EN ISO 7539-6:2003

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Korozija kovin in zlitin - Preskušanje napetostne korozije - 6. del: Priprava in uporaba preskušancev z umetno razpoko za preskuse pri konstantni obremenitvi ali konstantni deformaciji (ISO 7539-6:2003)Korrosion der Metalle und Legierungen - Prüfung der Spannungsrisskorrosion - Teil 6: Vorbereitung und Anwendung von angerissenen Proben für die Prüfung unter konstanter Kraft oder konstanter Verformung (ISO 7539-6:2003)Corrosion des métaux et alliages - Essais de corrosion sous contrainte - Partie 6: Préparation et utilisation des éprouvettes préfissurées pour essais sous charge constante ou sous déplacement constant (ISO 7539-6:2003)Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and use of pre-cracked specimens for tests under constant load or constant displacement (ISO 7539-6:2003)77.060Korozija kovinCorrosion of metalsICS:Ta slovenski standard je istoveten z:EN ISO 7539-6:2003SIST EN ISO 7539-6:2003en01-december-2003SIST EN ISO 7539-6:2003SLOVENSKI
STANDARDSIST EN ISO 7539-6:19991DGRPHãþD

EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN ISO 7539-6February 2003ICS 77.060Supersedes EN ISO 7539-6:1995English versionCorrosion of metals and alloys - Stress corrosion testing - Part6: Preparation and use of pre-cracked specimens for tests underconstant load or constant displacement (ISO 7539-6:2003)Corrosion des métaux et alliages - Essais de corrosionsous contrainte - Partie 6: Préparation et utilisation deséprouvettes préfissurées pour essais sous chargeconstante ou sous déplacement constant (ISO 7539-6:2003)Korrosion der Metalle und Legierungen - Pru¨fung derSpannungsrisskorrosion - Teil 6: Vorbereitung undAnwendung von angerissenen Proben für die Prüfung unterkonstanter Kraft oder konstanter Verformung (ISO 7539-6:2003)This European Standard was approved by CEN on 7 February 2003.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Management Centre or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovak Republic, Spain, Sweden, Switzerland andUnited Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2003 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN ISO 7539-6:2003 ESIST EN ISO 7539-6:2003

2003-03-12ForewordThis document (EN ISO 7539-6:2003) has been prepared by Technical Committee ISO/TC 156"Corrosion of metals and alloys" in collaboration with Technical Committee CEN/TC 262"Metallic and other inorganic coatings", the secretariat of which is held by BSI.This European Standard shall be given the status of a national standard, either by publication ofan identical text or by endorsement, at the latest by August 2003, and conflicting nationalstandards shall be withdrawn at the latest by August 2003.This document supersedes EN ISO 7539-6:1995.According to the CEN/CENELEC Internal Regulations, the national standards organizations ofthe following countries are bound to implement this European Standard: Austria, Belgium, CzechRepublic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,Luxembourg, Malta, Netherlands, Norway, Portugal, Slovak Republic, Spain, Sweden,Switzerland and the United Kingdom.Endorsement noticeThe text of ISO 7539-6:2003 has been approved by CEN as EN ISO 7539-6:2003 without anymodifications.NOTE
Normative references to International Standards are listed in Annex ZA (normative).SIST EN ISO 7539-6:2003

Reference numberISO 7539-6:2003(E)© ISO 2003
INTERNATIONAL STANDARD ISO7539-6Second edition2003-02-15Corrosion of metals and alloys — Stress corrosion testing — Part 6: Preparation and use of pre-cracked specimens for tests under constant load or constant displacement Corrosion des métaux et alliages — Essais de corrosion sous contrainte — Partie 6: Préparation et utilisation des éprouvettes préfissurées pour essais sous charge constante ou sous déplacement constant
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ii © ISO 2003 — All rights reserved
ISO 7539-6:2003(E) © ISO 2003 — All rights reserved iii Contents Page Foreword.iv 1 Scope.1 2 Normative references.1 3 Terms and definitions.2 4 Principle.4 5 Specimens.5 6 Initiation and propagation of fatigue cracks.23 7 Procedure.25 8 Test report.33 Annex A (normative)
Use of notched specimens for stress corrosion tests.35 Annex B (normative)
Determination of crack growth velocity.38
