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    Vibration Fatigue Testing of SocketWelds

    TR-111188

    Interim Report, December 1998

    Prepared for

    EPRI3412 Hillview AvenuePalo Alto, California 94304

    EPRI Project ManagerR. Carter

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    DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

    THIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORKSPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI).NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) NAMED BELOW,NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

    (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITHRESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEMDISCLOSED IN THIS REPORT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULARPURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNEDRIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS REPORT ISSUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

    (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDINGANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THISREPORT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS REPORT.

    ORGANIZATION(S) THAT PREPARED THIS REPORT

    Structural Integrity Associates, Inc.Pacific Gas & Electric Company

    ORDERING INFORMATION

    Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box23205, Pleasant Hill, CA 94523, (925) 934-4212.

    Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc.

    Copyright 1998 Electric Power Research Institute, Inc. All rights reserved.

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    iii

    CITATIONS

    This report was prepared by

    Structural Integrity Associates, Inc.3315 Almaden Expressway, Suite 24San Jose, CA 95118-1557

    Peter C. RiccardellaStephen Pan

    Pacific Gas & Electric Company3400 Crow Canyon RoadSan Ramon, CA 94583

    Michael SullivanJohn Schletz

    This report describes research sponsored by EPRI. The report is a corporate document

    that should be cited in the literature in the following manner:

    Vibration Fatigue Testing of Socket Welds, EPRI, Palo Alto, CA: 1998. Report NumberTR-111188.

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    v

    REPORT SUMMARY

    Failures of small bore piping connections continue to occur frequently in nuclear powerplants of the United States, resulting in degraded plant systems and unscheduled plantdowntime. Fatigue-related failures are generally detected as small cracks or leaksbefore major pressure boundary ruptures occur. However, in many cases, the leaklocations are not isolable from the reactor pressure vessel and result in forced plantoutages. Because socket welds are used extensively for small bore piping and fittings

    (less than 2 inches nominal pipe size) in nuclear power plant systems, this study wasundertaken to improve socket weld design and fabrication practices to allow thesewelds to resist high-cycle fatigue.

    Background

    EPRI report TR-104534 indicated that the majority of fatigue failures are caused byvibration of socket welds. Analytical results reported in EPRI TR-107455 havedemonstrated that the socket weld leg size configuration can have an important effecton its high cycle fatigue resistance, with longer legs along the pipe side of the weldgreatly increasing its predicted fatigue resistance. Other potentially important factors

    influencing fatigue life include weld bead sequence, residual stress, weld root and toecondition, loading mode, pipe size, axial and radial gaps, and materials of construction.

    Objectives

    To confirm the analytical predictions reported in EPRI TR-107455.

    To develop appropriate fatigue strength standards for socket welds, reflecting theeffects of those factors listed above that prove to be significant.

    Approach

    Researchers examined the effect of weld leg size on fatigue resistance by testingsamples fabricated with oversized legs on the pipe side, and comparing them to controlsamples of nominal Code dimensions. The test program consisted of bolting severalsocket weld specimens to a vibration fatigue shaker table and shaking themsimultaneously, at or near their resonant frequencies, to produce the desired stressamplitudes and cycles.

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    vi

    Other issues that were addressed included the effects of residual stress, pipe size,materials, and last pass weld improvement, a technique in which a normal ASMECode socket weld is improved by adding a last pass on the pipe side of the weld.Tests were also conducted to determine the potential effect of eliminating the Code-required axial gap from an otherwise standard Code socket weld.

    Results

    On the basis of the testing completed to date, it is concluded that socket welds with a 2to 1 weld leg weld configuration (weld leg along the pipe side of the weld equal totwice the weld leg dimension) offer a significant high cycle fatigue improvement overstandard ASME Code socket welds (in which both weld legs are equal). Since vibrationfatigue of socket welds has been a significant industry problem, it is recommended thatthis improved configuration be used for all socket welds in vibration-criticalapplications.

    The majority of the test failures (12 out of 15) occurred due to cracks that initiated atweld roots. However, toe-initiated failures interceded in three tests, and producedfailures that were premature in comparison with identical tests where root failuresprevailed. Therefore, care must be taken with socket welds of any design to avoidmetallurgical or geometric discontinuities at the toes of the welds (such as undercut ornon-smooth transitions). Such discontinuities promote a tendency for toe failures,which greatly reduce fatigue lives. Because of this effect, the last pass improvementprocess (in which a final pass is added to the pipe side toe of a standard Code weld)cannot be given an unqualified recommendation at this time. Two of the threespecimens in which toe failures occurred were last pass improved welds.

