Detailed strain field analyses of fatigue cracks in friction stir welded joints ICAF 2009, Rotterdam H.J.K. Lemmen, R.C. Alderliesten, R. Benedictus Aerospace Structures and Materials, Faculty of Aerospace Engineering 6-7-2009 Delft University of Technology Challenge the future
Introduction ICAF 2007: Fatigue crack initiation behaviour of friction stir welded joints in aluminium alloy σ ∞ Hole Direction of crack growth Width of nugget Fracture σ ∞ Different material flows Width = 50mm How affects an FS weld the fatigue crack growth behaviour? Detailed strain field analyses of fatigue cracks in FS welded joints 2
Contents • Introduction • Friction Stir (FS) welding • Residual stress profiles • Yield strength profiles • Fatigue Crack Growth (FCG) test • Loads & specimen configuration • Digital Image Correlation (DIC) • FCG test results • FCG curves • Results from DIC • Conclusions • Questions Detailed strain field analyses of fatigue cracks in FS welded joints 3
Friction Stir Welding FS welding performed by EADS in Munich Downward crown Welding welding force direction AS RS Shoulder HAZ HAZ Pin Tool rotation TMAZ L Nugget LT root weld Process parameters: Alloys: Advancing Retreating Side (AS) Side (RS) AA2024-T3 AA7075-T6 AA6013-T4 Thickness: 2.5 mm 2.0 mm 1.8 mm Welding speed: 350 mm/min 300 mm/min 1000 mm/min Rotational speed: 550 rpm 280 rpm 1500 rpm Downward force: 19 kN 18 kN 14 kN Detailed strain field analyses of fatigue cracks in FS welded joints 4
Residual stress profiles in FS welds Tool shoulder 500 Tool Pin AS RS 400 AA7075-T6 stress [MPa] 300 200 100 0 AA2024-T3 AA6013-T4 -100 -20 -15 -10 -5 0 5 10 15 20 distance from weld centre [mm] Residual stresses measured by X-ray diffraction Detailed strain field analyses of fatigue cracks in FS welded joints 5
Yield strength profiles of FS welds Tool shoulder 500 Tool Pin AS RS AA7075-T6 400 stress [MPa] 300 AA2024-T3 200 AA6013-T4 100 0 -100 -20 -15 -10 -5 0 5 10 15 20 distance from weld centre [mm] results presented at AIAA/SDM conference 2008 Detailed strain field analyses of fatigue cracks in FS welded joints 6
FS weld properties & FCG behaviour yield strength: residual stress: Residual stress σ crack tip = K t * σ ∞ Profile: Yield strength profile: σ Fatigue . 2 y i e l d , 0 Undisturbed crack plastic zone Direction of crack growth FS weld σ crack tip centre K t * σ ∞ FS weld σ residual ?? FCG Direction in FS weld FCG retardation? Detailed strain field analyses of fatigue cracks in FS welded joints 7
Fatigue crack growth test Goal: to repeat and measure the behaviour observed in the FI specimen Constant amplitude loading; R = 0.1; σ a = 60MPa Three configurations of centre crack FCG test specimen: 45º 90º 0º LT LT L AS RS AS L a = 2.5mm 2a = 5mm y AS RS y x x RS d = 3mm L y LT x Combined effect & Effect of residual stress Effect of yield strength Change of FCG direction Detailed strain field analyses of fatigue cracks in FS welded joints 8
Digital image correlation Strains are determined by analysing the images of a test DIC software has been developed by the author Images after DIC analyses, with overlay of strain field X-Y-Z axes CCT specimen electronically controlled rack Digital camera Grid of images For examples of applications for this DIC tool have a look at the poster presentations of Mr. Wilson, Mr. Rodi, Mr. Khan, and Mr. Delgrange. Detailed strain field analyses of fatigue cracks in FS welded joints 9
Results DIC Details: alloy = AA2024-T3; 90º configuration; y= 6.5mm; Centreline FS weld Centreline FS weld Centreline FS weld Detailed strain field analyses of fatigue cracks in FS welded joints 10
