treatments of structural welds using fitnet fitness-for-service assessment procedure

106 TREATMENTS OF STRUCTURAL WELDS USING FITNET FITNESS-FOR-SERVICE ASSESSMENT PROCEDURE

TREATMENTS OF STRUCTURAL WELDSUSING FITNET FITNESS-FOR-SERVICE

ASSESSMENT PROCEDURE

Welding in the World, Vol. 51, n° 5/6, 2007

Doc. IIW-1768-06 (ex-doc. X-1604-06) recommendedfor publication by Commission X “Structural perfor-mances of welded joints – Fracture avoidance”.

Document presented at the International Conference onFitness-for-Service, FITNET 2006, 17-19 May 2006,Amsterdam.

M. Koçak 1 2 E. Seib 2* A. Motarjemi3

1 European Fitness-for-Service Network – FITNET, Coordinator2 GKSS Research Center, Institute for Materials Research (Germany)

3 TWI Ltd (United Kingdom)

ABSTRACT

Recent developments of the advanced welding processes such as laser beam welding (LBW), solid state friction stirwelding (FSW) and hybrid welding, numbers of advanced structures are being designed and constructed in indus-tries such as aerospace, power generation, oil and gas transmission and transportation. Development of new struc-tural aluminum and magnesium alloys as well as high strength steels provide further possibilities for the weldedstructures in similar and dissimilar (material-mix) configurations. Consequently, there is an increasing demand for“Fitness-for-Service” (FFS) assessment of those advanced welded structures by considering the specific features ofthese weld joints (such as narrow weld width, high strength mis-match, etc.). In year 1999, Structural IntegrityAssessment Procedure SINTAP has been developed for analysis of flaws to avoid fracture within the EuropeanCommission funded project SINTAP. Recently, the European Community funded project FITNET in the form of aThematic Network (TN) organisation has completed to review the existing FFS procedures and developed an updated,unified and verified European FITNET FFS Procedure to cover structural integrity analysis to avoid failures due tofracture, fatigue, creep and corrosion. The fracture Module (Section 6) of this new FITNET FFS Procedure, pre-pared and presented at this conference, FITNET 2006, in three volumes has adopted the SINTAP approach forassessing of the welded structures. This paper describes the FITNET FFS weld assessment route (Option 2 andOption 3) and also aims to demonstrate suitability of weld joint assessment route in prediction of the critical condi-tions of various advanced welded joints containing flaw. The welded specimens used in this work cover conventionalmulti-pass welded Inconel-718 turbine blade (T-joint), center cracked wide plates of electron beam welded 13 % Crsupermartensitic stainless steel, laser beam welded shipbuilding C-Mn steel and aluminum alloy as well as weldedpipe. The results are showing that the weld strength mismatch analysis option of the FITNET FFS is conservativeand degree of conservatism is similar to the analysis options for the homogeneous materials. This provides confi-dence in the use of the FITNET FFS procedure for assessing of the structural significance of flaws in welded struc-tures.

IIW-Thesaurus keywords: Aluminium alloys; Carbon manganese steels; Circumferential welds; Comparisons; EBwelding; Evaluation; Duplex stainless steels; Laser welding; Light metals; Photon beam welding; Prediction; Practicalinvestigations; Mismatch; Radiation welding; Reference lists; Stainless steels; Steels.

* Formerly at the GKSS Research Center, currently at theCONTINENTAL AG, Hannover, Germany.

TREATMENTS OF STRUCTURAL WELDS USING FITNET FITNESS-FOR-SERVICE ASSESSMENT PROCEDURE 107

1 INTRODUCTION

Structural integrity procedures [1-8] are the techniquesused to demonstrate the fitness-for-service of engineer-ing components (with and without welds) transmittingloads. They are of value at the design stage to provideassurance for new constructions, particularly where thesemay be innovative in the choice of materials or fabrica-tion methods, and at the operation stage to provideassurance throughout the life of the structure. They haveimportant implications for economic development inassuring the quality of engineered goods and services.Used correctly, they can increase efficiency by prevent-ing over design and unnecessary inspection and repairbut can also provide a balance between economy andconcern for individual safety and environmental protec-tion, where this is affected by component failure.

Welded engineering structures may contain imperfec-tions (pores, flaws, defects or cracks) during the fabri-cation stage or during the service life. The structural sig-nificance of such imperfections, particularly crack-likeflaws needs to be assessed to prevent failure of thewelded component during service. Therefore, FITNETFitness-for-Service (FFS) procedure [9] containing ana-lytical expressions is currently being developed to assess(primarily to provide conservative estimation of the criti-cal condition) the structural significance of the flaws ordamage in metallic structures with and without welds.Clear guidelines for purposes of design, fabricationsupport, flaw assessment of in-service componentsand failure analysis within the FITNET FFS technologyare given to avoid failures in conventional and advancedstructures. The overall structure and description ofFracture Module of the FITNET FFS procedure are givenin references [10-12]. The fracture Module provides aspecial analysis option (identical to the SINTAP proce-dure) to deal with the welded structures by consideringthe special features of the weld joints. Advanced joiningtechnologies such as laser and electron beam, hybrid,friction stir welding produce welds with considerably dif-ferent yield strengths compared to the base metal. Thisstrength mis-match in tensile properties effects the plas-tic deformation behaviour of the component with defect,and hence the crack driving force (CTOD or J).