    Other conclusions drawn from this program are that the Code-required axial gap insocket welds (1/16) appears to have little or no effect on high cycle fatigue resistance,and that post-weld heat treatment appears to have increased the fatigue resistance ofstandard ASME Code specimens.

    EPRI Perspective

    Vibration fatigue is the leading cause of piping failures in nuclear power plants of theUnited States, accounting for more than one-third of all piping failures. Such failurescause unplanned and/or extended outages and have a significant cost impact on theindustry. The results obtained in this study and further confirmatory testing areexpected to lead to improvements in socket weld design, fabrication, and integritymanagement.

    Interest Categories

    Piping, reactor vessel & internals

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    CONTENTS

    1 INTRODUCTION ................................................................................................................. 1-1

    2 TECHNICAL APPROACH................................................................................................... 2-1

    3 TEST PROGRAM................................................................................................................ 3-1

    4 TEST RESULTS.................................................................................................................. 4-1

    5 CONCLUSIONS AND RECOMMENDATIONS.................................................................... 5-1

    6 REFERENCES .................................................................................................................... 6-1

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    LIST OF FIGURES

    Figure 3-1 Overall Test Setup................................................................................................ 3-4

    Figure 3-2 Test Specimen Detail ........................................................................................... 3-5

    Figure 3-3 Test Specimen Configurations.............................................................................. 3-6

    Figure 3-4 Actual Test Apparatus .......................................................................................... 3-7

    Figure 4-1 Root Failure .......................................................................................................... 4-5

    Figure 4-2 Toe Failure ........................................................................................................... 4-6

    Figure 4-3 Socket Weld Vibration Tests (3/4" Stainless Steel Socket Welds)........................ 4-7

    Figure 4-4 Socket Weld Vibration Tests (2" Stainless Steel Socket Welds)........................... 4-8

    Figure 4-5 Socket Weld Vibration Tests (2" Carbon Steel Socket Welds) ............................. 4-9

    Figure 4-6 HCF Data on Socket Welds (Stainless Steel Data vs. Higuchi TrendCurves for Different Pipe Sizes) ......................................................................... 4-10

    Figure 4-7 HCF Data on Socket Welds (Carbon Steel - 2 in. NPS) ..................................... 4-11

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    LIST OF TABLES

    Table 3-1 First Year Test Matrix............................................................................................. 3-3

    Table 4-1 Test Results........................................................................................................... 4-4

    Table 4-2 Fatigue Strength Reduction Factor Computations................................................. 4-5

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    1-1

    1

    INTRODUCTION

    Failures of small bore piping connections continue to occur frequently in nuclear powerplants of the United States, resulting in degraded plant systems and unscheduled plantdowntime. Fatigue-related failures are generally detected as small cracks or leaksbefore major pressure boundary ruptures occur. However, in many cases, the leaklocations are not isolable from the reactor pressure vessel and result in forced plantoutages. Most of the recent failures were small bore piping connections to the primarycoolant system and were first noticed as an increase in unidentified primary coolantleakage. However, other systems, such as main steam and electro-hydraulic controlsystems, have also experienced similar failures.

    Prior research [1] has indicated that the majority of such failures are caused byvibration fatigue of socket welds. Work is underway to better understand andcharacterize this phenomenon, both in the U.S. [1,2,3] and overseas [4,5,6]. Analyticalresults reported [3] have demonstrated that the socket weld leg size configuration canhave an important effect on its high cycle fatigue resistance, with longer legs along thepipe-side of the weld greatly increasing its predicted fatigue resistance. Otherpotentially important factors influencing fatigue life include weld bead sequence,

    residual stress, weld root and toe condition, loading mode, pipe size, axial and radialgaps, and materials of construction.

    To study the importance of these factors, and to confirm the analytical predictionsreported [3], a test program was initiated in 1997, under the sponsorship of EPRI, inwhich a large number of socket weld samples were vibration-fatigue tested to failureon a high frequency shaker table. The objectives were to improve the industrysunderstanding and characterization of the high cycle fatigue resistance of socket welds,and to develop appropriate fatigue strength standards for such welds, reflecting theeffects of those factors listed above that prove to be significant. The ultimate goal of thisresearch was to develop recommended design and fabrication practices that could beused to enhance socket weld fatigue resistance in vibration-sensitive locations, as wellas to provide guidelines for screening out and preventing vibration-fatigue failures inexisting welds. This document represents an interim report, including a summary ofthe test program, and interim results and conclusions.

