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BRITISH STANDARD BS 7448 :Part 2 : 1997

ICS 25.160.40

NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW

Fracture mechanics toughnesstests

Part 2. Method for determination of KIc,critical CTOD and critical J values ofwelds in metallic materials

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BS

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BS 7448 : Part 2 : 1997

This British Standard, havingbeen prepared under thedirection of the EngineeringSector Board, was publishedunder the authority of theStandards Board and comes intoeffect on15 August 1997

BSI 1997

The following BSI referencesrelate to the work on thisstandard:Committee reference ISE/NFE/4Draft for comment 96/706339 DC

ISBN 0 580 27997 9

Amendments issued since publication

Amd. No. Date Text affected

Committees responsible for thisBritish Standard

The preparation of this British Standard was entrusted to Technical CommitteeISE/NFE/4, Mechanical testing of metals, upon which the following bodies wererepresented:

British Iron and Steel Producers Association

British Non-ferrous Metals Federation

British Railways Board

Copper Development Association

Electricity Association

ERA Technology Ltd.

GAMBICA (BEAMA Ltd.)

Institute of Materials

Ministry of Defence

National Physical Laboratory

Society of British Aerospace Companies Limited

United Kingdom Accreditation Service

University College London

Welding Institute

The following bodies were also represented in the drafting of the standard through

subcommittees and panels:

BEAMA Ltd.

Castings Development Centre

Lloyds Register of Shipping

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BS 7448 : Part 2 : 1997

BSI 1997 i

Contents

Page

Foreword ii

1 Scope 12 References 13 Definitions 14 Symbols and designations 25 Principle 36 Choice of specimen design, orientation and notch location 37 Pre-machining metallography 38 Machining 49 Specimen preparation 5

10 Test procedure for KIc, CTOD (or d) and J tests 511 Post-test metallography 512 Post-test analysis 613 Test report 7

AnnexesA (informative) Example notch locations 17B (informative) Example of pre-test metallography 19C (informative) Example of post-test metallography 21D (informative) Residual stress modification and precracking technique 22E (normative) Assessment of pop-in 24

Figures1 Test method flow chart for BS 7448 : Part 2 82 Crack plane orientation code for welded fracture toughness specimens

(defined relative to weld direction) 93 Proportional dimensions and tolerances for rectangular section bend

specimens and square section bend specimens 104 Acceptable tolerances for misalignment, distortion and curvature in single

edge notch bend specimens 115 Method for straightening bend specimen blanks 126 Notch placement procedure using construction lines in a through

thickness notched specimen 137 Notch placement procedure in a surface notched specimen 138 Post-test sectioning procedure to identify microstructure at fatigue crack

in a through-thickness notched specimen 149 Post test sectioning of a surface notched specimen 15

10 Measurement of s in an SM surface notched specimen 1511 Definition of h and 2h in double and single sided welds 16A.1 Examples of weld positional (WP) notch locations 17A.2 Examples of specific microstructure notch locations 18B.1 HAZ adjacent to columnar weld metal for idealized notch line on

macrosection 19B.2 Microstructural map of HAZ adjacent to columnar weld metal 20C.1 Post-test microstructural map at the crack tip of a specimen notched into

the HAZ in a carbon manganese steel 21D.1 Alternative local compression treatments 23E.1 Measurement of Dapop 24E.2 Post-test sectioning procedure for identifying fracture initiation

microstructure in a through-thickness notched specimen 25E.3 Post-test sectioning for identifying fracture initiation microstructure in a

surface notched specimen 26E.4 Measurement of d1 (along crack front) and d2 (not along crack front)

microstructure in section taken from through-thickness notched specimens(slice B in figure E.2) 27

E.5 Measurement of microstructure d1 and Dapop in section taken from asurface notched specimen, see figure E.3 (example given for HAZ) 27

List of references Inside back cover

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ii BSI 1997

BS 7448 : Part 2 : 1997

Foreword

This Part of BS 7448 has been prepared by Technical Committee ISE/NFE/4.

It gives a method for determining the fracture mechanics toughness of welded joints inmetallic materials under displacement controlled monotonic loading.

This Part of BS 7448 extends the following Parts of this standard to cover weldedjoints:

Part 1. Method for determination of KIc, critical CTOD and critical J values ofmetallic materials.

Part 4. Method for determination of fracture resistance curves and initiation valuesfor stable crack extension in metallic materials.

NOTE. Part 4 is in preparation and will be published soon.

It has been assumed in the drafting of this standard that the execution of its provisionsis entrusted to appropriately qualified and experienced people.

CAUTION. It is important to note that tests of the type described involve the use oflarge forces, and may involve the rapid movement of machine parts and fractured testspecimens. Therefore it is important to consider the safety of machine operators.

Compliance with a British Standard does not of itself confer immunityfrom legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i and ii, pages 1to 28, an inside back cover and a back cover.

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BSI 1997 1

BS 7448 : Part 2 : 1997

1)1 MJ/m2 = 1 MN/m.

1 ScopeThis Part of BS 7448 specifies the specimen design,preparation, method for notch placement and posttest validation of the notch placement applicable tospecimens prepared from welds (i.e. specimenstesting the weld metal or heat affected zone).

This method is complementary to BS 7448 : Parts 1and 4, which cover all aspects of the fracturetoughness test and which need to be used inconjunction with this document. The method usesfatigue precracked specimens which have beennotched after welding, into a specified target area inthe weld. The target area and notch orientation to beused is specified by the customer. The specimens aretested in displacement control under quasi-staticloading and at a constant rate of increase in stressintensity factor within the range 0.5 MPa´m0.5 s21

to 3.0 MPa´m0.5 s21 during the initial elasticdeformation. Methods are described to evaluate thesuitability of a weld for notch placement within thetarget area which is either within the weld metal orthe heat affected zone (HAZ) and then, whereappropriate, to evaluate the effectiveness of thefatigue crack in sampling these areas.

2 References

2.1 Normative references

This Part of BS 7448 incorporates, by dated orundated reference, provisions from otherpublications. These normative references are madeat the appropriate places in the text and the citedpublications are listed on the inside back cover. Fordated references, only the edition cited applies; anysubsequent amendments to or revisions of the citedpublication apply to this Part of BS 7448 only whenincorporated in the reference by amendment orrevision. For undated references, the latest edition ofthe cited publication applies, together with anyamendments.

2.2 Informative references

This Part of BS 7448 refers to other publications thatprovide information or guidance. Editions of thesepublications current at the time of issue of thisstandard are listed on the inside back cover, butreference should be made to the latest editions.

3 DefinitionsFor the purposes of this Part of BS 7448 thefollowing definitions apply.

3.1 stress intensity factor (K)

The magnitude of the elastic stress field singularityfor a homogeneous, linear-elastic body.NOTE. It is a function of applied force, crack length and specimengeometry, and is expressed in units of MPa´m0.5.

3.2 plane strain fracture toughness (KIc)

A measure of a material's resistance to crackextension when the stress state near the crack tip ispredominantly plane strain, plastic deformation islimited, and opening mode monotonic loading isapplied.

3.3 maximum fatigue stress intensity factor(Kf)

The maximum value of opening mode stress intensityfactor which is applied during the final stages offatigue crack extension.

