BS7910:2005 FAILURE ASSESSMENT DIAGRAM (FAD) ON CRACKED CIRCULAR HOLLOW SECTION (CHS) WELDED JOINTS

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1 Advanced Steel Constuction Vol. 5, No. 4, pp (2009) 406 BS7910:2005 ALURE ASSESSMENT DAGRAM (AD) ON CRACED CRCUL HOLLOW SECTON (CHS) WELDED ONTS S.T. Lie * and Z.M. Yang School of Civil and Envionmental Engineeing, Nanyang Technological Univesity 50 Nanyang Avenue, Singapoe , Singapoe *(Coesponding autho: cstlie@ntu.edu.sg) Received: 19 Mach 2008; Revised: 15 uly 2008; Accepted: 28 uly 2008 ABSTRACT: The failue assessment diagam (AD) has been widely accepted fo assessing the safety and egity of cacked and damaged metallic stuctues. Recently, the usage of this method fo offshoe stuctues has been validated extensively and included in the Annex B of BS7910 [1]. Howeve, the ecommended equations ae valid only fo uni-plana cicula hollow section (CHS) T, Y, and T tubula welded jos. n this pape, an actual assessment has been caied out on cacked (CHS) tubula -jo containing a suface cack located at the cown location. The fatigue cack depths a and lengths 2c vay fom 3.30 mm to mm and mm to mm espectively. The value of β, atio of bace to chod diamete, is 0.52, which is within the validity ange specified in the codes of pactice. The ecommended coection facto, is used to calculate the plastic collapse load P c, and the usage of BS7910 [1] Level 2A AD fo cacked CHS tubula -jo is demonstated accodingly. eywods: Cack; cicula hollow section (CHS); failue assessment diagam (AD); factue toughness; -jo; plastic collapse load; stess ensity facto 1. NTRODUCTON n pactice, the safety of any welded stuctue depends vey much on the usage of non-destuctive inspection to detect cack befoe it develops to a citical size, and hence to pemit component epai o eplacement befoe catastophic failue occus. To detemine the citical cack size, the stuctue should be assessed accoding to the knowledge of the sevice stesses and the knowledge of the factue popeties of the mateial. Theefoe, factue mechanics is an indispensable tool fo pefoming a citical assessment of a defect discoveed in any steel stuctue. The main objective of this assessment is to establish the maximum toleable defect size which would not compomise the sevice equiements. At the design stage, the ability to evaluate the toleance of a stuctue to possible defects may be used to optimize the design with espect to popeties of the mateial, geometic shape, and ease of inspection. Duing opeation an assessment may be used to eassess a stuctue that has been found to contain defects, thus helping in making ational epai o no-epai decisions and impoving the inspection stategy. Recently, Ameican Petoleum nstitute AP RP579 [2], Bitish Standad BS7910 [1] and Cental Electicity Geneating Boad CEGB R6 [3] pocedue give guidance fo assessing the acceptability of defects in welded stuctues based on the failue assessment diagam (AD) method. The AD method was oiginally deived fom the oiginal two-citeion appoach epoted by Dowling and Townley [4]. This appoach states that a stuctue can fail by eithe of two mechanisms, bittle factue o plastic collapse, and that these two mechanisms ae connected by an epolation cuve based on the stip yield model. f the sevice (assessment) po falls inside the assessment cuve, the stuctue is consideed safe, othewise, the stuctue is deemed unsafe. This method enables the The Hong ong nstitute of Steel Constuction

2 Potentialyunsevel2Bevel2af407 S.T. Lie and Z.M. Yang analyst to go diectly fom linea elastic factue mechanics (LEM) calculations to plastic instability calculations (Wiesne et al. [5]). The assessment cuves specified in the BS7910 [1] ae diffeent fo diffeent mateials and geometies. Howeve, the lowe bound cuves ae always used to assess all types of stuctues including the cacked cicula hollow section (CHS) T, Y, and T-jos. n this pape, the standad Level 2A AD cuve is used to assess a typical cacked tubula CHS -jo specimen. The plastic collapse coection facto, as ecommended in Annex B of BS7910 [1], is used to calculate the plastic collapse load P c. nowing the elastic stess ensity facto and the factue toughness C, the coesponding values of and L fo diffeent cack sizes ae plotted in the AD cuve accodingly. The loading paths ae then used to study the factue assessment sensitivity analysis as the flaw (cack) inceases with time. 2. ALURE ASSESSMENT DAGRAM (AD) Accoding to BS7910 [1], any uni-plana cacked tubula CHS T, Y, and T-jos can be assessed using the nomal assessment oute. The standad AD cuve has two cuves, namely Level 2A and 2B as shown in igue 1 espectively. = /C elafelasl max L = P/P c igue 1. BS7910 [1] Levels 2A and 2B AD Cuves The Level 2A and 2B cuves can be descibed espectively by the following equations: 2 6 (1 0.14L )[ exp( 0.65L )] (1a) and 1 3 E 2 ef L Y Y 2 (1b) L Eef whee E is Young s modulus, ef is efeence stain and Y is yield stess of the mateial, and (2) C