Principally, the flaw analysis routes of homogeneous com-ponent can be applied for the welded structures, if thetensile properties of the weakest material are used; forexample base metal properties of the overmatched laserwelded C-Mn steels. However, such a simplified approachleads to an unduly conservative result, and therefore aFitness-for-Service analysis route specific to the strengthmis-matched welds is needed to reduce excessive con-servatism. For example, number of existing flaw assess-ment methodologies have been reviewed and comparedwith experimental data in reference [13]. It was reportedthat a significant overprediction of failure strain occurredby using the existing FFS assessment routes in signifi-cantly strength undermatched high strength steel weldsafter the post weld heat treatment (PWHT).

The SINTAP procedure [6-7] was the first structuralintegrity assessment method, which has introduced a

comprehensive weld joint assessment route with con-sideration of weld strength mis-match effect. If thestrength mis-match ratio (M; defined by the ratio of theyield strength of the weld metal (σYW) to that of the basemetal (σYB); M = σYW/σYB) between the weld and basemetal yield strengths is larger than 10 %, the FITNETFFS procedure recommends to use the “Mis-matchroute” – Option 2 to take account of beneficial (in thecase of overmatching) or detrimental (in the case ofundermatching) effects of the weld metal strength onthe behaviour of cracked weld metal. Various investi-gators [14-15] have studied the effect of weld strengthmis-match on the applied fracture driving force devel-oped in a structural weld joint subjected to an appliedload. Two international mis-match conferences, Mis-match 93 and Mis-match 96 [14-15] have provided forumfor the development this analysis route [16]. During thedevelopment of this route and also after the completionof the SINTAP Procedure, numbers of validation workof the mis-match analysis level have been conductedon various advanced welded steel and aluminum jointsincluding thin-walled LBW and FSW welded wideplates [17-22]. However, there is still an increasing needfor methods of assessing the structural integrity ofadvanced welded high strength steels, aluminium alloys,dissimilar joints used in energy and transportation indus-tries. The FITNET FFS Fracture Module adopts the spe-cific assessment routes (Options 2 and 3) to cover weldstrength mis-matched components. Hence, this paperaims to give firstly, the details of the FITNET FFS pro-cedure for treatment of flaws in structural welds withrespect to fracture avoidance and secondly, it providesnumbers of verification cases for this route.

2 THE FITNET FFS PROCEDURE:WELD STRENGTH MIS-MATCH OPTION

The FITNET FFS Fracture Module provides differentanalysis options (see Table 1) depending on the extentand quality of the input data available. Options 0 and 1can be used with limited material data, such as onlyyield and tensile strength, whereas Option 3 requiresfull stress-strain data. The Option 1 analysis is devel-oped for homogeneous components, and Option 2 isthe extension of Option 1 for analysis of welded struc-tures with strength mis-matched welds by incorporatingthe strength mis-match effect. The analysis by Option 3includes both homogeneous and welded structures usingfull stress-strain curves of the base and weld metals.

When the strength mis-match in the welded componentis small, then it should be expected that the strengthmis-match effect is minimal, and hence the use of thehomogenous routes (Options 1 and 3) by consideringthe tensile properties of the weaker material in the analy-sis would yield satisfactory results. It should be notedthat the mis-match limit load (FYM) is the most importantinput parameter to the FITNET FFS Analysis for strengthmis-matched welded components. The primary sourceof information for obtaining the limit load solutions is theFITNET FFS Procedure Annex B [9]. The solutions


consider both homogeneous and strength mis-matchedcomponents. The strength mis-matched corrected limitload (FYM) is described as a function of:– the mis-match factor (M),– the ratio of uncracked ligament length (W – a) to theweld width (H), and– the welded component geometry/loading type.

Furthermore, strain hardening behaviours of the weldand base materials are equally significant with respectto mis-match effect on the crack driving force and henceused as input data, Figure 2. The FITNET FFS proce-dure Fracture Module for welds can be interpreted interms of either the Failure Assessment Diagram (FAD)or the Crack Driving Force (CDF) routes. The principlesof both routes are shown schematically in Figure 1. Bothroutes give identical results, as long as the same inputdata are used and since the failure assessment linesare based on the same plasticity correction function. Inthe CDF route, the material resistance against the crackgrowth (R-curve) is compared with the crack tip loadingin the wide plate, in terms of J or CTOD.

Figures 2 and 3 show the basic flow charts of Options 2and 3 with respect to the generation of the material dataand Figure 4 shows the needed data for these Options.

