Managing Welding Induced Distortion – Compar ison of d i f ferent computat ional approaches

Mehdi Ghanadi1) and Zuheir Barsoum2)

2) Department of Aeronautical & Vehicle Engineering KTH Royal Institute of Technology, Stockholm, Sweden zuheir@kth.se

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

1) Department of Virtual Product Development Structure and Durability Volvo Construction Equipment, Braås, Sweden

Background

•  Accurate and reliable distortion predictions are essential for structural integrity of welded structures

-  Structural integrity analysis (Fatigue, Fracture, Buckling) -  Manufacturing (Cutting, Forming, Joining) -  and Assembly (Tolerances)

•  Need for simplified methods for prediction of welding induced distortion

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Stockholm Waterfront Conference Center, 29 Sept2016

Methods for FE welding simulations for distortion predictions

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•  Thermo-elastic-plastic FEM simulations

•  Elastic FEM based on Inherent Strain -  plastic strain developed during the welding processes give rise to

distortion - Other variants are; Inherent deformation and Shrinkage force

•  This study aims to assess and compare these different approaches and validation with experimental data

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

What happens during welding?

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1

23

4σ

ε

Uniaxial test specimen SATOH test

Longitudinal stresses in the weld

Initial condition / length

HeatingElastic compressionTo initial length

More heatingPlastic compressionTo initial length

CoolingElastic tensionTo initial length

U1-2

Δε < 0U3-4

U2-3

1-2.

1.

2-3.

3-4.

Δε < 0

Δε < 0

CompressionCompression

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

What is inherent strain?

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Three bar model

Before heat input After cooling After cutting

ΔL: miss match produced by welding (inherent strain)

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Classification of welding distortion

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Cause of residual stress and distortion (Inherent strain)

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Visible strain (total strain): ε = ΔL/L

Total strain ε = Elastic strain εe + Thermal strain εT + Plastic strain εp + Creep strain εc + Transformation strain εt

ε = εe + εT + εp + εc + εt

Rearranging equation:

ε - εe = εT + εp + εc + εt = ε* (inherent strain)

If Inherent Strain is known; welding distortion can be computed by elastic analysis

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Specimens and experimental set up

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-  T-fillet joint with S355 steel •  D u r i n g w e l d i n g , t h e

baseplate has been clamped in one side and in 60 mm f r o m c e n t e r o f t h e baseplate.

-  Welding deformation

measurements •  device is moved on the

surface of the specimen and measures the deformation of some specified points both on baseplate and stiffener

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Nonlinear FEA for welding simulation

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- weld bead is divided into adequate number of volumes (blocks) which will be used in the implementation of the moving heat source and rapid dumping approach.

-  Non-linear, temperature dependent material properties were used

500mm

500mm

Stress extraction points

Welding direction

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Inherent strain zone

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-  Determined through the non-linear FEA •  Residual plastic strain created

due plastic deformation during welding

- Good approximation; Average value of the inherent strain distribution along the weld line

-  Elastic FEA may be used to predict the welding distortion when the inherent strain region and averaged magnitude is known for the particular joint.

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Elastic FEA

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Inherent strain approach - Magnitude of inherent strain ε, consists of bending (εb) and membrane (εm)

- The inherent strain components are integrated over thickness (t) and the results applies on each element in the inherent strain zone,

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Elastic FEA

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Inherent deformation approach - integration of inherent strain over the cross section of the plate and perpendicular to welding line - The inherent deformation may be divided into four components

•  Longitudinal shrinkage (δ*L)

•  Transverse shrinkage (δ*T)

•  Longitudinal bending (θ*L)

•  Transverse bending (θ*T)

The average value of inherent deformation components along welding length (Lw)

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Elastic FEA

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Inherent deformation approach -  Inheren t de fo rmat i on and

inherent bending in web-flange

-  Average values as straight lines

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Elastic FEA

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Shrinkage force approach -  Welding distortion can also be

calculated by applying equivalent force and moments

-  Applied on inherent strain zone: •  longitudinal shrinkage force (Fz) •  longitudinal moment (Mz) •  transverse shrinkage force (Fx) •  transverse moment (Mx)

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Comparsion of different approaches

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-  Transverse bending -  Longitudinal bending

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Comparsion of different approaches

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-  Longitudinal, transverse shrinkages and angular distortion

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Conclusions

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-  The determination of the inherent strain distribution is essential in order to carry out the elastic FEA

-  Large variation is observed in the start and stop position of the welding, i.e. edge effects. Average value of the inherent deformation should be calculated by neglecting the results from the nodes and elements locate near the plate edges.

-  It is observed that the inherent strain predicts the distortion with good accuracy as compared with the other approaches. The inherent deformation and shrinkage force requires an integration step, which will result in a poorer distortion approximation.

-  Inherent strain and inherent deformation approaches are suitable to predict transverse shrinkage and transverse bending

-  Shrinkage force approach is more suitable to predict the longitudinal shrinkage and longitudinal bending

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

Future work

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-  Further evaluation of approaches -  Application to large, real, welded structures in Construction Machinery

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016

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Thank you for your kind attention!

Questions?

3rd Nordic Conference on Design and Fabrication of Welded Structures Stockholm Waterfront Conference Center, 29 Sept2016