3 BS7910:2005 ailue Assessment Diagam (AD) on Cacked Cicula Hollow Section (CHS) Welded os 408 L P total applied load giving to stesses ( P) (3) plastic collapse load of flawed stuctue( P ) c The application of Level 2B equies the knowledge of a complete stess-stain cuve; in paticula the egion aound the yield po has to be available in a detailed manne. Howeve, thee ae many cases whee this infomation is not available to the uses. Theefoe, Eq. 1b is applied fo a numbe of mateials to geneate a mateial independent lowe bound cuve of Level 2A as shown in igue 1, which is the moe consevative cuve. As it can be seen, this method adopts the assessment cuve which uses the atio of the stess ensity facto to the factue toughness C, defined as as the vetical (factue) axis, and the atio of the applied load P to the plastic collapse load P c, defined as L as the hoizontal (plasticity) axis. f the sevice (assessment) po falls inside the assessment cuve, the stuctue is consideed safe, othewise, the stuctue is deemed unsafe. t is also impotant to ecognize that the paamete of the assessment diagam uses the linea elastic stess ensity facto with no allowance fo the effect of plasticity on the cack tip diving foce. As L value inceases, plasticity also inceases the effective cack tip diving foce. f it is consideed that factue actually occus when the total effective cack tip diving foce, the elastic plastic value of eaches a citical value equivalent to the factue toughness, then this will ep occu at E ep C. Since the applied linea elastic stess ensity facto is equivalent to E e whee e is the linea elastic -egal, then e (4) C ep As plastic inceases so the atio e / ep educes, and this defines the shape of the assessment cuve with the inceasing of L. The standad assessment cuve is oiginally deived fo the case of a lage plate unde tension loading with a cental cack (Dawes and Denys [6]). They have been shown to epesent a lowe bound cuves fo othe common simple geometies. 2.1 actue Axis o assessing the safety and egity of an existing cacked stuctue, the factue paamete given by Eq. 2 is usually used in pactice. The elastic stess ensity factos along the 3D cack font of the -jo specimen can be obtained fom a finite element analysis (Lie et al. [7]), and the mateial factue toughness C can be detemined fom the standad CTOD o values tests (BS [8]). When the jo is subjected unde a mixed mode condition, the effective stess ensity facto eff should be used to eplace the as (1 ) (5) eff

4 409 S.T. Lie and Z.M. Yang whee,, ae the Mode-, and stess ensity factos espectively, and is the Poisson s atio. Howeve, it was shown that the Mode- stess ensity factos ae the dominant ones (Lie et al. [9]), and they ae almost equal to eff fo the -jo subject unde axial (AX) and in-plane bending (PB) loads shown in igue 3. Axial (AX) n-plane Bending (PB) Cown 2.2 Plasticity Axis L igue 3. The ull-scale -jo Specimen As it is vey difficult to obtain the plastic collapse load of any offshoe cacked tubula welded jo, BS7910 [1] has ecommended that the plastic collapse load of the cacked geomety P c is detemined by educing the plastic collapse load fo the coesponding uncacked geomety P uncacked using the coection facto. The plastic collapse load of an uncacked -jo can be obtained fom the Health and Safety Executive (HSE) [10]. The coection facto fo axial load is given by mq cacked aea 1 1 esection length T (6) Q β whee T is chod thickness, Q is 1 fo 1.0 and m q is 1 fo (CHS) tubula -jos. o in-plane bending moment, the coection facto is given by

