UDK 621.791.05:539.42:620.179.1:669.14.018.298 Izvirni znanstveni članek ISSN 1318-0010 KZLTET 33(1-2)33(1999) FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS OCENITEV SPREJEMLJIVOSTI NAPAK ODKRITIH V ZVARNIH SPOJIH VISOKOTRDNOSTNIH JEKEL Inoslav Rak Univerza v Mariboru, Fakulteta za strojništvo, Smetanova 17, 2000 Maribor Prejem rokopisa - received: 1998-11-10; sprejem za objavo - accepted for publications: 1999-02-19 The flaw size in weld joint can be determined by non destructive examination (NDE). Because of different materials, and loading as well as because of the possible effect of corrosive envirnoment the question arises how to assess reliably the allowable flaw size in different weld joint parts. The presence of flaws is obvious but the possibilities of their revealing are limited and not always possible. The flaws size and distribution are the essential parameters for the structure capacity of bearing under high loading the weld joint. The larger is the allowable flaw size anticipated, the safer is the welded structure, and the easiest is the detection of the flaw size by NDE methods. Thus, for assessing the safety of complex loaded welded structure, machine parts or equipment life time, it is obligatory to consider the requirements of different "Fitness for Purpose" systems. The article presents the possibility of assessing the detected flaw by means of NDE if the material fracture toughness of the area where the fatigue crack tip located is known. The fatigue crack represents the severest discontinuity that can occur in a welded joint. The principles of IIW Guidance on Assessment of The Fitness for Purpose of Welded Structures - IIW/IIS-SST-1157-90 and BS PD 6493 and separately ETM that treats mis-matched weld joints are shown and used. Key words: weld joint, allowable flaw size, fracture toughness, strength mis-match, fitness for purpose Že desetletja lahko dovolj dobro in natančno z neporušnimi metodami določamo in diagnosticiramo napake v zvarnih spojih. Glede na raznolikost materialov in njih izkoriščenost ter vrste obremenitve ob prisotnosti različnih medijev v zahtevnih nosilnih konstrukcijah, stopa vse bolj v ospredje problem kako zanesljivo oceniti dopustno velikost napake v raznih delih zvarnih spojev. Vemo, da zvarni spoji niso brez napak, vendar je možnost njihovega odkrivanja omejena, odkrivanje pa ni vedno izvedljivo. Za nosilnost visoko obremenjene varjene konstrukcije je torej bistvena velikost dopustne napake. Cim večja je, tem varnejša je konstrukcija in tem lažje jo odkrijemo z neporušnimi preizkavami. Zato je za ocenitev varnosti zelo zahtevno obremenjene konstrukcije, strojnega dela ali opreme potrebno upoštevati priporočila, ki jih podajajo različni sistemi znani pod mednarodnim izrazom "Fitness for Purpose". V prispevku je prikazan primer, kako je možno na osnovi poznavanja lomne žilavosti materiala, v katerem se nahaja konica utrujenostne razpoke, ki predstavlja najostrejšo možno nezveznost, na osnovi poznavanja zakonitosti elasto-plasto mehanike loma, določiti, ali je dopustna z defektoskopskimi metodami odkrita napaka v zvarnem spoju. V ta namen so prikazani in uporabljeni principi priporočila BS PD 6493 in posebej še ETM (Engineering Treatment Model), ki obravnava trdnostno heterogene zvarne spoje (mis-matching). Ključne besede: zvarni spoj, dopustna velikost napake, lomna žilavost, trdnostna heterogenost, primernost za uporabo 1 INTRODUCTION At the present state of the art available NDE equipment enable to detect the flaws in welded joints by combination of one or more methods. The codes and standards for welded structures with high bearing capacity prescribe with respect to the loading and the utilisation of construction details or engine parts, the type and size of allowable flaws for quality control. Only separated pores or non-metallic inclusions are permitted. Planar discontinuities (cracks, lack of fusion, lack of penetration, etc.) are not permitted. Problem arises when the quality of a welded joint is limited by the possibilities and capabilities of NDE existing methods. Practical experience confirms that the confidence of flaw