weld detail fatigue life improvement techniques part 1 review

However, when this is not practicable, or is not su$cient, fatigue life improvement techniques which rely on improving the stress "eld in and around the weld can be bene"cial.. concentra

* Corresponding author Fax: #1-281-877-5931.

E-mail address: kma@eagle.org (K.-T Ma)

q The project upon which this paper is based was performed at MIL Systems, Ottawa, Canada under

a contract from the Ship Structure Committee The "rst, fourth and "fth authors were employed at MIL Systems at the time.

Weld detail fatigue life improvement techniques.

K.J Kirkhope !, R Bell", L Caron#, R.I Basu$, K.-T Ma$,*

!Atomic Energy Control Board, Ottawa, Canada

"Carleton University, Ottawa, Canada

#Davie Industries, Le&vis, Que&bec, Canada

$American Bureau of Shipping, Houston, TX, USA

Received 22 March 1998; received in revised form 11 February 1999; accepted 23 February 1999

Abstract

Fatigue cracks in fabricated steel structures often occur at welded joints where stress tions due to the joint geometry are relatively high In many cases the fatigue performance can be improved by employing good detail design practices However, when this is not practicable, or is not su$cient, fatigue life improvement techniques which rely on improving the stress "eld in and around the weld can be bene"cial While such techniques have been applied successfully in several industries, their application in ship structures is limited This is at least partly due to the lack of relevant guidance This paper provides a review of weld detail fatigue life improvement techniques, while a companion paper (Kirkhope KJ, Bell R, Caron L, Basu RI, Ma K-T Weld detail fatigue life improvement techniques Part 2: application to ship structures Submitted to Marine Structures) describes their application to ship structures ( 1999 Elsevier Science Ltd All rights reserved.

concentra-Keywords: Fatigue; Weld improvement techniques; Grinding; Peening; Weld toe remelting

1 Introduction

In many cases, the fatigue performance of heavily loaded details can be improved byemploying good detail design practices, for example by upgrading the welded detailclass to the one having a higher fatigue strength In some cases, however, there may be

0951-8339/99/$ - see front matter ( 1999 Elsevier Science Ltd All rights reserved.

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no better alternatives to the detail in question and modi"cation of the detail may not

be practicable As an alternative to strengthening the structure, possibly at great cost,procedures that reduce the severity of the stress concentration at the weld, removeimperfections, and/or introduce local compressive stresses at the weld can be used forimprovement of the fatigue life Similarly, these fatigue improvement techniques can

be applied as remedial measures to extend the fatigue life of critical welds that havefailed prematurely and have been repaired

To date, weld fatigue life improvement techniques have been successfully applied too!shore structures, steel bridges, rail cars and, to a limited extent, ship structures.While there has been increasing interest in the application of fatigue life improvementtechniques to ship structures, at present there is a lack of guidance on the use of suchtechniques for design, construction and repair A project was undertaken with theobjective of addressing this de"ciency The results of the project are described in thispaper and a companion paper [1]

This paper contains a compilation of available data on fatigue life-improvementtechniques Each technique is reviewed in detail The techniques that have potentialfor application to ship structure details are identi"ed

The aforementioned companion paper further discusses those techniques sidered particularly suitable for application to ship structures It addresses varioustopics including identi"cation of ship detail types that might be suitable for treatment,potential fatigue strength improvements, the in#uence of corrosion, production as-pects, and inspection and quality control considerations As part of this project, testswere conducted to examine practical aspects including the speed at which thetechniques can be applied, indirect measures of cost, and to provide information onthe implementation of the techniques; these are also discussed In addition, recom-mendations in regard to design, construction and repair requirements were made

con-2 Fatigue improvement techniques

In general, weld fatigue improvement methods can be divided into two maingroups: weld geometry modi"cation methods and residual stress methods The formerremoves weld toe defects and/or reduces the stress concentration The latter introduc-

es a compressive stress "eld in the area where cracks are likely to initiate A summary

of the various improvement techniques to be considered is shown in Fig 1 This paperprovides an overview of improvement techniques Readers are referred to a report [2]

on this subject for more detailed discussion

3 Weld geometry improvement methods

3.1 Grinding techniques

There are several techniques which rely on mechanical means to improve the weldpro"le thus reducing the weld stress concentration The main ones are burr grinding,disc grinding and water jet eroding

Fig 1 Classi"cation of some weld-improvement methods (modi"ed after [3] ).

