Fracture Behaviour of an Aluminium Laser Weld: A numerical Study

Transcription

1 Fracture Behaviour of an Aluminium Laser Weld: A numerical Study Patricia Nègre, Dirk Steglich GKSS Forschungszentrum Geesthacht FEnet Industrial Meeting - December 04 -

2 Motivation weight reduction compared to riveting High strength aluminium alloys Al 6000 alloys corrosion resistant Welds in aircraft Laser beam welding (LBW) Friction stir welding (FSW) Aircraft Industry Damage tolerant design - Prediction of fracture toughness Experimental Investigations Computational Finite Element Methods

3 Motivation (con t) Finite element methods Simulation of various geometries: specimens, structures... Simulation of various configurations: materials, crack position, size of zone, porosity content... Cost and time savings Focus Monotonic loading Crack initiation and crack extension Fracture resistance Crack trajectory

4 Characterisation of aluminium laser beam weld : base metal FZ: fusion zone HAZ: heat affected zone HAZ FZ HAZ Butt joint of similar material Base metal: Al 6000 alloy CO 2 laser source Post Weld Heat Treatment Micro Flat Tensile specimen Mismatch ratio: σ 0FZ σ 0 = m m 2 m m 5 m m Undermatched weld Tensile test results 0.5 mm

5 Experimental results: Fracture toughness of Al 6000 LBW Compact tension (C(T)) specimen B = 4.2 mm 2H FZ = 2.8 mm H HAZ = 9 mm

6 Crack path in the asymmetric configuration Initial crack within the HAZ Crack Path Deviation towards the softer fusion zone Fractured C(T)-specimen Fracture surface topography average value of crack deflection over several specimens: α = 17

7 Damage modelling L endommagement, comme le diable, invisible mais redoutable Damage in metal: [Lemaitre, Chaboche ] void initiation, growth and coalescence 20 inclusions F [kn] Crack Computational cell Principle Elastic plastic material homogenisation Constitutive equations D Cohesive law 5 0 Experiment FE Gurson FE el.-pl l [mm] uncoupled models: no effect of damage on deformation Process Zone D voided cells f 0 coupled models: damage reduces stiffness

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9 Determination of model parameters Gurson-Tvergaard-Needleman model (GTN) some model parameters are related to the material microstructure f 0 assumed equal to the volume fraction of coarse particles present in metal D, size of finite element at crack tip is function of the particles nearest neighbour distance FZ Cohesive Model (CM) T 0 2σ 0 for sheet materials Γ 0 = J initiation first estimation for T 0 = 600 MPa Γ 0 = 12.5 MPa Pvf NND 50 µm 10 µm

10 Crack extension in σ 0 =302 MPa, n=0.67 (n:ludwik s exponent) 3D-simulation CTOD-δ5 clip δ m m a a o parameters GTN: f 0 =0.0115, f c =0.0195, k=4, q 1 =1.5, q 2 =1.0, D=150 µm parameters CZM: T 0 =570 MPa, Γ 0 =30 N/mm [after Hellmann and Schwalbe, 1986]

11 Crack extension in FZ σ 0FZ =200 MPa, n=0.25 σ 0 =302 MPa, n= mm 1,4 mm fusion zone base metal el-pl heat affected zone el-pl 3 parameters GTN FZ: f 0 =0.035, f c =0.065, k=8, q 1 =1.5, q 2 =1.0, D=75 µm parameters CZM FZ: T 0 =440 MPa, Γ 0 =8 N/mm 2 1

12 Crack extension at an interface Bi- or Tri-material configuration? Bi-material Tri-material FZ 2hFZ HAZ FZ 2h HAZ 2h FZ HAZ 2hHAZ larger FE-mesh outside the FZ reduction of computational time for this particular weld, local and global mismatch identical M local = σ 0FZ σ 0HAZ1 = σ 0FZ σ 0 = M global probably too simple configuration? real configuration effect of local mismatch fine mesh in HAZ Bi-material configuration

13 Crack extension at a bi-material interface - GTN Transfer of model parameters from FZ crack configuration Numerical crack path 3D simulation FZ - von Mises plasticity, FZ - GTN potential parameters FZ: f 0 =0.035, f c =0.065, k=8, q 1 =1.5, q 2 =1.0, D=150 µm

14 Influence of local mismatch Tri-material configuration Indicates CPD FZ HAZ 2h FZ =2.8 mm 2h HAZ =5.6 mm σ 0FZ σ 0 σ 0HAZ M local M global M local small -von Mises - HAZ-von Mises FZ-GTN: σ 0 =200 MPa, f 0 =0.035, f c =0.1, k=4, q 1 =1.5, q 2 =1.0 less damage resistant weld favors CPD

15 Tri-material configuration Influence of FZ width δ 5 (mm) contours of plastic strain FZ - 2H=4.2 mm H=4.2 mm a (mm) -von Mises : σ 0 =302 MPa HAZ-von Mises: σ 0 =302 MPa 2H=1.4 mm 2H=2.8 mm FZ - 2H=1.4 mm HAZ FZ-GTN: σ 0 =200 MPa, f 0 =0.035, f c =0.1, k=4, q 1 =1.5, q 2 =1.0 a = 1.8 mm a = 4.5 mm Direction of crack extension HAZ a = 1.8 mm a = 4.5 mm ε pl = 0.1 ε pl = 0.05

16 Conclusions Fracture toughness and thus damage tolerance - of a weld depends on the initial position of the crack degree of strength undermatching size of the weld Strength mismatch may induce crack path deviation Stable crack extension was simulated in / FZ / at bimaterial interface Constitutive laws accounting for damage have been used Cohesive model provides fast and efficient solutions, but the crack path is given by the finite element pattern GTN-model, the crack trajectory is not known ab initio and crack path predictions are possible

17 Example of applications at GKSS Cup cone fracture cohesive model - GTN-model Fuselage panel Two Bay crack under internal pressure - cohesive model DAMAGE MODELLING Punch test Al alloy - GTN model Crack arises at 0 and 90 Contours constant damage