detection is about 60 to 70% of all present flaws in welded joints. If the flaw acceptance and quality of weld joints are assessed by the concept of "Fitness for Purpose" it has to be kept in mind that non detectable flaws are also present. For this reason, it is essential to know the critical flaw size (or at least the order of its size) which can cause non-stability or, in the worst case, a catastrophic fracture of a severe loaded welded structure. If the fracture toughness properties at the top of a sharp planar discontinuity are determined, an allowable flaw size can be predicted or the allowability of the detected flaw can be assessed.. An essential important understanding is the larger the allowable flaw determined by the "Fitness for Purpose" concepts, the higher the safety in the welded structure in case of a sudden over-loading. On the other hand, the confidence of detection of a flaw, which appears as consequence of a poor welding procedure, is easier and more efficient. To realise the explained concept it is necessary to determine the following parameters: to measure fracture toughness, to set the dimensions of the detected flaw, to analyse the stress state around the crack tip at the limit loading condition, and to take into account also the overloading stresses using the fracture mechanics rules and the safety factors, to determine through thickness half crack length and at the end to transform this value into an allowable planar crack size (length and depth of surface or embedded flaw). In this article the procedure to determine the mentioned parameters will be shortly explained with the final target to forecast the order of magnitude of allowable planar discontinuity in a welded joint. KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 33 I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS 2 FRACTURE TOUGHNESS DETERMINATION Usually, the fracture toughness of welded joints is measured on the whole thickness and at the lowest construction operating temperature. For ductile micro-alloyed and Q+T steels and their weld joints the elastic-plastic concept CTOD (Crack Tip Opening Displacement) is used. The fracture toughness parameter at the onset of crack initiation as Si or at the moment of instability as 5c or 5u is determined from R-curves provided by testing on specimens which size and shape are prescribed in the recently issued standard BS 7448:Part 2:1997 which addresses also the mis-matched weld joints1, and other standards valid for uniform materials (BM) in latest modification2,3. An example of specimen instrumentation for the CTOD test before testing is presented in Figure 1. The procedure requires first to saw cut the specimen at the weld joint desired area with the micro-structure of the weld metal (WM) and heat affected zone (HAZ) of interest, than to fatigue it to produce a sharp crack tip. Fracture toughness standards are useful for welded joints only under specific Table 1: Single and average CTOD values and WM/BM hardening exponents Tabela 1: Posamezne in povpre~ne CTOD vrednosti in koeficienti utrjevanja za SZ/OM Testing location CTOD(BS) CTOD(S5) Hardening exponent nw, nB (mm) (mm) a/W=0.5 a/W=0.26 a/W=0.5 a/W=0.26 (Sc) (Su) (Sc) (Su) Bx2B BxB Bx2B BxB WM 0.085 0.214 0.116 0.233 0.061 - cap 0.128 0.185 0.123 0.229 0.056 - root 0.090 (0.046 Si) 0.085 (0.026 Si) 0.098 0.303 0.099 0.366 0.104 (0.008 Sc) - (0.019Sc) 0.086 0.079 0.130 0.234 av. 0.117 0.276 av. 0.100 av. 0.103 av. 0.059 - av. BM 0.123 Si 0.150 Si 0.137 av. 0.211 Si 0.151 Si 0.181 av. 0.097 - av. 34 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS Figure 2: Planar flaw interactions Slika 2: Vzajemno delovanje planarnih napak corrections and additional measures such as: special procedure for obtaining the fatigue crack, yield stress determination of the region where the crack tip is located, the consideration of the mis-match properties between (WM) and (BM), and the crack depth a/W = 0.5. This matter is extensively described in ref.4,5,6. In Table 1 the fracture toughness CTOD results calculated by prescribed BS procedure1 and GKSS proposed direct CTOD-§5 measurement7 are