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A101657AFWAL.TR-T1.3041VOLUME 1
EFFECT OF VARIANCES ANDMANUFACTURING TOLERANCES ON THEDESIGN STRENGTH AND LIFE OFMECHANICALLY FASTENEDCOMPOSITE JOINTSVOLUME I - METHODOLOGY DEVELOPMENT AND
DATA EVALUATIONS.P. Garbo and J.M. O4onowski JMcDonnell Aircraft CormnveyMcDonnell Douglas CorporationP.O. Box 516St. Louis, Missouri 63166 .
April 1981
Final Rnport for Period 15 February 1978- 15 April 1981
-pwntr URbIftISS idt'IbUwid UUUI*
-__ T DYNAMICS LABGS :?ORY'In FORCE WRIGHT AERONAUTICAL LABSRACIESAIR FORCE SYSTEkMS COMMANDWRIGHT-PATTERSON AIR FORCE BASE, OHIG 4 8 7210381 7 21 009
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-D' NOTICEWhen Government drawings, specifications, or other data areused fo r any purpose other than in connection with a definitelyrelated Government procuremant operation, the United StatesGovernment thereby incurs no responsibility nor azrny obligation
whatsoever; anid the fact that the government may have formulated,furnished, or in any way supplied the said drawing, specifica-tions, or other data, is not to be regarded by implication orotherwise as in any manner licensing the holder or any otherperson or corporation, or conveying any rights or permission tomanufacture, use, or sell any patented invention that may in anyway be related theereto.Thie report has been reviewed by the Office of PublicAffairs (ASD/PA) and is releasable to the National TechnicalInformation Service (NTIS). At NTIS, it will be available to thegeneral public, including foreign nations.Thiis technical report has been reviewed and is approved forpublication.
ROBERT L. GALLO, Capt, USAF DAVEY L/ SAITH, ChiefProject Engineer Structural Integrity Branch
Structures & Dynamics Division
FOR THE COMMANDER
RALPH L. KUSTER, JR., Col, USAFChief, Structures and Dynamics Division
If your address "is changed, if you wish to be removed frontour mailing list, or if the addressee is no longer employed byyour organization please notify AFWAL/FIBEC, Wright-PattersonAFB, OH 45433 to help urs maintain a cu:rent mailing list. Copiesof this report should riot be returned unless return is requiredby security considerations, contractual obligations, or notice ona specific document.AIR FORCE/5678011 July 1981 - 50 0
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SECURITY q r6,CATION Of THIS PAGE (WPhen Da. Entered)"RPORT DOCU4ENTATION PAGE READ INSTRUCTIONSBEFORE COMPLETING FORM"GoV ACCESSION NO. IENT'S CATALOG NUMBERAFAL R-8--3,4 1' V I h V
tFFECT OF YARIANCES AN D MANUFACTURIW OLEIRANCE [Final Je_. 15TH SI6N TRENGTHq A t IFE OF rE&ANICALLY 115 Apr t@S8lflITT-.,ASTENED OMPOITE P, :NTS. REPORT NUMBE
AU S0IF-0ONTRACT O GRA~NT UMNER(a)
J. M. /Ogonowski -- 7ibe'oSwPt) G ORGANIZATION NAME AND ADDRESS 10 PROGRAM ELEMENT. PROJECT, TASKMcDonnell Aircraft Company AREA A WORK UNIT NUMBERSP.O. Bo x 516 P.E. 62201FSt. Louis, Missouri 63166 W.U. 24010110
I.CONTROLLING OFFICE NAME: AND ADDRESSFlight Dynamics Laboratory (AFWAL/FIBEC) [ ApAir Force Wright Aej:onautical Lab. (AFSC) Ur PAGESWright-Patterson AFB, Ohio 45433 144
14 MONITORING AGENCY NAME & Ar)ORESS(It different fron. Controllin4 Olfice) 15. SECURITY CLASS. (of it . report)7 Jnclassif edIS. DE C ASSIFICATION/ DOWNGRADINGSCHEDULE16 DISTRBUTION STATEMENT (of th
Approved for public release; distribution unlimited
17, DISTRIBUTION ST 4ENT (of abstract eentered in Block 20, If different froe. Report) 4
IS S U P P LEMEN TA RY IE S " ' C'19. KE V WORDS Zonllnuie or reverse side if nec i ies l i zn d Identify iiy black numbwr)
Bolted Joints Orthotropic Ltad DistributionsComposite Stress Concentrations Sl:ress AnalysisGraphite-epoxy Methodology Failure CriterLaFatigue Life0 ABSTRACT (Conlinu on reverse sde If necesesry an d Identify by block number)
The subject of this program was structural evaluation of mechanicallyfastened composite Joints. Program objectives were threefold: (1) develop-ment and verification by test of improved static strength methodology,(2) experimental evaluation of the effects of manufacturing anomalies onjoint itatic strength, and (3) experimental evaluation of joint fatigue life.-
DI I JAN 3o 1473S ECU RITt ' CLASSIFICATION OF THIS PAGE (When Data Entered)
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SECURITY CLASSIFICATION OF THIS PAGE(When Data Enfl.;d_
