CORPORATION SPACE OPERATIONS
TWR-60352Solid Rocket MotorsThiokol CorporationEVALUATION OF RSRMCASE HARDWARE FRETTINGCONCERNS11 October 1990Prepared for:NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONGEORGE C. MARSHALL SPACE FLIGHT CENTERMARSHALL SPACE FLIGHT CENTER. ALABAMA 35812Contract No.DR. No.3-5WBS.No.4C102CORPORATIONSPACE OPERATIONSP.O. Box 707, Brigham City, UT 84302-0707 (801) 863-3511FORM TC4677 (REV 1-881CMASA-CR-184287)E V A L U A T I O N C3F RSRH CASEHARDWARE F R F T T I M G CONCERNS {Thiokol Corp.)40 oCSCL 21HN92-20243G3/20Uncl s0377050
TWR-60352EVALUATION OF RSRM CASEHARDWARE FRETTING CONCERNS11 October 1990Prepared by:T. R. SwaugerMetal Component DesignApproved by:sperry, Supervisor4etal Component DesignJ. V. Daines, ManagerMetallic Component DesignJx/M. Seil f, Project Engineerise Project EngineerH. (?.' Rotter, Program ManagerSteel CaseB. HowardSR&QAT. K. LaiFracture Control BoardReleased by:Data ManagementSS2432
CORPORATIONSPACE OPERATIONSContents1.0Introduction1Field Joint FrettingFactory Joint FrettingOverview of Effort2352.0Conclusions and Recommendations63.0Discussion9Case Hardware Fretting in Detail93.1.13.1.210111.11.21.33.1Field Joint Fretting in DetailFactory Joint Fretting in Detail3.2Possible Fretting Scenarios113.3Case Refurbishment Requirements - Fretting123.3.13.3.212163.43.5Refurbishment of Field Joint FrettingRefurbishment of Factory Joint Fretting1718Metallurgical Evaluation of Fretted RegionFlight Safety l EffectsStress Corrosion CrackingFatigue Crack GrowthJoint SealingOther Concerns 192021232427Figures and TablesFig. 1:Fig. 2:Fig. 3:Field Joint Fretting LocationFactory Joint Fretting LocationField Joint Fret Locationwith Associated ScratchFig. 4: Inner Clevis Leg I.D. Seal ZoneFig. 5: Fret Repair Entirely in Seal ZoneFig. 6: Fret Repair, Blend Zone ProjectedInto Interference ZoneFig. 7: Fret Cross SectionTable I: Stress Under Flight LoadsTable II: Critical Crack Size, Fretted .,,ONDOCNOSECTWR-603521 PAGE l. VOL
CORPORATIONSPACE OPERATIONS1.0IntroductionFretting corrosion was first noted on Shuttle flight STS-26.This flight was the first usage of the Redesigned Solid RocketMotor (RSRM).The occurrence of fretting has since beenobserved on both the field and factory joints of the RSRM.defined by G.A. TomlinsonAs(1939), fretting is a form ofcorrosion that occurs at the interface between contacting,highly loaded, metal surfaces when exposed to slight relativevibratory motions.The resulting effect is a loss of metal(pitting) and accumulation of metal and metal oxides on theinterfacing surfaces.The magnitude of the defect is a functionof the interface loads and the number of vibratory cycles towhich the interacting surfaces are exposed.This report has been prepared to summarize the engineeringeffort performed to evaluate the effect of fretting on the RSRMcase hardware.Based on the results of this evaluation, severalconclusions have been drawn concerning flight safety.Also,recommendations have been made concerning trending the effectsof multiple generations of fretting damage, and furtherevaluation of case service life concerns.The remainder of this introduction will briefly address theoccurrence of fretting at both the field joint and factory jointregions, as well as give an overview of the engineering effortR6V.S.ON DOC NO.lieTWR-60352I PAGE VOt1
CORPORATIONSPACE OPERATIONSperformed to evaluate the effect of fretting on the RSRMhardware.1.1Field Joint FrettingThe field joint fretting occurs at the capture feature/innerclevis leg interface, see Figure 1 below.Primary 0-rtng.PinsFigure 1:Field Joint Fretting LocationThe capture feature interface did not exist prior to the ShuttleRocket Motor redesign and was the primary element in the casehardware field joint change.REVISIONThe capture feature was added toDOC NO.SECTWR-60352PAGEVOL2
CORPORATIONSPACE OPERATIONSthe field joint configuration to control relative motion betweenthe inner clevis leg and the tang sealing surfaces during motorpressurization.This motion was characteristic of the 51-L andearlier Space Shuttle missions and requires the joint 0-rings totrack the growing seal gap. This becomes an insurmountable taskat colder temperatures when the 0-ring has lost some of itsresiliency.To provide the minimum possible relative motionbetween these surfaces the capture feature was designed with aninterference fit over the inner clevis leg. Since the redesign,fretting has become a regular occurrence at the field jointswith every motor since STS-26 exhibiting at least slightfretting on a few of the joints.1.2Factory Joint FrettingAlso, recently becoming evident is the advent of fretting at thefactory joints.This occurrence had not been documented untilQM-7 and was first detected after flight on STS-29.However,once brought to light the post flight data from STS-26 and STS27 was reviewed and it was determined some light fretting didoccur at the factory joint on these flights also.The factory joint fretting occurs at the interface between theinner clevis leg and the tang surface at the area of the landbetween the primary and secondary 0-ring grooves.which illustrates this location.REVISIONSee Figure 2,Factory joint fretting is muchDOC NO.sicTWR-60352I PAGEI VOL1
