Capacity Of Fillet Welded Joints Made Of Ultra High .

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Capacity of fillet welded jointsmade of ultra high strength steelBjörk T., Toivonen J., Nykänen TLaboratory of Fatigue and StrengthLappeenranta University of TechnologyP.O. Box 20. FIN-53851 Lappeenranta. FinlandRuukki is a metal expert you can rely on all the way, whenever you need metal based materials,components, systems or total solutions. We constantly develop our product range and operatingmodels to match your needs.1

Capacity of fillet welded joints made of ultra high strength steel Abstractmetals [2]. Kuhlman, Günter, Collin & al. have investigated the ultimate strength of filled welded joints madesteel grades in this category [3, 4, 5]. However, there isno generally accepted design rules or published resultsavailable concerning the static strength of steel gradesover 700 MPa, which in this paper is set to be the lowerlimit strength value for UHSS.The ultimate load-bearing capacity of typical fillet weldedjoints made of ultra-high-strength steel (UHSS) S960has been investigated. The aim of the work has beento assess the validity of current design rules for UHSSand possibly define new design criteria. Experimentaltesting and nonlinear finite element analysis (FEA) wasapplied to define the capacity of fillet welded joints. Jointgeometries and material properties were measuredfor both filler and base materials. In comparison withcurrent design rules, the experimental results showedthat the fillet welded joints had adequate load carryingcapacity presuming that weld quality is proper. Loadcarrying capacities and rupture modes in welds definedby FEA agreed quite well with experimental results. Theexperimental deformation capacities of some joints werefound to be critical, but the capacities can be improvedby use of undermatched filler metals. Heat input controlis essential in fabrica-tion of welded connections madeof UHSS and thus an additional failure criteria should becon-sidered in design codes due to the softening effectin HAZ.The main goal of this study is to investigate the validation of the current design rules for welded joints whichare fabricated from direct quenched (untempered) ultrahigh strength steel S960. These design rules concern theassessment of throat thickness and other dimensions forfilled welds. The validity will be proved by experimentaltesting and applying of nonlinear finite element analysis (FEA). The experimental test results are comparedwith capacities calculated according to Eurocode 3. Thepaper is based on the Master’s Thesis by Joe Toivonen,which deals the subject in more details [6]. 2.Material propertiesThe chemical composition and mechanical properties ofthe base and filler materials are seen in Table 1 and 2.If available, both nominal and measured values for theused materials are pre-sented. IndroductionUsing of high strength steels for weight critical construction is one way to save energy and to minimize thecarbon foot print in future. Weight critical structures aretypical of all moving constructions like carriers frame,lifting and hoisting devices but also many predominantlystatically loaded structures, where the weight can important due to transportation, assembly and maintenance.Lighter construction means generally thinner wall thicknesses and thus smaller welds meaning savings alsofor fabrication. Independent on the nature of the serviceload (static or fatigue) the joints must be designedalways for static loading.The strength of the welded joint depends on the regionof the weld as illustrated in Figure 1.Several reports are published concerning the staticstrength of welded joints fabricated from conventionalstructural steel with yield strength equal or less than 460MPa. For this strength cate-gory of steels the designof welded connections is properly guided in relevantdesign codes like Eurocode 3 [1]. Eurocode 3 part 1-12extends the design rules to cover the steel grades upto S700 and allows also the use of undermatched fillerFigure 1. Strengths of filler metals in terms of joint region [7] Chemical composition of S960Table 1CMnSiPSTinominal 0.0120.0040.032