,'Program activit ies to accomplish these objectives were organized underfive tasks. Under Task 1 - Literature Survey, a surivey wae perform4 todetermine the state-of--the-art in design and analysisi of bolted coa.6 ,sitejoints. Expezimental evaluations of joint static strength were performedunder Tatsks 2 and 3. In Task 2 - Evaluation of Joint Design Variables,strength data were obtained through an experimental, program to evaluate th eeffects of twelve joint design variables. Tn Task 3 - Evaluation of Manu-facturing and Service Anomaelies, effects of seven anomailies on jo int strengthwere evaluated experimentally and compared with Task 2 strength data. Beltedcomposite joint durability was evaluated under Task 4 - Evaluation of Crii:tcalJoint Design Variables on Fatigue life. Seven critical design variables o0.manufacturing anomalies were identified based on Task 2 and 3 strength data.Under Task 5 - Final Analyses eind Correlation, required data reduction,methodology development and correlation, and necessary documentation wereperformed . -
This report documents all program act ivi t ics performed under Tasks 2, 3,4 and 5. Activities performed under Task 1 - L-iterature Survey, were pre-viously reported in AFFDL-TR-78-179. Static. strength methodology and evalua-tions of joint static and fatigue test data are reported. Analytic, studiescomplfment methodology developmeizt and illustrate: th e need fcr detailedstress analysis, th e utility of the developed "Bolted Joint Stress FieldModel" (BJSFM) procedure, and define i-odel limitatLins. For static strengthdata, correlations with analytic predicbtions are included. Data trends inall cases are discussed relative joint strz',agth and failure mode. Forjoint fatigue studies, data trends are discuesed relative to life, holeelongation, and failure mode behavior.
Tbis final report is organized in the oll]owin$ three volumes:Vlolume 1 - M1ethodology Development ard asta Eval.uationVilume 2 - lest Data, Equlpment and ProceduresVolume 3 - Belted Joint Stress Field Model (BJSFM) Camputr
FrL'gram User's Manual
SECURITY LASSIFICATION OF THIS PArkir-'When Date Entered)
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FOREWORD
The work reported herein was performed by th e McDonnellAircraft Company (MCAIR) of th e McDonnell Douglas Corporation(MDC), St. Louis, Missouri, under Air Force Contract F33615-77-C-3140, for th e P'light Dynamics Laboratory, W right-Patterson AirForce Base, Ohio.. This effort was conducted under Project No.2401 "Structural Mechanics", Task 240101 " Structural Integrityfor M ilitary Aeroxzpace Vehicles", Work Unit 24010110 "Effect ofVariances and Manufacturing Toleranceii on th e Design Strength andLife of Meclianically Fastened Composite Joints". The Air ForceProject Engineer at contract go-ahead was Mr. Roger J.Aschenbrenner (A'WAL/FIBEC); in Dec,-.mber 1979, Capt. Robert L.Gallo (AFWAL/FIBEC) assumed this assignment. The work describedwas conducted during th e period 15 February 1978 through 15 April1981.
Program Manager was Mr. Raman A. Garrett, Branch ChiefTerhnology, MCAIR Structural Research Department.. PrincipalInvestigator was Mr. Samual P. Garbo, MCAIR Structural ResearchDepartment.
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Di st ribution /AvailmbilitY Codes .nn," or
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TABLE OF COI4rENTSSection P_I INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 1I1 SUMMARY AND COMCLUSIONS . . . . . . . . . . . . . . . . 2IXcI METHODOLOGY DEVELOPMENT KID ANALYTIC STUDIES . . . . . 5
1. BACKGROUND ...... . . . . . . . . . . . . 52. METHODOLOGY DEVELCPMENT ........... ... 53. PREDICTIONS USING BJSFM APPROACH. . . . . . . ... 15a. Effects of Anisotropy on Stress
SDistributions .... ......... 15b. Hole Size Effects ... .......... . 16c. Finite Width Effects .............. 17d. Characteristic Dimension Sensitivity Study . . 19e. Biaxial Loading Effects .............. 20f. Pure Bearing Strength Study .... .......... . 24g. Effects of Enviromment . . .......... . 25
IV TEST DATA EVALUATION ..... .................. 26I. METHODOLOGY VERIFICATION: EVALUATION OF
JOINT DESIGN VARIABLES - TASK 2 ......... .26a. Task 2 Test Plan . ....... .......... 26b. Correlation of 3JSFM Predictions WithExperimental Results and Evaluations ...... .. 35
(i) Strength of Laminates With Unloadedand Unfil led Holes . . . ........... 35(2) Strength of Laminates With LoadedFastener Holes ......... ............. 44(a) Layup Variat ion .. .. ........ . 50(b) Load Orientat ion .... ....... . . . . 50(c) Edgt Distance ........... ............ 55k1) Widthi . . . . . . . . 57(e) Hole Size ....... .............. 57(f) Load Interaction .... ........... .. 64(g) Fastener Pattern . ....... ............ 73(h) Torque-up ......... . ............ 76(i) Fastener Countersink and
Laminate Thickness ... .......... .. 77(j) Stacking Sequence ............... 86(k) Single-Shear Loading ........... .. 86
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TABLE OF CONTENTS (Concluded)Section age
2. EV\LIUATION OF MANUFACTURING AN.) SERVICEANOMALIES - TASK 3 .......................... 93a. Task 3 Test Plan .......... ............... 93b. Experimental Results and Evaluation ..... ...... 94