ISO 7539-6:2003(E) iv © ISO 2003 — All rights reserved Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 7539-6 was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys, in collaboration with the National Physical Laboratory (United Kingdom). This second edition cancels and replaces the first edition (ISO 7539-6:1989), Clauses 1, 2, 3, 4 and 7; subclause 5.2.5 d); Figures 1, 2 d), 5 b), 8, 9, and 10; Annexs A and B of which have been technically revised. ISO 7539 consists of the following parts, under the general title Corrosion of metals and alloys — Stress corrosion testing: =Part 1: General guidance on testing procedures =Part 2: Preparation and use of bent-beam specimens =Part 3: Preparation and use of U-bend specimens =Part 4: Preparation and use of uniaxially loaded tension specimens =Part 5: Preparation and use of C-ring specimens =Part 6: Preparation and use of pre-cracked specimens for tests under constant load or constant displacement =Part 7: Slow strain rate testing =Part 8: Preparation and use of specimens to evaluate weldments =Part 9: Preparation and use of pre-cracked specimens for tests under rising load or rising displacement
INTERNATIONAL STANDARD ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 1 Corrosion of metals and alloys — Stress corrosion testing — Part 6: Preparation and use of pre-cracked specimens for tests under constant load or constant displacement 1 Scope 1.1 This part of ISO 7539 covers procedures for designing, preparing and using pre-cracked specimens for investigating susceptibility to stress corrosion. It gives recommendations for the design, preparation and use of pre-cracked specimens for investigating susceptibility to stress corrosion. Recommendations concerning notched specimens are given in Annex A. The term “metal” as used in this part of ISO 7539 includes alloys. 1.2 Because of the need to confine plasticity at the crack tip, pre-cracked specimens are not suitable for the evaluation of thin products such as sheet or wire and are generally used for thicker products including plate bar and forgings. They can also be used for parts joined by welding. 1.3 Pre-cracked specimens may be loaded with equipment for application of a constant load or can incorporate a device to produce a constant displacement at the loading points. Tests conducted under increasing displacement or increasing load are dealt with in ISO 7539-9. 1.4 A particular advantage of pre-cracked specimens is that they allow data to be acquired from which critical defect sizes, above which stress corrosion cracking may occur, can be estimated for components of known geometry subjected to known stresses. They also enable rates of stress corrosion crack propagation to be determined. The latter data may be taken into account when monitoring parts containing defects during service. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 7539-1, Corrosion of metals and alloys — Stress corrosion testing — Part 1: General guidance on testing procedures ISO 11782-2, Corrosion of metals and alloys — Corrosion fatigue testing — Part 2: Crack propagation testing using precracked specimens SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 2 © ISO 2003 — All rights reserved 3 Terms and definitions For the purposes of this part of ISO 7539, the definitions given in ISO 7539-1 and the following apply. 3.1 crack length a effective crack length measured from the crack tip to either the mouth of the notch or the loading point axis depending on the specimen geometry 3.2 specimen width W effective width of the specimen measured from the back face to either the face containing the notch or the loading plane depending on the specimen geometry 3.3 specimen thickness B side-to-side dimension of the specimen being tested 3.4 reduced thickness at side grooves Bn minimum side-to-side dimension between the notches in side-grooved specimens 3.5 specimen half-height H 50 % of the specimen height measured parallel to the direction of load application for compact tension, double cantilever beam and modified wedge opening loaded test pieces 3.6 load P that load which, when applied to the specimen, is considered positive if its direction is such as to cause the crack faces to move apart 3.7 deflection at loading point axis VLL crack opening displacement produced at the loading line during the application of load to a constant displacement specimen 3.8 deflection away from the loading line V0 crack opening displacement produced at a location remote from the loading plane, e.g. at knife edges located at the notch mouth, during the application of load to a constant displacement specimen 3.9 modulus of elasticity E elastic modulus (i.e. stress/strain) in tension SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 3 3.10 stress intensity factor KI function of applied load, crack length and specimen geometry having dimensions of stress × length which uniquely define the elastic stress field intensification at the tip of a crack subjected to opening mode displacements (mode I) NOTE It has been found that stress intensity factors, calculated assuming that specimens respond purely elastically, correlate the behaviour of real cracked bodies provided that the size of the zone of plasticity at the crack tip is small compared to the crack length and the length of the uncracked ligament. In this part of ISO 7539, mode I is assumed and the subscript I is implied everywhere. 