3 FITNET FFS FRACTURE MODULE:BASIC EQUATIONS

The FITNET FFS Procedure Fracture Module offers aCDF and a FAD routes, as mentioned above, to predictcritical condition of a component with postulated (in case

of design) or a real flaw (in case of defective in-servicecomponent). In the FAD route, a failure line is con-structed by normalizing the crack tip loading (or appliedstress intensity factor) with respect to the material’s frac-ture toughness. The assessment of the component isthen based on the relative location of an assessmentpoint with respect to this failure line. The basic equationof the FITNET FAD route is:

Kr = ƒ (Lr) (1)

The functions ƒ (Lr) at various assessment levels areidentical for both CDF and FAD routes.

Table 1 – Analysis options of the FITNET FFS Fracture Module

ANALYSIS OPTIONS DATA NEEDED WHEN TO USE

BASIC OPTION

OPTION 0 Basic YS or PS When no other tensile data available

STANDARD OPTIONS

OPTION 1 Standard YS or PF; UTS – For quick result– For M < 10 %

OPTION 2 Mis-match YS or PS, UTS, M, Limit loads – Allows for mis-match in YSs of weld and base metals

OPTION 3 Stress-strain Full σ-ε curves– More accurate and less conservative than Options 1and 2– Weld mis-match option included

ADVANCED OPTIONS

OPTION 4 J-Integral Analysis Needs numerical analysisof cracked body

OPTION 5 Constraint Analysis

Estimates of fracture toughness Allows for loss of constraint in thin sectionsfor crack tip constraint conditions or predominantly tensile loadingsrelevant to those of crackedstructure. Needs numericalcracked body analysis

YS: Yield Strength, FS: Proof Strength,UTS: Ultimate Tensile StrengthM: Weld Strength Mis-match Factor(M = YS of weld/YS of base metal)

Figure 1 – Principles of FAD (for crack initiation)and CDF (for ductile tearing) routes used

in FITNET FFS Fracture Module (Section 6)


Figure 2 – Flow chart for Option 2 (Mis-match) Figure 3 – Flow chart for Option 3

Figure 4 – Flow chart for the generation of the input parameters (used in Options 2 and 3) dependingon the base and weld material tensile properties


In the CDF route, the material resistance against thecrack growth (R-curve) is compared with the crack tiploading in the component. The FITNET-CDF expres-sions in terms of CTOD δ are:

δ = δe [ ƒ (Lr )] – 2 (2)

with

δe = K 2 / E’σY (3)

or with elastic parts of the J-integral, Je, and CTOD, δe:

Je =K 2

, δe = K 2

E’ mσY E’

K denotes the elastic stress intensity factor, Lr = F / FY

is the ratio of externally applied load, F, and the yieldload of the cracked component, FY, which is generallya function of the material’s yield strength, σY, and com-ponent/weld geometry. The parameter m (m = 1 forplane stress and m = 2 for plane strain) is considered aconstraint parameter, E’ = E for plane stress andE’ = E / (1 – ν2) for plane strain (E = Young’s modulus,ν = Poisson’s ratio) and ƒ (Lr) functions are as definedfor the FAD route.

4 ANALYSIS OPTION 1: STANDARD

For materials with a Lüders plateau, the ƒ (Lr ) functionis given by:

ƒ (Lr ) = [1 + 1 Lr2 ]1/2 for 0 ≤ Lr < 1 (4)

2

ƒ (Lr = 1) = λ +1 – 1/2

for Lr = 1 (5)[ 2λ ]where

λ = 1 + EΔε

(6)σY

and

ƒ (Lr ) = ƒ (Lr = 1) × Lr(N – 1) / 2 N for 1 ≤ Lr < Lr

max (7)

If the Δε is unknown it can be conservatively estimatedfrom an empirical correlation:

Δε = 0.0375 1 – σy (8)[ 1000 ]

The strain-hardening exponent is estimated from yieldto tensile strength data on a conservative, empiricalbasis:

N = 0.3 1 – σy (9)[ σu

]The plastic collapse limit is defined as:

Lrmax =

1 σY + σu (10)2 [ σY

]b) For materials exhibiting continuous yielding, ƒ (Lr ) isgiven by:

ƒ (Lr ) = 1 + 1

Lr– 1/2 × [0.3 + 0.7 exp (– μM Lr

6)][ 2 ]

for 0 ≤ Lr ≤ 1

with

0.001E

(11)μ = min [ σY[ 0.6

and

ƒ (Lr ) = ƒ (Lr = 1) × Lr(N – 1)/2N for 1 ≤ Lr < Lr

max (12)

N and Lrmax are as in Eqs. (9) and (10).