(1) O ut of Round Holes .. .......... 94(2) Broken Fibers on Exit Side of Hole 98(3) Porosity . . . . . .. . . . . . . . . 100(4) Improper Fastener Seating Depth .. 102(5) Tilted Countersinks. . . ............ 104(6) Interference Fit .................... . 106(7) Fastener Removal an d Reinstallation .... .. 1093. EVALUATION OF CRITICAL JOINT DESIGN VARIABLESON FATIGUE LIFE - TASK 4 ...... ........... . . . . IC
a. Task 4 Test Plan . . . . ............ . . 110b. Experimental Results and Evaluation . . . . . . 115(1) Layup Variation ...... .............. .. 117(2) Stacking Sequence . . . . ........... 125(3) Torque-Up.............. . . . . . . . . . 1.30(4) Joint Geometry . . . . . . . ... ... 135(5) Interference Fit ............. 138(6) Single-Shear Loading .............. . .. 138(7) Porosity .... ............ .142
V RECOMM>,NDATIONS ....................... 143VI REFERENCES .............. ....................... 144
vi
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LIST OF ILLUSTRATrIoNSFigure Page
1 Superposition of Linear-Elastic Stress Solut ions . 62 Uniaxially Loaded Infinite Plate ........ .......... 93 Assumed Cosine Bolt-Load Distribution ....... ...... 104 Characteristic Dimension Failure Hypothesis . . .. 125 Failure Criteria Comparison ....... ........... .. 136 Superposition of Solutions to Account
fo r Finite Width. . ............ .* . . . . 147 CircuMferential Stress Solutions at
Unloaded Holes ....... ................. . . . . 158 Circ mferential Stress Solutions at
Loaded Holes ...................... 69 Effect of Hole Size on Stress Distributions .... 17
10 Correlation of Loaded Hole Analyses ..... ........ 1811 Effects of Width on Stress Ccncentrations ....... 1812 Correlation of Loaded Hole Analysis fo r SmallEdge Distance Cases ....... ..................... 1913 Effect of Characteristic Dimension onPredicted Strength - 30/60/10 layup .. ........ .. 2014 Effect of Biaxial Loading on Predicted Strength 2115 Effect of Biaxial Loading on Failure Location . . . 2216 effect of Biaxial Loading on Ply Strain
Concentrations ............ ................... 2317 Effect of Bearing Load Direction on
Predicted Strength ..................... 2418 Effect of Moisture and Temperature on
Laminate Strength ........... ................. 2519 Task 2 - Joint Design Variables - Test Matrix . . 2720 MCAIR Evaluation of Unloaded Hole Specimens -
Test Matrix ............... .................... 29
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LIST OF ILLUSTRATIONS (Continued)Frigure P _
21 MCAIR Evaluation of Loaded Hole SpecimensTest Matrix ............. .................... 3022 Unloaded Hole Specimen Geometry ... .......... 3023 Loaded Hole Specimen Configuration ... ......... .. 3124 Specimen Loading Configurations ... .......... 3225 Effect of Laminate Stiffness on SpecimenBolt-Load Distributions ....... .............. 3326 Task 2 - Layup Number and Stacking Sequence ... 3427 AS/3501-6 Lamina Mechanical Properties ....... .. 352R Effect of Layup Variation on UnloadedHole Tensile Strength ......... .............. 3629 Effect of Hole Size on Unloaded HoleTensile Strength .......... .................. 3730 Effect of Variation of Rc on Predicted
Tensile Strength ............ .................. 3831 Predicted Failure Orientations Were Verified . 3932 Effect Off-Axis Loading on Failure Stress ...... 4033 Efrect of Variation of R, on PredictedCompressive Strength ........ ................ 4134 Effect of Hole Size on Unloaded HoleCompressive Strength ........ ................ 4235 Effect of Layup Variation on UnloadedHole Compressive Strength ....... ............. 4336 Effect of Temperature on Unloaded HoleCompression Strength ........ ................ 4437 Two-Bolt-In-Tandem Spezimen RaselineConfiguration ............. ................... 4538 Variations of Single 7nstener SpecimenConfigurations . . ..................... 4639 Variations of Two-Rolt-In-TandemSpecimen ConfiguratiLns ......... .............. 47
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LIST OF ILLUSTRATIONS (Cont inued)Figure Page
40 Variations of Load-Interaction SpecimenConfigurations ............ ................... 4841 Fastener Pattern Specimen Geometry ... ......... .. 4942 Effect of Layup Variation on JointTensile Strength ............ ................. .. 5143 Effect of Layup Variation on JointCompreasive Strength ........ ................ 5244 Effect of Off-Axis Loading on RTD PureBearing Strength. ......... ................. 5345 Effect of Temperature and Moisture onOff-Axis Strength ........... ................. 5446 Effect of Temperature and Moisture onCow.preusive Strength ........ ................ 5547 Effect of Edge Distance on JointTensile Strength ............ ................. .. 5648 Effect of Fastener Spacing on JointTensile Strength .......... .................. 5849 Effect of Edge Distance on PureBearing Strength .......... .................. 5950 Effect of Specimen Width on RTD Joint Strength . . . 6051 Effect of Temperature and Moistureon Joint Strength ........... ................. 6152 Joint Failures Changed at ETW Test Conditions. . . . 253 Effect of Hole Size on Joint Strength ....... 6354 Load Interaction Specimen and Test Setup ...... .. 6455 Correlation of Bearing-Bypass Strength of a