5 BS7910:2005 ailue Assessment Diagam (AD) on Cacked Cicula Hollow Section (CHS) Welded os 410 cos 1 sin (7) 2 2 whee the angle is defined in igue 2. Cack Tip 2 2c Oute weld path along the weld toe O Z Cack Tip 1 Y igue 2. Definition of the Angle Used in Equation 7 n Annex B of BS7910 [1], the L paamete fo any tubula jo subjected unde combined loads is given by the following equation: 2 σ Pa M ai M ao L (8) σ Y Pc M ci M co whee σ and σ Y ae the flow and yield stesses; in-plane bending and out-of-plane bending; and P c, P a, M ai and M ao ae the applied axial load, M ae the plastic collapse load M ci and co in the cacked condition fo axial load, in-plane bending and out-of-plane bending espectively. The plastic collapse load is obtained by educing the plastic collapse load of the coesponding uncacked geomety on the basis of the net load-beaing aea fo axial load and the effect of the flaw aea on the plastic collapse modulus fo bending moments. 3. RACTURE ASSESSMENT O A CRACED k-ont 3.1 Specimen Dimensions A full-scale cacked tubula -jo shown in igue 3 containing fatigue cacks will be assessed in this pape. n the fatigue test caied out ealie (Lie et al. [11]), an axial (AX) and an in-plane bending (PB) wee applied at the bace end. As the hot spot stess was located at the cown of the chod, and the applied loads wee symmetical, the cack was found to initiate and popagate symmetically fom this position. The notations used to descibe the jo paametes, namely = 2L/D, β = d/d, γ = D/2T, τ = t/t and 2ς = g/d ae given in igue 4, and the oveall dimensions ae tabulated in Table 1.

6 411 S.T. Lie and Z.M. Yang d 2 o d Bace 2 Bace 1 d 1 o d t 2 o t t 1 o t g Saddle Heel Cown e T D Chod L igue 4. Paametes of the -jo Specimen Table 1. Oveall Dimensions of the -jo Specimen Specimen D d T t g 1, L 2 -jo To captue the cack details duing the ealie test (Lie et al., [11]), an altenating cuent potential dop (ACPD) technique shown in igue 5 was used in the fatigue test (Dove et al. [12]). The esults showed that the captued cack pofile by the ACPD technique ageed quite well with the actual cack shape. t is especially so at the deepest pos whee the ACPD measuements ae capable of poviding accuate and useful infomation such as the stess ensity factos. The measued cack depth a and the coesponding cack length 2c ae tabulated in Tables 2 and 3 espectively. igue 5. ACPD Test Set-up and Location of the Cack The cack shapes at diffeent popagation stages wee plotted fom the ACPD eadings, and they wee compaed with the actual cack shape measued by manual method. The two types of cack shapes wee then compaed with a semi-elliptical shape having the same depth and length (igue 6). om the ealie fatigue test (Lie et al. [11]), it can be seen that it is easonable to assume a semi-elliptical cack shape in the numeical model.

7 BS7910:2005 ailue Assessment Diagam (AD) on Cacked Cicula Hollow Section (CHS) Welded os 412 2c Cack font a T 3.2 Mesh Geneation of Cacked -jo igue 6. Semi-elliptical 3D Suface Cack To geneate the mesh of the completed tubula -jo model with a suface cack which can be located at any position with any length and depth, five types of elements ae used in the pesent mesh geneation pocedue. Quate-po cack tip elements ae used along the cack font to simulate the displacements singulaity. o these elements, the mid-side nodes ae moved to the quate po fo the edge connected to the cack font. Pism elements ae employed to model the tansition zone between the egion nea the cack suface and the fa field egion. Tetahedal elements ae used to link the quate-po cack tip elements and othe types of elements which enclose the cack font. Pyamid elements ae used to connect the pism elements with tetahedal elements aound the cack font. n the fields fa away fom the cack, hexahedal elements ae used to model the emaining pat of the membes. The locations of these diffeent types of elements ae illustated in igue 7. Tetahedal Elements Suface Cack ont Pism Elements SBLOC-A Pyamid Elements Second ing (Pism Elements) ist ing (Quate- Po/Cack Elements) ace to be connected to DCUBE-A igue 7. Detailed Mesh Along the Cack ont n the mesh geneation, all the zones ae classified o thee main types: efined (Zone C, Zone C1 and Zone CRBLOC), coase zones (Zone A, Zone ER, Zone EL, EXTENCHL, EXTENCHR and Zone H) and tansition zones (Zone B, Zone D, Zone G1 and Zone G2) as shown in igue 8. n the zones with efined mesh, thee layes of elements ae geneated in the thickness diection so as to model the cack depths. The mesh of the suface cack is extacted fom Zone C1. n the coase mesh zones, only one laye of elements is geneated in thickness diection. n the tansition zones, Zone B connects Zone A and Zone C o Zone C1.

