understand Phenomenon
      – Why did the Cracks Turned?
      – Can it be Captured, and
        Controlled?
 • Via Experiments
      – FCGR and stable tearing
      – C(t) and M(t) coupons
      – Cracks to FSW @ 90º & 45º




ASIP 2006                       4
                             Outline
1.    Material and weld properties

2.    Fatigue tests
     – Geometry
     – Crack path
     – da/dN curves
     – Residual stress measurements

3.    Stable tearing tests
     – Crack path
     – Properties

4.    Fractography
ASIP 2006                       5
            Material and Welding Description
•   Aluminum 7xxx-T7 Sheet , t = 5mm.
•   Post Weld Aging heat treat to stabilize properties of the weld.



                                                   Profile of the Tool :
                                    side
                        ide

                                        g
                           s

                                   eatin
                       ing
                   anc


                               Retr
               Adv




ASIP 2006                                   6
                                                       Weld Properties
                                                        UTS                       Elongation at UTS          Elongation at
                                                       MPa (ksi)                         (%)                  rupture (%)
     Base metal (T-L)                                  552 (80)                                 8.1              13.9
     Weld (T-L)                                        477 (69)                                 5                5.9
                            600
                                                                                           80
                                                                                                        Weld UTS : 86%
                            500                                                                         of base material
 engineering stress (MPa)




                            400                                                            60




                                                                                                (ksi)
                            300
                                                                                           40            T
                            200                                                                              S
                                                                    welded sam ple         20
                            100
                                                                                                             L
                                                                    base m aterial
                              0                                                            0
                                  0   2   4       6     8      10       12       14   16
                                              engineering strain (% )

ASIP 2006                                                                    7
                                                Hardness Profile
                                                              (inches)
                                         -1.0         -0.5    0.0         0.5          1.0        1.5
                            200
       Microhardness (HV)




                            180

                            160

                            140
                                                                                 S
                            120                                                        T

                                  Advancing side                              Retreating side
                            100
                                   -30          -20     -10    0         10       20         30    40
                                     Distance from the centerline of the weld (mm)

ASIP 2006                                                      8
                             Outline
1.    Material and weld properties

2.    Fatigue tests
     – Geometry
     – Crack path
     – da/dN curves
     – Residual stress measurements

3.    Stable tearing tests
     – Crack path
     – Properties

4.    Fractography
ASIP 2006                       9
                           C(T) Samples

                                      Dimensions are in inches.




                                           L
   L
               90º orientation                   45º orientation
           T                               T

       θ
                                       d




ASIP 2006                        10
                Crack Path 90º Orientation
Fatigue cycling at constant ΔK = 15 MPa*m1/2
Propagation along straight line, slight deviation angle.
     Angle           13.7º                 15º         < 5º



     R=0.1

     T
            L

     R=0.5



      Angle          7º             < 5º                   < 5º
ASIP 2006                          11
                Crack Path 45º Orientation
Curved crack path.
             32º (local max)   38º (local max)     10º



 R=0.1

   L     T




 R=0.5



             17º (local max)   21º (local max)   11.5º

ASIP 2006                           12
                                                             Fatigue Results
R=0.5 ΔK=15MPa*m1/2
                                                                                                                                     L
                                                                       (inches)

                                       -0.5         0.0          0.5           1.0        1.5      2.0
                                                                                                                                         T
                          0.01

                                                   weld


                                                                              Cracking Direction
                                                                                                               1e-4
     da/dN (mm/cycle)




                                                                                                                      (inch/cycle)
                         0.001
                                                                                                 parent




                                                                                                               1e-5




                        0.0001
                                 -20      -10        0      10           20          30     40     50     60

                                                Distance from the centerline of the weld (mm)




ASIP 2006                                                                                 13
                                                               Fatigue Results
R=0.1 ΔK=15MPa*m1/2
                                                                (inches)                                                      L
                       1e-3
                                    -1                   0                  1                  2                                  T
                                    parent
                                                                                                        1e-5

                       1e-4
    da/dN (mm/cycle)




                                                                                                        1e-6




                                                                                                               (inch/cycle)
                       1e-5


                                                                                                        1e-7

                       1e-6
                                                                            Cracking Direction
                                                       weld                                             1e-8

                       1e-7
                              -30        -20     -10     0     10      20       30   40    50      60

                                               Distance from the centerline of the weld (mm)




ASIP 2006                                                                            14
                    Residual Stress Evaluation
By “Cut Compliance” technique - Measure Δε at each additional cut




Forward determination of the
stress intensity factor due to
residual stresses :
                    E ' dε
   K I res (a) =
                   Z (a) da
where Z(a): a geometrical function

ASIP 2006                            15
                                                                  Kresidual
                                                                  (inch)
                                                 -1                  0                1
                               15

                                                                                                           10
                               10
                                                                           Slot cutting direction
                                5                                                                          5
            K res (MPa*m1/2)




                                                                                                                 (ksi*inch1/2)
                                0                                                                          0

                                -5                                                                         -5

                               -10
                                                                                                           -10

                               -15
                                                                                                           -15
                               -20
                                     -40   -30        -20   -10     0      10    20       30     40   50
                                                 Distance from the centerline of the weld (mm)

  L                                                                         True Kmax applied at the crack tip,
       T                                                                    Kmaxtotal, is a summation of Kresidual
                                                                            and Kmax generated by loading.