3.4 crack tip opening displacement (CTOD)

The relative displacement of the surfaces of a cracknormal to the original (undeformed) crack plane atthe tip of the fatigue precrack, expressed inmillimetres.

3.5 critical CTOD

A value of CTOD associated with a particular type ofcrack extension (see clause 4).

3.6 J-integral

A line or surface integral that encloses the crackfront from one crack surface to the other, whichcharacterizes the local stress-strain field at the cracktip, expressed in MJ/m2 1).

3.7 J

An experimental equivalent of the J-integraldetermined by the method given in the presentstandard.

3.8 critical J

A value of J associated with a particular type ofcrack extension (see clause 4).

3.9 brittle crack extension

An abrupt crack extension which occurs with orwithout prior stable crack extension.

3.10 stable crack extension

Crack extension which, in displacement control,stops when applied displacement is held constant.

3.11 stretch zone width (SZW)

The length of crack extension that occurs duringcrack tip blunting; that is, prior to the onset of brittlecrack extension, pop-in (see 3.13) or slow stablecrack extension, and which occurs in the same planeas the fatigue precrack.

3.12 target area

The intended fatigue crack tip position within theweld.

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2) See BS 499 : Part 1 : 1991.

3.13 pop-in

A discontinuity in the force versus displacementrecord.NOTE. The pop-in corresponds to a sudden increase indisplacement, and, generally, a sudden decrease in force.Subsequently, the displacement and force increase relativelyslowly to above their respective values at pop-in.

3.14 local compression

Controlled compression applied to specimens in thethickness direction on the unnotched ligament bymeans of hardened steel platens prior to fatiguecracking, (see annex D).

3.15 welding

An operation in which two or more parts are united,by means of heat or pressure or both, in such a waythat there is continuity in the nature of the metalbetween these parts. A filler metal, the meltingtemperature of which is of the same order as that ofthe parent metal, may or may not be used2).

3.16 weld

A union of pieces of metal made by welding.

3.17 weld metal

All metal melted during the making of a weld andretained in the weld.

3.18 parent metal

Metal to be joined by welding.

3.19 heat-affected zone (HAZ)

The zone of the parent metal that is metallurgicallyaffected by the heat of welding.

3.20 fusion line (f)

The junction between the weld metal and the heataffected zone.

3.21 weld positional (WP)

Target position for the fatigue crack tip defined withrespect to a reference line (for examples, seefigure A.1).

3.22 specific microstructure (SM)

Target microstructure for the fatigue crack tip (forexamples, see figure A.2).

3.23 weld width (2h)

Shortest distance between adjacent fusion lines ofthe weld at the fatigue crack tip position measuredperpendicular to the crack plane, (see figure 11).

3.24 specimen blanks

Specimen prepared from weld prior to notching.

3.25 post-weld heat treatment

Heat treatment applied after welding for the purposeof reducing residual stresses or modifying weldproperties.

4 Symbols and designationsFor the purposes of this Part of BS 7448 thefollowing symbols and designations apply.

a nominal crack length, in millimetresao weighted average original crack length, in

millimetres (see BS 7448 : Part 1)B specimen thickness, in millimetresBN net specimen thickness after sidegrooving, in

millimetresd1, d2 lengths of microstructural features

associated with pop-in, in micrometresF applied force, in kilonewtonsFf maximum fatigue precracking force during

the final stages of fatigue crack extension, inkilonewtons

GCHAZ grain coarsened HAZHV Vickers hardness valuesJc critical J at the onset of brittle crack

extension or pop-in when Da is lessthan 0.2 mm

Jm value of J at the first attainment of a plasticmaximum force plateau for fully plasticbehaviour

Ju critical J at the onset of brittle crackextension or pop-in when the event ispreceded by Da equal to or greaterthan 0.2 mm

Kf maximum fatigue stress intensity factorapplied during the final stages of fatigueprecrack

N normal to welding directionP parallel to welding directionQ weld thickness directionRp0.2 0.2 % proof strength at the temperature of

the fracture test, in newtons per squaremillimetre or megapascals

Rp0.2b 0.2 % proof strength for the base material atthe temperature of the fracture test, innewtons per square millimetre ormegapascals

Rp0.2w 0.2 % proof strength for the weld metal at thetemperature of the fracture test, in newtonsper square millimetre or megapascals

Rm tensile strength at the temperature of thefracture test, in newtons per squaremillimetre or megapascals

Rmb tensile strength of the base material at thetemperature of the fracture test, in newtonsper square millimetre or megapascals

Rmw tensile strength of the weld metal at thetemperature of the fracture test, in newtonsper square millimetre or megapascals

s distance between crack tip and target areameasured in the crack plane, in millimetres

S span between the two outer points in a threepoint bend test, in millimetres

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T fracture toughness test temperature, indegrees centigrade

W specimen width, in millimetresX direction parallel to primary grain flow of

parent metalY direction transverse to primary grain flow of

parent metalZ direction through the thickness of parent

metaldc critical CTOD at the onset of brittle crack

extension or pop-in when Da is less than0.2 mm, in millimetres

dm value of CTOD at the first attainment of amaximum force plateau for fully plasticbehaviour, in millimetres

du critical CTOD at the onset of brittle crackextension or pop-in when the event ispreceded by Da equal to or greaterthan 0.2 mm, in millimetres

Da stable crack extension, including SZW, inmillimetres

Dapop maximum length of brittle crack extensionassociated with pop-in, in millimetres

l length of specific microstructure measuredin pre- or post-test metallography, inmicrometres

5 PrincipleThis standard prescribes procedures for themeasurement of fracture toughness on notchedspecimens taken from welds. It addresses twosituations: (i) where the notch is located in relationto specific weld features i.e. weld positional (WP),and (ii) where a target microstructure at the finalcrack tip location is defined, i.e. specificmicrostructure (SM). This may includemetallographic examination of the weld to confirmthat the target microstructure is present and insufficient quantity for testing.

The specimen geometry and notch orientation is thenchosen and a fatigue crack is extended into thetarget microstructure by applying an alternatingforce within controlled limits. Modifications to thefatigue precracking conditions may be necessary togrow a straight fronted crack and to minimize theeffects that welding residual stresses may have onthe result in as-welded or partially stress relievedwelds.

The fracture toughness test is performed andevaluated as described in BS 7448 : Part 1 or Part 4,but subject to additional requirements of thisstandard on the calculation of fracture toughness(see 12.3) and crack front straightness (see 12.4).

Post-test metallography may be required to establishwhether the final crack tip was located in the targetmicrostructure and to determine the significance ofpop-ins.

The methods used in this procedure are summarizedin figure 1.

6 Choice of specimen design,orientation and notch location

6.1 Classification of target area for notching

Specimens selected for WP testing shall be specifiedto test a defined weld region with respect to areference position (e.g. weld metal centreline).

Specimens selected for SM testing shall sample thespecified microstructure along the whole or part ofthe crack front length within the central 75 % ofspecimen thickness.NOTE. Some examples of WP and SM notch locations are given inannex A.

6.2 Specimen design

The specimen design shall be of compact (CT) orsingle edge notch bend (SENB) geometry as definedin 5.1 of BS 7448 : Part 1: 1991 or in 6.1 of BS 7448 :Part 4 : 1997 and shall be plain sided or sidegrooved.NOTE 1. The tolerances on specimen dimensions are morerelaxed than when testing plain metallic materials, see 8.1.