8 413 S.T. Lie and Z.M. Yang igue 8. Mesh Geneation of the Tubula -jo igue 9 shows the mesh of CRBLOC in a detailed view. CRBLOC is extacted fom Zone C1. t should be emphasized that the numbe and location of elements extacted fom Zone C1 will depend on the cack length and position. Once the cack length and the cack position ae detemined, the numbe of elements to be extacted will be calculated automatically. Theefoe, a suface cack with any length at any fixed position can be geneated automatically. DCUBE-B SBLOC-A DCUBE-B DCUBE-A SBLOC-B DCUBE-A igue 9. CRBLOC Extacted fom Zone C1 and Zone D Afte extacting CRBLOC fom Zone C1, all the elements used in Zone C1 ae hexahedal elements since no cack in this zone. The mesh in details of Zone C1 afte extacting CRBLOC is shown in igue 10.

9 BS7910:2005 ailue Assessment Diagam (AD) on Cacked Cicula Hollow Section (CHS) Welded os 414 igue 10. Mesh of Zone C1 Afte Extacting CRBLOC Afte the mesh of all the zones has been completed, they ae then meged togethe to fom the mesh of the entie stuctue. igue 11 shows the mesh of a -jo afte meging the mesh of all the zones. igue 11. Mesh of Tubula -jo Afte Meging All the Zones Accoding to the jo dimensions and the 3D cack size, a finite element model shown in igue 12 is geneated automatically (Lie at al. [7]) to calculate the stess ensity factos at the citical locations, namely, at the deepest po and at the cack tips. The -jo specimen is subjected with an axial load (AX) of 150 kn and an in-plane bending (PB) of 38 knm (13.5 kn m) espectively. P a = 150 kn M ai = 13.5 kn Cown igue 12. inite Element Mesh and Loading Conditions

10 415 S.T. Lie and Z.M. Yang 3.3 Stess ntensity actos To estimate the emaining life of tubula -jos, the factue paamete called the stess ensity facto (S) is fequently used by many eseaches. Thee ae seveal methods which can be used to evaluate this factue paamete. n pactice, the two most commonly used methods ae the -egal and the displacement extapolation. The -egal has been found to be insensitive to mesh efinement, but it can not be used diectly in the mixed mode poblems. Howeve, the Mode-, & Ss can still be obtained fom the -egal using an indiect way by oducing an eaction egal method (Shih and Asao [13]). t is also noted that -egal lacks path independence in the egion whee the cack meets the weld toe because the stess at the toe and the cack tip is singula. On the othe hand, displacement extapolation method is based on Westegaad s equations which elate the displacements in the vicinity of the cack font to the stess ensity factos. This method is not applicable fo inelastic behaviou egal method -egal method is then used to obtain the stess ensity factos along the cack font and at the two cack tips. Although this method can not be used diectly fo the mixed mode poblems, it can still be able to poduce the Mode-, & stess ensity factos though an indiect way by oducing an eaction egal method as poposed by Shih and Asao [13]. The elationship between the -egal and the Ss can be witten as 1 8 T B (9) whee ] T [,, and B is called the pe-logaithmic enegy facto matix. Shih and Asao [13] gave the elationship between the S and the eaction -egal, follow:, as 4 B (10) whee ] T [,,. Theefoe, once is obtained, can be easily calculated fom Eq. 10. The detailed calculations of can be found in the pape published by Shih and Asao [13], and this method has been implemented in the ABAQUS [14] geneal finite element softwae. The values of Mode- stess ensity factos coesponding to the cack depth a and cack length 2c at the deepest po and the cack tips ae tabulated in Tables 2 and 3 espectively. 4. ASSESSMENT PONTS O AND L n ode to assess the safety and egity of this cacked tubula -jo subjected unde the combined loads, BS7910 [1] Level 2A failue assessment diagam (AD) method is employed in this study. ADs ae used to conside failue by linea elastic factues as one limiting citeia and failue by plastic collapse as the second citeia. When pefoming a stuctual egity assessment of a flaw in a stessed stuctue, an assessment po is deived fom two diffeent calculations and