ASIP 2006                                                                  16
    Residual Stress Profile Calculated from K
                                                                       (inch)
                                                  -1               0                    1              2
                                150
                                                                                                                20


                                100
        Residual stress (MPa)




                                                                                                                10
                                 50




                                                                                                                      (ksi)
                                  0                                                                             0



                                -50
                                                                                                                -10
                                            Advancing side                Retreating side
                         -100
                                      -40   -30        -20   -10   0     10        20       30   40   50   60

                                                  Distance from the centerline of the weld (mm)
                                              L
                                                        T

ASIP 2006                                                                     17
                             Outline
1.    Material and weld properties

2.    Fatigue tests
     – Geometry
     – Crack path
     – da/dN curves
     – Residual stress measurements

3.    Stable tearing tests
     – Crack path
     – Properties

4.    Fractography
ASIP 2006                      18
            Stable Tearing - Parent Material




                                  Rolling Direction
                                  appears to be the
                                  Preferred Path for
                                  Crack Extension.




ASIP 2006                  19
            Stable Tearing - Welded Samples




        Crack turns so it extends along the L direction.
        Combined effect of the microstructure and the welding?

ASIP 2006                          20
                                Stable Tearing Data
                                                    (inch)
                   -0.1   0.1    0.3          0.5            0.7   0.9          1.1        1.3
                 12                                                                              2.7
                                                                   base metal
                 10                                                classic weld (12 ipm)         2.2
                                                                   slow weld (4 ipm)
                                                    L              Series2
                  8                                                                              1.7
                                                             T
     load (kN)




                                                                                                        (kip)
                  6                                                                              1.2


                  4                                                                              0.7


                  2                                                                              0.2
                                       weld


                  0                                                                              -0.3
                      0   5      10           15             20    25           30         35
                                              delta a (mm)

ASIP 2006                                                21
            Stable tearing data




ASIP 2006           22
                             Outline
1.    Material and weld properties

2.    Fatigue tests
     – Geometry
     – Crack path
     – da/dN curves
     – Residual stress measurements

3.    Stable tearing tests
     – Crack path
     – Properties

4.    Fractography
ASIP 2006                      23
            Fractography – Fatigue R=0.1
                                   SEM pictures of Fracture Surface




                          100 μm
                                   Inside the nugget
                                   Appears mostly inter-granular

                                                         S
                                                             T




                  10 μm

ASIP 2006                          24
            Fractography – Fatigue R=0.5
                              Inter-granular Fracture
                              Surface rougher than for R=0.1
                                                  S
                                                        T




ASIP 2006                25
            Stable Tearing – Nugget

                                S       SEM pictures
                                    T

                                R               A

                  100 μm




                20 μm                               5 μm

ASIP 2006                  26
                              Summary
• Weld characterization:
     – With appropriate welding parameters, weld UTS would be 86%
       of the base material.


• Crack path:
     – For 90º oriented fatigue samples, propagation along a straight
       line, with sometimes a slight deviation angle observed.
     – For 45º oriented fatigue sample, curved crack path.
     – In base metal stable tearing tests, the rolling direction is often a
       preferred path direction for the crack.


ASIP 2006                            27
                              Summary
• Fatigue crack growth rate :
     – Compared to parent material, reduced crack growth rate is
       observed in the HAZ. This reduction is higher for lower R.
     – Residual stress seem to have a predominant effect in this
       variation. Closure and microstructure might also be involved.


• Fractography:
     – In fatigue, fracture is mostly inter-granular. The surface is
       rougher for higher R.
     – In stable tearing, the fracture is also clearly inter-granular.


ASIP 2006                            28
                 Future Work
• Investigate Residual Stress Affect due to FSW
• Acquire Crack Growth Properties Along the
  Weld, HAZ from both the Advancing Side and
  the Retreating Side of the Weld
• Acquire Crack Growth Properties Across the
  Weld, HAZ from both the Advancing Side and
  the Retreating Side of the Weld
• Repeat Testing and Data Acquisitions for other
  Materials Commonly used in FSW
ASIP 2006              29

								
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