presented. KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 A good correlation between both CTOD determinations concepts is found. A large disagreement between CTOD values determined on specimens with the standardised crack (a/W=0.5) and specimens with shorter crack (a/W=0.26) can be also recognised. As already known, the loading constraint conditions are higher in small standardised specimens with standardised crack size (a/W=0.5) than in weld joints with planar flaws found in a construction loaded by yielding. To overcome this I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS problem and to handle with more realistic fracture toughness data the correlation between fracture toughness Kic and Charpy impact toughness energy valid for wide plate tested specimens8 was used to determine the critical CTOD-5c by following procedure. The improved Barsom-Rolfe correlation between KIC and the absorbed energy valid for wide plate test in original form is: K \ 2 100 = 300 / \ vE vG y y (1) In the correlation the following units are used: Kic - kp/mm3/2 vE, 2mm Charpy energy - kpm Gy - kp/mm2 is: By vE=60 J at -10°C and Gy=848 MPa the KIC value KIC=147 MP m1/2 at -10°C Introducing KIC into equation: 5 c = KfC Eg y (2) the WM CTOD valid for wide plate test can be obtained: and for BM 5c=0.121 mm at -10°C 5c=0.163 mm at -10°C Comparing these fracture toughness values with fracture toughness values obtained by testing of small standardised CTOD specimens, higher toughness is found as affect of lower constraint conditions in wide plate loaded specimens. It seems that this differences at moderate fracture toughness level are not significant. Presently a, project of measuring CTOD fracture toughness on Wide plate specimen, Small standardised CTOD specimens and Charpy toughness specimens to determine the correlations among them is in realisation. The calculated hardening coefficients n9,10 for BM and WM are added in Table 1 as well. 3 EFFECTIVE FLAW SIZE Single weld joints flaws are rarely found and few flaws can be found in a specific region. Flaws could be parallel to each other or can even overlap to some extent and influence load caring capacity differently. The recommendation PD 6493-917 distributes the discontinuities into coplanar and non-coplanar embedded or surface flaws, as shown in Figure 2a, 2b and 2c, for example. The interaction effect and the proposed new effective flaw size can be seen clearly. For flaw acceptance analysis it is necessary to know the following dimensions: 2a- for a trough thickness crack, a- and 2c-for a surface crack and 2a- and 2c- for an embedded crack (see the symbols in Figures 2). Figure 3: Schematic representation of stress distribution across section Slika 3: Shematski prikaz porazdelitev napetosti preko preseka 4 CIRCUMSTANCES DETERMINATION AT THE TRANSVERSALLY FULLY LOADEC CRACK TIP To assess the allowance of detected flaws or to predict the allowable flaw size it is necessary to determine the stress field in which the crack is situated. The stresses which should be taken into account are schematically presented in Figure 3 and are: - Membrane stress-Pm, - Bending stress-Pb, - Secondary stress-Q (residual and thermal stresses) - Peak stress-F (stresses due to concentrations at local discontinuities-nozzles, weld misalignment-angular distortion and offset, holes notches, sharp angles etc.). The resulting real stress is a the sum of stresses which can act at the planar crack tip. 5 ALLOWANCE OF PLANAR FLAWS For easier understanding a simplified assessment will be presented. The analysis shows (Level 1) whether the planar flaw is a risk for fracture appearance or it can be assessed as allowable without employing a more complex assessment of allowability (as Level 2 or Level 3). This access incorporates a safety factor of about 2. The allowable planar through thickness flaw size takes into account the loading conditions at the crack tip G1/Gy>0.5. It can be calculated from: 5E a =_mat__(3) "max / \ V-V 2p - 0.25 Gy G1= max. applied tensile stress (Pm+Pb+Q+F) in MPa Gy= yield stress or determined from Figure 4. The allowable trough crack size am is: 36 