CORPORATIONSPACE OPERATIONSsmaller in both magnitude and number than the fretting at thefield joint.The presence and/or awareness of the factory jointfretting can be attributed to two factors:1.The more severe fretting at the field joint brought usto a greater awareness of the occurrence of fretting.2.The field joint shims (pin retainer clips) were modifiedduring the SRM redesign to provide a "force" fit at theouter clevis leg/tang gap. This fit changes therelative positions of the surfaces exhibiting fretting.Secondary 0-ringPrimary O-ringShim—:Figure 2:Factory Joint Fretting LocationIt is not clear whether fretting at the factory joint wasoccurring prior to the redesign.The small indications couldhave easily been attributed to other causes (rust, scratches,etc.) and repaired per standard repair procedures without muchattention drawn to the occurrence.REVISIONThe design change toDOC NO.SECTWR-60352VOLPAGE
CORPORATION. . .SPACE OPERATIONSincrease factory joint shim thicknesses to obtain a "custom"tighter fit may result in having a shim which is too thickselected.By forcing a shim which is too thick into the shimgap, the tang and clevis are forced to shift slightly, withrespect to each other, upon insertion.The shift brings thesurfaces known to exhibit fretting together, providing a greateropportunity for local interface contact, as well as increasedinterface pressures.This condition is thought to aggravate thefretting problem, resulting in a larger number and slightly moresevere occurrences. Fretting at the factory joint is also now aregular occurrence,with fretting noted on each of the flightsets to date.1.3Overview of EffortIn an effort to provide an adequate understanding of thefretting problem, and to alleviate flight safety concerns overthe occurrence of fretting, this report'addresses the followingtopics:1.Case Fretting in Detail2.Possible Fretting Scenarios3.Fretting Refurbishment Criteria4.Metallurgical Concerns5.Structural Concerns - Including Thermal Effects6.Joint Seal ConcernsREVISIONDOC NO.sicTWR-60352I PAGE VOL
CORPORATION.SPACE OPERATIONSRecommendations as to the non-critical nature of the concernsare also made.2.0Conclusions and RecommendationsA thorough evaluation of fretting on the RSRM case hardware hasbeen performed.While we do not have a clear picture of theexact scenario in which fretting occurs on the RSRM we can,based on these evaluations, make an assessment of the resultingdamage and determine the effect on flight safety.A metallurgical evaluation of an actual piece of frettedhardware was performed to determine the extent of the damage tothe micro-structure of the material.This study revealed thatat each fret indication there is a very shallow (.0015 in. deepor less) heat affected zone, as was expected.the presence of surface micro-cracks.Also noted, wasThe micro-cracks are lessthan .001 in depth, and run parallel to the contour of the - pitalong the interface between the heat affected zone and theparent case material.Stresses in the region of the fretting pits were also evaluatedfor the flight load situation.The stresses all result in largepositive margins of safety, including the required 1.4 factor ofsafety. To evaluate stress corrosion cracking concerns, stressesin the region were evaluated for the as-assembled load case, asREVISIONPOC NO.sicTWR-60352I PAGE VOLfi
CORPORATIONSPACE OPERATIONSwell as residual stresses in the region of the frets.Assemblystresses are tensile, however, the magnitude is much less thanthat which would result in a concern.Residual stresses at thefrets are compressive as indicated by X-ray diffractionexamination of an actual fret indication.Compressive stressesdo not support a stress corrosion environment.A fracturemechanics analysis was performed for all of the regionsexhibiting fretting.each region.The critical crack size was calculated forThe critical crack size for each region is largerthan that which can be detected by Non-Destructive Examination(NDE) during refurbishment.Also, the number of pressure cyclesrequired to propagate a crack from an undetectable size to thecritical size, is far larger than the safe life requirement offour missions.The case refurbishment specification requires that 0-ringcompression be evaluated at each seal surface fret repairlocation, thus insuring adequate 0-ring squeeze is maintained.Repairs in the interference zone are limited to the removal ofraised metal which maintains the integrity of the interferencefit, causing no significant thermal flow paths through theinterference fit.All the.required inspections to the case have been implementedduring refurbishment which assures that any fretting defectwhich falls outside our experience base, will requireREVISIONDOC NO.SECTWR-60352:I PAGE VOI7