50/40/10 Layup ............ ................... 6556 Correlation of Bearing-Bypass Strength of a
30/60/10 Layup ............ ................... 6657 Effect of Bearing Loads on Specimen
Load-Deflection Behavior ...... .............. 67
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LIST OF ILLUSTRATIONS (Cont inued)Figure Pag~e
58 Predicted Critical Plies for Net-SectionFailures .............. ...................... 68
59 Predicted Critical Plies fo r Bearing-ShearoutFailures .............. ...................... 69E10 Tests Verify Predicted FailureInitiation Points ........... ................. 7061 Load Conditions for Off-Axis".~oad-Interaction Tests ........ ............... .. 7162 Effect of Off-Axis Bearing Loads onBypass Strength ........... ................. 7263 Fastener-Pattern Specimen Loading Configurations . . 7364 Effects of Environmnint on Fastener-PatternJoint Strength ............ ................... 7465 Joint Failures fo r Fastener-Pattern Specimens . . . 7566 Corrosion Result ing From Salt-Spray Exposure . . .. 7667 Effect of Lavup and Torque-Up on RTD PureBearing Strength ............ ... .................. 7868 Failures of Pure Bearing Specimens ........... .. 7969 Comparison of Loaded and Unloaded Hole
Specimen Strength ........... ................. 8070 Effect of Torque-Up on RTD Pure Bearing
Strength 50/40/10 Layup ....... .............. 8171 Effect of Toraue-Up on RTD Pure BearingStrength 70/26/10 Layurp ....... .............. 8272 Effect of Torque-Up on RTD Pure BearingStrength 30/60/10 Layup ....... .............. 8373 Comparison of Effects of Torque-Up onJoint Stru-':'th ................ . ................... 8474 Effects of Thickness and Countersink onJoint Tensile Strength ........ ............... 8575 Effect of Stacking Sequence on JointTensile Strength .......... .................. 87
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LIST OF ILLUSTRATIONS (Continued)Figure Paqe
76 Correlation of Joint RTD Tensile StrengthWith Grouped 0 Plies ...... ............... 88
77 Correlation of Joint ETW Tensile StrengthWith Grouped 0 Plies ...............78 Correlation of Joint RTD Compressive Strength
With Grouped 0 Plies .... ................ 9079 Comparison of Single and Double-Shear
Joint Strengths ......... .................. 9180 Correlation of Joint Strength With Edge Distance-to-
Width Ratios ........................ 9281 Task 3 - Evaluation of Manufacturing Anomalies -
Test Matrix ............ .................... 93S2 Specimen Moisture Conditioning Histories ...... .. 9583 Summary of Task 3 Strength Reduction Percents . . . 9684 Out-of-Round Holes - Specimen Details ....... 9785 Effect of Out-of-Round Holes on Joint Strength .7 . C86 C-Scans of Laminates With Delaminationsat Fastener Holes ........ ................. 9887 Effect of Delaminations on Joint Strength ...... 9988 Panel Fabrication Procedures Used toProduce Panel Porosity ........... ............... 13089 Examples of Panel Porosity .... ............. .. 10190 Freeze-Thaw Exposure Profile .... ............ 10291 Effect of Porosity Around Hole on Joini Strength . 10392 Effect of Improper Fastener Seating onJoint Strength ......... ................... 10493 rilted Countersink - Specimen Configuration .... 10594 Effect of Tilted Countersink on Joint Strength . . . 106
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LIST OF ILLUSTRATIONS (Continued)Figure Pag
95 Effect of Fastener Interference Fit onJoint Strength ........... .................. 10796 P'hotomicrographic Examination of LaminatesWith Interference Fit Holes .... ........... 10897 Effect of Fastener Removal and Reinstallationon Joint Strength ........ ................. 10998 Task 4 - Evaluation of Critical JointVariables on Fatigue Life - Test Matrix ...... ...... li1i99 Task 4 - Fatigue Specimen Configuration .......... 113100 Distribution of Hours and Exceedances .. ...... 114101 Measured Mix-Truncated Spectrum ... .......... 114102 Task 4 Layup Numbers and Stacking Sequence ..... .. 115103 Summary of Task 4 Specimen Static Strength ..... .. 116104 RTD Baseline Joint Fatigue Life
50/40/10 Layup ......... ................... 117105 RTD Baseline Joint Fatigue Life30/60/10 Layup ......... ................... 118106 RTD Baseline Joint Fatigue Life