5 ANALYSIS OPTION 2: MIS-MATCH

5.1 Case 1: When both base and weld metalexhibit continuous yielding (no Lüders strain)

For a weld strength mis-matched configuration the yieldload also depends on the yield strength of the weldmaterial and the parameter ψ = (W – a) / H whichdefines the ratio of the uncracked ligament length, W – a,and the weld width, 2H. The plasticity correction func-tion, ƒ (Lr ), is defined in the mis-match option of theFITNET FFS fracture Module (Section 6) as follows:

ƒ (Lr ) = 1 + 1

Lr– 1/2 × [0.3 + 0.7 exp (– μM Lr

6)] (13)[ 2 ]for 0 ≤ Lr ≤ 1

ƒ (Lr ) = ƒ (Lr = 1) × Lr(NM – 1) / 2NM for 1 < Lr ≤ Lr

max (14)

where

μM = M – 1

< 0.6(FYM / FYB – 1) / μW + (M – FYM / FYB) / μB

else μM = 0.6 (15)

μB = 0.001 EB < 0.6 else μB = 0.6 (16)σYB

μW = 0.001 EW < 0.6 else μW = 0.6 (17)σYW

Lrmax =

11 +

0.3(18)

2 ( 0.3 – NM)

where

M = σYW / σYB is the mis-match factor defining the ratioof weld (σYW) to base (σYB) metal yield strengths. EB andEW are the Young’s moduli of base and weld materials,respectively. Strain hardening exponents for mis-match,NM, base, NB, and weld materials, NW, are defined asfollows:

NM = M – 1

(19)(FYM / FYB – 1) / NW + (M – FYM / FYB) / NB

NB = 0.3 1 – σYB (20)( σUTS,B

)NW = 0.3 1 –

σYW (21)( σUTS,W)

σUTS denotes the ultimate tensile strengths of base (sub-script B) and weld (subscript W) materials. FYM and FYB

are the yield load solutions for the mis-match and basematerial plates, respectively.


5.2 Case 2: When both base and weld metalexhibit discontinuous yielding (Lüders strain)

ƒ (Lr ) = [1 + 0.5 Lr2 ]–1/2 0 ≤ Lr < 1 (22)

ƒ (Lr ) = ƒ (Lr = 1) Lr(NM – 1) / 2 NM Lr = 1 (23){ƒ (Lr ) = [λM + 0.5 / λM ]–1/2 1 ≤ Lr ≤ Lr

max (24)

ƒ (Lr ) = 0 Lr > Lrmax (25)

λM =(FY

M / FYB – 1) λW + (M – FY

M / FYB ) λB

(26)(M – 1)

N M = (M – 1)

(27)(FY

M / FYB – 1) / N W + (M – FY

M / FYB ) / N B

Lrmax = min

0.5 (1 + σmW / σY

W ) (FYM / FY

W )(28){0.5 (1 + σm

B / σYB ) (FY

M / FY )

μW = 0.001 E W / σYW (29)

μB = 0.001 E B / σYB (30)

N W = 0.3 (1 – σYW / σu

W ) (31)

N B = 0.3 (1 – σYB / σu

B ) (32)

ΔεW = 0.0375 (1 – σYW / 1000) (33)

λW = 1 + E W ΔεW / σYW (34)

Δε B = 0.0375 (1 – σYB / 1000) (35)

λB = 1 + EΔε B / σYB (36)

6 ANALYSIS OPTION 3: STRESS-STRAIN

Failure lines at this level are obtained from full knowl-edge of the tensile characterization of the componentmaterial, that is, the full true stress-strain curve. Thedefinition of failure is given by:

ƒ (Lr ) = Eεref +

Lr3 σY –1/2

=Eεref +

Lr2 σref –1/2

(37)[Lr σY 2 Eεref] [ σref 2 Eεref

]for Lr < Lr

max

and

ƒ (Lr ) = 0 for Lr > Lrmax (38)

7 APPLICATION OF FITNET FFSFRACTURE MODULE TO WELDEDSPECIMENS AND COMPONENTS

In the following, five welded configurations are assessedby using Fracture Module of the FITNET FFS Procedure.The cases are also covering advanced weldments suchas laser beam and friction stir welds as well as thin-walled component-like specimens. For each case, thepredictions are compared with the experimental datagenerated at the GKSS and TWI within various “WeldAssessment” projects to investigate the behaviours ofthe strength mis-matched welds and validation of theSINTAP/FITNET fracture assessment procedures.

7.1 CASE I: Welded thick section Inconel-718turbine blade

This case covers a large welded Inconel 718 impeller,which has been analysed by using FITNET FFSProcedure at design stage with two assumed crack loca-tions namely, the weld root (LOP) and the weld toe (onthe heavy gauge attachment plate) Figure 5. Conven-tional manufacturing route was to fabricate this compo-nent via machining from single material, which involvedtime consuming and costly milling process for this highstrength and specially manufactured material. Analysisresults showed that fabrication using welding technologyis possible and assumed flaw sizes at various locationsare smaller than the critical crack size under serviceconditions.

