1. No category

VTT TECHNICAL RESEARCH CENTRE OF FINLAND LTD
Fatigue affected by residual
stresses, environment and
thermal fluctuations (FRESH)
Heikki Keinänen
Safir 2014 final seminar, March 2015
FRESH 2014 Research topics
Task 1.0 Thermal fatigue simulation, subtasks 1 & 2
- computational models and material parameters for thermal fat. simulation
Task 2.0 Weld residual stresses, subtasks 1, 2 & 3
- more realistic estimates for welding residual stresses and their magnitude
under operational conditions to be utilised in structural integrity assessments
Task 3.0 Categorization of stresses
- to assess the level of conservatism in the existing plastic strain correction
factor definitions
Task 4.0 Fatigue caused by turbulent mixing
- computational methods to assess fatigue caused by turbulent mixing
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Trueflaw thermal fatigue data - use in FRESH
project (task 1 – thermal fatigue simulation)
Trueflaw manufactures cracks for NDE training and qualification using in-situ
thermal fatigue loading
Each produced crack can be viewed as a thermal fatigue test
Thousands of cracks produced, mostly nuclear cases
In FRESH, parametric models were built to simulate these tests using elastic-plastic
FE analyses
Aim of the work was to determine the strain amplitude data for each test and
combine the numerical and experimental results into a strain-life fatigue curve
Known temperature load
and crack nucleation
data (input)
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Simulated stress/strain
distribution (output)
Measured failure cycles and
simulated stress/strain distribution
and amplitudes (result)
3
Simulation models
Calculation results for each case
Example loading
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Calculated strain cycles for all cases
4
Result data as a strain-life curve
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Comparison of computed and measured
residual stresses in a mock-up pipe (task 2 –
weld residual stresses)
General details of the computational analyses
Summary of the analysis performed for the mock-up pipe
and comparison of the computational results to the
measured ones
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Analysis procedure
• Abaqus general purpose FE code was utilised in the
computation
• Sequential thermal and mechanical analyses, small strains and
displacements assumed
• Temperature dependent material properties
• Mixed hardening material model of Abaqus including both
isotropic and kinematic hardening
• Anneal temperature of 1100-1400 °C
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Analysis procedure
Welding
speed,
voltage,
current
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Properties for
isotropic/kinematic
hardening model
8
Comparison of computed and measured residual
stresses in a mock-up pipe
- TVO has manufactured mock-up plates and pipes
containing multiple butt-welds.
- In the DEMAPP MACY - project residual stresses have
been measured with multiple methods.
- Computational simulation of welding of the pipe was
performed 2 years ago, measurement results are available
now:
[1] Miikka Aalto, Residual stress relaxation due to thermal loads in boiling water
reactor nuclear power plant pipe welds, Licentiate’s thesis, 15.12.2014.
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-
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Pipe material: SA 376 TP 304
Weld material: AISI 318L
Pipe outer diameter 323.85 mm
Wall thickness was 17.45 mm
Length of mock-up 400 mm
10
Run
1
2
3
4
5
6
7
Voltage
Current
Torch
speed
Interpass
temperature
[V]
[A]
[mm/s]
[°C]
13.5
13.5
22
22
22
22
22
84
90
77
100
100
100
100
0.3289
0.3929
1.0221
1.2386
1.1923
1.4570
1.6201
25
56
43
21
70
48
88
- ¼ of the circumference
- Abaqus FE code
- Pass by pass modelling
- Temperature dependent mat. props.
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The properties of Eshete 1250, which is a fully austenitic chromium-nickel steel
having a rather similar chemical composition as AISI 304 steel, were utilised.
The reason for using the Eshete 1250 properties is the availability of material
properties for the isotropic/kinematic hardening material model of Abaqus.
For parent metal the
utilised stress-strain
properties are
considerable higher than
presented in [1].
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Computed and measured [1] temperatures at weld root during welding of
passes 6 - 9. Time scales has been adjusted.
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Computed and measured [1] axial residual stresses at weld root.
Measurements are done by hole drilling method. Computational results at the
middle of the circumference.
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Axial residual stresses as a function of depth in the roots of three pipe butt
welds before thermal cycling. Pipes W5 and W6 were exposed to pressure
testing. Weld W6-7 is measured with ring-core method from the depth of 1 to 6.5
mm. All other measurements are done by hole drilling method [1].
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Computed and measured [1] hoop residual stresses at weld root.
Measurements are done by hole drilling method. Computational results at the
middle of the circumference.
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Hoop residual stresses as a function of depth in the roots of three pipe butt welds
before thermal cycling. Pipes W5 and W6 were exposed to pressure testing. Weld
W6-7 is measured with ring-core method from the depth of 1 to 6.5 mm. All other
measurements are done by hole drilling method [1].
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t = 17.45 mm, Q/t = 78…102 MJ/m2
Computed axial and circumferential (hoop) residual stress (MPa) after
welding at the middle of the weld. Three dimensional and axisymmetric
results are shown. Inner surface corresponds to coordinate value of 0 mm.
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t = 17.45 mm, Q/t = 78…102 MJ/m2
Computed axial and circumferential (hoop) residual stress (MPa) after
welding at the middle of the weld. Three dimensional and axisymmetric
results are shown. Inner surface corresponds to coordinate value of 0 mm.
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Conclusions
• The computed axial and hoop as-welded stresses are
tensile at the inside surface in the weld root area
• The computed hoop stresses are tensile through the wall at
the middle of the weld
• The computed axial stresses are tensile on the inner
surface and compressive on the outer surface at the middle
of the weld
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Conclusions
• There is large variation in the measured stresses. Some
discrepancies exist, e.g. negative hoop stresses at the weld
root?
• Additional measurement points or even other methods, e.g.
iDHD would be useful/necessary?
• The computations could be re-run with lower stress-strain
curve for the parent material and maybe with a 180deg
model (at least). Optimal situation would be that the real
cyclic stress strain curves are measured
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Conclusions
• In case of welding residual stresses there is
uncertainty in computations and measurements.
Both methods should be utilised to support each
other to get more reliable results.
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Reference:
• Computation of Welding Residual Stresses in a Multi-Pass Welded Mockup Pipe, Research Report VTT-R-08364-12
Other performed analyses:
•
•
•
•
•
•
•
Comparison of computed residual stress states by 3D and 2D models and evolution of
residual stresses due to operational loading (in feedwater nozzle), Research Report VTT-R04886-14.
Computation of (RPV) cladding welding stresses, Research Report VTT-R-06020-14.
Simulation of weld overlay, Research Report VTT-R-00485-14.
Computation of Welding Residual Stresses in a Multi-Pass Welded Mock-up Pipe. Keinanen,
H. 22nd International Conference on Structural Mechanics in Reactor Technology 18-23
August, San Francisco, California, USA.
Keinänen, H. Weld repair simulation for the Mock-up 2 of EU FP7 STYLE Project. Baltica
IX. International Conference on Life Management and Maintenance for PowerPlants. Pertti
Auerkari & Juha Veivo (eds.). VTT Technology 107. VTT. Espoo (2013), 24.
Keinänen, H., Alhainen, J., Karppi, R. & Verho, M. 2009. Control and Exploitation of Thermal
Distortions in Welded T-joints. 20th International Conference on Structural Mechanics in
Reactor Technology (SMiRT 20), Espoo, Finland, August 9.
Computation of welding residual stresses in a mock-up nozzle, Research Report VTT-R02265-11.
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