Centre for Marine Technology John D McVee Technical Manager

Numerical Predictions of Residual Stresses in Welded Steel Submersible Hulls 1 G.P. Campsie, 2 A.C. Ramsay and 1 J.D. McVee Centre for Marine Technology QinetiQ Rosyth 1 QinetiQ Future Systems Technology Division, Centre for Marine Technology, Rosyth 2 MSC Software Ltd., Frimley

3 Contents 1Cracking in structures 2Model tests 3Numerical modelling 4Representation of joint configurations 5Heating and cooling 6Predicted results 7Comparison with experiment 8Conclusions

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

7 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

8 Large Fatigue Chamber Facility

9 Schematic of Large Fatigue Chamber

10 Internal View of Large Fatigue Chamber

11 Model Tests 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 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 Strains measured before and after welding 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 An extensive database now exists of experimentally determined residual stresses

15 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

16 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

17 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

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

20 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

21 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

22 Representation of Joint Configuration 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

23 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 noded, iso-parametric, arbitrary hexahedral lower order reduced integration MSC.Marc elements

24 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

25 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

26 Experimental circumferential and longitudinal elastic strains

27 Comparison of Results Predicted circumferential and longitudinal elastic strains extracted from Finite Element model internal and external surfaces Magnitude of predicted and experimental strains in reasonable agreement Distribution along internal and external surfaces of predicted and experimental strains in reasonable agreement

28 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

29 Predicted Results - Through Thickness Extracted through thickness longitudinal residual stress distribution at toe of the weld Bending moment calculated – directly from distribution opposite –using experimental method applied to predicted elastic strains

30 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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