Since full stress-strain curve data of both base and weldmaterials were available, the impeller was assessedbased on the FITNET FFS Fracture Module AnalysisOption 3. The yielding behaviour of the weld and basematerials were continuous yielding and limit load solu-tion of the component was determined using finite ele-ment analysis. The output of the assessment in form ofa (FAD) plot is shown in Figure 6. Figure 6 a) is theresult for rotation speeds of 360 m/s and 450 m/s basedon the both Von-Mises / Maximum Principal Stress cri-terion for the weld root crack (LOP), whereas Figure 6 b)is for the same stress criteria / rotation speeds but for theweld toe crack. Both FADs are showing the maximum(critical) crack sizes (2 a max) predicted for both loading(rotation speed) conditions of the impeller blade andused stress criteria. Summary of the defect assessmentresults are listed in Table 2.

The lack of penetration (LOP) and weld toe at attachmentwere assumed to contain flaws (white lines) for FFS analysisat design stage of this component.

Figure 5 – Asymmetrically welded Inconel 718 impeller

CrackPredicted maximum crack sizes (mm)

position U = 360 m/s U = 450 m/s

Von-Mises Max. Prin. Von-Mises Max. Prin.

Weld root 35.8 38.5 24.2 33.5

Weld toe 16.8 20.2 9.6 16.0

Table 2 – Predicted critical crack sizes (2amax)for weld root and weld toe of the welded impeller

for two rotation speeds


This welded impeller case is an example for the use ofthe Fitness-for-Service Procedure FITNET at the designstage of an advanced welded structure using postulatedcrack/fracture locations. Such an analysis provide engi-neering critical assessment for a new product designand cost-effective fabrication route by taking account ofthe typical fabrication flaw, imperfection or possiblecrack-like defect which may occur during the service lifeof the component.

7.2 CASE II: Electron beam welded 13 % Crsupermartensitic stainless steel wide plate

This case includes 13 % Cr supermartensitic stainlesssteel Middle Tension M(T) panels containing throughthickness centre cracks under tension [17, 18, 23](Table 3). These high strength stainless steels are anadequate substitute material for the conventional car-bon and duplex stainless steel pipes for mild corrosiveenvironments in the oil and gas industries. These steelsbecame interesting for use as flowline materials in off-shore applications, due to high strength at elevated tem-perature, substantial corrosion resistance, good fracturetoughness and low cost compared to duplex stainlesssteels. Some modified Super 13 % Cr steels have beenmanufactured and proposed for flow-line applications inCO2/H2S environment. Some have already beeninstalled as the sub-sea flow line in the North Sea.Recent development activities in the area of steel gradesand consumables are aimed at the optimization of com-positions and processing routes to meet the require-ments for weld joint quality. By development of these

new steels and respective welding technologies (includ-ing electron beam, SAW, hybrid etc.) structural integrityanalysis of the longitudinal and girth welded pipes, isessential and a challenging task. Depending on thewelding process, filler wire used, the deformation (dur-ing reeling process) and failure behaviours in service ofthe welded pipes could be different due to the changesin strength mis-match levels.

The experimental results in the form of load versusCMOD plots, for the three panels (BM, WM and HAZnotched) are shown in Figure 7. It can be seen that themaximum load values obtained from the experimentsare about 2 200 kN, for the panels with cracks at theWM and HAZ (the latter is not considered in this paper),and 1 970 kN for the BM notched one.

Hence, the use of FITNET FFS Procedure FractureModule can provide good predictions with increasingaccuracy of the input parameters following by applyinghigher analysis options as the procedure designed todo so. Figure 8 shows a decrease of conservatism with

b) Postulated crack at the attachment’s weld toe

a) Crack at the weld root (LOP)

Figure 6 – FITNET FFS Fracture Modulepredictions using Option 3 for the welded

Inconel 718

Geometry/ Material** / FITNETCrack Weld strength ANALYSISlocation mis-match OPTION

M(T)* Tension 13 % Cr 2.5 % MoBase Metal Supermartensitic OPTION 1:

stainless steel plates STANDARDt = 12 mm

M(T) * Tension YS BM = 615 MPaWeld Metal YSWM = 835 MPa OPTION 2:

(EBW) M =1.36 MIS-MATCHOvermatching

M(T)*: 2W = 200 mm, a/W = 0.1, t = 12 mm, 2H = 6 mm(weld width).** Material Behaviour: BM and WM Continuous.

Table 3 – Detailed information of the 13 % Crstainless steel wide plates used for validation

of the FITNET FFS Procedure

Figure 7 – Load vs. CMOD test results of the basemetal and welded supermartensitic M(T) panels

with 20 mm centre crack, testedat – 40 °C temperature


increasing analysis options (called as “Levels” in SINTAPprocedure).

In this analysis, the fracture toughness estimation wasmade using SINTAP procedure. The FITNET FFSProcedure, however, provides updated correlationsbetween Charpy and toughness as described in thepaper “Charpy Correlations and the Master CurveApproach in the FITNET Procedure”, by Lucon andWallin et al. [24] and following section gives short guid-ance for derivation of the fracture toughness data (ifactual toughness tests results are not available or can-not be produced) needed such an analysis on ductilematerial systems.