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS a = C matE ö V sy y and C is calculated for ferritic steel as: C =_1_ ^ S X 2p ^ - 0.25 vGy (4) (5) interaction between tension and bending which effects the collapse behaviour: Sr = ^ s / (10) or determined graphically, as shown in Figure 4. The factor C represents the loading conditions of a weld joint. The calculated value should be checked for the possible plastic collapse. The planar flaw is allowable if: •^§7 < < 0.707 a/2 By the CTOD fracture ratio of: &=lf V§ mat with §i as applied CTOD (driving force), and §mat as the measured CTOD by specimen testing (6) (7) with On=net section stress in MPa and Sf=flow stress of the material in MPa The flow strength is the average of the yield stress and of the tensile strength up to a maximum 1.2sy. For the net section stress simple equations are derived, which take into account a straight plate, a shell of an penstock, or a pressure vessel for planar through thickness as welded surface or and embedded flaw. For a bended plate which represents a shell of a pressure vessel the on in accordance with the appendix of as a reference11 can be determined as: On=1.2MTPm (11) The non-dimensional factor for stress raising MT is calculated as: Mt= <|1 + 3.2 yDBy §1 = K Es y v° 1 0 ^ -0.25 Vs y with Sy=weld metal yield stress in MPa and Ki = s j V(pa) (8) (9) and the material flow strength sf as: s / = S y +S 2 (12) (13) with S1 as max. applied tensile stress (Pm+Pb+Q+F) in MPa and the a according to equation (3) the calculation is acceptable by the ratio of plastic collapse Sr<0.8, as it is shown in Figure 5. To determine the ratio Sr it is necessary to take into account the stress sn, acting as net- section stress and the Determination o/ allowable /law size in the weld joint o/ a severe loaded penstock As an example the pressurised penstock assembled by SMAW and SAW welding procedures on quenched Figure 4: Values of constant C for different loading conditions-level 1 Slika 4: Vrednosti za konstanto C za različne obremenitvene primere-stopnja 1 Figure 5: Level 1 and level 2 failure assessment diagram Slika 5: Ocenitveni diagram napak po stopnji 1 in stopnji 2 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 37 2 a s y I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS and tempered (Q+T) steel grade HT80, was treated. The main data are following: - Thickness, B: 40 mm - Pipe diameter, D: 4200 mm - BM yield stress : Oy=693 MPa - BM tensile strength: om=838 MPa - WM yield stress: Oy=848 MPa - WM tensile strength: om=917 MPa - Tested BM CTOD at -10°C: Smat=0.163 mm - BM impact toughness at -40°C: ak=50 J/cm2 - Membrane stress: Pm=315MPa - Bending stress: Pb=100MPa - Residual stresses: Q=700MPa - Local stress concentrations: F=150 MPa - Tested WM CTOD at -10°C: Smat=0.121 mm - WM impact toughness at -40°C: ak=40 J/cm2 s y 1.21 - Mis-match factor: M= s y BM Due to beyond equations the values for allowable WM planar flaws size are as follows: - Allowable planar through thickness flaw size am =3.8 mm, derived from equation (3) and Figure 4, by C=0.128 and the tensile stress ratio Pm+Pb+Q+F/sy=1.492 - The planar through thickness flaw size of 2 mm is chosen, because of a possible plastic collapse and in accordance with the calculation by using equations (6), (7), (8), (9) and (10) - Plastic collapse ratio from equation (10) Sr=0.43 - CTOD fracture ratio ^/§7=0.623, by using equations (7), (8) in (9) - The planar flaw size according to Level 1 is allowable because Sr<0.8 and -^/ô^<0.707. As shown in Figure 5 the flaw size is in the permitted field framed by the assessment line and no additional partial safety factors are required. Determination of the equivalent part thickness flaw size The transformation from through thickness to part thickness is obtained according to reference11 after having obtained am and if the term am >a is fulfilled according to parameters in Figure 6. If the ratio a/B=2.0/40=0.05 is assumed the allowable dimension of the a planar surface flaw using Figure 6 is as shown in Table 2. From Table 1 it can be recognised that the allowable planar flaw size, as crack, lack of fusion or lack of root penetration are small and not easy detectable by NDE methods and particularly difficult by X ray wxamination, if the locations of the cracks in WM and HAZ are inclined to the X-ray beam by an angle larger than 20° and the thickness is higher than 20 mm. Table 2: Allowable planar surface flaw sizes Tabela 2: Dopustne dimenzije površinskih napak a/2c a/B a allow. 