3.1.1 Burr grinding

Weld burr grinding is carried out using a high-speed pneumatic, hydraulic orelectric grinder driving rotary burrs at a rotational speed of between 15,000 and40,000 rpm In full pro"le burr grinding the complete weld face is machined to removesurface defects and to blend the weld metal with the base plate This gives the weld

a favourable shape which reduces the local stress concentration In weld toe burrgrinding only the weld toe is machined to remove weld toe defects and reduce the weldtoe angle which results in a decrease in the weld toe stress concentration For bothprocedures it is essential that all defects and undercuts are removed from the weldtoe Therefore, material is removed to a depth of at least 0.5 mm (0.02 in) below anyvisible undercut, but should not exceed 2.0 mm (0.08 in) or 5% of the plate thickness.The speci"cations for performing weld toe burr grinding are outlined in a recent IIWWorking Group document [4]

The grinding process can be performed at the rate of about 1 m/hr by a equipped operator, however, the process is noisy and the operator must wear heavyprotective clothing to protect against the hot sharp cuttings The burrs have a limitedlife and must be changed regularly therefore the process is time consuming andexpensive [5] Inspection of the ground welds should include the weld toe radius, andthe depth of material removed at the weld toe The improvement in fatigue strengthresulting from the toe burr grinding is lower than that obtained by full pro"legrinding However, the cost for toe grinding is substantially less From the publisheddata [6] the range in fatigue strength improvement at 2]106 cycles is between 50 and200% depending on the type of joint.

well-3.1.2 Disc grinding

When a disc grinder is used to remove slag inclusions and undercuts and modify theweld shape the process is less time consuming and thus less costly, however, aninexperienced operator may remove too much material In addition, disc grinding hasthe disadvantage of leaving grinding marks which are normal to the stress direction in

a transversely loaded weld, which serve as initiation sites for fatigue cracks Thus, thefatigue improvement results obtained using disc grinding are somewhat less thanthose obtained for burr grinding and the results also have a larger scatter The fatiguestrength improvement obtained for disc ground welded joints at 2]106 cycles is in therange of 20}50% [6]

3.1.3 Water jet eroding

The water jet eroding technique involves directing a jet of high-pressure waterwhich contains abrasive particles at the weld The abrasive particles erode the weldface material removing the weld toe area including undercuts and slag inclusions Thephysical mechanisms for the resulting improvement in fatigue strength are similar toother weld toe treatments, namely, the weld toe angle is reduced to provide a smoothtransition to the base plate, and weld toe inclusions and undercuts are removedresulting in a reduction in the weld toe stress concentration It is reported by Harris[7] that this technique can be applied more rapidly and thus more cost e!ectively thanother toe dressing treatments such as grinding, TIG or Plasma dressing The rate oferosion is recorded as 20}46 m/h (65}150 ft/h) as compared to 0.5}2.5 m/h (1.5}8 ft/h)for grinding and 0.9 m/h (3 ft/h) for TIG dressing However, this fast rate of erosionrequires special operator training and control since there can be a risk of removingtoo much material in a relatively short time

3.2 Weld toe remelting techniques

Using these techniques the weld toe region is remelted to a shallow depth whichresults in a weld joint with a substantially increased fatigue strength This increaseresults from an improved weld toe shape with a reduced stress concentration factor,the removal of slag inclusions and weld toe undercuts and a higher hardness in theheat a!ected zone as discussed by Kado et al [8] The remelting or weld toe dressingprocess is carried out using Tungsten Inert Gas (TIG) or Plasma welding equipment

Fig 2 E!ect of TIG dressing on the fatigue strength of a medium strength steel [11].

