A SIMPLIFIED STRESS INTENSITY FACTOR RELATION FOR ASSESSMENT OF EXTERNAL AXIAL FLAWED SAFETY-RELATED NUCLEAR POWER PLANT HDPE PIPING 1

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1 Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference PVP2011 July 17-21, 2011, Bltimore, Mrylnd, USA PVP A SIMPLIFIED STESS INTENSITY FACTO ELATION FO ASSESSMENT OF EXTENAL AXIAL FLAWED SAFETY-ELATED NUCLEA POWE PLANT HDPE PIPING 1 S. Klynm, P. Krishnswmy, M. Uddin, D-J. Shim Engineering Mechnics Corportion of Columbus (Emc 2 ), Columbus, Ohio, USA ABSTACT PE piping hs been used successfully for over 50 yers in the nturl gs distribution, wter systems, petrochemicl industry piping, nd specilty pplictions for corrosive fluids. The ppliction of PE pipes to sfety-relted service-wter nucler power plnt (NPP) piping, demnds superior mteril nd frcture toughness performnce t elevted stresses nd therml lods/grdients. The ASME Boiler nd Pressure Vessel Code Cse N-755 ws developed to identify requirements tht PE piping must meet nd mteril qulifiction bsed on llowble stress levels in the presence of crcks/flws nd mteril resistnce to crck growth. The PE nd NPP industry intend to use PE pipes in sfety-relted, service-wter pplictions nd hve conducted mteril evlution tests tht re similr in nture to those used erlier for qulifying PE mterils for other pplictions listed bove. Coupon nd PENT tests tht include controlled notches tht simulte the stress-intensity fctor (K I ) of typicl xil flw in PE pipe form the bsis of current ssessment procedures for PE mteril resistnce to slow-crck growth (SCG). The pplicbility of these ssessment methods, nd experience from other pplictions, to specifying llowble flw depths is of interest to the industry nd is the subject of much discussion. In this study, detiled investigtion ws conducted to ssess K I for PE pipes with vrious outside dimeters (OD) nd rnge of OD xil surfce crck/flw depths, different SCG test specimens with prescribed initil notch, notched-pipe tests (NPT) prescribed by interntionl stndrds. A simplified eqution to represent the SIF vrition with crck/flw depths for lrge group of PE pipes (with sme D vlue) nd sme xil semi-ellipticl surfce flws ws developed. INTODUCTION PE piping is going through rpid phse of development from the resin developers nd plstics industry to meet future infrstructure nd energy demnds. PE piping offers lrge set of dvntges, such s resistnce to corrosion, microbiologicl induced corrosion (MIC), brsion resistnce, much lower weight when compred to steel, high flexibility, ductility nd resistnce to soil movement nd superior flow chrcteristics due to low hydrulic friction. Hence, they re considered s potentil replcement option for steel piping in sfety relted service-wter pplictions in nucler power plnts (NPP). While, PE pipes hve been used for severl decdes for wter distribution nd in nturl gs pipelines, their ppliction to service-wter piping in NPP requires more stringent nd demnding performnce under opertionl conditions tht include high pressure nd elevted tempertures. The Americn Society of Mechnicl Engineers (ASME) Boiler nd Pressure Vessel Code Committee (BPVC) formed Specil Working Group on Polyethylene Pipe (SWG-PP) to develop code cse, N755 [1] tht ddresses the HDPE piping 1 This pper ws prepred s n ccount of work sponsored by n gency of the United Sttes Government. Neither the United Sttes Government nor ny gency thereof, nor ny of their employees, mkes ny wrrnty, expressed or implied, or ssumes ny legl libility or responsibility for ny third prty s use, or the results of such use, of ny informtion, pprtus, product, or process disclosed in this pper, or represents tht its use by such third prty would not infringe privtely owned rights. The views expressed in this pper re not necessrily those of the U. S. Nucler egultory Commission. Furthermore, this mteril is declred work of the U.S. government nd is not subjected to copyright protection in the United Sttes. Approved for public relese; distribution is unlimited. 1

