Prediction of Load-Displacement Curve for
Weld-Bonded Stainless Steel Using Finite
Element Method
Essam Al-Bahkali
Jonny Herwan
Department of Mechanical Engineering
King Saud University,
P.O.Box 800, Riyadh 11421, Saudi Arabia
Multiphysics Dec 09-11, 2009
Lille, France
Background
• It is one of the oldest cost effective, less labor and readily
automated electric welding techniques that is used to join
similar and dissimilar metals.
• Introducing an adhesive layer in conjunction with a spot
weld nugget helps strengthening welded joints and
balancing stresses in the weld nugget area.
• Optimum welding quality of a spot welded or a weldbonded joints, Required optimum welding parameters, i.e.
welding current, electrode force, and welding time.
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• The advantages of resistance welding process include
– short process times
– heat focused at the material interfaces.
– easily be automated
• While experimental work provides the necessary physical
insight about the behavior of spot-welded joints,
predictive tasks such as design; analysis and evaluation of
spot-welded structures are often carried out by
computational methods
• In predicting stress distribution, stress concentration and
failure modes of a weld-bonded or a spot welded nugget,
a finite element modeling can do an excellent job in this
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regard.
• Engineers use the process of adhesive bonding to join
materials
• Adhesive bonding can be used to adhere a wide range of
materials such as metal to metal, metal to ceramic, metal
to polymer, … etc.
• Some advantages of adhesive bonding
–
–
–
–
Reducing weight
Uniform stress distribution
Fatigue resistance
Ability to join thick with thin materials as well as the ability to
join dissimilar materials.
– No stress concentration
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• There are two types of Bonded structure
– Purely Adhesive
– Adhesive/Mechanical Connection.
• Bonded-Welded
• Bonded-Riveted
• Bonded Screwed connections
• The combined connections ensure high fatigue strength of
the structures.
• Single lap welded joint is one of the important method
used to join two plates together.
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Objective
• In present work
– Manufacturing point of view
• To develop a finite element model of weld-bonded that can
represent a complete load-displacement curve
• Minimize the experimental or trial in industrial application
– Design point of view
• To obtain the representative weld-bonded model which has
reasonable deformation shape and fracture initiation.
• To design the position and the effective number of these joints
in the mechanical structures
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The scope of research
Process Parameters :
• welding current
• welding time
• electrode force
• etc
Size,
Properties of
Nugget, and
HAZ
Already done by some researchers:
- A. De, et al [2003]
- E. Feulvarch, et al [2004]
- J. Z. Chen, et al [2006]
- There was Sorpass Software that can analyze the
temperature distribution, size of nugget and Heat Affected
zone (HAZ)
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Deformation and
Fracture of the Joints
Our research !
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Experimental Analysis
Material Verifications
- Base Metal Tensile Test
- Ductile Fracture Limit of AISI 304 steel
- Mechanical Properties of Adhesive
Adhesive
Bonding
Spot Weld
Micro-hardness & Spherical
Indentation
Lap Shear Tensile Test
Weld-Bonding
Finite Element Analysis
Modeling of Material Tensile Test
Modeling of Lap Shear Tensile
Test of Spot Welding
Modeling of Lap Shear Tensile
Test of Adhesive Bonding
Modeling of Lap Shear Tensile
Test of Weld-Bonding
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FE Modeling and Boundary
Conditions
100 mm
30 mm
x
z
25 mm
F
Bonded Model (T.V)
Strip - A
Adhesive
F
Strip - B
y
100 mm
x
Bonded Model (F.V)
Nugget
F
5 mm
y
x
Weld-Bonded Model (F.V)
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Assumptions
• 3-D FE model
• Half of the model was considered to save computation
time
• Isotropic adhesive layer
• The elastic-plastic FE model was considered in current
analysis for verifying the model with the experimental
test
• There is no adhesive layer in a zone 1 mm around the
circumference of the welds
• The damage evolutions were chosen arbitrary in term of
displacement because the failure propagation is not
considered
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Finite Element Meshes
•
The FE mesh was modeled using eightnode linear brick reduced integration
elements for strips and nugget
Transverse
direction
Longitudinal direction
Bonded Model
Strip - A
 The FE mesh for adhesive
layer was modeled using
eight-node 3-D cohesive
element
Strip - B
Adhesive Part
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Partial of the Bonded Model
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• Fine mesh was used in strips around the nugget, nugget,
and adhesive layers
Partial of the Weld-Bonded Model
Nugget Part
Partial of the Adhesive Model
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Stress Triaxiality
• In Abaqus, the ductile fracture limits are in term of stress triaxiality
• The stress triaxiality equation can be written as:
• Where
• Stress triaxiality can be obtained through equivalent fracture strain.
• This can be done by conducting standard tensile test to record the
true strains at the fracture limit.
• The value of the stress triaxiality were calculated numerically using
finite element modeling of the notch tensile test
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Hardness Measurement and Indentation
• Micro-hardness test was usedTest
to define the location of the weld
nugget and heat affected zone (HAZ)
• The hardness measurement started from the center of the nugget and
move a way from the center to the heat affected zone with a step of
0.25mm.
• To obtain the plastic properties of each region, spherical indentation
( 2mm diameter) was carried out at several loads.
• Spherical indentation data can be transferred to true stress-true strain
curve using Ahn-Equation:
• Where
σ is the true stress, ε is the true strain, φ is the plastic constrain factor
=3.6, P is the load, Pm is the mean pressure, α is the adjustment
constant = 0.14, ac is the contact
radius between the indenter and 15
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Micro-hardness and Spherical Indentation
Results
250
230
HV
210
190
170
Nugget
HAZ
HAZ
150
-8
-6
-4
-2
0
2
4
6
8
Position from center of nugget (mm)
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Tensile Test Comparison between
Experimental and Finite Element Model
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Experiment
FE Model
Load (kN)
8
6
4
2
0
0
10
20
30
40
50
Displacement (mm)
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Load- Displacement Curve of Spot
Welding
10
Experiment-1
9
Experiment-2
8
Experiment-3
Experiment-4
Load (kN)
7
FE Model
6
5
4
3
2
1
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Displacement (mm)
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Initial Failure Comparison between
Experimental and Finite Element Model
Failure Initiation from Experimental
and Finite Element Model at HAZ
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Load- Displacement Curve of Adhesive
Bonded
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Experiment-1
Experiment-2
Experiment-3
Experiment-4
FE Model
10
Load (kN)
8
6
4
2
0
0
0.5
1
1.5
2
2.5
3
Displacement (mm)
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Load- Displacement Curve of WeldBonded
12
Experiment-1
Experiment-2
Experiment-3
Experiment-4
FE Model
10
Load (kN)
8
6
4
2
0
0
0.5
1
1.5
2
2.5
3
Displacement (mm)
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Conclusions
• The ductile fracture limit criteria were developed to
predict the deformation and fracture initiation of the
model.
• Detailed material properties of each zone of resistance
spot welding (nugget, heat affected zone, and base metal)
are essential to accurately simulate the model.
• Reverse engineering analysis is introduced to get those
material properties by modeling the indentation process
using finite element software, and conduct some iteration
of models until the load-displacement curve of
indentation agree with the experimental curve.
• The results in general for the load-displacements curve
from finite element model shows a good agreement with
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the experimental data.MULTIPHYSICS 2009