computational weld mechanics: an approach towards simplified...

Computational weld mechanics: An

approach towards simplified and

efficient welding simulations

Ayjwat Awais Bhatti

PhD Student

Departmen of Aeronautical and Vehicale Engineering

KTH Royal Institute of Technology, Stockholm, Sweden

Supervisor: Dr. Zuheir Barsoum

‘’Incorporating each and every detail of the welding process in the FE

simulation makes it complex, costly and time consuming. Such

simulation is an ‘unintelligent solution’. An intelligent solution would

be to incorporate only those parameters that can possibly influence

the ultimate outcome’’.

Summary

Introduction: Weld induced stresses and distortions

The non-uniform expansion and contraction of the weld due to localized heat input, results in welding residual stresses and distortions.

• Tensile and compressive welding residual stresses

• In fatigue loaded structures tensile residual stresses are regarded as detrimental and often equal to material’s yield strength. While compressive residual stresses are considered as beneficial.

The welding distortions can result in the degradation of dimensional tolerances of the geometry followed by costly rectifications and possible delays in production line.

Welding residual stresses can influence the fatigue and buckling strength of the product.

Weld induced residual stresses

Weld induced residual

stresses in a butt joint

The longitudinal residual

stresses i.e. the stresses along

the weld line can reach upto the

material yield strength

Weld induced distortions

Different types of distortions can occur due to the welding process

and it will influence the dimensional tolerances of the component

Fatigue strength of welded joints

The fatigue strength of welded joints do not increase with the

increase in the yield stress of the base material

In a welded component the

bulk of the fatigue life is spent

in propagating a crack

Weld defects, sharp transition

between weld and base plate,

residual stresses

Fatigue strength comparison

LTT filler wires will only reduce

the magnitude of residual

stresses at the weld toe.

Peening treatment at the weld

toe will not only smoothens the

transition between weld and

base metal but also induce

compressive residual stresses

at the weld toe.

Assessment of welding residual stresses

Many experimental techniques are available that can be used to

measure the welding residual stresses and distortions but these

are expensive, requires certain level of expertise and sometimes

difficult to carry out especially in large complex welded structures.

In the last three decades, with the evolution of computing

capabilities, finite element (FE) method has proved itself as an

alternative and acceptable tool for prediction of welding residual

stresses and distortions.

• It is an effective tool for prediction of residual stresses

• Really handy during the initial stages of product

development.

• Different alternatives can be tried.

Why we need FE simuations?

Time

Money

Negative aspects of welding simulations

Highly nonlinear and transient nature of welding process makes finite element simulations computationally intensive.

Moreover, an accurate representation of welding residual stress in a structure demands three dimensional FE simulation as well as incorporation of the entire structure surrounding the local weld zone.

And a large welded structure would make FE simulations more complex and time expensive.

Simplifications have been developed for welding simulations but they are made at the cost of accuracy.

Main aim: Reduce computational time for residual stress estimation

Simplify the input parameters involved in welding simulations (Material properties)

Reducing computational time

FE simulation methodology

Transient temperature field at

each time step is used as thermal load

Temperature dependent thermal properties

• Thermal Conductivity

• Heat Capacity

Thermal

Simulation

Simulation is validated by

temperature measurements

using thermocouples

Mechanical

Simulation

Simulation is validated by

Residual Stress and

distortion measurements

Temperature dependent mechanical properties

• Yield Stress

• Young’s Modulus

• Thermal Expansion

Sequentially coupled thermo-mechanical simulation is carried out i.e. firstly

thermal simulation is performed then the mechanical simulation is carried

out.

Gradual weld bead simulation appraoch

This approach is

computationally

intensive but it

produces more

accurate results since it

is much closer to reality.

Block dumping simulation approach

This approach is

computationally efficient

but less accurate

results are produced.

Substructuring: small scale specimen

500mm

500mm

Stress

extraction

points

Welding

direction

During the welding process the region close to the heat source is highly

nonlinear while remaining region in the structure behaves nearly elastic.