SPACE OPERATIONSCORPORATION .Material Review Board (MRB) action prior to usage.With allthese factors considered, fretting does not represent a safelife concern.This eliminates the need to address this issue atthe Flight Readiness Reviews and allows added devotion to othermore pertinent items.The issue of case attrition should not, however, be consideredclosed.While we can assure flight safety on a one-by-one basisthru refurbishment requirements, multiple generations offretting may eventually cause the dimensional requirements(specifically those features which affect the interference fit)to begin falling out of specification.At this point in time wehave very limited data concerning the effects of multipledamage, therefore, useful life predictions are impossible tomake.It is with this knowledge in mind that we make the followingrecommendations.We must continue to closely monitor multiplegenerations of fretting.This will provide engineering therequired information to identify possible trends which wouldcause decreases in case life.It is also recommended that wedevelop an automated system which is capable of inspecting, indetail, local defect and repair zones.The data generated bythis equipment could then be put into a working data base tosupport the trending effort.REVISIONDOC NO.sicTVR-60352I PAGE VOL8
CORPORATIONSPACE OPERATIONSWe must concurrently pursue load characterization and testing tohelp evaluate the case service life concern.This would be bestaccomplished by taking steps to instrument during the variousloading scenarios which are thought to be contributors in theoccurrence of fretting.Then institute a comprehensive testingprogram, which more accurately simulates actual conditions.Information gathered in this testing could be used two fold:1.Testing would give us insight into the case life concern.2.Testing would help characterize the actual mechanism offretting and may lead to a possible fix. Thus, totallyavoiding the service life concern.As a last recommendation, the Quality Organization must movequickly in completing their probability of detection study tosubstantiate the magnetic particle inspection sensitivity level. This would insure that inspection requirements are met.3.0 Discussion3.1Case Hardware Fretting in DetailBoth the fretting which is occurring at the field joint and thefactory joint have been identified as flight safety issues.better understand the phenomena each have been described indetail in this report. Also, TWR-61070 "RSRM Joint FrettingSummary" (preliminary) provides a flight by flight history ofthe fretting damage which has been encountered.ooc NO. TVR-60352SicPAGE9To
CORPORATIONSPACE OPERATIONS3.1.1Field Joint Fretting in DetailThe fretting which is occurring at the capture feature of thefield joint has very defined characteristics.A typical fret onthe clevis leg side of the interface is a small pitapproximately .004 to .005 inch deep and approximately .020 to.060 inch wide.The most severe pit was .017 inch deep (STS-29) which is a great deal larger than any other occurrence.Thecapture feature side of the interface is-characterized by verylight pitting and the presence of slightly raised material.During disassembly of the field joint, the raised metal presenton the capture feature leaves a small scratch trailing from thepit on the clevis.This scratch is typically .0015 inch or lessdeep, and less than .020 inch wide.Figure 3 below illustratestypical field joint fretting as described.Capture FeatureClevisInner LegScratchFrettingIndicationsFigure 3:REVISIONField Joint Fret Indication withAssociated ScratchDOC NO.licTWR-60352PAGE
CORPORATIONSPACE OPERATIONSFigure Al in Appendix A, shows an actual photograph of a typicalclevis leg fret, note the scratch or "tail" trailing from thepit towards the end of the part.3.1.2Factory Joint Fretting in DetailThe fretting characteristic of the factory joint is a veryshallow pit usually only .001-.002 inch deep and approximately-.030 inch wide.inch deep.The worst case instance is documented at .004Occasionally very slight raised metal is associatedwith these pits but is hardly detectable.Factory jointfretting has been much less severe than that noted on fieldjoints.The number of indications is also much less thanfretting -on the field joint.3.2Possible Fretting ScenariosThe exact scenario for the occurrence of fretting on the RSRM isnot clearly understood.A number of possible scenarios havebeen defined, however, it is not known which is the culprit, andin fact it may be the combined effect of all, or an isolated fewof them.What is clear, however, is that no matter what thescenario, two basic elements are required: contact pressure andvibration.It is not the intent here to discuss each of thesepossible scenarios in detail in this report.It does however,seem prudent to outline them with a few brief conclusions.REVISIONDOC NOSECTVR-60352I PAGE VOL4f