The test specimen shall have the dimension B equalto the full thickness of parent material adjacent tothe weld to be tested (i.e. excluding the thickness ofthe weld overfill).

B shall be measured in the direction Q shown infigure 2.NOTE 2. Testing of sub-size (i.e. B < thickness in direction Q infigure 2) and/or sidegrooved specimens is permitted, provided thatthis is reported and justified for the same circumstances that aregiven in 5.1.2 of BS 7448 : Part 1 : 1991. Testing of sub-sized and/orsidegrooved specimens may give values of fracture toughnessdifferent from full thickness specimens owing to size effectsand/or because different microstructural regions have been tested.

6.3 Specimen and crack plane orientation

The specimen and crack plane orientation relative tothe weld direction shall be defined using theidentification system described in figure 2.

7 Pre-machining metallography7.1 Microstructural assessment ofmacrosections

When the notch target area is defined as SM,macro-sections shall be prepared with the plane ofthe section perpendicular to the welding direction.At least two macrosections shall be prepared frompositions bounding the length of weld to be tested toensure that the target microstructure is present atthe expected crack tip position and suitable fortesting. The macrosections shall be polished andetched for metallography and examined at amagnification suitable to identify the target areaprior to making the fracture toughness specimens.

The positions of macrosections shall be recorded.

The examination of the macrosections shall be usedto establish whether the crack tip in athrough-thickness notched specimen is likely to belocated in the target area which is within thecentral 75 % of thickness. When a surface notchedspecimen is used, the target area shall be ahead ofthe final crack tip by no more than 0.5 mm.

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If the specified microstructure is not present, orthere is insufficient quantity to test reliably, or thecrack tip position tolerances cannot be achieved, theweld shall be rejected as unsuitable for testing tothe SM criteria. (If the weld is rejected, the targetarea criteria can be modified, or a new weldprepared).

7.2 Additional requirements for heat affectedzone tests

When the target area is SM in the visibly transformedHAZ, the following microstructure examinations,additional to those of 7.1 shall be conducted on thepolished and etched macrosection.

The examination shall be conducted along the likelycrack tip position to assess whether the targetmicrostructure is within the central 75 % of thicknessand in sufficient quantity for a successful test.

The measured positions and lengths of the targetmicrostructure shall be presented in map form (anexample is shown in annex B). The map shall bedrawn to include the full macrosection thickness,showing the positions of the target microstructure.The percentage of the target microstructure shall becalculated over the middle 75 % of the specimenplate thickness.

Where surface notched specimens are specified, themacrosection shall be examined to confirm that thetarget microstructure is present within the ratios ofao/W defined in 6.4.7 of BS 7448 : Part 1 : 1991or 7.4.6 of BS 7448 : Part 4 : 1997, as appropriate.

The suitability of the weld for the location of thefatigue crack tip within the target area shall beassessed. If a successful placement of the fatiguecrack tip with respect to the SM acceptance criteriais not likely, then consideration shall be given torevising the definition of the target area or preparinga new weld.

8 Machining

8.1 Tolerances on specimen dimensions

Prior to notching, specimen blanks shall beprepared.

NOTE 1. To facilitate testing of the original product using singleedge notch bend specimens, the shape and surface finish, thetolerances on specimen dimensions, squareness and straightnessare more relaxed than in 5.1 of BS 7448 : Part 1 : 1991.

Compact specimens shall be fully machined to meetthe geometry and tolerances on specimen shapedefined in 5.1.1 of BS 7448 : Part 1 : 1991 and 6.1.1of BS 7448 : Part 4 : 1997. The tolerances forrectangular and square section bend specimens aredefined in figure 3. Tolerances for weldmisalignment, distortion, and specimen blankcurvature (when removing specimen blanks frompipe sections) shall be as defined in figure 4.

NOTE 2. The required straightness tolerance of 2.5 % W on thespecimen blank sides will allow for pipe curvature for ratios ofpipe radius to weld thickness $ 10.

When full section thickness specimen blanks areused, machining shall be kept to a minimum in orderto conform to the tolerance requirements and therequirements for local compression (see note 2to 9.1). The weld overfill shall be machined flushwith the original product surface.

When the material thicknesses either side of theweld differ by 10 % or more, the blank shall bemachined down to the thickness of the thinnermaterial. In such cases the original and finalspecimen blank dimensions shall be reported.

Welded joints not meeting thestraightness/misalignment requirements identifiedabove shall be straightened prior to notching bylocal bending. The load bearing points and plasticdeformation shall be located at a distance $ B fromthe region to be notched.

NOTE 3. A method for straightening specimen blanks fromdistorted or curved sections is illustrated in figure 5.

For intended crack plane orientations NP and NQ(see figure 2), the crack plane shall be parallel to theweld direction P.

8.2 Notch placement for through-thicknessnotched specimens

When through-thickness notching is specified, thesurface of the specimen blank to be notched, and theopposite surface, shall be ground and etched toreveal the weld and HAZ. After identifying the targetarea, a line shall be scribed onto the surface to markthe position for the machined notch.

The scribed line shall be made on both preparedsurfaces in a manner which best represents thenotch line on the pre-test macrosections. The lines(on each of the prepared surfaces) extended downthe perpendicular surfaces of the specimen blankshall be averaged and, if necessary, a new line drawnon the specimen blank to indicate the position forthe machined notch (see figure 6), which is mostlikely to result in the crack tip being located in thetarget area.

To ensure that the final crack tip tests the targetarea, the angle between the scribed line on thesurface to be notched and specimen blank axis shallbe 90Ê ± 5Ê (maximum), see figure 6.

8.3 Notch placement for surface notchedspecimens

When surface notching is specified, the surfaces ofthe specimen blank at right angles to the surface tobe notched shall be ground and etched to reveal theweld metal and HAZ. The target area on each of theprepared surfaces shall be identified and linesscribed from the target area to the surface to benotched. Any lateral displacement of the two linesshall be averaged and, if necessary, a new line drawnon the surface to be notched to indicate the positionfor the machined notch (see figure 7).

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To ensure that the final crack tip tests the targetarea, the angle between the scribed line on thesurface to be notched and the specimen blank axisshall be 90Ê ± 5Ê (maximum), see figure 7.

8.4 Machine notching

The machine notching procedure and requirementsshall be as described in 5.1.3 and 6.3 of BS 7448 :Part 1 : 1991.

9 Specimen preparation9.1 Fatigue precracking

The general fatigue precracking requirements shallbe the same as those specified in 6.4 of BS 7448 :Part 1 : 1991, but with the following exceptions. Forweld metal tests, the calculation of the maximumfatigue precracking force, Ff, and maximum fatiguestress intensity factor, Kf, shall be based on thetensile properties of the weld metal, i.e. the region inwhich the fatigue crack will be located. In all othercases the properties of the adjacent material whichhas the lowest tensile properties shall be used.

Any post-weld or stress relief heat treatment shall becompleted before commencing fatigue precracking.NOTE 1. When possible, the use of the shortest fatigue cracklength permitted in 6.4.7 of BS 7448 : Part 1 : 1991 isrecommended. This minimizes fatigue crack front bowing andproblems of the crack deviating from the specified target area.