Estimation of the crack resistance (J-R) curve

A conservative estimate of the J-R curve based on theCharpy upper shelf energy (USE) [25] is expressed bythe following relationship:

J = J1 mm ⋅ Δa m (39)

where

J1 mm is the J-integral corresponding to 1 mm of ductilecrack extension (Δa) and

m is the exponent of the power law.

The two parameters which are needed for Eq. (39) aregiven by:

J1 mm = 0.53 ⋅ USE 1.28 ⋅ exp – T – 20

(40)( 400 )m = 0.133 ⋅ USE 0.256 ⋅ exp –

T – 20–

σy + 0.03 (41)( 2000 ) 4664

7.3 CASE III: CO2-laser beam welded C-Mnsteel wide plates

This case covers defect assessment of a laser weldedC-Mn CORUS steel (Lloyd’s D) wide plate, applicable inthe shipbuilding industry. The laser welding of 12 mmthick wide plate was carried out at the FORCE Institute-Denmark using 17 kW CO2 laser within the completedEuropean Community funded ASPOW project. The wideplates (Table 4) had the following dimensions: thickness,t = 12 mm and width, 2W = 200 mm. Small through thick-ness fatigue cracks (a / W = 0.1) were produced at thetip of short machined notches in tension. The fatiguecracks were located in the fusion zone (FZ) and HAZregions and tests were conducted at room temperature(the HAZ case is not studied in this paper). In thesetests, local CTOD (with four CTOD δ5 clip-on-gauges atthe original fatigue crack tip over a gauge length of5 mm), CMOD and overall elongation (gauge length of300 mm) as a function of the applied load were mea-sured.

Application of the FITNET FFS Fracture Module-FADRoute to this case by using the analysis options 2 and3 (both mis-match option) is shown in Figure 9. Theinput data for the FFS analysis of the highly over-matched (88 % OM) welded panels are shown in Table 4The straight lines on the plots in Figure 9, are the load-ing path obtained after conducting sensitivity analysisfor the applied load.

Moreover, numbers shown on the plots in “kN” are thepredicted maximum load carrying capacity of the laserwelded cracked plates by the FITNET FFS Procedure.Comparison of the predicted maximum load carryingcapacity values of the welded panels with the experi-ments (Fmax = 792 kN), is shown in Figure 10. This com-parison shows that FITNET predictions are conserva-tive for highly overmatched C-Mn steel laser welds. Thedegree of conservatism was decreased from 16 % forOption 2 (Level II of SINTAP procedure) – Mis-matchto 9 % for Option 3-Mis-match. A reduction in conser-vatism with an increase of analysis option is obviouslyapplies to advanced welded steel structures such asanalyzed in this case.

Note: “LEVEL” refers to “OPTION” in FITNET FFS Procedure.

Figure 8 – Comparison between the FITNET FFSpredictions and experimental results

a) For BM

b) For WM panel

Geometry/ Material / FITNETCrack Weld strength ANALYSISlocation mis-match OPTION

LB welded C-MnM(T) * Ferritic steel plates OPTION 2:

Tension t = 12 mm MIS-MATCHWeld YS BM = 284 MPaMetal YSWM = 535 MPa OPTION 3:(LBW) M = 1.88 MIS-MATCH

Overmatching

M(T)*: 2W = 200 mm, a/W = 0.1, t = 12 mm, 2H = 3 mm(weld width).Material Behaviour: BM and WM Discontinuous.

Table 4 – Detailed information of the laser beamwelded steel wide plates used for validation

of the FITNET FFS Procedure


7.4 CASE IV: CO2-laser beam weldedthin-walled aluminium alloy wide platesunder tension

Contrary to the Cases II and III, in which the highlystrength overmatched steel welds are covered, this sec-tion aims to demonstrate the applicability of FITNETFFS Fracture Module to the highly strength under-matched and thin-walled aluminium alloy welds. Thewelds considered in this section are unique for theadvanced welded aerospace structures called “integralstructures”.

The advanced welding technologies such as laser beam(LBW) and friction stir welding (FSW) together with newweldable 6xxx series Al-alloys have recently providednew options for welded light-weight structures. Theseweld joints exhibit considerable strength undermatchingand hence advanced flaw assessment procedure FIT-NET FFS is being considered to predict the critical con-ditions of these highly undermatched and thin-walledpanels. This case therefore is discussed in detail in thissection.

The application of the FITNET FFS procedure to predictthe failure load of the LBW panel requires materials aswell as geometry related input parameters, as explainedin the description of the procedure and previous cases.