19/76/5 Layup .......... ................... 118107 Comparison of R = +0.1 Joint Fatijue Life Trends . . 119108 Comparison of R = -1.0 Joint Fatigue Life Trends . 119109 Hole Elongation Fatigue Life Trends
50/40/10 Layup ................. ................... 120110 Hole Elongation Fatigue Life Trends
30/60/10 Layup ................. ................... 121iii Hole Elongation Fatigue Life Trends19/76/5 Layup .......... ................... 121112 Representative Specimen Fatigue Failures ...... .. 122113 Summary of Effects of Environment onJoint Spectrum Fatigue Life ...................... 123
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LIST OF ILLUSTRATIONS (Concluded)Figure Page11, Effect of R - +0.1 Loading on Hole Elorgation -Baeie50/40/10 Layup .............. 124aseline 0//lay..........................2115 Effect of R = +0.1 Loading on Joint Spring Rate -Baseline 50/40/10 Layup . . . . . . . 125116 Effect of Stacking Sequence on Joint Life -
50/40/10 Layup .*. . ......... ........... 126117 Effect of Stacking Sequence on Joint Life -
19/76/5 Layup ........ . ............... 127118 Stacking Sequence - Comparison With R = -1.0Baseline Life Trends ...................... . . . . 128119 Su.acking Sequence - Comparison With R = +0.1
Baseline Life Trends . . . . . .............. 128120 Effect of Stacking Sequence - Hole ElongationLevels - 50/40/10 Layup ........... . . . . 129121 Effect of Stacking Sequence - Hole Elongation
Levels - 19/76/5 Layup ............. ........ 130122 Summary of Effects of Stacking Sequence on
Joint Spectrum Life . . . . . ............... 131123 Effect of Torque-Up on Joint Fatigue "Lie50/40/10 Layup ............ ................... 132124 Effect of Torque-Up on Joint Fatigue Life
19/76/5 Layup ............. ................... 133125 Specimen Failures fo r Torque-Up Conditions ..... .. 134126 Effect of Geometry on Joint Fatigue Life
50/40/10 Layap . . .................... 135127 Effect of Geometry on Joint Fatigue Life
19/76/5 Layup ................. ................... i36128 Changes in Failure Due to Specimen Geometry . . . . 137129 Effect of 0.005 Inch Interference Fit on Joint
Fatigue Life - 50/40/10 Layup .... .......... 139130 Effect of Single-Shear Loading on JointFatigue Life - 50/40/10 Layup ..... ........... .. 140131 Failures of Single-Shear Specimens ... ......... .. 141132 Effect of Porosity on Joint Fatigue Life -50/40/10 Layuip ............ ................... 142
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SECTIONII TRODUCT ION
The subject of this program was structural evaluation ofmechanically fastened composite joints. Program objectives werethreefold: (1) development and verification by test of improvedstatic strength methodology, (2) experimental evaluation of theeffects of manufacturing anomalies on joint static strength, and(3) experimental evaluation of joint fatigue life.Program activities to accomplish th'ese objectives wereorganized under five tasks. Under Task 1 - Literature Survey, asurvey was performed to determine the state-of-the-art in designand analysis of bolted composite joints. Experimental evalua-tions of joii~t static strength were performed under Tasks 2 and3. In Task 2 - Evaluatior~ of Joint Design Variables, baselinestrengt~h data were obtained through an experimental program toevaluate the effects of twelve joint design variables. In Task 3
- Evaluation of Manufacturing and Service Anomalies, effects ofseven anomalies on joint strength were experimentally evaluatedand compared with baseline Task 2 strength data. Bolted compos-ite joint durability was evaluated under Task 4 - Evaluation ofCritical Joint Design Variables On Fatigue Life. Seven criticaldesign variables or manufacturing anomalies were ide'nLified basedon Task 2 and 3 strength data. Under Task 5 - Final Analyses andCorrelation, required data reduction, methodology development andcorrelation, and necessary documentation were performed.
This report documents all program acti- 'ities performed underTasks 2, 3, 4 and 5. Activities performed under Task 1 - Litera-ture Survey, were previously reported in Reference 1. The mainbody of this report (Sections III and IV) documents developedstatic strength methodology and evaluations of joint static andfatigue test data. Associated analytic studies have beenincluded in Section III to complement methodology developmentdiscussions and to illust-ate: the need for detailed stressanalysis, the utility of the developed "Bolted Joint Stress FieldModel" (BJSFM) procedure, and to define model limitations. Forstatic strength data reported in Section IV, correlations withanalytic predictions, where applicable, are included. DataJtrends in all cases are discussed relative to joint strength andfailure mode and compared to baseline data. For joint fatiguestudies, data trends are discussed relative to life, hole elonga-tion, and failure mode behavior.