Problems can occur in meeting the fatigue crackfront straightness requirements of 12.4 particularlywith specimens prepared from as-welded or partiallystress relieved welds. In these instances theprocedures given in annex D shall be considered.NOTE 2. The magnitude and distribution of residual stresses inas-welded and partially stress relieved specimens depend on thematerial, welding procedure, the degree of restraint, and thepost-weld specimen preparation.

NOTE 3. Residual stresses may (or may not) contribute to unevenfatigue crack extension, but they will have an effect on theresulting fracture toughness values determined from the testspecimens.

NOTE 4. Clause 12.5 provides a description of the parametersused to identify fracture toughness obtained from specimens takenfrom welds.

If the specimen is prepared from a stress relievedweld, or if the specimen is shown to contain lowresidual stresses (see D.5), then the proceduresin D.1 to D.4 may not be necessary. The fracturetoughness determined from the test shall beidentified as specified in 12.5a.

If the residual stresses cannot be shown to be low,but the fatigue precrack meets the straightnessrequirements of 12.4, the fracture toughnessdetermined from the test shall be identified asspecified in 12.5b.NOTE 5. A straight fatigue crack front may be due to the crack tipsampling:

a) low residual stresses; or

b) a uniform level of tensile or compressive residual stresses.

If the fatigue precrack does not meet thestraightness requirements of 12.4, then modificationsto the fatigue-precracking procedure shall be madein accordance with annex D. If the specimen then

meets the crack front straightness requirementsof 12.4 and the residual stresses are shown to below in accordance with D.5, the fracture toughnessvalue shall be identified as required in 12.5c. Whenthe requirements of D.5 cannot be met, the resultsshall be identified as specified in 12.5d.

9.2 Sidegrooving

Where sidegrooving is specified, this shall beconducted in accordance with the requirementsof 6.2 of BS 7448 : Part 4 : 1997.

10 Test procedure for KIc, CTOD (or d)and J testsThe procedure for KIc, CTOD (or d) and J testingshall be as described in clause 8 of BS 7448 : Part 1 :1991 or clause 9 or clause 10 of BS 7448 : Part 4 :1997, as appropriate.

11 Post-test metallography

11.1 General

Post-test metallography shall be applied tospecimens designated for SM testing in order toestablish if the crack tip has been successfullylocated in the target microstructure. A slicecontaining the fracture face shall be cut from thespecimen. When the target area is the HAZ, the sliceshall be removed from the side of the specimencontaining the weld metal. This slice shall be usedfor the post-test sectioning analysis described in 11.2and 11.3 to establish the microstructure at thefatigue crack tip.

Post-test sectioning is not required on tests in whichthe target area is WP to meet the requirements ofthis standard.

NOTE. In the case of brittle fracture, confirmation that the cracktip sampled the specified microstructure does not guarantee thatcleavage initiation occurred in that microstructure. Furthersectioning and metallography may be necessary (when requestedby the customer) to identify the microstructure at fractureinitiation. The sectioning procedures are the same as described forthe assessment of pop-in, and are given in annex E.

11.2 Through-thickness notched specimens

11.2.1 Sectioning

The slice of the fracture surface from athrough-thickness notched specimen shall besectioned on a plane perpendicular to the fracturesurface and original parent material surfaceimmediately behind the fatigue crack tip (seefigure 8, slice A). This shall be at a positionwithin 2 mm of maximum crack depth and shallinclude the fatigue crack over the central 75 % ofspecimen thickness (B, or BN in the case ofsidegrooved specimens). The cut surface of thesectioned slice nearest the machine notch (slice A infigure 8) shall be prepared for metallography toestablish whether the fatigue crack successfullysampled the specified microstructure.

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11.2.2 Assessment

The prepared metallographic section shall beexamined, at a suitable magnification using opticalmicroscopy, to establish if the fatigue crack tip frontsampled the SM and to confirm that this was locatedwithin the central 75 % of specimen thickness (B orBN). A microstructural map shall be prepared whichrecords the positions and lengths of the specifiedmicrostructure within the central 75 % of specimenthickness (B or BN). An example for a specimennotched into the transformed HAZ is shown inannex C.

Sections taken at the brittle fracture initiationposition (when requested by the customer) shall beexamined and the initiation microstructure recorded.Sectioning procedures are the same as described forthe assessment of pop-in and are given in annex E.

11.3 Surface notched specimens

11.3.1 Sectioning

At least one section shall be taken within themiddle 75 % of specimen thickness (B or BN). Theplane of the section shall be perpendicular to thenotched surface and the crack plane (see figure 9).

11.3.2 Assessment

The prepared metallographic section shall beexamined at suitable magnification under an opticalmicroscope to establish if the fatigue crack tipsampled the specified microstructure. If the specifiedmicrostructure is ahead of the fatigue crack tip, theminimum separation distance, s, shall be measured,with an accuracy of ± 0.05 mm, (see figure 10). (Itmay be necessary to section both fracture surfacesto establish this distance).

11.4 Assessment of pop-in

The weld shall be assessed for pop-in as described inannex E.

Pop-ins giving both force drops and displacementincreases of less than 1 % shall be ignored. All otherpop-ins shall be considered significant unless shownto be insignificant by the fractographic andmetallographic procedures described in annex E.NOTE. The criteria for the assessment of pop-in described inBS 7448 : Part 1 are intended for testing homogeneous material,and may be inappropriate when applied to welds. Experienceindicates that with weld testing, the size of the pop-in may berelated to the length of brittle material present at the crack tip.Small changes in crack tip position can alter the size of the pop-in.

12 Post-test analysis12.1 Choice of tensile properties

The tensile properties of the region in which the cracktip lies are required, at the fracture toughness testtemperature, for the calculation of fracture toughnessand qualification assessment. When the crack tip islocated completely in the weld metal, the tensileproperties shall be those measured using an all-weldmetal specimen. For a crack tip located in, or partiallyin, the transformed HAZ, the higher of the parentmaterial or weld metal strengths shall be used.

For carbon and carbon manganese steels, wheretensile properties of the weld metal and parentmaterial cannot be measured directly, estimates fromcorrelations with hardness tests may be used. Thefollowing correlations are appropriate for estimatingroom temperature yield strength in N/mm2 or MPa:

Parent material, Rp0.2b = 3.28HV2 221,for 160 < HV < 495

where HV is the measured diamond pyramid number.

Weld metal, Rp0.2w = 3.15HV2 168,for 150 < HV < 300

Parent material and weld metal room temperaturetensile strength can be estimated from:

Rmb or Rmw = 3.3HV 2 8, for 100 < HV < 250

and,

Rmb or Rmw = 3.15HV + 93, for 250 < HV < 400

Yield strength (in N/mm2) obtained for ferritic steelsat room temperature may be estimated for lowertemperatures from the following equations:

Rp0.2 = Rp0.2 (at room temperature) + 2 189105

(491 + 1.8T)

where T is the temperature in ÊC in the fracturetoughness test.

12.2 KIc

Interpretation of the test record to determine KIcshall be in accordance with 9.2 of BS 7448 : Part 1 :1991, but with the additional requirements of 12.1 ofthis Part of BS 7448.