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    2-1

    2

    TECHNICAL APPROACH

    The test program sought to provide experimental confirmation of several importanteffects observed in the analytical studies reported [3], which could have a majorinfluence on how socket welds are designed and fabricated for vibration-criticalapplications. Based on the results, the project team proposed practical evaluation tools,design and fabrication methods, and remedial measures for socket welds in vibration-critical applications.

    One of the most important factors, the effect of weld leg size on fatigue resistance, wasstudied by testing samples fabricated with oversized legs on the pipe-side, andcomparing them to control samples of nominal Code dimensions. Other questionsaddressed in the first year of the program include the effects of residual stress, pipesize, materials, and last pass weld improvement, a technique in which a normal Codesocket weld is improved by adding a last pass on the pipe-side of the weld. Testswere also conducted to determine the potential effect of eliminating the Code-requiredaxial gap from an otherwise standard Code socket weld.

    Three series of tests were conducted to study the effects of these factors, using three sets

    of nine specimens each. Each specimen set was fabricated from a different nominal pipesize (NPS) and/or material (3/4 and 2 NPS stainless steel plus 2 NPS carbon steel).Loading amplitudes were selected based on literature data and analysis with a target ofgenerating failures in approximately 10

    7cycles. Test data were plotted on conventional

    S-N plots and compared to socket weld data from the literature as well as to standardmaterial S-N curves. Observations were then made relative to the effect of the variousfactors on high cycle fatigue performance of the welds and conclusions were drawn asto the effectiveness of each as a proposed remedial action.

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    3-1

    3

    TEST PROGRAM

    An efficient testing scheme was devised for the program, which consisted of boltingseveral socket weld specimens to a vibration fatigue shaker table (as illustratedschematically in Figure 3-1), and shaking them simultaneously at or near their resonantfrequencies to produce the desired stress amplitudes and cycles. A cantileveredspecimen of the type illustrated in Figure 3-2 was used, with the test weld being theweld at the lower end of the specimen, between the pipe and the flange used to bolt thespecimen to the table. With this test technique, different load amplitudes could beapplied to different samples in the same test by fine-tuning the specimens naturalfrequencies relative to the shaker table excitation frequency. The flange configurationswere modified to produce socket weld details typical of the socket weld fittings usedon small bore piping in nuclear plants (Tees, Elbows, Weld-o-lets, Couplings, and soon). All specimens were fabricated from Schedule 80 piping and compatiblecomponents.

    Each specimen was instrumented with an accelerometer and a pressure gauge, asillustrated in Figure 3-2, and the instruments were monitored continuously duringtesting. The specimens were pressurized to a moderate pressure with air (approx. 50

    psig). When depressurization occurred, indicating a failure, the specimen was removedfrom the table at the next convenient test stoppage. The testing was then resumed withonly the remaining, unfailed specimens. This test arrangement permitted the efficienttesting of a large number of specimens. Each test series included nine specimens, testedat approximately 100 Hz, yielding 10

    7cycles on each specimen in less than a week. This

    test method is directly compatible with the plant loading mechanism of concern(vibration fatigue), whereas conventional fatigue testing techniques (rotating beam orfour point bend) can have considerable variability with respect to each other [5,6], andpossibly with respect to in-plant vibration.

    The various socket weld configurations (specimen types) used to test the potentiallysignificant factors cited in Section 1.0, are illustrated in Figure 3-3. The complete firstyear test matrix, summarized in Table 3-1, included three test series of nine specimenseach, for a total of 27 tests. The first test series included two standard ASME Codespecimens (1 x 1 leg sizes), three weld specimens with pipe-side legs extended to twicethe ASME Code requirement (2 x 1 leg sizes), two last pass improved specimens, oneno-gap specimen, and one post-weld heat-treated specimen. All test series 1specimens were NPS stainless steel. Test series 2 was identical to 1, but utilized 2

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    Test Program

    3-2

    NPS stainless steel specimens. Test series 3 was similar, but with seven 2 NPS carbonsteel specimens and two 2 NPS stainless steel specimens (repeats of test series 2 tests).