This final report is crganized in three volumes: Volume 1-Methodology Development and Data Evaluation, Volume 2 -- TestData, Equipment and Procedures, Volume 3 - Bolted Joint StressField Model (BJsFM) computer Program User's Manual.
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SECTION IISUMMARY AND CONCLUSIONS
Analytic methods were developed which permit strengthanalysis of bolted composite joints with minimal test datarequirements. Methodology was based on anisotropic theory ofelastici ty; classical lamination plate theory, and a character-istic dimension (R ) failure hypothesis. The princAple ofelastic superpositioAc was used to obtain laminate stress distr ibu-t ions due to combined loadings of bearing and bypass. Test datarequirements for general method use were minimized by extendingth e characteristic dimension failure hypothesis to a ply-by-plyanalysis in conjunction with known material failure criteria.Unidirectional (lamina) stiffness and strength data were usedwith an empirical value of R to predict stress distributions,critical plies, failure locatfon, and failure load of arbitrarylaminates under general loadings. The developed analysisprocedre is entitled th e "Bolted Joint Stress Field Model"
The BJSFM methodology was originally developed for unloadedhole analysis by McDonnell Aircraft Company (MCAIR) under an in-house research and development program. The methodology waxfurther verified and extended to th e analysis of loaded holeaunder this program.
Under Task 2 - Evaluation of Joint Design variables, effectsof twelve joint design variables on static strength were experi-mentally evaluated. The BJSFM procedure was used to predict th eeffects of five of the twelve design variables (layups, loadinteraction, off-axis loading, hole size, and width). Only exper-imental evaluation of th e remaining variables was possible. Therange of test conditions in each design variable was identif iedin th e Task I - Literature Survey.Initial verification of analysis was obtained by correlatingstrength predictions with data b-i-- iti Lcfsws of specimenswith unfilled fastener hole,. Specimenn were tested to fai lurein tension and compressio[z. Values for R of .02 inch for ten-sile strength predictions and .025 inch f8r compressive strengthpredictions were empirically determined fo r one laminate, each of
which were fabricated using the Hercules AS/3501-6 graphite-epoxysystem. With these values of R &nd unidirectional ply (lamina)mechanical properties, BJSFM predictions were correlated withunloaded hole strength data fo r an extensive range of layup varia-t ions and, in general, were accurate to within + 10%. Predictedfailure initjAtion points were visually verified: Correlation ofstrength predictions with data indicated that only knowledge oftemperature-altered lamina properties is required to predicteffects of temperature on general laminate strength; R remainedunchanged. C
2
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Joint load-deflection plots for specimens with either shear-out or bearing failures indicated nonl inear mechanical behavior,with ult imate failure occurring at loads considerably above ini-tial nonlinearities. Strength predictions obtained using th eBTSFM procedure correlated with initial points cf joint nonlineardeflection behavior. Ultimate strength predictions using th el inear-elart ic BJSFM procedure became increasingly conservativeas this joint nonlinear load-deflection behavior occurred. How--ever, predicted linear-elastic failures of critical plies weredue to exceeding lamina fiber or shear strength and implied th etype of localized joint failures and load redistribution typicalof the failure modis observed in tests.
Under Task 3 - Evaluation of Manufacturing and Service Anom-alies, effects of seven anomalies on joint static strength wereexperimentally evaluated and compared with baseline (no anomaly)'strength data. The seven anomalies were aelected based on MCAIRexperience and an available industry-wide survey (Reference 2) .Results fell into two groups: (1) anomalies which resultedin strength reductions of more than 13 percent (porosity around
hole, improper fastener seating depth, and tilted countersinks),and (2) anomalies which resulted in strength reductions of lessthan 13 percent (out-of-round holes, broken fibers on exit sideof hole, interference fit tolerances, removal and reinstalla-tion). For anomalies of the first group, current industryinspection and acceptance criteria would have detected all threeand resulted in part rejection or required repair.Unuer Task 4 - Evaluation of Critical Joint Design Variableson Fatigue Life, th e influence of seven design variables and ano-malies on joint durability were evaluated. Tests were performedto provide data on joint fatigue life performance, hole elonga-t ion, and failure mode behavior. Single-fastener pure bearingspecimens were cycled, under tension-tension (R = +0.1) and ten-
sion-compression (R = -1.0) constant amplitude fatigue loading,and under spectrum fatigue loadings.During fatigue testi'ng, load-deflection data were obtainedat specified increments of accumulated hole elongation. Severaltests were performed at environmental conditions previouslydefined in Task 2. Static and residual strength tests wereperformed on selected specimens at each test condition.Joint fatigue life was defined to occur at specimen fai lureor when hole elongations of .02 inch were measured. Based onthis definition, little difference was noted in relative fatiguelife between 50/40/10, 30/6G/10 and 19/76/5 layups (percent 00,+450, 90 plies respectively) under tension-tension (R = +0.1)