12.3 CTOD (or d) and J

Interpretation of the test record to determine CTOD(or d) and J shall be in accordance with 9.3.2and 9.4 of BS 7448 : Part 1 : 1991 or 11.2 and 11.3 ofBS 7448 : Part 4 : 1997, as appropriate, but subject tothe additional requirements in 12.1.

For specimens testing weld metal, the CTOD (or d )estimation procedures for the initiation of fracture inBS 7448 : Part 1 and Part 4 shall be considered to bequalified for the purposes of the present standardwhen the following conditions are met.

a) For cases where the crack is in the centre ofthe weld: the ratio of weld width (over thecentral 75 % of thickness) to the ligament length isgreater than 0.2, i.e. 2h/(W2 ao) > 0.2 (seefigure 11a and b).

b) For cases where the crack is offset from theweld centre line: the ratio of the shortest distancebetween the crack plane and the weld fusionboundary (over the central 75 % of thickness) tothe ligament length is greater than 0.1,i.e. 2h/(W2 ao) > 0.1 (see figure 11c and d).

c) For both cases a) and b) above, an additionalrequirement is that the ratio of weld metal yieldstrength to parent metal yield strength shall be inthe range 0.50 to 1.50 when determining CTOD(or d), i.e:

0.50 < < 1.50Rp0.2w

Rp0.2b

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BSI 1997 7

BS 7448 : Part 2 : 1997

For specimens testing weld metal, the J estimationprocedures for the initiation of fracture in BS 7448 :Part 1 and Part 4 shall be considered qualified for thepurposes of the present standard when the ratio ofweld metal yield strength to parent metal yieldstrength is in the range 0.50 to 1.25, i.e:

0.50 < < 1.25Rp0.2w

Rp0.2b

For specimens testing the HAZ, the CTOD (or d )and J estimation procedures in BS 7448 : Part 1 andPart 4 shall be used (see 12.1 for choice of yieldstrength when calculating CTOD). When reportingresults the yield strengths of the parent material andweld metal shall be stated.

NOTE 1. The estimation procedures will result in less than ± 10 %error in CTOD or J. Overestimates of CTOD occur ifRp0.2w/Rp0.2b > 1.50, or 1.25 for J; underestimates occur ifRp0.2w/Rp0.2b < 0.50 for CTOD and J.

NOTE 2. The CTOD and J equations in BS 7448 : Part 4 may be inerror when applied to R-curve estimation on welds with weldmetal yield strength mismatch when CTOD and J values exceeddm or Jm , respectively.

12.4 Qualification requirements

12.4.1 General

All the qualification checks listed in clause 10 ofBS 7448 : Part 1 : 1991 shall be applicable but withthe following modifications.

Where CTOD and J are assessed using bendspecimens, the fatigue crack front straightnessrequirements in 8.7.2b and 10.2.3c of BS 7448 :Part 1 : 1991 shall be increased from 10 % to 20 %(viz. no two of the inner seven crack lengthmeasurements shall differ by more than 20 % ao). Thefatigue crack straightness requirements for compactspecimens and KIc tests (both compact and bendspecimens) shall be the same as specified in 8.7.2band 10.2.3c of BS 7448 : Part 1 : 1991.

NOTE. In order to achieve the requirements of SM and WPtesting, it may not be possible to allow a relaxation to the fatiguecrack front straightness requirements, and the more stringentrequirements of BS 7448 : Part 1 : 1991 may be necessary.

12.4.2 Through-thickness notched specimens

Where post-test sectioning and metallographyconducted on SM specimens in accordance with 11.2shows that the fatigue crack front sampled thedesignated target area and, where specified, thedesignated lengths of specified microstructure,within the central 75 % of specimen thickness (B orBN), the fracture toughness result shall beconsidered qualified. When these requirements arenot achieved, the fracture toughness of the specifiedmicrostructure has not been measured and the testresult shall be considered not qualified.

12.4.3 Surface notched specimens

Where post-test sectioning and metallographyconducted on SM specimens in accordance with 11.3shows that the fatigue crack tip has sampled thespecified microstructure, or the dimension s

(see 11.3.2) is < 0.5 mm, the fracture toughnessresult shall be considered qualified. When theserequirements are not achieved, the fracturetoughness of the specified microstructure has notbeen measured and the test result shall beconsidered not qualified.

12.5 Symbols used to identify fracturetoughness values from welds

The following symbols shall be used to identify thefracture toughness of welds. These shall be inaddition to those required in 12.2 and 12.3.

a) K, J and d (with no superscripts) shall be usedwhen the weld is stress relieved or in as-welded orpartially stress relieved specimens where it can bedemonstrated that low residual stresses arepresent (see D.5).

b) K*, J* and d* (asterisk as superscript) shall beused to identify specimens from as-welded orpartially stress relieved specimens where nomodified precracking procedure ( in accordancewith annex D) has been used.

c) KM, JM and dM (M as superscript) shall be usedto identify results for specimens when a modifiedprecracking procedure in accordance withannex D has been used to provide a straight crackfront and the residual stresses have been shown tobe low ( see D.5).

d) KM*, JM* and dM* (M as superscript followed byan asterisk) shall be used to identify results forspecimens when a modified precracking procedurein accordance with annex D has been used but therequirements of D.5 are not achieved.

13 Test reportThe test report shall be in accordance with clause 11of BS 7448 : Part 1 : 1991 with the followingadditions:

a) record of weld positional (WP) or specifiedmicrostructure (SM) notching;

b) specimen geometry and dimensions;

c) crack plane orientation in accordance withfigure 2;

d) original thicknesses of the weld and parentmaterial adjacent to the weld;

e) pre-test metallography results on macrosectionexamination (if appropriate);

f) tensile properties of the weld and parentmaterial and method used to derive the values;

g) weld width, 2h or h, as appropriate;

h) method used to achieve a straight fatigue crackfront, if appropriate;

i) assessment of pop-in significance (ifappropriate);

j) qualification of result with respect to the cracksampling the designated target area.

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8 BSI 1997

BS 7448 : Part 2 : 1997

Choose target area

Choose specimen size and geometry

Choose notch orientation

Specific microstructure, SM

Pre-test metallographyWeld positional,WP

Prepare blank

Mark notch location

Notch

Modify residual stresses ?

Choose annex Dprocedures

Fatigue pre-crack specimen

Test specimen

Measure specimen dimensions and crack length

Post test metallography

WP Pop-in (WP or SM)

No metallography Metallography Metallography

Assess pop-in significance

Evaluate result

Report

Can SM be tested ?

SM

Y

Y

N

N

Figure 1. Test method flow chart for BS 7448 : Part 2

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BSI 1997 9

BS 7448 : Part 2 : 1997

Q

P

NP

NQ

PQ

PN

N

N = Normal to weld directionP = Parallel to weld directionQ = Weld thickness direction

First letter: the direction normal to the crack planeSecond letter: the expected direction of crack propagation

A subscript X or Y is added to the N direction to indicate whether this direction is parallel ortransverse to the primary grain flow direction in the parent material when testing the HAZ,e.g. if N is parallel to the primary grain flow direction, the NP specimen is defined as NXP, andNYP if the primary grain flow direction is perpendicular to N.

Specimen orientations NP and PN shall be referred to as through-thickness notched, whilstspecimen orientations NQ and PQ shall be referred to as surface notched.