Plastic zone behaviour 550 AA7075-T6 Y = 6.5 mm 450 stress [MPa] AA2024-T3 350 Details: 250 > AA2024-T3; > 90º conf.; Small butterfly wings AA6013-T4 150 > y= 6.5mm; -15 -10 -5 0 5 10 15 distance from weld centre [mm] σ ∞ a=6.5mm Weld σ ∞ centre Crack path a=9mm a=11mm Large butterfly wings Detailed strain field analyses of fatigue cracks in FS welded joints 11
FCG behaviour 90º conf. 5.E-03 Details: base AA2024-T3 > AA2024-T3; FS weld, 90º config, spec 1, y = 0.0mm FS weld, 90º config, spec 2, y = 3.0mm 4.E-03 > 90º conf.; FS weld, 90º config, spec 3, y = 6.5mm da/dN [mm/cycle] FS weld, 90º config, spec 4, y = 6.5mm 3.E-03 6.5mm σ ∞ 0.0mm 6.5mm 2.E-03 base 1.E-03 3.0mm 0.E+00 0 5 10 15 20 σ ∞ a [mm] FCG acceleration FCG retardation Detailed strain field analyses of fatigue cracks in FS welded joints 12
Plastic zone behaviour Y= 3.0mm Details: Y= 0.0mm base > AA2024-T3; > 90º conf.; σ ∞ a=5.0mm a=8.0mm a=10.5mm σ ∞ Detailed strain field analyses of fatigue cracks in FS welded joints 13
FCG behaviour 0º configuration 5.E-03 Tool edge Weld centre Weld centre Tool edge Details: Tool edge base AA7075-T6-3, 1 saw cut FS welded AA7075-T6; 0º conf; spec 1 > AA7075-T6; FS welded AA7075-T6; 0º conf; spec 2 4.E-03 > 0º conf.; da/dN [mm/cycle] σ ∞ 3.E-03 acceleration 2.E-03 Base material 1.E-03 0.E+00 σ ∞ 0 5 10 15 20 25 rapid acceleration a [mm] retardation Detailed strain field analyses of fatigue cracks in FS welded joints 14
FCG behaviour 45º behaviour Details: > AA2024-T3; > 45º conf.; σ ∞ σ ∞ Detailed strain field analyses of fatigue cracks in FS welded joints 15
FCG behaviour 45º behaviour n o i t c e r Details: i d g n > AA2024-T3; i d l e W > 45º conf.; σ ∞ a=5mm a=7.5mm a=20mm Crack path a=12mm σ ∞ Tool edge Weld centre Detailed strain field analyses of fatigue cracks in FS welded joints 16
FCG behaviour 45º behaviour 1 2 3 7 4 5 6 8 Details: Failure after > AA7075-T6; several > 45º conf.; centimetres σ ∞ σ ∞ Detailed strain field analyses of fatigue cracks in FS welded joints 17
FCG behaviour 45º behaviour Material flows W e in Nugget Details: l d i n g > AA7075-T6; d i r e c > 45º conf.; t i W o n e l d σ ∞ c e n t r Crack path e T o o l e σ ∞ d g e Detailed strain field analyses of fatigue cracks in FS welded joints 18
FCG behaviour 45º behaviour n o Material flows i t c e r in Nugget i d g Details: n i d l e > AA7075-T6; W > 45º conf.; σ ∞ e r t n e e c g d d l e e W l o σ ∞ Crack path o T Detailed strain field analyses of fatigue cracks in FS welded joints 19
Introduction ICAF 2007: Fatigue crack initiation behaviour of friction stir welded joints in aluminium alloy σ ∞ Hole Direction of crack growth Width of nugget Fracture σ ∞ Different material flows How affects an FS weld the fatigue crack growth behaviour? Detailed strain field analyses of fatigue cracks in FS welded joints 20
Conclusions • The yield strength affects the FCG rate, depending on the orientation of the FS weld. • The residual stresses affect the FCG rate, with the largest effect found for the 90º configuration • The micro structure in the nugget of the FS weld provides a path with low fatigue resistance • Digital image correlation has a large potential in future research in fatigue. Detailed strain field analyses of fatigue cracks in FS welded joints 21
22 Detailed strain field analyses of fatigue cracks in FS welded joints Questions?
FCG behaviour 0º configuration Details: Weld centre Tool edge > AA2024-T3; Welding direction > 0º conf.; σ ∞ Tool edge a=5mm a=22mm a=7.5mm a=10mm a=15mm Crack path σ ∞ Detailed strain field analyses of fatigue cracks in FS welded joints 23
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