cycling. Under fully reversed load cycling, (R = -1.0), holeelongations of .02 inch occurred more rapidly in th e matrix-dominant 19/76/5 layup, followed by th e 30/60/10 and 50/40/10layups. Under spectrum fatigue tests of a'1 three layups, no
3
S i. II I
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hole elongation occurred after 16,COO spectrum hours at testlimit loads (TLL) of 891 of static strength values. This agreedwith constant amplitude fatigue test zesults; 16,000 hours ofspectrum loading at tbese levels did not produce enough cycles ofhigh loads to produce hole elongation.Effects of faistener torque--up on Joint fatigue life werepronounced. Joints with torque-up Paltien exhibited increasedstrength and life fo r all layups testad, inder both R - +3.1 andR - -1.0 cycling. While failure modoas wore the same, aireas ofdamage were more pronounced fo r specimern with torque-up andfailure occurred more abruptly (rates of ',ole elongation, onceinitiated, were faster).Effects of joint eccentricity and geometry also infLuencedjoint fatigue life characteristics. hingle.-shear specimensexhibited lower fatigue life as compared to the double-shearbaseline configuration. Further reductions occurred wit-hflush-head fasten-ars relative to pzotrtuding head fastenerii;attributed to jincreased fastener f.tex;.bility and nspecimnn
bending. For tTi_ 19/76/5 layup, chianrtges in specieren width causedchanges in failuxe modes and marked changeii in both strength andlife. However, for the 50/40/10 layup, variationa in width andedge distance did not alter failure modes, and strergth and lifewere virtually unchanged.Effects of remaining v_4riables on joint fatigue life wereminor, with failure modes and durability essentially .he satme asfo r baseline configurations.
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SECTION IIIMETHODOLOGY DEVELOPMENT AN D ANALYTIC STUDIES
1. 0ACKGROUND - As described in th e Literature Survey, analysisof t-ei -omposite joints in aircraft structural componentsthroughout the industry proceeds from overal l structural andbolt-load distribution analyses, to assessment of stress distri-butions and strength predict ions at individual fas tener holesthrough utilization of joint failure analysis .
Methodi currently used to deternine detailed stress distribu-tions in the immediate vicinity of th e fastener hole include boththeoretical and empirical approaches. Theoretical approachesinclude analytic, finite element, and strength of materialsapproximation methods. Analytic methods, preferred because oftheir potential generality, economy, and exactness, are princi-pally formulated from two-dimensional anisotropic elasticitytheory. Empirical approaches lack generality at high cost.Joint failure analyses, in current use, include: (1) empiri-cal approaches, (2) elastic and inelastic failure analyses,(3) phenomenological failure analysis, and (4) fracture mechanicsmodels. Physical variables considered for accurate solutionswere generally agreed upon throughout th e industry. However, th edegree to which variables were accounted fo r was different inparticular methods. No single methodology accounted for all ofth e important variables (e.g. orthotropy, finite geometry, non-linear or inelastic material behavior).In each joint failure analysis approach, after detailedstress distr ibutions are determined, strength is assessed byusing some material failure criteria; however, no single material
failure criterion is uniformly endorsed. Studies of utilizationof various material failure criteria for joint failure analysisare very limited.Detailed stress analysis performed at individual fastenerholes and associated application of failure criteria representsth e primary area of analytic development in this program. Themethodology developed requires only unidirectional materialproperties and minimal laminate test data to calculate laminatestrength with an arbitrary in-plane loading. Because th e methoduses closed-form solutions, parametric studies are easily andinexpensively performed.
2. METHODOLOGY DEVELOPMENT - The goal of th e methodology develop-ment. was to provide a technique for predicting th e strength of ananisotropic plate with a stress concentration. A closed-formanalytic approach was developed to predict stress distr ibutionsand perform failure analysis of an anisotropic plate with aloaded or unloaded fastener hole. This section describes th e
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analytical development of th e Bolted Joint Stress Field Model(BJSFM). An associated computer program is described in Volume3.The method of analysis is based on (1) anisotropic theory of
elasticity, (2) lamination plate theory and (3) a failurehypothesia. The principle of elastic e,uperposition is used t:obtain laminate stress distributions due to combined loadings ofbearing and bypass (Figure 1) . The leveloped analysis can beused with various material failure criteria and the failurehypothesis to predict laminate load carrying capability.
T - -- --
ProblemrInt tmtUlZ)e Ol addHoeOnly
Flgut. 1. Suputpcehmdtn of Llnear-Elastkc Strses Solutions
0 01
The elastic solution for the stress field in a homogeneous,anisotropic infinite plate with a stress concentration was solvedusing two-dimensional anisotropic theory of elasticity (Reference3). Equilibrium and compatibility requirements a~e satisfied bya stress function, F, which satisfies the generalized biharmonicequation fo r anisotropic materials,a4F __4F __4
2S6+ 21 +66
$22 Ed463 IyX 2 M2
_4_ a4F~l6PaxaY' U
04W&
Fgr 1.L': SI.ipsto ofIewEa i Stres Solutions..