Figure 2. Crack plane orientation code for welded fracture toughness specimens(defined relative to weld direction)

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10 BSI 1997

BS 7448 : Part 2 : 1997

����

����

2.5% W

0.2 % W

A

A

AB 5%+-

B 5%+-

W

2.5%

+ -

W 2.5%+-

2.3W 2.3W

2.3W 2.3W

a

a

N

60° nom.

0.8 µm

2.5% W

0.2 % W

A

A

A0.8 µm

0.8 µm

0.8 µm

A

A

Detail A

60° nom.

Detail A

For nominal crack length, a = 0.45W to 0.70W(see BS 7448 : Part 1 : 1991 and BS 7448 : Part 4 : 1997 as appropriate)

For nominal crack length, a = 0.45W to 0.70W(see BS 7448 :Part 1 : 1991 and BS 7448 : Part 4 : 1997 as appropriate)

Specimen length straightness:10%W along length, see fig. 4

Specimen side straightness:2.5%W, see fig. 4

Specimen length straightness:10%W along length, see fig. 4

Specimen side straightness:2.5%W, see fig. 4

Width = WThickness = B = 0.5WLoading span, S = 4WNotch width = 0.065W max.

Width = WThickness = B = 0.5WLoading span, S = 4WNotch width = 0.065W max.

Section throughcrack plane

Section throughcrack plane

a) Rectangular section bend specimens

b) Square section bend specimens

Figure 3. Proportional dimensions and tolerances for rectangular section bendspecimens and square section bend specimens

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BSI 1997 11

BS 7448 : Part 2 : 1997

a) Misalignment b) Misalignment and/orangular distortion

c) Curvature

d) Curvature

Loading points

Curved surface dueto tube radius

B

B BB

W

Span = 4W

Spa

n =

4W

≤ 2.5%W

≤ 10%W

≤ 10%W ≤ 10%W

Figure 4. Acceptable tolerances for misalignment, distortion and curvature in singleedge notch bend specimens

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12 BSI 1997

BS 7448 : Part 2 : 1997

Weld

Load bearingpoints

> B

> B

a) To reduce angular distortion

b) To reduce curvature of pipe specimen

c) Resultant 'gull wing' specimen from pipe section

Figure 5. Method for straightening bend specimen blanks

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BSI 1997 13

BS 7448 : Part 2 : 1997

Side B

Side A(notched side)

Constructionline side A

Constructionline side B

Fusion line

90° 5°+-W

B

Adjusted notch line

Direction ofmachined notch

Figure 6. Notch placement procedure using construction lines in athrough-thickness notched specimen

Direction ofmachined notch

a

B

W

90° 5°+-

Figure 7. Notch placement procedure in a surface notched specimen

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14 BSI 1997

BS 7448 : Part 2 : 1997

���������������

Saw cuts

Slice B

Slice A

Slice A

Polish and etch

Machine notch

Fatiguepre-crack

Slice B

Fracture

≤ 2m

m 0.75B

B

W

Figure 8. Post-test sectioning procedure to identify microstructure at fatigue crackin a through-thickness notched specimen

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BSI 1997 15

BS 7448 : Part 2 : 1997

B

W

Saw cut

Machined notch

Fatigue pre-crack

Polish and etch����

Figure 9. Post-test sectioning of a surface notched specimen

SM

W

as

Fatigue pre-crack tip

Figure 10. Measurement of s in an SM surface notched specimen

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16

B

SI

1997

BS

7448

:P

art

2:

1997

2h

2h

h

h

B o

r W

B o

r W

B o

r W

B o

r W

0.12

5B0.

125B

0.75

B

0.12

5B0.

125B

0.75

B

Crack along weldcentre line

Crack off weldcentre line

a)

b)

c)

d)

Figure 11. Definition of h and 2h in double and single sided welds

Licensed Copy: Rupert Heygate-Browne, Agip KOC, 08 March 2004, Uncontrolled Copy, (c) BSI

BS 7448 : Part 2 : 1997

BSI 1997 17

Annexes

Annex A (informative)

Examples of notch locationsExamples are given in this annex of typical locations which can be used when testing weld metal and HAZ withthrough-thickness and surface notched bend specimens. Figure A.1 shows weld positional (WP) notch locations,whilst figure A.2 shows specific microstructure (SM) notch locations.

Orientation Geometry Notch location

(i)

B

NP B3B or B32B Weld metal centre line

(ii)

W

a

NQ B3B Weld metal centre linefrom weld root

(iii)

B

B/2

NP B3B or B32B HAZ with notchintersecting fusion line atmid-thickness

(iv)

B

B/4

NP B3B or B32B HAZ with notchintersecting fusion line atquarter thickness

(v)

������

W

B

a

PQ B3B Transverse to weld

(vi)

������

B

a PN B3B Transverse to weld, weldcentre line

Figure A.1 Examples of weld positional (WP) notch locations

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18 BSI 1997

BS 7448 : Part 2 : 1997 Annex A

Orientation Geometry Notch location

(i)

W

a

NQ B3B Columnar weld metal onweld centre line

(ii)

W

aNQ B3B Weld root of first side

welded

(iii)

W

a

NQ B3B GCHAZ adjacent tocolumnar weld metal

(iv)

B

NP B3B or B32B Maximum volume ofas-deposited columnarweld metal

(v)

B

NP B3B or B32B Maximum volume ofcolumnar weld metal

(vi)

B

NP B3B or B32B Crack front to sample atleast 15 % GCHAZ with agrain size > 50 mm(or 15 % HAZ adjacent tocolumnar weld metal)

(vii)

B

5mm NP B3B or B32B HAZ at fusion line+ 5 mm

Figure A.2 Examples of specific microstructure notch locations

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BSI 1997 19

Annex B BS 7448 : Part 2 : 1997

Annex B (informative)

Example of pre-test metallographyPre-test metallography is necessary when SM testing is specified for the HAZ. Figures B.1 and B.2 give anexample of the method of quantifying the amount of HAZ microstructure, in this case HAZ adjacent to columnarweld metal, present in a macrosection prepared for metallography. Figure B.2 shows how to prepare a map of thetarget microstructure identified in the macrosection (see figure B.1) within the central 75 % of specimenthickness. The individual lengths of SM (l) along a line representing the idealized notch are summed to give thepercentage SM present.

��������������������

���������

Columnar weldmetal

Idealized notchline

HAZ adjacent tocolumnar weldmetal

Figure B.1 HAZ adjacent to columnar weld metal for idealizednotch line on macrosection

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20 BSI 1997

BS 7448 : Part 2 : 1997 Annex B

0.75

B

B

Cap

Root

TargetHAZ

λλ

λλ

12

34

% specified microstructure (over middle 75 % of thickness)

= 3 100l∑ll

ln

0.75B

Figure B.2 Microstructural map of HAZ adjacent to columnar weld metal

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BSI 1997 21

Annex C BS 7448 : Part 2 : 1997

Annex C (informative)

Example of post-test metallographyPost-test metallography is necessary when SM testing is specified to confirm that the target microstructure waspresent close to the fatigue crack tip. Figure C.1 shows mapping of the lengths of target microstructure (l) in thiscase grain coarsened HAZ, present in a macrosection removed from a through-thickness notched specimen,e.g. slice A in figure 8.