3.11 initial stress intensity factor KIi stress intensity applied at the commencement of the stress corrosion test 3.12 plane strain fracture toughness KIc critical value of KI at which the first significant environmentally independent extension of the crack occurs under the influence of rising stress intensity under conditions of high resistance to plastic deformation 3.13 provisional value of KIc, KQ KQ = KIc when the validity criteria for plane strain predominance are satisfied 3.14 threshold stress intensity factor for susceptibility to stress corrosion cracking KISCC that stress intensity factor above which stress corrosion cracking will initiate and grow for the specified test conditions under conditions of high resistance to plastic deformation, i.e. under plane strain predominant conditions 3.15 provisional value of KISCC, KQSCC KQSCC = KISCC when the validity criteria for plane strain predominance are satisfied 3.16 maximum stress intensity factor Kmax in fatigue highest algebraic value of the stress intensity factor in a cycle, corresponding to the maximum load 3.17 0,2 % proof stress Rp0,2 stress which must be applied to produce a plastic strain of 0,2 % during a tensile test 3.18 applied stress σ stress resulting from the application of load to the specimen 3.19 stress intensity factor coefficient Y factor derived from the stress analysis for a particular specimen geometry which relates the stress intensity factor for a given crack length to the load and specimen dimensions SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 4 © ISO 2003 — All rights reserved 3.20 load ratio R in fatigue loading algebraic ratio of minimum to maximum load in a cycle: minminmaxmaxPKRPK== 3.21 crack velocity instantaneous rate of stress corrosion crack propagation measured by a continuous crack monitoring technique 3.22 average crack velocity average rate of crack propagation calculated by dividing the change in crack length due to stress corrosion by the test duration 3.23 specimen orientation fracture plane of the specimen identified in terms of firstly the direction of stressing and secondly the direction of crack growth expressed with respect to three reference axes identified by the letters X, Y and Z where Z is coincident with the main working force used during manufacture of the material (short-transverse axis); X is coincident with the direction of grain flow (longitudinal axis); Y is normal to the X and Z axes 4 Principle 4.1 The use of pre-cracked specimens acknowledges the difficulty of ensuring that crack-like defects introduced during either manufacture or subsequent service are totally absent from structures. Furthermore, the presence of such defects can cause a susceptibility to stress corrosion cracking which in some materials (e.g. titanium) may not be evident from tests under constant load on smooth specimens. The principles of linear elastic fracture mechanics can be used to quantify the stress situation existing at the crack tip in a pre-cracked specimen or structure in terms of the plane strain-stress intensity. 4.2 The test involves subjecting a specimen in which a crack has been developed by fatigue, from a machined notch, to either a constant load or displacement at the loading points during exposure to a chemically aggressive environment. The objective is to quantify the conditions under which environmentally-assisted crack extension can occur in terms of the threshold stress intensity for stress corrosion cracking, KISCC, and the kinetics of crack propagation. 4.3 The empirical data can be used for design or life prediction purposes in order to ensure either that the stresses within large structures are insufficient to promote the initiation of environmentally-assisted cracking at whatever pre-existing defects may be present or that the amount of crack growth which would occur within the design life or inspection periods can be tolerated without the risk of unstable failure. 4.4 Stress corrosion cracking is influenced by both mechanical and electrochemical driving forces. The latter can vary with crack depth, opening or shape because of variations in crack-tip chemistry and electrode potential and may not be uniquely described by the fracture mechanics stress intensity factor. 