    Figure 3-4 illustrates the actual test apparatus in almost test-ready condition. Six of nine2 specimens are shown mounted on the shaker table in this figure. The tubes and

    wires are leads for the strain gages and pressure transducers, which were ultimatelyrelayed to the test control computer (right side of figure). Shaker table amplitude wascomputer-controlled to achieve a pre-set accelerometer amplitude on one of the ninesamples. The accelerometer amplitude was recorded for all nine samples and used as ameasure of stress amplitude for each specimen.

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    Test Program

    3-3

    Table 3-1First Year Test Matrix

    Test Series 1 - 3/4" NPS - SS

    Spec # NPS Material Specimen

    DescriptionB1-3/4SS-1 3/4" Stainless Steel Standard Code

    B1-3/4SS-2 3/4" Stainless Steel Standard Code

    B1-3/4SS-3 3/4" Stainless Steel 2 x 1 Leg Sizes

    B1-3/4SS-4 3/4" Stainless Steel 2 x 1 Leg Sizes

    B1-3/4SS-5 3/4" Stainless Steel 2 x 1 Leg Sizes

    B1-3/4SS-6 3/4" Stainless Steel PWHT

    B1-3/4SS-7 3/4" Stainless Steel Last Pass Improved

    B1-3/4SS-8 3/4" Stainless Steel Last Pass Improved

    B1-3/4SS-9 3/4" Stainless Steel No Axial Gap

    Test Series 2 - 2" NPS - SS

    Spec # NPS Material SpecimenDescription

    B2-2SS-1 2" Stainless Steel Standard Code

    B2-2SS-2 2" Stainless Steel Standard Code

    B2-2SS-3 2" Stainless Steel 2 x 1 Leg Sizes

    B2-2SS-4 2" Stainless Steel 2 x 1 Leg Sizes

    B2-2SS-5 2" Stainless Steel 2 x 1 Leg Sizes

    B2-2SS-6 2" Stainless Steel PWHT

    B2-2SS-7 2" Stainless Steel Last Pass Improved

    B2-2SS-8 2" Stainless Steel Last Pass Improved

    B2-2SS-9 2" Stainless Steel No Axial Gap

    Test Series 3 - 2" NPS - CS/SS

    Spec # NPS Material SpecimenDescription

    B3-2CS-1 2" Carbon Steel Standard Code

    B3-2CS-2 2" Carbon Steel Standard Code

    B3-2CS-3 2" Carbon Steel 2 x 1 Leg Sizes

    B3-2CS-4 2" Carbon Steel 2 x 1 Leg Sizes

    B3-2CS-5 2" Carbon Steel 2 x 1 Leg Sizes

    B3-2CS-6 2" Carbon Steel PWHT

    B3-2CS-7 2" Carbon Steel No Axial Gap

    B3-2SS-8 2" Stainless Steel PWHT

    B3-2SS-9 2" Stainless Steel No Axial Gap

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    Test Program

    3-4

    Figure 3-1Overall Test Setup

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    Test Program

    3-5

    Figure 3-2Test Specimen Detail

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    Test Program

    3-6

    Figure 3-3Test Specimen Configurations

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    Test Program

    3-7

    Figure 3-4Actual Test Apparatus(six of nine specimens shown - test control computer on right)

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    4-1

    4

    TEST RESULTS

    A tabulation of applied stress amplitudes and resulting cycles to failure for all three testseries is given in Table 4-1. In test series 1 (the NPS specimens), all nine specimensfailed at cycles ranging from 1 x 10

    6to 1.15 x 10

    7. Seven of the nine specimens exhibited

    root failures, originating at the weld roots and propagating to the outside surface ofthe specimen on the socket-side of the weld, as illustrated in Figure 4-1. The other twospecimens exhibited toe failures, initiating at the outside surface of the specimen nearthe pipe-side toe of the weld, and propagating to the inside surface (see Figure 4-2).The 2 NPS specimens, test series 2 and 3, also produced mostly root failures, withonly one toe failure, but exhibited a number of runouts. A runout is defined as a testconducted to a large number of cycles (approx. 2 x 10

    7), in which no evidence of

    specimen failure is observed but the test is terminated because of time constraints.