0.75

B

B

Cap

Root

TargetMicrostructure

λλ

λλ

12

34

% specified microstructure (over middle 75 % of thickness)

= 3 100l∑ll

ln

0.75B

Figure C.1 Post-test microstructural map at the crack tip of a specimen notched intothe HAZ in a carbon manganese steel

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22 BSI 1997

BS 7448 : Part 2 : 1997 Annex D

Annex D (normative)

Residual stress modification andprecracking technique

D.1 General

One of the following techniques, as given in D.2, D.3or D.4, shall be used when testing as-welded orpartially stress relieved specimens. The technique usedshall be described when reporting the test results, andshall include any new or previously documentedevidence that the residual stresses have been reducedto low uniform levels, as specified in D.5.

D.2 Local compression

Experience of testing specimens representing fullsection thickness welds in as-welded and partiallystress relieved conditions indicates that the applicationof local compression to the ligament below themachine notch is often sufficient to reduce the weldingresidual stresses to low and uniform levels and resultin the growth of an acceptably straight fatigueprecrack ([1], [2] and [3]). Furthermore, the reductionin residual stress ensures that any residual stressremaining will have a minimal effect on fracturetoughness.

Local compression is applied prior to fatigueprecracking and side grooving across 88 % to 92 % ofthe ligament (W2 a) in front of the machined notch,and shall encompass the notch tip. Indentation iscarried out using hardened steel platens, to produce atotal plastic strain of up to 1 % of the specimenthickness (see notes 1 and 2). Guidance on the forcesthat need to be applied are indicated in figure D.1.Depending on the thickness (B), local compressionmay be applied from one side only or, up to 0.5 % B toeach side of the specimen simultaneously, (seefigure D.1).

Multiple indents may be used with lower compressionforces. For this case, the platens shall have nodimension in the plane of the indent less than 0.5 B,see figure D1. In addition, the final indent shall bemade nearest to the notch tip.

A number of force applications may be necessary toachieve the required plastic deformation. This shall bemeasured to ± 0.025 mm or ± 0.1 % B, whichever islarger.

For specimens that have been locally compressed, thedimension B used for the calculation of fatigue forceand stress intensity factor shall be B in the region ofthe notch measured after local compression.NOTE 1. Local machining of the ligament to be compressed onboth sides of the specimen may be necessary to ensure a smoothbearing surface for the platen and to achieve uniform deformation.

Any bulging of the back face of the ligament leading todistortion in three point bend specimens at the loadingpoint shall be removed by machining.NOTE 2. Experience indicates that 1 % B total deformation may betoo much for some welds and materials, and straighter crackfronts may be obtained with less. Trials may be necessary toestablish the optimum conditions.

NOTE 3. Local compression is normally unnecessary for weldswhich have been stress relieved by post weld heat treatment.

Post weld heat treatment shall only be carried out onthe specimen if the final condition of the weld is in theheat treated condition. Any heat treatment shall becompleted prior to fatigue precracking.

D.3 Reversed bendingThe specimen is loaded to compress the machinednotch (i.e. reverse bending of the specimen), beforeconventional fatigue precracking [3]. The aim is todeform plastically the material at the notch root andproduce a uniform tensile residual stress there. This isdone using a single cycle of bending load applied tothe notched side of a single edge notch bendspecimen. The maximum value of this `reversedbending load' is determined from the followingequation:

Krb = LRp0.2 √8vrb

pwhere

Krb is the reverse bend stress intensity factor;

L is the notch constraint factor (typically 2.3for a rectangular specimen);

vrb is the plastic zone size resulting fromreverse bending.

Experience indicates that reverse bending is notalways successful and does not reduce significantly thelevel of residual stress at the fatigue crack tipposition [4]. The presence of residual stresses canaffect the test result.

D.4 Stepwise high R-ratioIn the stepwise high R-ratio technique [5], fatigueprecracking consists of two steps, each at differentfatigue stress ratios (R). For the first step, the stressratio R = 0.1 is used (i.e. the conventional R value) untilthe fatigue precrack has grown to a length ofabout 1 mm. In the second step, R is increased to 0.7and the fatigue precrack grown to the desired length.The same Kf is used in both steps.NOTE. Use of R > 0.1 is inconsistent with the fatigue precrackingrequirements in 6.4 of BS 7448 : Part 1 : 1991.

Although experience indicates that the high R-ratiotechnique can result in acceptably straight fatigueprecracks, it does not reduce significantly the residualstresses in the ligament (W2a) ahead of the fatiguecrack tip and this can affect the test result [4].Nevertheless, the technique has been used as analternative to local compression when testing thicksection high strength steels and when the forcesnecessary for local compression are not readilyattainable.

D.5 Evidence of low uniform residual stresses

D.5.1 GeneralThe results for all specimens meeting the requirementsof this standard shall include documentary evidencethat any residual stresses ahead of the precrack tipsare at low uniform levels. Where such evidence is notavailable for the particular combination of technique(D.2, D.3 or D.4), metal and weld used (see D.5.3), asacrificial test to indicate the level of any residualstresses shall be carried out on a representativespecimen as described in D.5.2.

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BSI 1997 23

Annex D BS 7448 : Part 2 : 1997

����

����

����

A

A

B

B

C

C

P

P

P

Ø B/2

W-a W-a

Ø BCC

B B

≤ 1%

B

≤ 0.

5%B

≤ 0.

5%B

≤ 0.

5%B

≤ 0.

5%B

(or 0.5%B on each side)

A - A B - B C - C

a) b) c)

B/2

W - a

B

P = 1.4 B2Rp0.2 P = 0.8 B2Rp0.2 P = 0.3 B2Rp0.2

C = 8 % to 12 % of (W 2 a)

Figure D.1 Alternative local compression treatments

D.5.2 Sacrificial test

D.5.2.1 Regardless of the technique used to preparethe specimen for the sacrificial test (D.2, D.3 or D.4),the initial fatigue precrack shall be extended byadditional fatigue cracking with R# 0.1, and Ff inaccordance with 6.4 of BS 7448 : Part 1 : 1991, until theaverage of the two crack length measurements on thesurfaces of the specimen, measured to ± 0,05 mm, is$ {a + 0.4 (W2 a)}.

D.5.2.2 The specimen shall be broken open to revealthe fatigue crack surfaces using the procedures in 8.7.1of BS 7448 : Part 1 : 1991.

D.5.2.3 The weighted nine-point average crack lengthto the tip of the original fatigue crack (ao) shall bemeasured as specified in 8.7.2 of BS 7448 : Part 1 :1991. Similar measurements shall be made of theweighted nine-point average crack length to the tip ofthe additional fatigue crack (aaf).

D.5.2.4 Residual stresses in the uncracked ligamentahead of the notch tip in the specimen shall bedeemed to be acceptably low and uniform providedthat:

a) the values of ao/W are within the acceptable rangefor the appropriate specimen in BS 7448 : Part 1 orPart 4;

b) aaf $ {ao + 0.4 (W2 ao)};

c) for KIc tests on three point bend and compactspecimens, and CTOD (or d) and J tests on compactspecimens, the difference between any two of thenine-point crack length measurements for ao is# 10 % ao, and similarly, for aaf is # 10 % aaf ;

d) for CTOD (or d) and J tests on three point bendspecimens, the difference between any two of theinner seven crack length measurements for ao is# 20 % ao, and similarly, for aaf is # 20 % aaf.