4.5 The mechanical driving force includes both applied and residual stresses. The possible influence of the latter shall be considered in both laboratory testing and the application to more complex geometries. Gradients in residual stress in a specimen may result in non-uniform crack growth along the crack front. SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 5 5 Specimens 5.1 General 5.1.1 A wide range of standard specimen geometries of the type used in fracture toughness tests may be applied. The particular type of specimen used will be dependent upon the form, the strength and the susceptibility to stress corrosion cracking of the material to be tested and also on the objective of the test. 5.1.2 A basic requirement is that the dimensions be sufficient to maintain predominantly triaxial (plane strain) conditions in which plastic deformation is limited in the vicinity of the crack tip. Experience with fracture toughness testing has shown that for a valid KIc measurement, both the crack length, a, and the thickness, B, shall not be less than 2Icp0,22,5KR and that, where possible, larger specimens where both a and B are at least 2Icp0,24KR shall be used to ensure adequate constraint. From the view of fracture mechanics, a minimum thickness from which an invariant value of KISCC is obtained cannot be specified at this time. The presence of an aggressive environment during stress corrosion may reduce the extent of plasticity associated with fracture and hence the specimen dimensions needed to limit plastic deformation. However, in order to minimize the risk of inadequate constraint, it is recommended that similar criteria to those used during fracture toughness testing should also be used regarding specimen dimensions, i.e. both a and B shall be not less than 2Ip0,22,5KR and preferably should be not less than 2Ip0,24KR where KI is the stress intensity to be applied during testing. The threshold stress intensity value eventually determined should be substituted for KI in the first of these expressions as a test for its validity. 5.1.3 If the specimens are to be used for the determination of KISCC, the initial specimen size should be based on an estimate of the KISCC of the material (in the first instance, it being better to over-estimate the KISCC value and therefore use a larger specimen than may eventually be found necessary). Where the service application involves the use of material of insufficient thickness to satisfy the conditions for validity, it is permissible to test specimens of similar thickness, provided that it is clearly stated that the threshold intensity value obtained, KQSCC, is of relevance only to that specific application. Where determining stress corrosion crack growth behaviour as a function of stress intensity is required, the specimen size shall be based on an estimate of the highest stress intensity at which crack growth rates are to be measured. SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 6 © ISO 2003 — All rights reserved 5.1.4 Two basic types of specimen can be used a) those intended for testing under constant displacement, which are invariably self-loaded by means of built-in loading bolts; b) those intended for testing under constant load, for which an external means of load application is required. 5.1.5 Constant displacement specimens, being self-loaded, have the advantage of economy in use since no external stressing equipment is required. Their compact dimensions also facilitate exposure to operating service environments. They can be used for the determination of KISCC by the initiation of stress corrosion cracks from the fatigue pre-crack, in which case a series of specimens must be used to pin-point the threshold value, or by the arrest of a propagating crack since under constant displacement testing conditions the stress intensity decreases progressively as crack propagation occurs. In this case a single specimen will suffice in principle, but in practice the use of several specimens (not less than 3) is often recommended, taking into account the disadvantages described in 5.1.6. 5.1.6 The disadvantages of constant displacement specimens are a) applied loads can only be measured indirectly by displacement changes; b) oxide formation or corrosion products can either wedge open the crack surfaces, thus changing the applied displacement and load, or can block the crack mouth, thus preventing the ingress of corrodent and impairing the accuracy of crack length measurements by electrical resistance methods; c) crack branching, blunting or growth out of plane can invalidate crack arrest data; d) crack arrest must be defined by crack growth below some arbitrary rate which can be difficult to measure accurately; e) elastic relaxation of the loading system during crack growth can cause increased displacement and higher loads than expected; f) plastic relaxation due to time-dependent processes within the specimen can cause lower loads than expected; g) it is sometimes impossible to introduce the test environment prior to application of the load, which can retard crack initiation during subsequent testing. 