2 performnce requirements from stndpoint of mteril resistnce, design, fbriction nd instlltion, nd nondestructive exmintion of PE piping for structurl integrity, especilly in fusion joint regions of PE pipe nd fittings before they cn be pproved for use in NPP. PE pipe industry hs used mximum of 10% (of wll thickness) depth of flw for integrity ssessment s rule of thumb for qulifying PE mterils using the time to filure from ASTM 1473 PENT test [2] s n indictor of the service life. Although, the code cse N-755 specifies the llowble surfce flw in PE piping during mnufcture nd instlltion to be 10% of the pipe wll thickness per Section 2310 in CC N-755 ev. 0 [1], there is no informtion on the permissible length, shpe, loction, or notch cuity. There is lso lck of experimentl dt to support the 10% llowble limit of flw in lrge dimeter (greter thn 12 inches or mm) PE piping t elevted tempertures up to 140 F (60 C) for the expected service lods nd life tht is typicl for service-wter pplictions. Code cse N755 ev 1 is being developed to ddress some of the dditionl needs identified by the industry for use in sfetyrelted nucler power plnt pplictions [3]. Flw ssessment criterion for PE piping requires the identifiction of driving force nd chrcteriztion of the crck growth resistnce of the piping mterils. Of prticulr concern is the premture filure from slow crck growth (SCG) phenomen exhibited by polyethylene (PE) mterils under sustined pressure nd temperture lods similr to those experienced by service-wter piping in NPP [4, 5]. Stress intensity fctor (SIF) solutions hve been employed by the plstic pipe industry to chrcterize the slow crck growth driving force resulting from lods, component geometry, nd flw geometry [6]. Different test specimens hve been used by vrious orgniztions to chrcterize the SCG resistnce, which is sensitive to the pplied SIF tht hs direct bering on the time to filure [5,7]. This pper presents snpshot of the driving force (SIF) for ongoing tests t vrious orgniztions chrcterizing PE mterils for their SCG resistnce in the fusion joints nd prent mterils. It compres the SIF vlues for vrious externl xil surfce flwed PE pipe cses with the SIF of ASTM 1473 PENT specimen, nd the Notched Pipe Test (NPT) s per ISO13479 [8], Since the current efforts in the PE pipe evlution rely on the SIF, the bove mentioned prgrphs ws envisged to provide n overview s to where ech of the testing efforts fll when probing PE pipe mteril performnce under sustined pressure nd therml lods nd how they ll compre with the stipulted minimum requirements proposed in ASME Code Cse N-755 [1]. While the ASME SWG, USNC, NPP industry ctively revise nd incorporte chnges into the CC N-755 [1], bsed upon ongoing efforts from industry nd confirmtory orgniztions tht chrcterize PE piping mnufcture nd mteril performnce, the current study is step in the direction of unifiction of diverse pproches (specimens nd tests) nd stndrds tht re in vogue to chrcterize the mteril resistnce nd performnce of these rther complex viscoelstic PE mterils [9]. In this study, mtrix of vrious PE pipe dimeters nd Dimension tio [D = Averge Outer Dimeter(OD)/ Minimum Thickness(t)] vlues were considered long with prescribed