Capacity of fillet welded joints made of ultra high strength steel Mechanical properties of materialsgroupmaterial codebase material(BM)S960nominalX96filler metal weld (WM)Table 212.6413.31ƒy MPaƒu MPaA5 %KV [J]9601000750 (-40 C)measured1014107612.5nominal9309801440 (-40 C)measured99012452670 (-30 C)1850 (-30 C)nominal470500 [2]measured580690nominal850890measured790915Figure 2. Hardness (HV5) – distributions in a butt weld of S960 & X96 (0 centreline of the weld) [8]Figure 3. Strength of S960 versus cooling time t8/5 [9] 3. Experimental testsFor matched filler metals the weakest strength appearsin HAZ but for undermatched electrodes in the welditself. According to the IIW recommendation the carbonequivalent can be calculated using the following formulaCEV C Mn6 Cr Mo V5 Ni Cu153.1 Joint types and parametersA typical drawing for test joint fabrication is illustrated inFigure 4. For each joint the heat input is controlled according welding process specification (WPS) preparedfor each joint. The lap joints were welded using a narrowgap between the parallel plates in order to avoid contactwith plates and thus eliminating the friction effect on thejoint behaviour.,which obtains in this case the value Cekv 0.51. A typical hardness distribution for a butt weld of S960 withX96 filler metal is seen in Figure 2.The used fully-mechanised welding process ensured tominimize the variation of throat thickness and penetration for each weld. In order to obtain theoretically correctthroat thicknesses for studied joints the penetration inthe root of the weld was adjusted as illustrated in Figure5. This is a delicate requirement consider the usedprocess (GMAW-process) and its susceptibility for incomplete fusion. The throat thicknesses were measuredusing manual calibre and laser distance transducer.The weldability of the steel is good and no preheating(welding in elevated temperature) is needed for the platethickness up to 8 mm. On the contrary, the material isprone for softening effect due the heat input and thusthe heat input by welding should be limited to minimumlevel. Opposite to conventional structural steels thereseems to be no upper limit for cooling rate and the bestmechanical properties for direct quenched steel S960are reached, if the t8/5-times could be less than 10 s asillustrated in Figure 3. By using GMAW process (MAG)for joining of the current plate thicknesses of 8 mm, theoptimal cooling rate is difficult to reach and small heatinput increase also the risk of fusion failure by welding.The studied joints are load carrying (L-. T-. LT- andX-series) and non-load carrying (X0-series) joints asillustrated in Tables 3-7. Each joint type includes several3

Capacity of fillet welded joints made of ultra high strength steeljoint andwelds underinvestigationgap arrangedby a thin threadbetween theplateshole for applying of loadhole for pin end fixingFigure 4. A typical test specimen (T-series)ano lack of fusion neither extrapenetration is allowed in rootFigure 5. A fillet weld of test specimens. transverse view and longitudinal distribution for a.thickness was defined my measuring the profile of thethroat thickness along the weld length as illustrated inFigure 5. The value from manual throat thickness calibrewas applied as reference dimension for fixing the profilehigh.parameters such as type of the filler metal, length l andthroat thickness a of the welds. When possible the startand end parts of the fillet weld were machined away inorder to keep the effective length unambiguous. In thecases this tooling was not applicable the effective throat4

Capacity of fillet welded joints made of ultra high strength steel Longitudinal loaded cruciform joint (L-series)IDfiller metalULTable 3URDLDRlalalalaL1 4.78111.924.41L12 –163.764.15––159.384.44 Transverse load carrying lap joint (T-series)IDfiller metalTable 4UDlalaT1X96100.33.01100.33.00T2 2X96100.04.64100.05.00100.03.00100.02.85T3T4 112.5198.55.0498.54.83T5 T1113.31100.04.27100.04.315

Capacity of fillet welded joints made of ultra high strength steel Transverse and longitudinal load carrying lap joint (LT-series)IDfillermetalUU LlalU RaTable 5DlaD LlalD 5.2873.04.8672.364.18LT3 44.3373.544.38100.155.1377.03.8871.544.64LT7 .89307.03.77308.63.96 Load carrying transverse cruciform joint (X-series)Table 6IDfiller metal1234lalalalaX1 1X9699.63.499.63.499.63.299.63.3X2 1X9686.383.8286.724.1185.203.8087.804.30X3 1X9669.684.4074.704.3770.704.5774.684.80X4 60.684.3362.144.6899.63.199.53.099.63.099.53.2X7 1X96X8 1X9671.424.3273.984.4471.524.4573.584.14X9 7666.564.2170.124.196