2c allow. (mm) (mm) 0.0 0.037 1.48 ¥ 0.1 0.044 1.76 17.6 0.2 0.055 2.20 11.0 0.3 0.062 2.48 8.30 0.4 0.090 3.60 9.00 0.5 0.113 4.52 9.10 6 USE OF ENGINEERING TREATMENT MODEL (ETM) FOR MIS-MATCHED WELD JOINTS By high BM tensile strength (800 MPa) a high WM toughness is generally not obtained (higher than BM) and the reliability of a welded joint is assessed by means Figure 6: Relationship between actual flaw dimensions and the parameters of surface flaws Slika 6: Odnos med dejansko velikostjo napake in parametri površinske napake 38 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS of ETM developed by K.-H. Schwalbe at the GKSS (12,13). The principle of the model is the mis-match ratio between yield stress of WM and BM which results in a different hardening ability of both materials. In the treated case the mis-matching factor M>1 (M is the ratio between weld metal yield stress and base material yield stress) and the weld joint is in over-matched condition. This behaviour can be used for the assessment of small WM planar flaws in elastic stress in over-loading condition, while the BM is strained. The size of the acceptable planar flaw can be larger than that determined using reference11. This difference will grow in proportion to the mis-matching factor M. In Figure 7, an example of mis-matching loading ranges and of the WM fracture toughness requirements according to mis-match condition M (1>M>1) in each range is shown. The formulations for the calculation of the driving force Sw are added. Driving force ratio Sr for the over-matched weld joint The crack driving force ratio for a weld metal §r=§W/§B can be calculated using the equations (14), (15), and (16) from Figure 7, while the base metal driving force is expressed as Sb=15 paeB. For three loading ranges and for an over-matched weld joint the crack driving force ratio can be expressed as function of the lower and the upper limit loading as follows: Loading range 1: For the lower limit e/eyB^0 1 S R = — < 1 R M and for the upper limit e/eyB^1 s 2M2 +1 , Sr =-^ < 1 R 3M3 (17) (18) Loading range 1: Base material and weld metal material are deformed bellow their respective yield strength, 0FyW. The weld metal CTOD driving force is S w =-^ • My E (14) (15) (16) Figure 7: ETM for mis-matched weld joints and crack driving force Slika 7: ETM za zvarne spoje s trdnostno heterogenostjo in gonilna sila odpiranja razpoke KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 39 I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS Loading range 2: The lower limit is equal to the upper limit of loading range 1 1 e — For the upper limit-= MnB e -yB ô R = M (1—) < 1 (19) Loading range 3: The lower limit is equal to the upper limit of loading range 2. If the strain ew in weld metal is used as the global strain e=eB, the plastic properties of WM and BM yield the ôR as term of the normalised applied strain according to the following equations: ee w _ (__) n ee :w :B syB nw (1 -1) ÔR _ M 'yB H nw J-1 J (20) These equations give the required minimum toughness of the WM compared to the BM if the following solution is satisfied: ôô Ô cB ô B (21) conditions the driving force ratio ôR is smaller than the measured material fracture toughness ratio and the requirement in equation (21) is fulfilled. Normalised driving force ô^* for the over-matched weld joint For the design curve consideration the formalism proposed in the British CTOD Design Curve1113 can be applied. The normalised applied CTOD is defined in weld metal as the driving force ôc related to the local weld metal stress S1. When ôc is the critical CTOD and am is the defect size equal to one half of the defect length ac: ôc _ ô CE 2pamsy / \2 s Vs y ; for — < 05 (22) and the normalised applied CTOD in the weld metal