But during the FE simulation the whole structure is treated as nonlinear

model.

Substructuring: large scale specimen

The computational time can be reduced by using sub-structuring technique in

which the linear region is condensed into a single element matrix called super

element matrix and only element matrices for nonlinear portion is evaluated at

the end of every equilibrium iteration during the nonlinear solution.

Linear Region (Substructure)

Nonlinear Region

The original bogie beam structure was meshed with 60139 elements

and after the sub-structuring the elements are reduced to 48731.

Rapid dumping simulation approach

Butt welded joint

Stresses extracted at the center of joint

480mm

10mm 240mm

Comparison of experimental and predicted longitudinal residual stresses in butt

weld using different simulation approaches.

Rapid dumping simulation approach

130mm

300mm

Stress extraction Welding Direction

Comparison of experimental and predicted transverse residual

stresses in T-joint using rapid and block dumping.

T-fillet welded joint

Simplifying the input parameters (Material properties)

in welding simulations

FE simulation methodology

Transient temperature field at

each time step is used as thermal load

Temperature dependent thermal properties

• Thermal Conductivity

• Heat Capacity

Thermal

Simulation

Simulation is validated by

temperature measurements

using thermocouples

Mechanical

Simulation

Simulation is validated by

Residual Stress measurements

Temperature dependent mechanical properties

• Yield Stress

• Young’s Modulus

• Thermal Expansion

Sequentially coupled thermo-mechanical simulation is carried out i.e. firstly

thermal simulation is performed then the mechanical simulation is carried

out.

Thermo-mechanical material properties of S355 steel grade

The temperature dependent thermo-mechanical properties

for S355 steel

T-fillet joint

X (Transverse) Z (Longitudinal)

Y (Vertical)

Investigations are carried out on a T-fillet joint

Different cases for thermal properties

Influence of thermal material properties

Temperature histories at point A and B using test cases (TP 1-5) for T-fillet joint welded with S355 steel grade.

The influence of different thermal cases on temperature distributions is

investigated by comparing the temperature histories

Different cases for thermal properties

Different cases for mechanical properties

Influence of mechanical properties

Comparison of experimental and numerical transverse residual stresses using mechanical cases (MP 1-8) for S355 steel grade.

Different cases for mechanical properties

Influence of mechanical material property

Comparison of angular deformation predicted by different mechanical cases (MP 1-8) in T-fillet joint welded with different steel grades.

Different cases for mechanical properties

Generalization of yield stress

The present study has shown that the temperature dependent yield stress is the

most important material property in the mechanical analysis. Therefore, in order

to develop generalized semi-empirical expressions for temperature dependent

yield stress, piece-wise linear equations are used to describe it for a wide range

of steel grades.

1 1

1 2 2 1 1 2

2 1

2 3 3 2 2 3

3 2

3 3

( )

1( ) ( ) ( )

( )

1( ) ( ) ( )

( )

( )

RT RT f

f f f f f

f f

f f f f f

f f

f f MT

T T T T T

T T T T T T T TT T

T T T T T T T TT T

T T T T

Tf1, Tf2 and Tf3 represent the temperatures at 500°C, 800°C, and 1100°C

respectively

Generalization of yield stress

Conclusions An efficient sequential thermo-mechanical welding simulation approach

called rapid dumping is developed in which it is suggested to use

moving heat source method in thermal analysis and, in mechanical

analysis use final cooling load step for entire weld bead instead of using

final cooling load step for individual activated block. By using rapid

dumping the CPU computational time is reduced by 90-95% as

compared to gradual weld bead deposition.

For assessment of longitudinal as well as transverse residual

stresses with acceptable accuracy, all of the mechanical material

properties except temperature dependent yield stress can be taken

as constant (room temperature value can be used).

In mechanical analysis temperature dependent yield stress is the

most important material property for estimation of angular distortions.

For accurate predictions of angular distortions, it is suggested that

yield stress and thermal expansion coefficient must be temperature

dependent.

Questions??