5.1.7 Constant load specimens have the advantage that stress parameters can be quantified with confidence. Since crack growth results in increasing crack opening, there is less likelihood that oxide films will either block the crack or wedge it open. Crack length measurements can be readily made via a number of continuous monitoring methods. A wide choice of constant load specimen geometries is available to suit the form of the test material, the experimental facilities available and the objectives of the test. This means that crack growth can be studied under either bend or tension loading conditions. The specimens can be used for either the determination of KISCC by the initiation of a stress corrosion crack from a pre-existing fatigue crack using a series of specimens, or for measurements of crack growth rates. Constant load specimens can be loaded during exposure to the test environment in order to avoid the risk of unnecessary incubation periods. 5.1.8 The principal disadvantage of constant load specimens is the expense and bulk associated with the need for an external loading system. Bend specimens can be tested in relatively simple cantilever beam equipment, but specimens subjected to tension loading require constant load creep rupture or similar testing machines. In this case, the expense can be minimized by testing chains of specimens connected by loading links which are designed to prevent unloading on the failure of specimens. The size of these loading systems means that it is difficult to test constant load specimens under operating conditions, but they can be tested in environments bled off from operating systems. 5.2 Specimen design 5.2.1 Figure 1 shows some of the pre-cracked specimen geometries which are used for stress corrosion testing. SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 7
NOTE Stress intensity factor coefficients for the specimens shown above are available in the published literature. Figure 1 — Pre-cracked specimen geometries for stress corrosion testing SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 8 © ISO 2003 — All rights reserved 5.2.2 Constant load specimens can be of two distinct types: a) those in which the stress intensity increases with increasing crack length; b) those in which the stress intensity is effectively independent of crack length. Type a) is suitable for KISCC determinations and studies of crack propagation rates as a function of KI, while type b) is useful for fundamental studies of stress corrosion mechanisms. 5.2.3 Increasing K constant load specimens can be subjected to either tension or bend loading. Depending on the design, tension loaded specimens can experience stresses at the crack tip which are predominantly tensile (as in remote tension types such as the centre-cracked plate) or contain a significant bend component (as in crackline loaded types such as compact tension specimens). The presence of significant bending stress at the crack tip can adversely affect the crack path stability during stress corrosion testing and can facilitate crack branching in certain materials. Bend specimens can be loaded in 3-point, 4-point or cantilever bend fixtures. 5.2.4 Constant K constant load specimens can be subjected to either torsion loading as in the case of the double torsion single edge cracked plate specimen, or tension loading as in the case of contoured double cantilever beam specimens. Although loaded in tension, the design of the latter specimens produces crackline bending with an associated tendency for crack growth out of plane, which can be curbed by the use of side grooves. 5.2.5 Constant displacement specimens are usually self-loaded by means of a loading bolt in one arm which impinges on either an anvil or a second loading bolt in the opposite arm. Two types are available: a) those which are (W-a) dominated such as the modified wedge opening loaded (modified WOL) specimen in which the proximity of the back face to the crack tip influences the crack tip stress field; b) those which are (W-a) indifferent, such as the double cantilever beam (DCB) specimen in which the back face is sufficiently remote from the crack tip to ensure that its position has a negligible effect on the crack tip stress field. 5.2.6 A number of the specimen geometries described above have specific advantages which have caused them to be frequently used for stress corrosion testing. These include: a) cantilever bend specimens, which are easy to machine and inexpensive to test under constant load; b) compact tension (CTS) specimens, which minimize the material requirement for constant load testing; c) self-loaded double cantilever beam (DCB) specimens, which are easy to test under constant displacement in service situations; d) modified wedge opening loaded (modified WOL) specimens, which are also self-loaded and minimize the material requirement for constant displacement testing; e) C-shaped specimens, which can be machined from thick-walled cylinders in order to study the radial propagation of longitudinally oriented cracks under constant load. Details of standard specimen designs for each of these types of specimen are given in Figures 2 to 6. 