    The test results are shown graphically in Figures 4-3, 4-4, and 4-5 for the three testseries, respectively (except that the two 2 NPS stainless steel specimens from test series3 are plotted in Figure 4-4 with the other 2 stainless steel specimens from test series 2,rather than with the carbon steel specimens in Figure 4-5). Trend curves from socketweld fatigue testing reported in [4,5,6] are also shown on these figures, labeled

    Higuchi Curves, for comparison. Appropriate trend curves for each pipe size andmaterial were selected. The figures illustrate the following trends:

    In general, the testing of nominal Code dimension (1 x 1) specimens yielded dataright on, or slightly above, the corresponding Higuchi Curve from the literature.With one exception, these were root failures (open points in the figures).

    Occasionally, specimens exhibited toe failures (solid points in the figures), whichtended to fail somewhat prematurely relative to the more common root failures.

    The enhanced 2 x 1 specimens all exhibited runouts in the larger pipe size, even

    though tested at stress amplitudes 30% to 60% higher than those applied to thestandard Code specimens. The 2 x 1 specimens did fail, but at stress levels about15% to 20% higher than would be predicted by the corresponding Higuchi trendcurve.

    The last pass improved specimens yielded somewhat mixed results. Some failuresof these specimens occurred on or even below the Higuchi trend curve for normal

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    Test Results

    4-2

    Code specimens (for example, the toe failure at 106cycles in Figure 4-3). Other last

    pass improved specimens performed significantly better (for example, the runout inFigure 4-4). In general, where premature failures occurred in last pass improvedspecimens, they were due to toe failures, indicating that the last pass welding mighthave left a discontinuity or stress raiser at the toe.

    Post-weld heat treatment appears to have increased the fatigue life of the standardCode specimens. The only exception to this observation was the specimen inFigure 4-3 that failed right on the trend curve. This specimen was heat-treated at toolow a temperature and thus, the heat treatment might have been ineffective atsignificantly relieving residual stresses.

    The ASME Code-required gap appears to have no effect on high cycle fatigueresistance. No Gap specimens failed both on and above the trend curve, with noconsistent trend.

    Figures 4-6 and 4-7 present the current socket weld data compared to the ASME Codemean failure curves for stainless steel and carbon steel, respectively [7,8]. The Higuchitrend curves for the appropriate materials and pipe sizes are also shown. In Figure 4-6,both the and 2 NPS data for stainless steel are presented. The runouts for the 2NPS 2 x 1 weld specimens in this figure occur at 13 ksi, and it is fair to assume thathigher stress levels than that would be required to produce failures. Similarly, the 2 x 1runouts for 2 carbon steel specimens in Figure 4-7 occur at 10-12 ksi and higherstresses than that would be required to produce failures. Additional testing might beperformed to determine the exact stress magnitudes that cause failures at 2 x 10

    7cycles

    but, until that is complete, preliminary estimates of endurance limit and fatigue

    strength reduction factors (FSRFs) are shown in Table 4-2, based on ratios of theendurance limits from the ASME Code mean failure curves to the apparent endurancelimits from the current tests. (Since the testing was terminated at 2 x 10

    7cycles, the

    alternating stress corresponding to this stress amplitude was taken as the endurancelimit for this calculation.)

    Except for the stainless steel standard Code specimens, a consistent trend isobserved with a FSRF of approximately 4 for standard Code specimens and about 2 for2 x 1 leg size specimens. The only apparent break in this trend is that, as observedpreviously [5], normal Code specimens appear to have significantly greater fatigueresistance than larger specimens. This effect is counteracted somewhat, however, by the

    smaller improvement in this smaller pipe size for a 2 x 1 specimen. Nonetheless, as ageneral rule of thumb, it would be reasonable to use a FSRF of 4 for standard Codesocket welds and a FSRF of 2 for 2 x 1 leg size socket welds in vibration fatigueapplications, independent of pipe size. The EPRI Fatigue Management Handbook [1]recommends 2.6 for Good welds and 4.2 for Fair welds. The use of a 2 x 1 weld legconfiguration would, in essence, move a weld from the Fair to the Good category.

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    Test Results

    4-3

    The above results apply only to welds with no root defects or lack of fusion. Priortesting and analysis [3,6] have shown that root defects can have a significantdetrimental effect on socket weld high cycle fatigue resistance and many field failureshave been affected by such defects. The EPRI Handbook recommends a FSRF of 8.0 forPoor welds, which is intended to encompass welds with root defects. Thus, unless

    one can demonstrate that root defects do not exist, it might be advisable to use a FSRFof 4 for 2 x 1 welds and 8 for standard Code welds (allowing for root defects).