The material parameters are tensile (yield and ultimate)strengths of both weld and base materials and the frac-ture resistance property (R-curve) of the region wherethe crack is located. Geometry related input parametersare the Mode I elastic stress intensity factor, KI, and themis-match limit load, FYM, of the component to beassessed. The material properties were obtained exper-imentally, whereas the geometry related input parame-ters for an M(T) panel are available in closed form solu-tions in the SINTAP compendium [6] as currentlyupdated for the development of the FITNET FFSFracture Module (Annex A and Annex B).

7.4.1 Material related input data

The macrosection of the weld joint in 2.6 mm thick Al-alloy 6013 in T6 heat treatment condition is shown inFigure 11.

Tensile properties of narrow weld area were determinedfrom testing of micro-flat tensile specimens (0.5 mmthick, 1.5 mm wide) as shown in Figure 12. These spec-

a) Analysis Option 2 – Mis-match

b) Analysis Option 3 – Mis-match

Figure 9 – FITNET FFS Fracture Module – FADpredictions for laser beam welded 12 mm

thick M(T) plate

Figure 10 – Comparison of the FITNET FFSFracture Module predictions

with the experimental load-carrying capacityof a laser welded steel wide plate

Figure 11 – The macrosection of the CO2-laserbeam welded 2.6 mm thick Al-alloy 6013


imens (developed at the GKSS) are designed to obtain“intrinsic” properties of small zones, which need to beused as input parameters. The engineering stress-straincurves together with geometrical dimensions of thesespecimens are shown in Figure 13.

As depicted by the microtensile test technique, the weldmaterial shows significantly low strength compared tothe base metal. The strength mis-match factor, M,defined as the ratio of weld metal yield strength, σYW, tothat of the base metal, σYB, is for this particular caseM = σYW / σYB = 0.42. The CTOD δ5 R-curves weredetermined for the weld and base materials using stan-dard C(T) specimens (W = 50 mm, B = 2.6 mm). The R-curves in terms of CTOD δ5 using multiple specimenstechnique are shown in Figure 14. The weld materialshows a lower R-curve compared to the base material.

7.4.2 Component related input data

The geometry of the LB welded thin-walled M(T) panelis shown in Figure 15. The stress intensity factor solutionfor such M(T) panel is available in closed form [26] as:

KI = F √πa × √Sec

πa(42)

2WB 2 W

The L and S indicate the longitudinal and short orientations,respectively.

Figure 12 – Schematic showing the micro-flattensile (MFT) specimen extraction and testingtechnique for determination of the “intrinsic”

tensile properties of the very narrow laser beamwelds in thin-walled structures

Figure 13 – Engineering stress-strain curvesobtained from micro-flat tensile specimens

for base and LBW fusion zone (FZ) showingthe significant undermatching nature

of the weld metal

Figure 14 – The R-curves in terms of CTOD δ5

using multiple specimens technique determinedfor base and weld metal (Fusion Zone, FZ) regions

Figure 15 – Configurations of the middle crackedM(T) base material and LBW specimens tested

for verification of the FITNET FFS FractureModule in application to the thin-walled

advanced welded structures

The limit load solution for an undermatched M(T) platewith a crack located in the center of the weld is definedas [6, 15]:

FYM = M2 – 2 – √3 1.43 for ψ > 1.43 (43)

FYB[ √3 ( √3 ) ψ ]

where

ψ = (W – a) / H is the ratio of the ligament size, W – a,and the weld width, 2 H. The limit load of a homoge-neous middle cracked base plate, FYB, under planestress condition is given by:

FYB = 2 B (W – a) σY (44)

7.4.3 Application of FITNET FFS Fracture Modulefor laser beam welded thin-walled strengthundermatched Al-Panels

Using all input data described in the previous section,the FITNET FFS Procedure Fracture Module analysis


Option 2 is applied to the thin-walled LBW cracked plate.Figure 16 shows the comparison of the load-deformationbehaviour between the FITNET FFS prediction and theexperimental results. The maximum load carrying capac-ity, which coincides with the failure load of the LBWpanel in this case, is predicted by the FITNET FFS pro-

cedure conservatively being 5 % lower than the exper-imental value. Figure 16 presents the sensitivity of thepredictions to the weld width (2 H), strain hardeningexponent of the weld material (N W), and the constraintfactor m of Eq. (3).

The weld width, 2H, appears only (through the para-meter ψ = (W – a) / H) in the limit load solution through-out the entire FITNET FFS Fracture procedure. Sincethe uncracked ligament size is relatively large resultingin the very large ψ, the influence of 2H on the mis-matchlimit load in Eq. (43) is negligibly small. Thus, the weldwidth, 2H, is an insensitive parameter as long as ψremains large [see Figure 16 a)]. However, the struc-tural significance of the weld width, 2H, should be takeninto account due to the occurrence of confined plastic-ity within the weld zone which increases the crack tipconstraint and, hence, reduces the structural stability ofthe cracked weld joint.