is defined as: ô E ô E ô w* _ _ (23) 2ps y 2pMs yB Loading range 1: Combining equation (14) from Figure 7 and equation (18) and having in mind a small e/eyB ratio it can be set: ôw* _ J_ M2 / \2 e VeyB 0 (24) In such a case the toughness performance of weld joint with over-matching WM is equal to or even better than that of BM. In Figure 8 the driving force ratio ôr is shown as function of the normalised strain e/eyB for a treated over-matched weld joint. For all over-matched For the upper limit given by e = eB the solution is: ôw*_ J_ M2 1 + - 1 2M2 (25) Table 3: Driving force ratio ôr, driving force ôw and normalised driving force of weld metal CTOD ôCw* Tabela 3: Odnos gonilne sile ôr, gonilna sila ôw in normalizirana gonilna sila strjenega zvara CTOD ôcw* n B s y Loading stress (MPa) e/eyB M ôR ôW (mm) ôw* Loading range 1 0.1 1 1 0.0030 0.0100 0.1 1.1 0.909 0.0026 0.0080 lower limit 0.1 1.2 0.833 0.0022 0.0069 ss> yw> > yB 6.589 1.2 0.182 0.598 1.5 15.077 1.3 0.086 0.647 1.5 Loading range 3 10 1 4.410 21.90 66.17 10 1.1 0.959 4.345 13.08 s ^ s yW< > yB 10 1.2 0.238 0.989 2.978 50 1.3 0.186 0.292 0.761 40 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS Toughness of a over-matched weld metal is equal to or even better than that of base material if the following toughness requirements is met: Scw >Sw = gs ScB SB This requirement is met at the loading range 1 (when e/ey=1 or bellow), at the loading range 2 (when e/ey=M1/nB=7.76 or bellow) and at the loading range 3 (when for instance e/ey=10 or bellow). Data: weld joint mis-match M=1.21, syB=639 MPa, syw=848 MPa, Scw=0.121 mm and ScB=0.163 mm at -10°C, nB=0.097, nw=0.059. Figure 8: Driving force ratio as a function of normalized strain e/ey for treated over-matched weld joint Slika 8: Odnos gonilne sile v odvisnosti od normalizirane deformacije za obravnavani zvarni spoj e/EyB Figure 9: Assessment of WM critical crack size in loading ranges 1, 2 and 3 in the as-welder over-matched weld joint M=1.21 Slika 9: Ocenitev velikosti napake v strjenem zvaru v področjih obremenitev 1, 2 in 3 zvarnega spoja s trdnostno heterogenostjo M=1.21 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 41 I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS Loading range 2: Combining equation (15) for §w from Figure 7 and equation (18) we obtain for the upper limit given by e/eyB = M1/nB the constant value: § w = 15 (26) Loading range 3: Combining equation (16) from Figure 7 and equation (18) the normalised form is obtained as: SI, = 15 " 1 " 1 nw e _ M _ _eyB _ (27) By fully plastic condition §w can be written as §w = 1.5paew. The normalisation of the equation (18) leads to S*w = 15 Ve yw 0 (28) The values for normalised weld metal CTOD §w* as the function of the applied normalised global strain for treated over-matched weld joint is presented in Figure 9. In Table 3 all data for the driving force ratio §R, the driving force §w and the normalised driving weld metal CTOD §cw* for different weld joint over-matching conditions are given as function of the applied normalised global strain. 7 CRITICAL CRACK LENGTH ESTIMATION Inserting the measured §W=0.121 mm and yield stress sy = 848 MPa for over-matched weld metal into the normalised CTOD driving force expression §W*(18) the normalised critical crack length a* can be derived: S E § E § w* = (29) 2ps y paMs yB = ac Ps yw a* = - § E (30) ac =- p§ §* yww (31) §W*=§WE/apsyw = 1.514 valid for CTOD §c=0.121 mm at -10°C if a = 6.3 mm §W*=§WE/apsyw = 3.170 valid for CTOD §c=0.121 mm at -10°C if a = 3 mm The allowable planar trough thickness flaw sizes shown in Figure 9 are due to three different weld joint loading ranges. By transforming this flaw size into a part through flaw size, as mentioned above, the WM allowable planar crack size in the weld joint operating in mis-matched condition can be determined. In Table 4 the allowable planar surface crack size for through flaw size a=6.3 mm (for a/B=6.3/40=0.158) and overloading by Pm+Pb+Q+F/sy=7.6 is presented. By comparing allowable surface crack sizes in Table 2 and Table 4 one can recognise that a 6-8 times larger flaw size much easier to detect by