5.2.7 If required, e.g. if fatigue crack initiation and/or propagation is difficult to control satisfactorily, a chevron notch configuration as shown in Figure 7 may be used. If required, its included angle may be increased from 90° to 120°. 5.2.8 Where it is necessary to measure crack opening displacements, as during the application of deflection to constant displacement specimens, knife edges for the location of displacement gauges can be machined into the mouth of the notch, as shown in Figure 8 a). Alternatively, separate knife edges can either be screwed or glued onto the specimen at opposite sides of the notch, as shown in Figure 8 b). Details of a suitable tapered beam displacement gauge are given in Figure 9. SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 9 Dimensions in millimetres,
surface roughness values in micrometres
Width = W Thickness, B = 0,5 W Notch width, N = 0,065 W maximum (if W > 25 mm) or 1,5 mm maximum (if W u 25 mm) Effective notch length, l = 0,25 W to 0,45 W Effective crack length, a = 0,45 W to 0,55 W
Figure 2 — Proportional dimensions and tolerances for cantilever bend test pieces SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 10 © ISO 2003 — All rights reserved Dimensions in millimetres, surface roughness values in micrometres
Net width = W Total width, C = 1,25 W minimum Thickness, B = 0,5 W Half height, H = 0,6 W Hole diameter, D = 0,25 W Half distance between hole outer edges, F = 1,6 D Notch width, N = 0,065 W maximum Effective notch length, l = 0,25 W to 0,40 W Effective crack length, a = 0,45 W to 0,55 W
Figure 3 — Proportional dimensions and tolerances for compact tension test pieces SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 11 Dimensions in millimetres, surface roughness values in micrometres
Key 1 Screw tip radius 12,5 - 50 Half height = H Thickness, B = 2 H Net width, W = 10 H minimum Total width, C = W + d Screw diameter, d = 0,75 H minimum Notch width, N = 0,14 H maximum Effective notch length, l = 2 H NOTE 1 “A” surfaces should be perpendicular and parallel as applicable to within 0,002 H TIR. 2 At each side point “B” should be equidistant from the top and bottom surface to within 0,001 H. 3 The bolt centreline should be normal to the speciment centreline to within 1°. 4 The bolt material should be similar to that of the specimen, fine-threaded with a square or Allen-screw head. Figure 4 — Proportional dimensions and tolerances for double cantilever beam test pieces SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 12 © ISO 2003 — All rights reserved Dimensions in millimetres, surface roughness values in micrometres
a All over Thickness = B Net width, W = 2,55 B Total width, C = 3,20 B Half height, H = 1,24 B Hole diameter, D = 0,718 B ± 0,003 B Effective notch length, l = 0,77 B Notch width, N = 0,06 B Thread diameter, T = 0,625 B Distance from hole centreline to notch centreline, F = 0,239 B NOTE 1 Surface should be perpendicular and parallel as applicable to within 0,002 H TIR. 2 The bolt centreline should be normal to the specimen centreline to within 1°. 3 The bolt material should be similar to that of the specimen, fine-threaded with a square or Allen-screw head. Figure 5 — Proportional dimensions and tolerances for modified wedge opening loaded test pieces SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 13 Dimensions in millimetres, surface roughness values in micrometres
Net width = W Thickness, B = 0,50 W ± 0,01 W Distance from the hole axes to a tangent with the inner surface, X = 0,50 W ± 0,005 W Notch width, N = 1,5 mm minimum (0,1 W maximum) Notch length, l = 0,3 W Distance from the hole axes to face of specimen, Z = 0,25 W ± 0,01 W Distance from the hole axes to outer surface, T = 0,25 W ± 0,01 W Diameter of holes, D = 0,25 W ± 0,005 W NOTE All surfaces should be perpendicular and parallel as applicable to within 0,002 W TIR and “E” surfaces should be perpendicular to “Y” surfaces to within 0,02 W TIR. Figure 6 — Proportional dimensions and tolerances for C-shaped test pieces SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 14 © ISO 2003 — All rights reserved Dimensions in millimetres
a Mill with a 60° cutter, notch root radius 0,3 mm maximum for all test piece sizes Figure 7 — Chevron notch SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 15
a)
Integral type
b)
Screw-on type NOTE Provided adequate strength can be assured, the above knife edges may be fixed using adhesive. Figure 8 — Knife edges for location of displacement gauges SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 16 © ISO 2003 — All rights reserved Dimensions in millimetres