externl xil surfce flws with depths rnging from 2.5% to 50% of the pipe wll thickness (t). API 579-1/ASME FFS-1 [10] ws used to determine SIF for pipes subjected to internl pressure tht were identified in the mtrix identified for the current study. SIF solutions for few specific bounding cses of pipe outer dimeter (OD), D, nd crck/flw depths (/t) were benchmrked ginst SIF predictions from 3D finite element nlysis (FEA) simultions. Aside, from the SIF solutions developed to provide chrts for picking notched pipe test (NPT) cses for structurl integrity evlutions, the influence functions determining the SIF were obtined for ech D of PE piping hving the simplified semi-ellipticl nd long xil flw considered in this study. A single curve essentilly represents the vrition in normlized crck depth with thickness (/t) nd the influence coefficients determining K I for rnge of pipe dimeters from 1 to 36 inch tht is commonly produced by the plstics industry. Similrly, curves relting K I to /t were developed for other D vlues (7, 9, 11, nd 13) of PE pipes. Using the curves developed for vrious D vlues, simplified verge curve ws postulted for PE pipe with semi-ellipticl xil surfce crck flw (2c/=5) tht covers flw evlution for rnge of currently mnufctured/considered HDPE pipes for instlltion in NPP. DESCIPTION OF ANALYSES SIF Solutions from API 579-1/ASME FFS The tsk t hnd ws to develop opening mode stress intensity fctors, K I for PE pipe with n externl, xil surfce (prt-through wll) flw. For simplicity, we ssume semiellipticl flw with n spect rtio of crck length to depth (2c/) to be 5 s shown in Figure 1. The pipe is subjected to internl pressure loding. The stress intensity fctor t the deepest point of the semi-ellipticl crck/flw is given by Eq. (1) which lso corresponds to the loction with the mximum vlue of K I long the crck front nd hence provides conservtive bsis for flw evlution. 2

3 2c obtined for vrious crck depths in the rnge from /t 0.5 for PE pipes with D rnging from 7 to 13 s shown in the next section nd OD rnging from 1 to 36 inch. Internl pressure, p Figure 1 PE pipe with externl, xil surfce (prtthrough) flw K I 2 p i = 2G G o i, where 1.65 o + 3G 2 Q = for / c 1. 0 c o 2 + 4G 3 o 3 + 5G 4 o 4 (1) G 0, G 1, G 2, G 3, nd G 4, re geometry dependent influence functions - from API 579-1/ASME FFS Hndbook; i nd o re inner nd outer rdius of pipe respectively p = internl pressure in the pipe Using these equtions in Section C.5.10 nd the Tble C-13 from API 579-1/ASME FFS , vlues for K I were π Q To develop simplified eqution tht reltes the SIF vlue to the flw depth, nd function of the pipe/flw geometry, the D-11 PE pipe ws chosen with vrious ODs. For the D-11 pipe, vlidtion study ws conducted for four bounding cses using FEA simultions. The FEA ws to model semiellipticl crck to obtin the crck opening-mode SIF s vlidtion of the SIF vlues obtined using the API 579-1/ASME FFS-1 hndbook solutions. SIF from FEA using PipeFrcCAE nd ABAQUS In this effort computer code, PipeFrcCAE [11], ws employed to develop FEA models for four bounding cses, which included PE pipes with lrge (D) nd smll dimeters (D), nd hving lrge (/t=0.2) nd smll (/t=0.025) depths of crck when compred to the wll thickness of the pipe. The FEA ws performed using the commercil code ABAQUS [12] to obtin the SIF vlues, K I t the deepest point of the crck. Figure 2 shows the finite element mesh employed nd the boundry conditions used in the nlysis. Since the xil crck considered is symmetric with respect to the crck mid-length (X-xis) nd the pipe is symmetric bout the Z- nd X- xes, the FEA model developed considered only onequrter of the pipe. c Symmetric bout X-xis Applied internl pressure Symmetric bout Z-xis Figure 2 FEA model used for determintion of SIF 3

