Computational weld mechanics: An approach towards simplified...
Transcript of Computational weld mechanics: An approach towards simplified...
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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
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‘’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
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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.
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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
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Weld induced distortions
Different types of distortions can occur due to the welding process
and it will influence the dimensional tolerances of the component
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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
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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.
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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.
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Why we need FE simuations?
Time
Money
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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)
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Reducing computational time
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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.
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Gradual weld bead simulation appraoch
This approach is
computationally
intensive but it
produces more
accurate results since it
is much closer to reality.
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Block dumping simulation approach
This approach is
computationally efficient
but less accurate
results are produced.
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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.
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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.
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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.
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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
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Simplifying the input parameters (Material properties)
in welding simulations
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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.
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Thermo-mechanical material properties of S355 steel grade
The temperature dependent thermo-mechanical properties
for S355 steel
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T-fillet joint
X (Transverse) Z (Longitudinal)
Y (Vertical)
Investigations are carried out on a T-fillet joint
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Different cases for thermal properties
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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
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Different cases for thermal properties
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Different cases for mechanical properties
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Influence of mechanical properties
Comparison of experimental and numerical transverse residual stresses using mechanical cases (MP 1-8) for S355 steel grade.
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Different cases for mechanical properties
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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.
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Different cases for mechanical properties
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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
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Generalization of yield stress
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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.
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Questions??