D.5.3 Previously documented evidence

When the local compression technique (D.2) has beenused, and the crack front straightness requirements forao in D.5.2.4 have been met, reference [4] shall beaccepted as representing the required documentaryevidence of low uniform residual stresses in specimensfrom multi-pass arc welds in carbon manganese steels.

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24 BSI 1997

BS 7448 : Part 2 : 1997 Annex E

Wa

or

(a +

∆a)

o

o

∆a po

p

Figure E.1 Measurement of Dapop

Annex E (normative)

Assessment of pop-in

E.1 General

This procedure shall be used to assess theacceptability of pop-ins classed as significant accordingto 11.4.

The first step is to assess the significance of the pop-inin accordance with 9.1, 9.3 and 9.4 of BS 7448 : Part 1.If the pop-in is assessed as significant according toPart 1, it is also significant according to therequirements of this standard and post-testfractography and metallography is not required.However, if the pop-in is assessed as acceptableaccording to Part 1, the actual significance, withrespect to this Part can be determined from thefractographic and metallographic assessmentprocedures described in E.2 to E.5.

E.2 Fractography

Both fracture faces shall be carefully examined forevidence of an arrested brittle crack generally in theplane of the fatigue crack, and the maximum crackextension (Dapop) shall be measured, (see figure E.1).Where no evidence of such an arrested brittle crackcan be found, the significance of the pop-in shall beassessed in accordance with 9.1, 9.3 and 9.4 ofBS 7448 : Part 1.

NOTE. Pop-in can be caused by an arrested crack runningperpendicular to the plane of the fatigue precrack; this issometimes referred to as a `split'. The fracture toughness at pop-incaused by a split needs to be reported. However, the assessmentof the structural significance of the split is outside the scope ofthis standard.

E.3 Sectioning and metallography

One or both fracture surfaces containing the arrestedbrittle crack shall be examined with optical and/orscanning electron microscopy, to identify the primaryfracture initiation position. When the crack tip islocated in the HAZ, the fracture surface adjacent to theweld shall be examined. After marking the initiationposition, a metallographic section shall be takenthrough the initiation point on a plane perpendicular tothe fatigue crack plane, as illustrated in figure E.2 for athrough-thickness notched specimen, and figure E.3 fora surface notched specimen. The sections shall bepolished and etched according to usual metallographicpractices for microstructural examination.

E.4 Assessment

The metallographic section taken from athrough-thickness notched specimen (see figure E.4)shall be examined and the length of the specificmicrostructure parallel to the crack front at initiation(d1) shall be measured. The lengths of similarmicrostructures present in the section within thecentral 75 % of B (or BN, in the case of sidegroovedspecimens), but not intersected by the crack front shallbe measured and the maximum individual length d2,recorded (see figure E.4). If the section is beyond thefatigue crack tip, a further section behind the fatiguecrack tip may be necessary to measure d2.

The metallographic section taken from a surfacenotched specimen (see figure E.5) shall be examinedand the total length (d1) of the microstructural regionin which the pop-in initiated, shall be measured. Thislength (d1) shall only include the microstructuralregion ahead of the fatigue crack tip, (see figure E.5).(More than one section may be taken to assess thedimension d1).

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Annex E BS 7448 : Part 2 : 1997

���������������

Saw cuts

Slice B

Slice B

Slice A

Slice A

Polish and etch(see fig E4)

Fatiguepre-crack

B

W

Initiation

Arrested crack

Arrested crack

Figure E.2 Post-test sectioning procedure for identifying fracture initiation microstructurein a through-thickness notched specimen

E.5 Pop-in significance

All pop-ins shall be considered significant unless it canbe demonstrated otherwise by metallographicexamination. Values of CTOD and J measured at thefirst pop-in event shall be designated dpop or Jpop, asappropriate.

Following metallographic examination, a pop-in shallnot be considered significant if:

a) dn % F1, calculated in accordance with clause 9 ofBS 7448 : Part 1 : 1991, is less than 5 % and d1 $ d2,for a through-thickness notched specimen; or

b) dn % F1 is less than 5 % and Dapop # d1 for asurface notched specimen.

The pop-in shall be considered significant whend2 > d1 or d1 > Dapop because a larger pop-in mayhave occurred if more of the brittle microstructure hadbeen sampled or had been present ahead of the cracktip. Further tests may be necessary to confirm or rejectthis possibility.

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26 BSI 1997

BS 7448 : Part 2 : 1997 Annex E

B

W

Saw cut

Machined notch

Fatigue pre-crack

Polish and etch����

Initiation

Arrested crack

Figure E.3 Post-test sectioning for identifying fracture initiation microstructure in a surfacenotched specimen

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Annex E BS 7448 : Part 2 : 1997

BB

dd

1d

12

d 2

Fatigue crack

Fatigue crack

HAZ test

Weld metal test

d1 d2Fatigue crack cuts through HAZ but not HAZ

d1 d2Fatigue crack cuts through weld bead but not weld bead

Figure E.4 Measurement of d1 (along crack front) and d2 (not along crack front) microstructurein section taken from through-thickness notched specimens (slice B in figure E.2)

W

d1

a or

(a

+ ∆

a)∆a

pop

Figure E.5 Measurement of microstructure d1 and Dapop in section taken from a surfacenotched specimen, see figure E.3 (example given for HAZ)

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BS 7448 : Part 2 : 1997

3) In preparation.

List of references (see clause 2)

Normative references

BSI publications

BRITISH STANDARDS INSTITUTION, London

BS 7448 Fracture mechanics toughness testsBS 7448 : Part 1 : 1991 Method for determination of KIc, critical CTOD and critical J

values of metallic materialsBS 7448 : Part 43) Method for determination of fracture resistance curves and

initiation values for stable crack extension in metallic materials

Informative references

BSI publications

BRITISH STANDARDS INSTITUTION, London

BS 499 Welding terms and symbolsBS 499 : Part 1 : 1991 Glossary for welding, brazing and thermal cutting

Other references

[1] DAWES, M. G. Fatigue precracking weldment fracture mechanics specimens. Metal Construction & BritishWelding Journal, February 1971, pp.61-65.[2] TOWERS, O. L. and DAWES, M. G. Welding Institute research on the fatigue precracking of fracturetoughness specimens. Elastic plastic fracture test methods. ASTM STP 856, 1985, pp.23-46.[3] MACHIDA, S., MIYATA, T., TOYOSADA, M. and HAGIWA, Y. Study of methods for CTOD testing ofweldments. Fatigue and fracture testing of weldments. ASTM STP 1058, 1990, pp.142-156.[4] REEMSYNDER, H. S., PISARSKI, H. G. and DAWES, M. G. Residual stresses and fatigue precrackingtechniques for weldment fracture toughness specimens. Journal of Testing and Evaluation, November 1991,pp.416-423.[5] KOCË AK, M., SEIFERT, K., YAO, S. and LAMPE, H. Comparison of fatigue precracking methods for fracturetoughness testing of weldments. Proc. Int. Conf. Welding 90 Ð Technology, Material, Fracture Ð GKSS,Geesthacht, Germany, October 1990. i.i.t.t International, France.

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