4 SIF USING EMPIICAL SOLUTIONS AND FEA SIF from Empiricl eltions in API 579-1/ASME FFS- 1 Stndrd To begin with, K I ws computed for vrious pipe dimeters (1 Tble 1 provides mtrix of PE pipes nd externl xil OD 36 inch) nd thickness (7 D 13) nd flw depths surfce crcks/flws tht were evluted. The current study (0.025 /t 0.5). ws restricted the semi-ellipticl flw to n spect rtio of 2c/=5. Tble 1 SIF cses for prt-through wll externl xil crck in PE pipes Nominl Dimeter, mm 25.4 (1) 50.8 (2) (4) (8) (12) (18) (24) 762 (30) Outer Dimeter OD, mm (1.315) (2.375) (4.5) (8.625) (12.75) (18) (24) (30) D t minimum OD/D /t minimum, % Semi-ellipticl flw length = 5*depth 2c = 5 Internl pressure p, kp (psi) (100) (36) (36) Eqution (1) ws used to determine K I vlues using the elliptic function vlues (Q) nd vrious influence coefficients (G i, i=0 to 4). Note tht the elliptic function, Q hs the sme vlue of for ll the cses considered in this study, since the sme spect rtio (2c/=5) of the semi-ellipticl crck/flw ws ssumed. The following Eq. (2) provides more concise representtion of the Eq. (1) in tht it reduces the dependence of K I to three primry prmeters, nmely (i) hoop stress from the internl pressure, (ii) flw depth, nd (iii) influence function tht is dependent on flw nd pipe geometry. K I = Stress * (flw depth) 1/2 * Influence Function(geometry) (2) Figure 3 shows the SIF clculted for vrious dimeters (OD) of PE pipes for D-11. The CC N-755 specifies n llowble limit for the xil flw to be 10% of the pipe wll thickness (i.e. /t=0.1) nd hence we focus our ttention to the region with flw depths in the rnge /t 0.2 but evlute the K I vlues in the rnge from /t 0.5. Figure 3 SIF for D-11 for vrious dimeters PE piping with semi-ellipticl xil OD surfce flw (2c/=5) nd subjected to internl pressure = 100 psi. Note: SIF of PENT for fixed notch depth of 35% nd pplied stress of 2.4 MP. SIF of n ISO NPT specimen t flw depth of 20%. 4

5 Figure 3 lso shows K I vlues for n ASTM 1473 PENT test specimen nd for n ISO notched-pipe test (NPT) specimen. The PENT nd ISO NPT tests probe the mteril toughness for n pplied K I of MP m ( psi inch) nd MP m (274.4 psi inch), respectively. If K I indeed controls both initition nd propgtion of SCG, then the lines in Figure 3 indicte tht testing of ll pipes with xil flw tht re below the blck line (PENT test) would result in filure time tht re lower thn tht predicted from using the ASTM 1473 stndrd PENT test. All pipes with xil flws tht re bove the blck line in Figure 3 would be expected to fil in shorter times thn tht seen from PENT test. Hence, Figure 3 provides visul ide of the SIF t which pipe with n OD surfce xil flw test, PENT, nd ISO NPT re conducted when evluting HDPE pipe resin mterils. More specificlly, the stress field hed of the notch in PENT specimen is bove the stress field hed of 11% semiellipticl nd xil flw in D-11 pipes with 36 inch OD or smller. SIF from 3D FEA of PE Pipe with OD Axil Surfce Flw The K I vlues obtined from the API 579-1/ASME FFS-1 hndbook were checked with others published in the literture [6] using SIF hndbook solutions [13]. Since this forms the bsis for the development of the simplified eqution in the next phse of the study, further vlidtion of the K I vlues were performed using 3D FEA. Tble 2 shows the four cses chosen for detiled 3D FEA simultions. As discussed erlier in Section 3.2, qurtersymmetric model of the pipe with pplied internl pressure loding is simulted. The K I vlues clculted