Capacity of fillet welded joints made of ultra high strength steel Non load carrying transverse cruciform joint (X0-series)Table 7IDfiller .099.55.33.2 Test set upA typical test set up with boundary conditions and applyof force is seen in Figure 6. Quasi-static loading was increased by displacement control until failure took place.For LT-series the displacements were measured separately for longitudinal and transverse welds. For L-seriesthe transducers were fixed in the middle and in the endof the longitudinal weld. The displacement transducers were fixed on the weld toes in order to define thedeformations in the weld only and to eliminate the displacements in the base plates. In all the cases also thetotal deformation of the specimen was measured by themovement of the hydraulic jack in the loading rig.displacementtransducer forlongitudinal weldfixing platesjoints under investigationapplied loaddisplacement transducerfor transverse weldpin end fixingFigure 6. Test set up for a LT-test7

Capacity of fillet welded joints made of ultra high strength steel3.3 Experimental test resultsA typical load-displacement-curve for a test specimen isseen in Figure 7. The total (δt) and plastic (δp) displacements are defined for the critical welds. Also the appliedload F versus total dis-placement on the specimen (δF)were recorded in tests. The results from the experimental tests are presented in Table 8.cessing was carried out with Femap 10.0.2 software. NXNastran 6 was used as a solver and calculations werebased on non-linear Newton-Raphson method [10].Four different load carrying joint types (L, T, LT andX) from experimental test series were analysed. Themeasured minor values of joint geometry were used formodelling the chosen test specimens: L15, L16, T1, LT2and X1 1. Symmetry was utilized to simplify the modelsand Hex-Mesh solid-elements with eight nodes wereused for meshing. A typical FE-model for weld geometryis plotted in Figure 8. 4. Finite element analysis4.1 Modelling of specimensThe nonlinear finite element analysis (FEA) was appliedto evaluate the joint behaviour and to compare theresults with experimental tests. Modelling and post-pro-Figure 7. Load –displacement -curve for transverse and longitudinal weldsof a test specimenFigure 8. A part of FEA-model for a cruciform L-jointultimatetrue stessFigure 9. The used true-stress-strain-curve for base and filler materialsFigure 11. Von Mises stresses in weldFigure 10. A typical load-displacement curve from FEA (test LT2)8

Capacity of fillet welded joints made of ultra high strength steel Experimental test resultsTable 8IDδp [mm]δt [mm]δF [mm]Fu [kN]place of fractureW weldBL base plateFLF fusion line failurenote!LOF lack of fusionL1 1L2L3L4L7L10L11L12 1L13L14L15L16T1T2 1bT3T4 1T7T10T11X1 1X2 1X3 1X4 1X5X6X9 30.20.214.010.60.814.21.8δF 874769872840WWWWWWWW and BLWWWWWWWWWWWW, FLFW, FLFW, FLFW, FLFWWW, FLFW, FLFW, 14954560WWWWWHAZLT1LT2LT3 1.21.21.90.49LOF

Capacity of fillet welded joints made of ultra high strength steel Deformation and load carrying capacities from FEATable 9IDδp [mm]δt [mm]Fu [kN]L150,81,01270L161,21,31170T10,70,81020LT2L:1,1 T:0,3L:1,2 T:0,42400X1 11,41,5900 5.DiscussionThe capacity of the fillet weld can de calculated according to Eurocode 3 using the stress component in thecritical plane of weld throat thickness as illustrated inFigure 12.Using von Mises stress criteria the theoretical strengthof the fillet weld joint can be calculatedWhere the σ and τ are stress components according toFigure 12, ƒu is tensile strength of the base material andβw is the ratio for tensile strengths of weld and basematerial and γM2 safety factor 1.25. Eurocode 3, part1 – 12 allows the use of undermatched filler materialand this is assumed to valid also for the current steelS960. In consequence the capacity of the joint can becalculated by replacing the tensile strength of the basematerial by tensile strength of the filler metal feu. Therequired joint load bearing capacities F can be definedfor joints of L-series,Figure 12. Stress components in the plane of throat thicknessFor non load carrying of X0- series where the softeningcan take place next the weldwhere ƒu is the tensile strength either of the base material or the softened HAZ whereas t and b are the platethickness and width, respectively. Comparison betweenexperimental and theoretical load bearing capacities ofthe joints are seen in Figure 13. The theoretical capacities of joints are calculated using measured tensilestrength (engineering values) either of filler metal orbase material, depending on the failure place. Thenominal ultimate strength is nominal value of the basematerial except in the case of undermacthed filler metal,where the nominal strength is defined according Eurode3, part 1 – 12. The black columns relate the safety levelavailable by designers and the grey columns refer to thereal safety level of the applied procedure. The safetylevels are depending on the joint type: the experimentaltest results of T-series prove the highest extra strengthwhereas the tested joint capacities of L- and X-seriesmatch quite well with theoretical evaluationand for joint of T and X-seriesand for joint of LT-series10