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    Table 4-1Test Results

    Test Series 1 - 3/4" NPS - SS

    Specimen Sa(ksi) Nf Comments

    1 - Code 16.17 1.15E+07 Root Failure2 - Code 16.17 3.60E+06 Toe Failure

    3 - 2x1 21.17 3.30E+06 Root Failure

    4 - 2x1 20.14 1.95E+06 Root Failure

    5 - 2x1 19.17 3.13E+06 Root Failure

    6 - PWHT 17.14 2.44E+06 Root Failure

    7 - LP 18.11 6.50E+06 Root Failure

    8 - LP 18.09 1.06E+06 Toe Failure

    9 - NoGap 17.17 2.84E+06 Root Failure

    Test Series 2 - 2" NPS - SSSpecimen Sa(ksi) Nf Comments

    1 - Code 10 2.28E+07 Runout (No Failure)

    2 - Code 10 9.80E+06 Root Failure

    3 - 2x1 13 2.28E+07 Runout (No Failure)

    4 - 2x1 13.1 2.28E+07 Runout (No Failure)

    5 - 2x1 12.9 2.28E+07 Runout (No Failure)

    6 - PWHT 10.1 2.28E+07 Runout (No Failure)

    7 - LP 11.1 2.28E+07 Runout (No Failure)

    8 - LP 11 1.02E+07 Toe Failure

    9 - NoGap 10 8.10E+06 Root Failure

    Test Series 3 - 2" NPS - CS/SS

    Specimen Sa(ksi) Nf Comments

    1 - Code 7.5 1.08E+07 Root Failure

    2 - Code 7.5 6.70E+06 Root Failure

    3 - 2x1 10 2.40E+07 Runout (No Failure)

    4 - 2x1 11 2.40E+07 Runout (No Failure)

    5 - 2x1 12 2.40E+07 Runout (No Failure)

    6 - PWHT 8.5 2.40E+07 Runout (No Failure)

    7-NoGap 7.5 2.40E+07 Runout (No Failure)

    8 - PWHT 11 2.40E+07 Runout (No Failure)

    9 - NoGap 10 7.00E+06 Root Failure

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    4-5

    Table 4-2Fatigue Strength Reduction Factor Computations

    Apparent Endurance Limit(ksi @ 2 x 10

    7Cycles)

    FSRF(Specimen/ASME Mean)

    SS 2 SS 2 CS SS 2 SS 2 CSStandard Code

    Specimen14.1 8.25 6 2.3 4 3.9

    2 x 1 Leg SizeSpecimen

    ~16 >13 >12 ~2.1

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    4-6

    Figure 4-2Toe Failure

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    10.00

    12.00

    14.00

    16.00

    18.00

    20.00

    22.00

    24.00

    26.00

    28.00

    30.00

    1.00E+06 1.00E+07

    Cycles

    AlternatingStress(ksi)

    2x1 leg

    sizes

    1x1 (Code)

    Dimensions

    Last Pass

    Improved

    Solid Points = Toe Failures

    Open Points = Root Failures

    NoGap

    Figure 4-3Socket Weld Vibration Tests(3/4" Stainless Steel Socket Welds)

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    4

    6

    8

    10

    12

    14

    16

    18

    20

    1.00E+06 1.00E+07 1

    N(Cycles)

    Sa(ksi)

    Solid Points = Toe Failures

    Open Points = Root Failures

    2x1 leg

    sizes

    1x1 (Code)

    Dimensions

    Last Pass

    ImprovedNo GapPWHT

    Runouts

    Figure 4-4

    Socket Weld Vibration Tests(2" Stainless Steel Socket Welds)

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    4

    6

    8

    10

    12

    14

    16

    18

    20

    1.00E+06 1.00E+07

    N(Cycles)

    Sa(ksi)

    Solid Points = Toe Failures

    Open Points = Root Failures

    1x1 (Code)

    Dimensions

    2x1 leg

    sizes

    Runouts

    NoGap

    PWHT

    Figure 4-5Socket Weld Vibration Tests(2" Carbon Steel Socket Welds)

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    1

    10

    100

    1.00E+04 1.00E+05 1.00E+06 1.00E+07

    N(Cycles)