3.2.1 Tungsten inert gas (TIG) dressing

The TIG welding process is also known as gas tungsten arc welding (GTAW) asde"ned by the American Welding Society In this technique, standard TIG weldingequipment is used without the addition of any "ller material, at typical heat inputs of1.0}2.0 kJ/mm (25,000}50,000 J/in) Optimum conditions for TIG dressing have beenproposed by Kado et al [8] The depth of penetration of the arc is approximately

3 mm (0.12 in), however, in some cases a deeper penetration of 6 mm (0.25 in),produced by higher heat inputs, has been used to remove 4 mm (0.16 in) deep fatiguecracks, as noted by Fisher and Dexter [9]

In older C}Mn steels with a relatively high carbon content the remelting processproduces excessive hardness levels in the heat a!ected zone To remedy this problem

a second TIG run procedure was developed to temper the weld toe region andproduce acceptable hardness levels of 300 HV using 10 kg load [10] This second TIGrun also contributes to a better transition between the weld and the base plate but theoverall economy of the dressing process is adversely a!ected

The success of TIG dressing is very sensitive to operator skill and requires ensuringproper operating conditions such as cleanliness of weld and plate, welding current,welding speed and gas shield #ow rate for optimum results In addition, the positionand angle of the torch relative to the weld toe is critical to obtain an optimum weld toeshape For this reason and the complexity of the optimization process it has beensuggested by Haagensen [11] that the procedure be validated through a TIG dressingprocedure quali"cation test similar to welding procedure quali"cation tests Typicalresults [11] obtained from weld joints treated by this process are shown in Fig 2 Theincrease in fatigue strength at 2]106 cycles is approximately 50%

Fig 3 The AWS improved pro"le weld and the `Dime Testa [13].

3.2.2 Plasma dressing

Plasma dressing is similar to TIG dressing, the main di!erence being higher heatinput of about twice that used in TIG dressing The higher heat input produces alarger weld pool which results in a better transition between the weld material and thebase plate Also the larger weld pool makes this procedure less sensitive to electrodeposition relative to the weld toe

It has been found that the improvements in fatigue life obtained from plasmadressing are generally greater than for TIG dressing particularly for higher strengthsteels, Haagensen [3] The cost of TIG and Plasma dressing is relatively inexpensive,however, the heavy cumbersome equipment and accessibility may limit use

3.3 Special welding techniques

Special welding techniques are fatigue improvement methods that are applied aspart of the welding process and attempt to eliminate costly post-weld "nishing Thisapproach is attractive because at the production stage costs are lower and qualitycontrol is simpler than for post-weld procedures The goal of these procedures is

to produce improved weld shapes and thus reduce the stress concentration at theweld toe

3.3.1 AWS improved proxle welds

In the AWS structural welding code (1996), a reduction in the stress concentrationfactor in multipass welded joints of the type shown in Fig 3 is obtained by controllingthe overall weld shape In this procedure, a concave weld pro"le is speci"ed as shown

in the "gure and a smooth transition at the weld toe is ensured by the use of the`dimetesta As shown in the "gure, the pro"le radius `Ra recommended is dependent on the

plate thickness`ta The weld toe pass (butter pass) is laid down before the capping

passes and the weld toe is inspected using a `dimea of diameter equal to theattachment thickness (to a maximum diameter of 50 mm or 2 in) If the weld does not

Fig 4 Improved pro"le weld results for a 370 MPa yield strength steel [15].

pass the dime test, remedial grinding at the weld toe and at inter-bead notches can becarried out The fatigue strength of weld joints can be increased by weld pro"ling, thebene"t being attributed mainly to the stress concentration being moved to a lowerstress region by an increase in weld leg length [14] Typical reductions in stressconcentration factor are from 3.3}5.1 for as-welded joints to 1.36}1.56 for AWSpro"led joints [14] Haagensen et al [15] present results, shown in Fig 4, fortransverse welded plates with improved welds tested in bending which show anincrease in fatigue strength of 25}30% The results emphasize the importance of goodworkmanship in providing a long leg length and a low weld toe angle

The e!ect of pro"ling will generally reduce the throat thickness In some cases thismay be severe enough to a!ect the static strength of the joint In this case there is

a trade-o! between static strength and fatigue strength

In the API-RP2 guidelines [16] for the design of tubular joints, the use of improved pro"les are discouraged by the use of a lower S}N curve, Fig 5 If pro-

non-"le control is carried out the designer may use the X1 curve; if not, the lower X2curve must be used Tests on tubular joints have shown the bene"cial e!ects of pro"lecontrol, but more consistent improvements in fatigue life are obtained if the weldtoe region is carefully ground

Fig 5 The AWS/API design curve [16].