Capacity of fillet welded joints made of ultra high strength steelTensile test / measured engineering strength of critical materialTensile test / nominal strength of base materialFigure 13. Comparison of tests results with theoretical resultsFigure 14. Fracture plane including lack of fusionFigure 15. Extra penetration in rootside of weld increase the load carryingcapacity of the jointIn the joints where the load carrying capacities fromexperimental tests are below the theoretically definedvalues appeared locally lack of fusion in welds or failurein fusion line. The previous one can be find out bymeans of visual inspection of welds before or after thetest but the later one after test as illustrated in Figure 14.longitudinal direction of weld (L and LT-series) agreeingwith the theoretical assump-tion. However, in the jointssubjected to transverse loading (T, LT and X-series)rupture followed either the fusion line of the weld or failure plane formed an angle α 20 degrees in the weld.The experimental findings agreed well with results fromnonlinear FEA, as illustrated in Figure 16, where alsothe distribution of the plastic strain in weld are plotted forcomparison.In the case the load bearing capacity of the joint exceeded the theoretical value considerable, one reason is theextra penetration in the root of the weld found in somejoints as seen in Figure 15.The load carrying capacities depend on length-throatthickness-ratio (l/a) of the weld in longi-tudinal directionloaded joint are seen in Figure 17. It can be noticedthat the capacity will be de-crease if l/a-ratio exceedthe value of 50, which can thus set to be allowable limitwithout strength reduction in calculations. More tests willbe needed in order to define the reduction factor for l/a 50.The more accurate comparison between tested andcalculated results can be carried out after the areas offailure surfaces are measured. This procedure is quitearduous but it will consider the effects involving in lackof fusion and extra penetration.Typically the failure took place in the angle of 45degrees in welds subjected to pure shear stresses in11

Capacity of fillet welded joints made of ultra high strength steelFigure 16. Fracture planes from experimental tests with comparison of FEA-results (T-series)Figure 17. Effect of the l/a-ratio on the joint strengthX02, a 4.5X010, a 5X05, a 3 3X04, a 6Figure 18. Failure modes in non load carrying joints (X0-series)12

Capacity of fillet welded joints made of ultra high strength steelThe failure and capacity of non load carrying joints(X0-series) seems to be depend on the heat input dueto welding. If the heat input is low, the softening is localand it has no effect on the capacity of the joint and thefailure take place outside the joint (typically tilting in angle of 30 degrees as illustrated in Figure 18). If the heatinput will be increased the softened width/plate thickness-ratio increases and the critical ratio ( 0.2) can beexceeded and consequently failure takes place in HAZnext to weld. This phenomena is not necessary to consider when using the conventional steels but it typicallyaccompanied welded joints made of aluminium alloy.For the X0-joint with 8 mm plate thickness the limit heatinput value seems to 4.5 mm in terms of throat thickness. The failure took place in HAZ in joints with 5 mmand 6 mm throat thickness but in base plate outside theHAZ, if the throat thickness was less than 4.5 mm or if itwas larger but executed by multi pass welding.0.5 mm in transverse direction and 1.0 mm in longitudinal directions are typical requirements. However, someoverall plasticity in the weld occured which is an essential criterien concerning the basic assumptions for theapplied design approach of welded joints. In generallythe longitudinal fillet welds have a little bit more plasticdeformation capacity compared to transversely loadedwelds. Using an undermatched filler metals betterplastic deforma-tion capacity can be reache

Capacity of fillet welded joints made of ultra high strength steel Transverse and longitudinal load carrying lap joint (LT-series) Table 5 ID filler metal U U_L U_R D D_L D_R l a l a l a l a l a l a LT1 X96 100.2

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