Figure 16 c) also shows the effect of the variation of theparameter m in Eq. (3), which can be interpreted as aconstraint parameter, on the load vs. deformation curve.In the case of strong undermatching weld, the plasticzone in front of the crack tip is entirely confined to theweld metal. Due to the very narrow weld (2H is small),the softer weld metal cannot freely deform in thicknessdirection, thus, exhibiting an out-of-plane constraint. Thestress state within the weld, therefore, tends to be closeto the plane strain condition although the overall thick-ness of the structure is considered thin. Also the deter-mination of the mis-match limit load for an undermatchedbutt weld needs to consider a plane strain condition forlarge ψ = (W – a) / H ratio (i.e. long ligament, W-a, andsmall weld width, 2H). For large ψ the yield load solu-tion approaches a plateau with a value equal to the yieldload of an all-weld-metal cracked plate under planestrain condition [6, 15] [see also Eq. (43)]. This fact jus-tifies the use of m = 2 in Eq. (3) when estimating theCTOD δ5 crack driving force for thin-walled highlystrength undermatched laser beam weld joints for thedefect assessment using the FITNET FFS Fracture pro-cedure as described in Ref. [11].

The successful application of the FITNET FFS FractureAnalysis Procedure to the thin-walled highly under-matched plates as shown in Figure 16 yielded a verygood prediction of the residual strength as well as theload-deformation behaviour. Further details of these pre-dictions are reported in [19-22].

7.5 CASE V: Girth-welded pipeunder four point bending

This case V validation work was conducted at the TWIand deals with the assessment of two girth-welded steelpipes under four-point bending. It is assumed that thepipes contain two types of cracks: through-wall and sur-face breaking.

7.5.1 Input data for assessment

All the required input data for the assessment of thepipes are collected from [27]. Those include the infor-mation listed below.

a) For weld width (2H)

b) For strain hardening exponent of the weld material

c) For constraint factor m

Figure 16 – The comparison between experimentsand FITNET predictions with sensitivity analysis


Tensile properties

Japanese carbon steel was used to manufacturing thepipes. The pipes were girth-welded using GMAWprocess. Tensile properties of both base and parentmaterials are reported in Table 5.

The Table also included Ramberg-Osgood fitting para-meters.

Fracture toughness of the pipe materials

Maximum load value of J, Jm, was calculated based onthe information given in [27] and used in the currentstudy. Values of maximum-load fracture toughness forthe weld metal were estimated 1 744 N/mm [27].

Mis-match yield load solutions

Solutions provided in Appendix B of the FITNET proce-dure for a welded centre-cracked plate and a girth-welded pipe containing a circumferential surface crackat the weld centre-line are used in the calculations.

Pipes geometry

Table 6 shows crack geometry and dimensions of thepipes tested/assessed. As it can be seen, all the pipeshave similar outside diameter and wall thickness but dif-ferent crack length/geometry. For better traceability,pipes are numbered 01 and 02 (Table 6).

7.5.2 Experimental results

Table 7 shows results of the pipe fracture test.

7.5.3 Prediction of the fracture load

Values of fracture load in kN for pipes with through-thickness and surface cracks are shown in Figure 17. Asit can be seen that the FITNET Option 2 (mis-match)procedure provides a reliable and safe prediction forboth pipes under four point bending loading condition.

8 CONCLUSIONS

Basic features of the weld strength mis-match Option 2of the FITNET FFS Fracture Module (Section 6) havebeen presented and its applications to various structuralwelds demonstrated.

The comparison between predicted and the experimentalresults of the advanced welds covering highly mis-matched cases provided always conservative results.

The prediction for thin-walled and highly undermatchedaerospace Al-alloy welds was based on the assumptionof a plane strain condition in the weld joint due to theoccurrence of confined plasticity in the weld area.

The assessment of a through-wall-cracked and a surfacecracked pipe under four-point bend loading conditionwere conducted based on Section 6 of the FITNET FFSOption 2 procedure. The predictions showed reasonableamount of conservatism compared with the experimen-tal data.

ACKNOWLEDGEMENTS

The financial contribution of the European Communityto conduct this thematic network project (Contract No:GIRT-CT-2001-05071) is greatly appreciated. Authorswish to thank FITNET TN members for their valuableefforts for the development of this section. Thanks toMr. H. Mackel for the technical support for conductingthe tests at the GKSS.

σ0.2, MPa σU, MPa σ0, α n

BM WM BM WM Mpa

272 350 465 545 272 7.65 4.55

Table 5 – Tensile properties of parentand weld metal [27]

Pipe No. Outer diameter Wall thickness Crack type Crack length, Crack height,(mm) mm 2 c mm a mmW

01 318.50 10.70 Through-wall 170.80 –

02 318.30 10.70 Surface 51.52 6.05

Table 6 – Crack geometry and dimensions of the pipes [27]

Crack geometry Fracture load kN

Through-thickness 415.0

Surface 734.0

Table 7 – Results of pipe fracture tests [27]

Figure 17 – Comparison of the maximum loadprediction for the through-wall pipe

at room temperature


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treatments of structural welds using fitnet fitness-for-service assessment procedure

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