11 BS7910:2005 ailue Assessment Diagam (AD) on Cacked Cicula Hollow Section (CHS) Welded os 416 plotted on the diagam. The stuctue is deemed unsafe if the po calculated lies on the cuve o falls outside it, and it is safe if the po is within the cuve. o offshoe tubula jos, the plastic collapse loads fo the cacked geomety P c ae detemined by multiplying the plastic collapse loads fo the coesponding uncacked geomety P uncacked with the eduction facto. The plastic collapse loads of uncacked -jo P uncacked can be obtained fom the Health and Safety Executive (HSE) [10], and the coection factos fo axial load and in-plane bending ae given by Eqs. 6 and 7 espectively. The bace and chod membes wee fabicated fom standad AP 5L Gade B specifications pipes, and the factue toughness measued at oom tempeatue is appoximately 147 MPa.m 1/2 (Someday [15]). Because the mateial exhibits significant stain-hadening, the flow stess will be used and it is given by Y u (11) 2 to obtain the L paamete. om the standad coupon tests, the measued yield stess 352 MPa and u 493 MPa espectively. Y Then, the coesponding values of and L fo diffeent cack sizes can be computed and they ae tabulated in Table 2 at the deepest pos and Table 3 at the cack tips subsequently. The assessment pos of this cacked -jo ae plotted in the assessment cuve shown in igues 13 and 14 espectively. o the suface cacks in the -jo, the maximum cack diving foce (Ss o CTOD) seems to be at the cack tips. Theefoe, the Ss o CTOD at this po should be used to calculate the values. 1.2 Assessm ent po 1.0 Unacceptable Acceptable 0.4 C u t o ff fo m ild stee l L igue 13. Plot of Assessment Pos at the Deepest Pos

12 417 S.T. Lie and Z.M. Yang Unacceptable Assessm ent po Acceptable 0.4 C u t o ff fo m ild ste e l 0.2 a igue 14. Plot of Assessment Pos at the Cack Tips Table 2. Calculated Values of a/t c 2c and L at the Deepest Pos MPa.m 1/2 L C P P c a a/t Table 3. Calculated Values of c 2c and L at the Cack Tips MPa.m 1/2 L C P P c L n accodance with the ADs shown in igues 13 and 14, it can be seen that only thee assessment pos fall inside the standad Level 2A cuve, i.e. fo a T is less than 0.4. Theefoe, the citical load value is less than the cack initiation load value and the cack -jo is still safe. When a T goes beyond 0.4, it is noted that the L values ae substantially lage, and theefoe the cacked -jo is deemed to fail beyond this cack size.

13 BS7910:2005 ailue Assessment Diagam (AD) on Cacked Cicula Hollow Section (CHS) Welded os 418 The incease of is moe gadual compaed to the incease of L as the stess ensity factos, both at the deepest pos and cack tips, do not incease dastically as the cack size inceases. An inceased load o lage cack size will move the assessment po along the loading path towad the failue line as shown in igues 13 and 14. The eliability of the method depends on how accuately the esection zone is descibed by the failue cuve which depends on the stuctual geomety, type of loading and cack size. 5. CONCLUSONS This pape demonstates the usage of BS7910 [1] to assess the safety and egity of a typical cacked tubula CHS -jo whee failue is chaacteized by two citeia, namely, cack tip failue whee failue occus when the applied load equals the LEM failue load, i.e. load 1 (12) LEM failue load C and failue by plastic collapse of the ligament given by P load L 1 (13) P ligament aea c whee is the mateial flow stess. This appoach enables the egity of cacked cicula hollow section (CHS) tubula jos to be assessed though two sepaate calculations based on the two extemes of factue behaviou, linea elastic and fully plastic. A design cuve is used to epolate between the two failue citeia. The elative position of the assessment po on the diagam, deived fom the two sepaate calculations, detemines the egity of the stuctue. f the assessment po falls inside the failue cuve, the stuctue is deemed safe; if the assessment po is on o outside the cuve, then failue is pedicted to occu. o the cacked -jo consideed, it is found that only thee assessment pos fall inside the standad Level 2A cuve, and hence this damaged jo is still safe if a T is less than 0.4 opeating unde the specified conditions. NOMENCLATURE a = cack depth of suface cack B = pe-logaithmic enegy facto matix d = bace diamete D = chod diamete E = Young s modulus = coection facto g = gap distance between two baces e = value of detemined using an elastic analysis = value of detemined using an elastic-plastic analysis ep = -egal matix of = -egal matix of T [,, ] [,, ] T