CORPORATIONSPACE OPERATIONSFor those scenarios which occur without motor pressure thecapture feature interface pressures would be a direct result ofthe assembly loads induced due to the interference fit.However, in those scenarios which assume the fretting isoccurring during motor pressure, the resulting interface loadwould be the combined effect of assembly loads and much higherloads induced due to internal motor pressure.Even the lowerassembly loads can generate locally high interface pressures,since the load is reacted out along a line of contact with verylittle contact area.If you add to this effect the localasperities on each of the contact surfaces, and possible caseout of roundness, very localized high pressure points aredefinitely a possibility, even for the assembly load frettingscenarios.Given the varied vibration environments and thepossi-bility of high local interface loads for both thepressurized and non-pressurized cases it has proven verydifficult to determine the exact scenario for the occurrence offretting.Table Bl in Appendix B lists each possible scenariowith a brief statement where appropriate.3.3Case Refurbishment (Re-use) Requirements - FrettingThe major concern generated by fretting is the reuse of frettedhardware.The case hardware has a reuse goal of 19 flights.The cost effectiveness of the entire RSRM program is a directresult of the ability to reuse this critical case hardware.DOC NO. TVR-60352PAGEI *" In
CORPORATIONSPACE OPERATIONSorder that we understand the concerns of flying fretted casesegments we must first have a thorough understanding of howfretted indications are repaired and inspected duringrefurbishment operations.The following data is compiled fromthe RSRM Case Segment Refurbishment Specification (STW7-2744).3.3.1Refurbishment of Field Joint FrettingThe field joint fretting occurs at the capture featureinterface.Due to the interference, the inner clevis leg isforced to bend slightly.This bending action forces hardcontact, and thus the major occurrence of fretting to be at theextreme end of the interface.This area is extremely close tothe capture feature 0-ring seal surface.Jn fact, givenassembly tolerances what may be the interference zone for oneparticular joint mate, may actually stray into the capturefeature seal zone for another mate.The refurbishmentspecification must and does account for the worst case toleranceseal zone, resulting in some fretting pits occurring in thedefined seal zone of the clevis leg. Fretting which occurs inthe seal zone is blended using an abrasive Dremel tool and afinal hand polish.As part of the repair procedure, 0-ringsqueeze requirements are verified for the reworked region toinsure minimum squeeze requirements have not been violated.Also, the surface finish in the rework zone is reverified toassure that original surface finish requirements are maintained.REVISIONDOC NO.lieTVR-60352I PAGE VOL13
CORPORATIONSPACE OPERATIONSFretting pits which occur in the seal zone but so near theinterference zone that the projected blend zone would cross intothe interference zone are feathered out in the axial directionat the seal/interference zone boundary.By not allowing theblend to extend into the interference zone the interference fitis not compromised.Those pits which are entirely in theinterference zone are repaired by only breaking sharp edges andremoving raised metal.The entire capture feature tang frettingregion falls into this category of surface.Figures 4 thru 6identify the seal and non-seal zones as well as illustrate therequired rework for the various situations we have describedabove .— 0.997 —.66SEAL SURFACEINTERFERENCEZONEIFigure 4:REVISIONInner Clevis Leg I.D. Seal ZoneDOC NO.SECTVR-60352VOLPAGE
CORPORATIONSPACE OPERATIONSNON-SEALZONE NON-SEALZONENOTE:INITIAL DEFECT0 INITIAL DEPTH OF DEFECTW INITIAL WIDTH OF DEFECTBLENO ZONESEAL SURFACEBLENO TO FULL.DEPTH OF DEFECTENTIRE REPAIR ZONE TOBE BLENDED SMOOTHLYFigure 5:Fret Repair Entirely in Seal Zone NON-SEAL ZONESEAL 20NEBOUNOARYNOTE:D INITIAL DEPTH OF DEFECTW INITIAL WIDTH OF DEFECTINITIAL DEFECTBLEND ZONESEAL SURFACEBLEND TO FULLDEPTH OF DEFECTINSIDE THE SEAL ZONE THEENTIRE REPAIR ZONE SHALLBE BLENDED SMOOTHLYFigure 6:REVISIONFret Repair, Blend Zone ProjectedInto Interference ZoneDOC NO.SECTVR-60352[ VOLPAGE15
CORPORATIONSPACE OPERATIONSIt should also be noted that no pitting in excess of .010 inchdeep is permitted by the specification.Any pit which exceedsthis limitation must be evaluated through formal Material ReviewBoard (MRB) action.The segment is proof tested at 1.12 times the maximum expectedoperating pressure prior to each usage.After the proof test amagnetic particle inspection is performed to detect surfacecracks. The Refurb Specification requires that no cracks greaterthan .100 long are acceptable.3.3.2Refurbishment of Factory Joint FrettingAs indicated earlier, fretting at the factory joint oc
TWR-60352 VOL sic I PAGE fi. CORPORATION SPACE OPERATIONS well as residual stresses in the region of the frets. Assembly stresses are tensile, however, the magnitude is much less than that which would result in a concern. Residual stresses at th
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