with the API 579-1/ASME FFS-1 stndrd empiricl reltions were found to be within 3% of the vlues clculted using FEA for the 2.5% deep crcks nd within 0.2% for the 20% deep crcks. Tble 2 SIF from 3D FEA simultions of prt through-wll xil crck on the OD surfce Nominl Dimeter, inch Outer Dimeter o, mm (1.315) (1.315) (36) (36) Crck depth, mm (0.003) 0.61 (0.024) 2.08 (0.082) (0.655) Crck length 2c, mm (0.015) 3.05 (0.120) (0.409) (3.273) /c /t t/ i Internl Pressure p, MP (psi) (100) (100) (100) (100) K I FEA, MP m (psi inch) (36.24) (111.3) (189.5) (583.27) K I API-579/ ASME FFS-1, MP m (psi inch) (37.33) (111.17) (195.33) (581.67) % Difference 2.92% -0.12% 2.98% -0.28% CONCISE EPESENTATION of SIF FO PE PIPE WITH AN AXIAL SUFACE FLAW Although the results presented in Section 4.1 in the form of chrt of K I versus /t for D-11 pipes, hve been vlidted nd re pplicble to rnge of PE pipe dimeters, it becomes cumbersome to clculte these for specific cse of, t, i, nd o. Also, it is cumbersome to develop detiled plots over vrious rnges of these prmeters nd D vlues before using grphicl pproch to interpolte nd obtin K I vlues. From design nd mteril ssessment stndpoint, it would be useful to hve concise representtion for the clcultion of SIF for certin groups of PE pipes nd xil surfce flw spect rtio (2c/=5). Towrds such n end, we rewrite Eq. (1) in terms of [(/t)(t/ o )], which in essence is [(/t)(2/d)], where the D vlue (i.e. D o /t) is used to chrcterize PE pipes by the plstic pipes industry. The following Eq. (3), uses D, nd the nondimensionl K I /σ π, where K I is expressed s function of (/t). (3) Using Eq. (3), ll curves presented in Figure 3 for D-11 PE pipes collpse to single curve tht shows the vrition of non-dimensionl K I /σ versus /t (see Figure 4). Similrly, Figure 4 shows the vrition of K I /σ versus /t developed for other D vlues (D=7 to 13) for PE pipes with externl xil surfce flws. Upon closer observtion of Figure 4, it ppers tht for given spect rtio of the semi-ellipticl flw/crck (2c/=5.0), the curves to obtin SIF vlues s function of (/t) for vrious D vlues of PE pipe re closely spced. Hence, simplified verge curve, shown in Eq. (4) could be used to represent rnge of D vlues nd cn be employed to obtin K I vlues for ny input pressure lod, flw depth, nd nominl pipe dimeter. 5

6 = = stress t pipe OD = = Flw depth = p = internl pressure For specific D of PE pipe, we now hve single curve tht represents the vrition of K I /σ with /t. The SIF vlues obtined from simplified Eq. (4) ws used to clculte the (4) SIF for D-11 PE pipes with n OD surfce xil flw tht is 10% deep (/t=0.10) nd t hoop stress level of MP (500 psi) present in the OD surfce of the PE pipes. Figure 5 shows the K I vlues obtined from clcultions in eference [6], the API 579-1/ASME FFS-1 hndbook empiricl solutions, nd using the simplified Eq. (4) tht ws developed in the current study. Clerly, they re ll in very good greement for rnge of PE pipe dimeters from 2 to 36 inch (considered for NPP sfety relted pplictions). Similrly, verge curves could be developed for other semiellipticl OD surfce xil crcks/flws tht hve different spect rtios from wht ws considered (2c/=5) in the current study. Influence Function, K I /(σ ) D 7 D 9 D 11 D 13 Averge Poly. (Averge) y = 2.681E-04x E-04x E+00 ² = 9.990E Flw Depth /t, % Figure 4 Effect of D on SIF for externl, xil surfce crck/flw in PE pipe under internl pressure 6