Fig 6 Fatigue strength improvements obtained by improved pro"le and shot peening [18].

a reduction in the stress concentration factor The best improvements in fatigueperformance using these special electrodes have been obtained with high strengthsteels with 500}800 MPa (70}115 ksi) strength Bignonnet et al [18] reported im-provement results using these electrodes which are shown in Fig 6 A relatedtechnique is to use special electrodes only for the "nishing pass at the weld toe asdescribed by Kado et al [8]

Fig 7 Weld geometry data for specimens with improved weld pro"les [19,20].

Fig 8 Plot of fatigue strength versus stress concentration for specimens with normal welds and welds prepared with an improved electrode [19,20].

The improvement in weld toe parameters as a result of the use of special electrodes

is shown in Fig 7 based on data of Kobayashi et al [19] and Bignonnet et al [20].The increase in fatigue strength as a result of the reduction in stress concentrationfactor in these weld specimens is shown in Fig 8

4 Residual stress methods

Improve-4.1.1 Shot peening

Shot peening is a process similar to sand blasting with the sand replaced by smallcast iron or steel shot The shot is propelled against the surface by a high-velocity airstream and causes yielding of the surface layer which builds up compressive residualstresses of about 70}80% of the yield stress The e!ectiveness of shot peening isa!ected by many variables, the control of which are cumbersome and impractical,therefore only two parameters are used to specify the process These parameters arethe Almen intensity and the coverage The intensity of the peening which is related tothe depth of plastic deformation is measured by Almen strips which are attached tothe surface and exposed to the same peening intensity The Almen strips develop

a curvature due to the surface deformation on the exposed side and the curvature ofthe strip of a given material and thickness de"nes the Almen intensity The coverage isrelated to the area covered by the dimples produced by the shot on the surface

A hundred percent coverage is obtained when visual examination at a 10] magnition of the surface reveals that all dimples just overlap To produce 200% coverage thetime required to produce 100% coverage is doubled

"ca-The major advantage of shot peening is that it covers large areas at low cost,however, care must be taken to ensure that the shot size is small enough to reachthe bottom of all undercuts and weld inter-pass notches Typical shot size is in therange of 0.2}1.0 mm (0.008}0.04 in) and the velocities of projection are in the range of40}60 m/s (130}200 ft/s)

Results obtained from tests performed on shot peened welded joints show tial improvements in the fatigue strength for all types of joint, with the magnitude ofthe improvement varying with the type of joint and the yield strength of the steel.Maddox [21] reported an increase of 33% in the fatigue strength at 2]106 cycles of

substan-joints with longitudinal attachments and fabricated from 260 and 390 MPa(36}56 ksi) yield strength steel while the improvement was 70% for higher strength

QT steels with yield strengths of 730 and 820 MPa (105 and 120 ksi) Bignonnet et al.[18] report typical improvements produced by shot peening as shown in Fig 6,however these joints were also fabricated with improved pro"les using special elec-trodes

4.1.2 Hammer peening

Hammer peening is carried out manually using a pneumatic or electrical hammeroperating at approximately 5000 blows/min Hardened steel bits are used which haverounded hemispherical tips with diameters of between 6 and 18 mm (0.25 and 0.75 in).The hammer peening tool should be held approximately normal to the weld face andinclined at 453 to the base plate surface, and should be moved along the toe at a rate ofabout 25 mm/s (1 in/s) Knight [22] investigated the relationship between the severity

of the deformation and the e!ectiveness of the hammer peening and found that fourpasses along the weld toe produces an indentation of about 0.6 mm (0.025 in) in mildsteel and 0.5 mm (0.020 in) in high-strength steel which represents optimum treatment

He also showed that, within reasonable limits it was not possible to over-peen,reporting that after nine passes there were no deleterious e!ects from the peening.This type of peening produces much higher improvements in fatigue strength thaneither shot peening or needle peening due to the large amount of cold workingproduced, which results in the compressive residual stresses penetrating to a greaterdepth in the plate The hammer peening treatment also reduces the stress concentra-tion at the weld toe by modifying the weld toe angle and the weld toe radius It hasbeen reported by Gurney [23] that the improvement produced by this treatment hasbeen so large that the weld is no longer critical and the failure initiates in the baseplate away from the weld Results [3] produced from a number of test programs havealso shown that the largest improvements are obtained for higher strength steels,Fig 9, and typical improvements in fatigue strength [24] are shown in Fig 10