NDE is permissible due to over-matching condition M=1.21. Table 4: Allowable part thickness planar flaw sizes determined by ETM for weld joint with M>1 Tabela 4: Dopustna velikost površinske napake določena po ETM za zvarni spoj z M>1 a/2c a/B a allow. (mm) 2callow. (mm) 0.0 0.105 4.20 8 0.1 0.131 5.24 52.4 0.2 0.168 6.72 33.6 0.3 0.215 8.60 28.6 0.4 0.255 10.2 25.5 0.5 0.320 12.0 24.0 Hence, the absolute value ac can be derived with §W* for the appropriate loading ranges. a§ cwE In Figure 9 the loading ratios e/ey are presented as function of a different selected critical crack length (a = 32, 10, 6.3 and 3 mm) by as welded over-matched condition M=1.21 and by the critical weld metal CTOD value §cw=0.121 mm. The values for §w* are as follows: §W*=§WE/apsyw = 0.298 valid for CTOD §c=0.121 mm at -10°C if a = 32 mm §W*=§WE/apsyw = 0.953 valid for CTOD §c=0.121 mm at -10°C if a = 10 mm 8 CONCLUSIONS The following conclusions are proposed: - The acceptability of planar discontinuities in a weld joint can be determined on the basis of the knowledge of the material properties and of the stress field in which the discontinuity is located. - By using of recommendations, such as BS PD 6493-91, IIW Guidance on Assessment of the Fitness for Purpose of Welded Structures and ETM the detected weld joint flaws can be assessed and the allowable flaw size before NDE can be determined. The larger is the determined allowable flaw size, the safer is the welded structure and at the same time the higher is the certainty of revealing the flaw size by the NDE inspection. - Usually, (due to codes and standards roles) planar discontinuities are not permitted because due to a poor welding procedure or incorrect welding technique used. In case of impossibility of repairing the flaw, the fracture mechanics assessment is very valuable. - Especially important and pretending is the assessment of planar flaw acceptance of 42 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 n B n e m I. RAK: FLAW ACCEPTABILITY ASSESSMENT DETECTED IN HSLA STEEL WELD JOINTS mis-matched welded joints. In such a case the assessment in accordance with ETM is unavoidable. 9 REFERENCES 1 BS 7448: Part 2:1997. Method for Determination of kic, critical CTOD and critical J values of welds in metallic materials 2 ASTM E 1290-91. Standard Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement 3 European Structural Integrity Society, ESIS Recommendation for Determining the Fracture Resistance of Ductile Materials, ESIS P1-92 4 K. H. Schwalbe, M. Koçak: Fracture Mechanics of Weldments: Properties and Application to Components, Keynote Lecture on the 3rd International Conference on Trends in Welding Research, June 1-5, 1992, Gatlinburg, Tennessee, USA 5 Y. Mukai, A. Nishimura: Fatigue Crack Propagation Behaviour in the Hardness Heterogeneous Field; Transactions of the Japan Welding Society, 14, (1983) 1 6 M. Koçak, K. Seifert, S. Yao,H. Lampe: Comparison of Fatigue Precracking Methods for Fracture Toughness Testing of Weldments: Local Compression and Step-Wise High R-ratio, Proc. of the Int. Conf. Welding-90, Oct. 1990, Geesthacht, FRG (ed. by M. Koçak), 307-318 7 GKSS Forschungszentrum Geesthacht GMBH Bulletin: GKSS-Displacement Gauge System for Application in Fracture Mechanics 8 T. Ito, K. Tanaka, M., Sato: Study of Brittle Fracture Initiation from Surface Notch in Welded Fusion Line, IIW Doc. X-794-73 9 ASTM E 646-91: Standard Test Method for Tensile Strain-Hardening Exponents (n-Values) of Metalic Sheets materials 10ESIS Procedure for Determination the Fracture Behaviour of Materials, ESIS P2-92 11 BS PD 6493: 1991: Guidance on Methods for Assessing the Acceptability of Flaws in Fusion Welded Structures 12K.-H. Schwalbe: Effect of weld metal mis-match on toughness requirements: Some simple analytical consideration using Engineering Treatment Model (ETM), International Journal of Fracture, 56 (1992) 257-277 13 K.-H. Schwalbe: Welded joint with non-matching weld metal-crack driving force consideration on the basis of the Engineering Treatment Model (ETM), Bulletin GKSS 93/E/66 KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 1-2 43