14 419 S.T. Lie and Z.M. Yang = Mode- stess ensity facto = Mode- stess ensity facto = Mode- stess ensity facto = effective stess ensity facto eff = factue toughness C = atio of elastic stess ensity facto to factue toughness T = stess ensity facto matix of [,, ] L = half chod length L = atio of applied load to plastic collapse load m = a facto equal to 1.0 fo cicula hollow section (CHS) tubula jos q M ai = applied in-plane bending M = applied out-of-plane bending ao M = in-plane bending plastic collapse ci M co = out-of-plane bending plastic collapse P = total applied load P = applied axial load a P = plastic collapse load c P = plastic collapse load of the uncacked jo uncacked Q = a facto equal to 1.0 fo t = bace thickness T = chod thickness = atio of twice chod length to chod diamete = atio of bace to chod diamete = atio of bace diamete to twice chod thickness ef = efeence stain = atio of gap distance of two baces to chod diamete, 1 2 = bace to chod angles = Poisson s atio P = total applied stess = flow stess of the mateial = ultimate stess of the mateial u Y = yield stess of the mateial = atio of bace thickness to chod thickness = extended angle of suface cack

15 BS7910:2005 ailue Assessment Diagam (AD) on Cacked Cicula Hollow Section (CHS) Welded os 420 REERENCES [1] BS7910, Guide to Methods fo Assessing the Acceptability of laws in usion Welded Stuctues, Bitish Standads nstitution, London, U, [2] AP RP579, itness-fo-sevice, Ameican Petoleum nstitute, Washington, USA, [3] CEGB R6, Assessment of the ntegity of Stuctues Containing Defects, Revision 4, Bitish Enegy, Glouceste, U, [4] Dowling, A.B. and Townley, C.H.A., The Effect of Defect on Stuctual ailue: A Two-Citeia Appoach, ntenational ounal of Pessue Vessels and Piping, 1975, Vol. 3, No. 2, pp [5] Wiesne, C.S., Maddox, S.., Xu, W., Webste, G.A., Budekin,.M., Andews, R.M. and Haison,.D., Engineeing Citical Analyses to BS7910 the U Guide on Methods fo Assessing the Acceptability of laws in Metallic Stuctues, ntenational ounal of Pessue Vessels and Piping, 2000, Vol. 77, No , pp [6] Dawes, M.G. and Denys, R., BS 5500 Appendix D: An Assessment Based on Wide Plate Bittle actue Test Data, ntenational ounal of Pessue Vessels and Pipings, 1984, Vol. 15, pp [7] Lie, S.T., Lee, C.., Chiew S.P. and Shao, Y.B., Mesh Modelling and Analysis of Cacked Uni-plana Tubula -jos, ounal of Constuctional Steel Reseach, 2005, Vol. 61, No. 2, pp [8] BS7448-1, actue Mechanics Toughness Tests Pat 1: Method fo Detemination of C, Citical CTOD and Citical Values of Metallic Stuctues, Bitish Standads nstitution, London, U, [9] Lie, S.T., Lee, C.., Chiew, S.P. and Shao, Y.B., Estimation of Stess ntensity actos of Weld Toe Suface Cacks in Tubula -jos, Poceedings of the 10th ntenational Symposium on Tubula Stuctues, Madid, Spain, 2003, pp [10] Health and Safety Executive (HSE), Offshoe nstallation: Guidance on Design, Constuction and Cetification, 4th Edition, Thid Amendment, He Majesty s Stationay Office, London, U, [11] Lie S.T., Chiew S.P., Lee C.. and Shao, Y.B., Validation of a Suface Cack Stess ntensity actos of a Tubula -jo, ntenational ounal of Pessue Vessels and Piping, 2005, Vol. 82, No. 8, pp [12] Dove, W.D., Dhamavasan, S., Bennan,.P. and Mash,.., atigue Cack Gowth in Offshoe Stuctues, Engineeing Mateials Advisoy Sevices (EMAS) Ltd., Chameleon Pess, London, U, [13] Shih, C.. and Asao, R.., Elastic Plastic Analysis of Cacks on Bimateial nteface: Pat -Small Scale Yielding, ounal of Applied Mechanics, ASME, 1998, Vol. 51, pp [14] ABAQUS, Use s Manual, Vesion 6.5, Hibbit, alsson and Soensen nc., Povidence, USA, [15] Someday, B.P., Technical Refeence on Hydogen Compatibility of Mateials Cabon Steel: C-Mn Alloys (Code 1100), Sandia National Laboatoy, Livemoe, Califonia, USA, 2007.