7 K I, psi inch API-579/ASME FFS-1 Hndbook Simplified Emc2 Eqution Nominl Dimeter, inch Figure 5 CONCLUSIONS SIF comprison between the hndbook solution nd the simplified eqution for D-11 PE pipes with vrious dimeters hving externl, xil surfce crck/flw of 10% depth of the wll thickness nd hoop stress on OD surfce of 500 psi. In this pper, SIF vlues for PE pipes with OD surfce xil flws were developed. Closer observtion of the nture of equtions reveled tht single eqution could be used to cpture the SIF vlue for certin D vlue ( o /t) nd spect rtio of semi-ellipticl flw (/t) considered. It hs been demonstrted tht the SIF predictions using the simplified reltion re in good greement with those obtined using widely used SIF hndbooks. Within smll mrgin of error, the compiled SIF curves for vrious D vlues of PE pipes tht re commonly mnufctured by the plstics industry could be represented by n verge curve. This demonstrted vible pproch to clculte SIF for given semi-ellipticl flw in PE pipe. The simplified reltion is useful tool to develop bsis for conducting confirmtory notched pipe tests (NPT) used for flw tolernce evlution, mteril ssessment, nd PE pipe cceptnce. It lso provides for rpid mens to compre the SIF of PE pipe hving n xil flw with tht of n equivlent coupon or PENT specimen used in SCG test evlutions for service life estimtion. Clerly, the simplified eqution is significnt dvncement over using severl grphicl chrts nd interpoltion techniques before rriving t SIF vlue for given PE pipe nd xil OD surfce crck/flw. Although, it is t the initil stge of development, such simplified equtions re envisged to be hndy for PE pipe design nd mteril evlution methodologies. ACKNOWLEDGMENTS This work hs been supported by the U.S. Nucler egultory Commission (NC), Division of eserch. The uthors wish to cknowledge NC-ES Progrm Mnger Mr. Eric Focht for his support nd input for this effort. EFEENCES 1 ASME Code Cse N-755, Use of Polyethylene (PE) Plstic Pipe for Section III, Division I, Construction nd Section XI epir/eplcement Activities, Mrch ASTM Stndrd , Stndrd Test Method for Notch Tensile Test to Mesure the esistnce to Slow Crck Growth of Polyethylene Pipes nd esins, Americn Society for Testing nd Mterils. 3 Krishnswmy, P., Shim, D.-J., A eview of Service Life Prediction Models for High Density Polyethylene Piping for Nucler Sfety-elted Applictions, Plstic Pipes XV, Vncouver, Cnd, September 20-22, Krishnswmy, P., Focht, E., Shim, D.-J., Zhng, T., Use of Polyethylene (PE) Pipe in Sfety-elted, Clss 3, 7

8 Service-Wter Piping, ICONE , Proceedings of the 16 th Interntionl Conference on Nucler Engineering, ICONE16, My 11-15, 2008, Orlndo, Florid, USA. 5 Shim, D.-J., Krishnswmy, P., Focht, E., Comprison of Prent nd Butt Fusion Mteril Properties of High Density Polyethylene, PVP , Proceedings of PVP2009 ASME Pressure Vessels nd Piping Division Conference, July 26-30, 2009, Prgue, Czech epublic. 6 Zhou, Z.J., Chng, D., Dependence of Slow Crck Growth esistnce on Pipe Sizes for Scrtched Polyethylene Nucler Wter Pipe, Proceedings of the 17 th Interntionl Conference on Nucler Engineering, ICONE17, July 12-16, 2009, Brussels, Belgium. 7 EPI DAFT eport No ISO 13479, Polyolefin Pipes for the Convergence of Fluids Determintion of esistnce to Crck Propgtion Test Method for Slow Crck Growth on Notched Pipes (Notch Test), Yoshikiyo, T., High performnce PE100 esin with Extrordinry esistnce to Slow Crck Growth, Proceedings of the Plstics Pipes XIII, Session 2B, October, API 579-1/ASME FFS-1, Fitness-For-Service, Second Edition, June PipeFrcCAE, Engineering Mechnics Corportion of Columbus (Emc 2 ) Proprietry Softwre for Pipe Frcture Modeling nd Anlysis, ABAQUS Version 6.8. User s mnul. ABAQUS, Inc., I; ooke, D.., Crtwright, D.J., Compendium of Stress Intensity Fctors, Her Mjesty s Sttionry Office, London 247,