4.1.3 Needle peening

Needle peening is a similar technique to hammer peening except that the solid tool

is replaced by a bundle of steel wires of approximately 2 mm (0.08 in) diameter withrounded ends The improvement obtained from this treatment is generally slightly lessthan that obtained from single point hammer peening

4.1.4 Ultrasonic impact peening

Ultrasonic impact peening is a recently developed technique to apply compressiveresidual stresses to weldments described by Trufyakov et al [25] In this process

a 4}7 mm (0.16}0.28 in) wide zone at the weld toe is treated with an ultrasonichammer The equipment consists of a magneto constriction transducer, an ultrasonicwave transmitter and a peening tool The tool is either a single ball element with

a 16 mm (0.625 in) diameter or multiple needles which vibrate at 27 kHz The process

is completed in a single pass by moving the tool along the weld toe at a rate of 0.5 m/s(1.6 ft/s) The tool holder isolates the operator from the vibration and there is little

Fig 9 Variation in fatigue strength improvement due to hammer peening as a function as a base material strength [3].

Fig 10 Improvement in fatigue strength due to hammer peening [24].

audible noise during the application The mechanism by which the improvementoccurs is the same as for hammer peening The area along the weld toe is deformed to

a depth of about 0.5}0.7 mm (0.020 to 0.028 in) so as to induce compressive residualstresses which introduce a substantial initiation period into the fatigue life Inaddition, the abrupt transition and weld undercuts at the weld toe are smoothed outresulting in a reduction in the weld toe stress concentration factor

Fig 11 Improvement in fatigue strength due to ultrasonic impact peening [25].

Trufyakov et al [25], have reported that improvements in fatigue strength

produc-ed by this technique for butt and overlap joints are in the order of 50}200%, as shown

in Fig 11 These authors also report that this technique has been used in theshipbuilding industry of the Confederation of Independent States (the former USSR).Other researchers, Castellucci et al [27], have reported similar results for transverse

"llet-welded joints fabricated from high-strength steel

4.2 Overloading treatments

Overloading treatments are not considered practicable for ship structures Again,however, for reasons of completeness a brief summary follows A more completedescription is given in [2]

4.2.1 Prior static overloading

This treatment relies on the introduction of compressive residual stresses at theweld toe as a result of local yielding The positive e!ect of prior overloading has beenobserved in joints with both mild and severe stress concentrations Results reported

by Trufyakov et al [26] show that the fatigue limit of a load-carrying member with anattached transverse sti!ener is increased by 10, 35 and 65% under applied tension

loading of R"0 as a result of overloading to levels of 0.5, 0.7 and 0.9 times the yield

stress, respectively It should also be noted that regular periodic tensile overloading ofthe structure throughout its life may also be bene"cial but the fatigue damage caused

by these overload cycles must also be considered in the life calculation

4.3 Stress relief methods

There are several stress relief methods but most are not considered practicable forship structures Nevertheless, for the sake of completeness a brief overview of thesemethods is provided below More detail is provided in [2]

4.3.1 Thermal stress relief

This method, also called post-weld heat treatment (PWHT), relies on the removal ofwelding residual stresses, rather than the introduction of compressive stresses, fromthe welded joint and to some degree tempers the microstructure possibly increasingthe material toughness A typical post-weld heat treatment for an o!shore qualitysteel would be to heat the joint to a high temperature, maintain this temperature forseveral hours, depending on thickness, and then allow cooling slowly in still air Theprimary e!ect is the reduction of compressive residual stresses There is apparentlyinsu$cient data on the e!ects of this method and hence, no design rules have beenformulated

4.3.2 Vibratory stress relief

Vibratory stress relief is a process whereby residual stresses are relieved by vibratingthe component at frequencies often close to the resonant frequency as described byGnirss [28] The success of this technique has not been proven for welded structuresand it has been pointed out by Booth [29] that vibratory stress relief techniques mayuse up a considerable portion of the fatigue life of the structure themselves