Transcript No Slide Title

Centre for Marine Technology
John D McVee
Technical Manager
Numerical Predictions of Residual Stresses
in Welded Steel Submersible Hulls
1G.P.
Campsie, 2A.C. Ramsay and 1J.D. McVee
Centre for Marine Technology
QinetiQ Rosyth
1QinetiQ
Future Systems Technology Division, Centre for Marine
Technology, Rosyth
2MSC Software Ltd., Frimley
3
Contents
1 Cracking in structures
2 Model tests
3 Numerical modelling
4 Representation of joint configurations
5 Heating and cooling
6 Predicted results
7 Comparison with experiment
8 Conclusions
4
Cracking in Structures
• Cracks in a structure are formed
– during fabrication
– during service
• Cracks in a structure are spread by mechanisms such as
– fatigue
– stress corrosion cracking
– fast fracture
• These mechanisms are heavily influenced by
– material and environmental variables, and/or
– applied and fabrication stresses (residual stresses)
5
The Military Context
• Compared to a commercial structure, a military structure
– may use novel materials
– will operate at high stress levels
– will operate in harsh environments
– will have additional requirements to resist weapon attack
• The consequences of cracks in a military structure are thus
– at best, loss of availability
– at worst, catastrophic failure and loss of life
6
Cracked T-Butt Weld
Technical Issues for Military Submersibles
• Pressure Hull
– internal ring-stiffened cylindrical structure
– high strength steel, which is difficult to fabricate
– tensile residual stresses locked in during welding ring stiffeners to hull plating
– applied stress levels are a high percentage of yield
• Current knowledge of Pressure Hull fatigue based on
– model tests
– experimental determination of residual stress
– fracture mechanics based fatigue crack growth predictions
7
8
Large Fatigue Chamber Facility
9
Schematic of Large Fatigue Chamber
Internal View of Large Fatigue Chamber
10
11
Near full scale thickness Q1N steel hull plate
•
Much reduced diameter compared to full scale hull
•
Welded to exacting Naval Engineering Standards
•
Closure domes attached
•
Externally pressurised
•
Soft cycled
0 .5 L
t
R
•
0 .5 L
t
Model Tests
Supporting non destructive evaluation performed after pre-determined
numbers of cycles gives a direct indication of fatigue crack growth rates
12
Typical Fatigue Model
13
Fatigue Model being Lowered into Chamber
14
Experimental Determination of Residual Stress
•
Weldable strain gauges mounted on hull
FORWARD
•
Strains measured before and after welding
GAUGES AT POSITIONS 1,3
•
Bending moment at toe of weld estimated by extrapolation of strain gauge data
•
Extrapolation based on elastic shell theory
•
Detailed through thickness residual stress distribution not known
•
But, assume residual stress peaks at yield at toe of weld
•
Postulated distribution then set to have same net bending moment as that deduced
by extrapolation of elastic strain gauge data
AFT
76
127
178
PLATING GAUGES - RIGHT ANGLED PAIRS
INTERNAL AND EXTERNAL BACK TO BACK
GAUGES AT POSITION 2
178
178
127
127
76
76
FORWARD
AFT
CROWN
POSN. 2
LOOKING AFT
STIFFENER
POSN. 1
An extensive database now exists of experimentally determined
residual stresses
POSN. 3
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Fracture Mechanics based fatigue crack growth predictions
•
Fatigue crack growth propagation material constants
•
Stress intensity factor solutions
•
Geometry, R/t ratio, stiffeners, weld profile
•
Crack shape and crack gradient
•
Applied stresses
•
Residual stresses
a
n
 C K 
N
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Present Work
• Carried out under auspices of DERA Corporate Research Programme (CRP)
• Funded by UK MoD Stakeholders (primarily Submarine and Armoured Fighting Vehicle
communities)
• Recognition that :
– a validated numerical modelling methodology for prediction of residual stress levels would
improve current fracture mechanics based fatigue crack growth predictions
– a validated numerical modelling methodology for prediction of residual stress levels would
allow rapid assessment of proposed changes in fabrication practice
Numerical Modelling Methodology
- Initial Studies
• Coupled thermo-mechanical analyses using MSC.MARC
• Issues included:
– 2-D axi-symmetric representation of the ring stiffener to cylinder joint configuration and weld cross section
– an explicitly defined and idealised fusion profile, extending into the ring stiffener and cylinder plating
– weld passes defined individually within preprocessor, but lumped together for analysis
– nodal temperatures of 1550C and 120C prescribed as initial conditions to weld and parent plate respectively
– kinematic constraints applied between weld and parent plate
Comparison with experimental results revealed a reasonable correlation
A number of improvements were identified for subsequent implementation to permit
3-D analysis of thick section multi-pass welds
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18
Overview of Improvements - 1
• Acquisition of accurate material property data
• Physical:
– variation of specific heat capacity with temperature
– variation of coefficient of thermal expansion with temperature
– variation of thermal conductivity with temperature
– variation of density with temperature
– latent heat of fusion
• Mechanical:
– variation of Poisson’s ratio with temperature
– variation of elastic modulus with temperature
– series of stress-strain flow curves, obtained at different temperatures and at different strain rates
Temperature range from ambient up to a nominal
solidus ( 1500 C)
19
Overview of Improvements - 2
• Development of a tool based on MSC.Marc subroutines
• Functions include:
– control of the heat input, i.e., proportion of heat directed at weld and parent plate
– moving heat flux
– control of filler material element activation
• User definable inputs include:
– number of weld passes
– definition of weld pass start locations
– definition of weld paths in cartesian or cylindrical co-ordinates
– variation of power input per weld pass
– Variation of welding torch travel speed per weld pass
– Radius of weld pass and hot zone per weld pass
Permits 3-D thermal analysis by accounting for the pre-heating effect of the welding process on
material ahead of the welding torch
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Overview of Improvements - 3