a)
Displacement gauge mounted on a test piece
b)
Dimensions of beams SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 17
c)
Bridge measurement circuit a This dimension should be 3,8 × the minimum initial gauge length b Beam thickness taper 0,5 to 0,8 NOTE Strain gauges and materials should be selected to suit the test environment. Figure 9 — Details of tapered beam displacement gauge 5.3 Stress intensity factor considerations 5.3.1 It can be shown, using elastic theory, that the stress intensity factor acting at the tip of a crack in specimens or structures of various geometries can be expressed by relationships of the form: I=QaKσ×× where Q is the geometrical constant; σ is the applied stress; a is the crack length. 5.3.2 The solutions for KI for specimens of a particular geometry and loading method can be established by means of finite element stress analysis or by either experimental or theoretical determinations of specimen compliance. 5.3.3 Stress intensity factors can be calculated by means of a dimensionless stress intensity coefficient, Y, related to crack length expressed in terms of a/W, or a/H for (W-a) indifferent specimens, where W is the width and H is the half height of the specimen, through a stress intensity function of the form: IYP=KBW for compact tension or C-shaped specimens or IYP=KBa for T-type wedge opening loaded specimens or IYP=KBH for double cantilever beam specimens. SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 18 © ISO 2003 — All rights reserved 5.3.4 Where it is necessary to use side-grooved specimens in order to curb crack branching tendencies, etc., shallow side grooves (usually 5 % of the specimen thickness on both sides) can be used. Either semi-circular or 60° V-grooves can be used, but it should be noted that even with semi-circular side grooves of up to 50 % of the specimen thickness, it is not always possible to maintain the crack in the desired plane of extension. Where side grooves are used, the effect of the reduced thickness, Bn, due to the grooves on the stress intensity can be taken into account by replacing B by nBB× in the above expressions, however, the influence of side grooving on the stress intensity factor is far from established and correction factors should be treated with caution, particularly if deep side grooves are used. 5.3.5 Solutions for Y for specimens with geometries which are often used for stress corrosion testing are given in Figures 10 to 14.
Dimensions in millimetres
23yLLI323(0,6)4(0,6)EVHHaHHKaHHa×++=++ NOTE This expression was derived from elastic compliance theory and although its inaccuracy and validity limits are not well-defined, it has been used over the range 25aHuu. For greatest confidence it is recommended that an empirical compliance be used. Figure 10 — Stress intensity factor solution for double cantilever beam specimen [(W-a) indifferent] SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 19
a V, the crack-opening displacement (COD) for a rigid bolt, is a constant (g − gi) IYPKBa= where 234530,96195,8730,61186,3754,6aaaaaYWWWWW=−+++ NOTE This expression was derived from elastic compliance theory and although its inaccuracy and validity limits are not well-defined, it has been used over the range 30,8.aWuu For greatest confidence, it is recommended that an empirical compliance be used. Figure 11 — Stress intensity factor solution for modified wedge opening loaded specimen SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 20 © ISO 2003 — All rights reserved
IYPKBW= where 3316,2111aYWaW=−+− in the case where S = 1,5 W NOTE This expression was originally derived from the combined techniques of stress analysis and compliance and although its inaccuracy and validity limits are not well-defined, it has been used over the range 0,20,6.aWuu For greatest confidence it is recommended that an empirical compliance be used. Figure 12 — Stress intensity factor solution for cantilever bend specimens SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 21
IYPKBW= where 234320,8864,6413,3214,725,61aaaaaWYWWWWaW+=+−+−− NOTE The inaccuracy of this expression is considered to be no greater than ± 0,5 % over the range0,21,0.aWuu Figure 13 — Stress intensity factor solution for compact tension specimens
ISO 7539-6:2003(E) 22 © ISO 2003 — All rights reserved