    Sa(ksi)

    Figure 4-6HCF Data on Socket Welds(Stainless Steel Data vs. Higuchi Trend Curves for Different Pipe Sizes)

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    1

    10

    100

    1.00E+04 1.00E+05 1.00E+06 1.00E+07

    N(Cycles)

    Sa(ksi)

    Solid Points = Toe Failures

    Open Points = Root Failures

    = No Failure Runout

    Figure 4-7HCF Data on Socket Welds(Carbon Steel - 2 in. NPS)

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    5

    CONCLUSIONS AND RECOMMENDATIONS

    On the basis of the testing completed to date, it is concluded that socket welds with a 2to 1 weld leg weld configuration (weld leg along the pipe-side of the weld, equal totwice the Code-required weld leg dimension) offer a significant high cycle fatigueimprovement over standard ASME Code socket welds (in which both weld legs areequal to the Code-required dimension). Since vibration fatigue of socket welds has beena significant industry problem, it is recommended that this improved configuration beused for all socket welds in vibration-critical applications.

    The majority of the test failures (12 out of 15) occurred due to cracks that initiated atweld roots. However, toe-initiated failures interceded in three tests and producedfailures that were premature in comparison to identical tests in which root failuresprevailed. Therefore, care must be taken with socket welds of any design to avoidmetallurgical or geometric discontinuities at the toes of the welds (such as undercut ornon-smooth transitions). Such discontinuities promote a tendency for toe failures,which greatly reduce fatigue lives. Because of this effect, the last pass improvementprocess (in which a final pass is added to the pipe-side toe of a standard Code weld)cannot be given an unqualified recommendation at this time. Two of the three

    specimens in which toe failures occurred were last pass improved welds.

    Other conclusions drawn from this program are that the Code-required axial gap insocket welds (1/16) appears to have little or no effect on high cycle fatigue resistance,and post-weld heat treatment appears to have increased the fatigue resistance ofstandard Code specimens.

    The test data support the use of a fatigue strength reduction factor of approximately 4.0for standard Code welds. They also indicate that this factor might be reduced toapproximately one-half that value for 2 to 1 leg size specimens. Both of these values areappropriate only if the weld roots are free of defects such as lack of fusion or lack ofpenetration. Although not tested in this program, prior work has indicated that weldroot defects can cause as much as a factor of 2 increase in fatigue strength reductionfactor.

    Further testing will be performed under Phase 2 of this program in 1998 to addressuncertainties in the current results and to further develop the aforementioned conceptsinto practical field tools. Additional testing might include:

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    5-2

    Demonstrating the effect of upgrading existing Code welds to a 2 x 1 configuration,after some fatigue cycling in their original configuration.

    Defining acceptance standards for toe discontinuities.

    Developing and testing repair concepts for leaking socket welds that would allowplants to continue operating until the next outage before implementing a permanentrepair.

    Providing specific guidance on weld procedures/critical variables to ensure goodhigh cycle fatigue performance of socket welds.

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    REFERENCES

    1. EPRI Fatigue Management Handbook,EPRI, Palo Alto, CA: December, 1994.TR-104534-V1, -V2, -V3.

    2. J. K. Smith, Vibrational Fatigue Failures in Short Cantilevered Piping with Socket-Welding Fittings,ASME PVP. Vol. 338-1, (1996).

    3. Vibration Fatigue of Small Bore Socket-Welded Pipe Joints,EPRI, Palo Alto, CA: June,

    1997. TR-107455.

    4. M. Higuchi et al, Fatigue Strength of Socket Welded Pipe Joints,ASME PVP. Vol.313-1, (1995).

    5. M. Higuchi et al, A study on Fatigue Strength Reduction Factor for Small DiameterSocket Welded Pipe Joints,ASME PVP.Vol. 338-1, (1996).

    6. M. Higuchi et al, Effects of Weld Defects at Root on Rotating Bending FatigueStrength of Small Diameter Socket Welded Pipe Joints,ASME PVP. Vol. 338-1,(1996).

    7. Criteria of the ASME Boiler and Pressure Vessel Code for Design by Analysis,Sections III and VIII, Division 2, ASME, (1969).

    8. C. E. Jaske and W. J. ODonnell, Trans. ASME, J. Pressure Vessel Technology, 1977,pp. 584-592.