• Introduction of Filler Material
– elements defining the filler material are present at commencement of analysis
– elements are inactive prior to being reached by welding torch
– elements are activated at the correct melt temperature
– heat flux is corrected to allow for specific heat capacity introduced
– inactive elements take up a configuration based on current location of nodes attached to activated
elements and the original configuration of nodes attached only to further inactive elements
Automation of filler material element activation permits
reduced modelling and analysis time
Numerical Modelling Methodology
- Current Studies
•
Improvements implemented
•
Applicability to multi-pass welding simulations
•
3-D mesh of parent plate and weld passes required - elements defining each weld pass grouped separately
•
Loadcase 1
– temperature of parent plate raised automatically from preheat at rate dependent on user defined inputs
– at suitable timestep, initial filler material elements activated as liquid at appropriate temperature, mechanically attached to parent plate
– with continued timesteps, additional filler material elements activated
•
Loadcase 2
– “composite” structure cools and shrinks, after all filler material elements defining weld path length have been activated
•
Loadcases 1 and 2 sequentially repeated
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Representation of Joint Configuration - 1
• 2-D Visualisation
– 14 weld passes laid on first side
– 13 weld passes laid on second side
• References include weld history sheets and
macrographs of cross section
• Created on CAD as an IGES file
• Input to MSC.Marc F.E. pre-processor
MSC.Mentat
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Representation of Joint Configuration - 2
• Symmetry condition imposed at bulkhead mid-thickness
• 2-D planar axi-symmetric mesh created
• Rotation of 2-D planar axi-symmetric mesh
• 3-D solid mesh representing a half length 4° segment of structure
created
• 9510 8-noded, iso-parametric, arbitrary hexahedral lower order
reduced integration MSC.Marc elements
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Heating Load-cases - Weld pass 2
• Pre-heating effect of welding torch prior to deposition provided by user
subroutine based tool
– rapid heating of parent plate
– diffusion of heat into parent plate
– stresses induced, as natural thermal expansion inhibited by cooler, stiffer surrounding
material
• Activation of filler material elements provided by user subroutine based tool
– filler material has no strength at elevated temperature
– will contract under expansion of parent plate
– stress fields induced by pre-heating overtaken by thermal stresses due to differential
cooling
•
Three increments selected to move the welding torch through one plane of newly
activated elements
–
maintains stability of the analysis
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Cooling Load-cases - Weld pass 13
• Film coefficient prescribed to all external surfaces
• Natural contraction of filler material again restrained by cooler, stiffer
surrounding material
• Residual plastic strains give rise to
– external distortion
– a system of self-equilibrating locked-in residual stresses
• Predicted circumferential and longitudinal elastic strains shown
opposite as output for comparison with experimental circumferential
and longitudinal elastic strains
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Experimental circumferential and longitudinal elastic strains
Position
Forward
1
2
3
Aft
2
Distance from Edge
of Stiffener [mm]
76
127
178
76
127
178
76
127
178
76
127
178
Internal
Long. [  ]
766
192
-139
808
83
-263
882
232
-93
587
-17
-270
Internal
Circ. [ ]
-1040
-762
-518
-728
-475
-219
-1352
-968
-669
-526
-350
-137
External
Long. [  ]
-163
271
432
-153
351
550
-221
268
423
38
423
489
External
Circ. [ ]
-674
-474
-249
-1190
-928
-594
-605
-396
-222
-1149
-879
-542
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Comparison of Results
1.50E-03
1.00E-03
Predicted Internal
Exp. Ext. (f w d)
0.00E+00
0
100
200
300
400
500
Exp. Ext. (af t)
Exp. Int. (f w d)
-5.00E-04
• Magnitude of predicted and experimental strains
in reasonable agreement
Exp. Int. (af t)
-1.00E-03
-1.50E-03
Distance f rom centre of bulkhead [mm]
1.50E-03
1.00E-03
Predicted External
5.00E-04
Mic rostrain
• Distribution along internal and external
surfaces of predicted and experimental strains in
reasonable agreement
Predicted External
5.00E-04
Micr os tra in
• Predicted circumferential and longitudinal elastic
strains extracted from Finite Element model
internal and external surfaces
Predicted Internal
Exp. Ext. (fw d)
0.00E+00
0
100
200
300
400
500
Exp. Ext. (aft)
Exp. Int. (fw d)
-5.00E-04
Exp. Int. (aft)
-1.00E-03
-1.50E-03
Distance from centre of bulkhead [mm]
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Predicted Results - Residual Stresses
• Circumferential residual stresses in cylindrical structure
analogous to longitudinal residual stresses in flat plate leads to tensile yield magnitude stresses in global
circumferential direction and balancing compression in
parent material, as confirmed opposite
• Longitudinal residual stresses - tensile yielded zone at toe
of weld, with compressive zone on outer surface of the
cylindrical structure
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Predicted Results - Through Thickness
• Extracted through thickness longitudinal residual
stress distribution at toe of the weld
– directly from distribution opposite
– using experimental method applied to predicted elastic
strains
600
400
Comp. 33 of Stress [MPa]
• Bending moment calculated
800
200
0
-200
-400
-600
-800
I.D.
Through Thickness, t
O.D.
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Conclusions and Recommendations
• Computations reveal that the previous experimental method (based on extrapolation of
surface strains) overestimates the bending moment and assumed through thickness
longitudinal residual stress distribution by 30%
• Correction factor has been applied to improve the accuracy of fatigue crack growth
predictions
• Numerical modelling methodology developed could be applied for parametric surveys of weld
induced residual stresses in submersible hulls and surface ships
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