IYPKBW= where 35791218,23106,2397,7582,0369,111,540,510,2211aaaaaXaarYWWWWWWWWr=−+−×++×+−− NOTE The inaccuracy of this expression is considered to be no greater than 1 % over the range 0,450,55aWuu. However, it can be used over the wider range 0,30,7aWuu when 00,7xWuu and 1201,rruu in which case the accuracy is believed to be no greater than 2 %. Figure 14 — Stress intensity factor solution for C-shaped specimens SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) © ISO 2003 — All rights reserved 23 5.4 Specimen preparation 5.4.1 Residual stresses can have an influence on stress corrosion cracking. The effect can be significant when test specimens are removed from material in which complete stress relief is impractical, such as weldments, as-quenched materials and complex forged or extruded shapes. Residual stresses superimposed on the applied stress can cause the localized crack-tip stress intensity factor to be different from that computed solely from externally-applied loads. The presence of significant residual stress often manifests itself in the form of irregular crack growth, namely excessive crack front curvature or out-of-plane crack growth, and generally indicates that residual stresses are affecting behaviour. Measurement of residual stress is desirable. 5.4.2 Specimens of the required orientation (see Figure 15) should, where possible, be machined in the fully heat-treated condition. For specimens in material that cannot easily be completely machined in the fully heat-treated condition, the final heat treatment may be given prior to the notching and finishing operations provided that at least 0,5 mm per face is removed from the thickness at this finish machining stage. However, heat treatment may be carried out on fully machined specimens in cases in which heat treatment will not result in detrimental surface conditions, residual stress, quench cracking or distortion. 5.4.3 After machining, the specimens shall be fully degreased in order to ensure that no contamination of the crack tip occurs during subsequent fatigue pre-cracking or stress corrosion testing. In cases where it is necessary to attach electrodes to the specimen by soldering or brazing for crack monitoring by means of electrical resistance measurements, the specimens shall be degreased following this operation prior to pre-cracking in order to remove traces of remnant flux. 5.5 Specimen identification Specimen identification marks may be stamped or scribed on either the face of the specimen bearing the notch or on the end faces parallel to the notch. 6 Initiation and propagation of fatigue cracks 6.1 The machine used for fatigue cracking shall have a means of loading such that the stress distribution is symmetrical about the notch and the applied load shall be known to an accuracy of ± 2,5 %. 6.2 The environmental conditions apparent during fatigue pre-cracking, as well as the stressing conditions, can influence the subsequent behaviour of the specimen during stress corrosion testing. In some materials, the introduction of the stress corrosion test environment during the pre-cracking operation will promote a change from the normal ductile transgranular mode of fatigue cracking to one that more closely resembles stress corrosion cracking. This may facilitate the subsequent initiation of stress corrosion cracking and lead to the determination of conservative initiation values of KISCC. However, unless facilities are available to commence stress corrosion testing immediately following the pre-cracking operation, corrodent remaining at the crack tip may promote blunting due to corrosive attack. Furthermore, the reproducibility of results may suffer when pre-cracking is conducted in the presence of an aggressive environment because of the greater sensitivity of the corrosion fatigue fracture mode to the cyclic loading conditions. In addition, more elaborate facilities may be needed for environmental control purposes during pre-cracking. For these reasons, it is recommended that, unless agreed otherwise between the parties, fatigue pre-cracking shall be conducted in the normal laboratory air environment. 6.3 The specimens shall be pre-cracked by fatigue loading with an R value in the range 0 to 0,1 until the crack extends at least 2,5 % W or 1,25 mm beyond the notch at the side surfaces, whichever is greater. The crack may be started at KI values higher than the expected KISCC but, during the final 0,5 mm of crack extension, the fatigue pre-cracking shall be completed at as low a maximum stress intensity as possible (less than 60 % of the expected KISCC). NOTE Load shedding procedures as described in ISO 11782-2 may be helpful when the KISCC values are expected to be low. SIST EN ISO 7539-6:2003

ISO 7539-6:2003(E) 24 © ISO 2003 — All rights reserved
a)
Basic fracture plane identification: rectangular section
1) Radial grain flow — Axial working direction 2) Axial grain flow — Radial working direction b)
Basic fracture plane identification: cylindrical sections
c)
Non-basic fracture plane identification a Grain flow Figure 15 — Fracture plane ident
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