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wher,- Sjk are laminate compliance coefficients. The generalexpresafion for the function F depends upon th e roots of theassociated characteristic equation. Solving th e characteristicequation yiel(s a set of complex conjugate roots (RI, RA, R2 ,R2 ). The stress function ca n be expressed as
F - 2Re {F 1 (Z) + F 2 (Z 2 ))
where Fl(ZI), F2(Z 2 ) are analytic functions of th e complex coordi-nates Z1 = X + RIY and Z2 X + R2 Y respect ively . Introducingth e functionsaF(Z1 aF(Z)
lZ =z1 __2(z2z2#2
general expressions for th e stress components are obtained:Ox= 2R e {R I O (Z ) + R 2 O (Z 2 )1
C, = 2Re {1 j (z 1 ) + 4 (Z 2 )1
Oxy -2Re {R 1 i (ZI) + R2 ; (Z 2 )}
Superscr ipt primes represent derivat ives with respect to th e com-plex arguments. Displacements, ignoring terms for rigid bodyrotation and translation which in this problem do r~ot affect solu-tions, ca n be expressed as:
U = 2Re {P I (zI) + P2 cb 2 (Z 2 ))
V 2Re {Q I I (z 1 ) + Q2 2 (Z 2 )}where
2P 1 =SI R1I + S 12 - S16 R1I
P S R2 + S -S R2 11 2 12 16 2
7
..k.1 Ii l I I Ill l
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S22 + S 1 2 RP . S2 6
Q222 + S12 R2 - S2 62
To obtain exact solutions fo r an infinite platk with acircular hole (loaded or unloadad) and uniform stresses atinfinity, conformal mappinq techniques were used. A mappingfunction was used to map the physical circular boundary ofradius, a, in th e Zk plane (k - 1,2) onto a unit circle in th e &kpl e. The mapping function is given by2 _72 a2 k 1,2
k a(l - iUV) k = 1,2
The sign of the square root is chosen such that th e exterior of ahole is mapped to th e exterior of a unit circle.The above equations contain unknown stress functions *1 (Zl)and 2 (Z 2 ). For infinite plate problems with a stress concentra-t ion, these functions will have th e general form:
cc -M(Z) B Z + A In 1+ AM =I
(Z 2 ) B2 Z + A in2 + E A2 M 2-MM=ILinear Z1 and Z2 terms are required fo r a uniform stress atinfinity. Terms with In &1 and In F2 are present whenever theresultant of the applied stresses on the circular boundary arenonzero. Boundary conditions on th e circular hole are satisfiedby the AIM and A2M series coefficients.
To obtain th e solution fo r a plate with an unloaded holesubjected to a remote uniaxial in-plane stress field, P, a t anarbitrary orientation, a, with the X-axis (Figure 2), th e imposedboundary conditions at infinity and on th e circular boundaryresult in the stress functions:
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2i
iPa 2 (1-iR1 )i(z)= x1 4(R 1 -R 277
[R2 sin 2a + 2 co 2a+ (2-R2a2 J02 2 2
iPa2 (1-iR 2 )2 (z 2) " 4(R 1 - R2 )
R n2a2 coo2 a + i (2R sin 2a + sin 2a)][Z2 + 2 -a2 _R2 2 a22 2 22
2< 2
y-
900400
--- 45k)IpFigure 2. Unlaxiafly Loaded Infinite Plate
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Only th e linear terms and first coefficient of the eummation areused fo r th e unloaded hole solut. ion,Loaded hole analys is was performed by specifying a radialstress boundary condition varyina as a cosine over half th e hole
(Tigure 3). Boundary conditions at infinity required to satisfyequilibrium result in stress free conditions since th e finitefrorce required to balance th e bolt load is applied to an infinite)V.undary. Thus, the l i rear terns are not required.
Y PR _CO$O
Figure 3. Assumned Cosine Bolt-Load Distribution
Since the specified hole loading is not self-equilibratingon th e boundary, single-valued displacement conditions areimposed to determine th e log term coefficients. The followingset of simultaneous equations are solved fo r the A1 and A2complex coefficients.A A 1 + A2 - A2 = Py/2 Yri
R A1 R A + R A2- R A = -P/2 7i11 22 2 2 X
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2 - + 2 - 2- S12 Y siC: Px1 R1 R2 A2 2 2 2 w S 2 2S1 P + $26P
A /R A /R + A /R. _/ SI2 x y1 1 2 2 2 2 ni S 2 2
'The terms Px and P are net load resultants on the internalboundary in th e X anc Y directions respectively.Expressing th e radial stress boundary conditions on th e holein terns of a Fourier series and equating th e series representa-
tion of the solution, th e unknown AlM and A2M coefficients areobtained (Reference 4). The resulting expansion can be written
A 1 2 = aP i (I + iR 2 )/[16 (R 2 - RI)]
A2 2 = -aPi (I + iR 1 )/[16 (R 2 - RI)]for M = 4,6,8. . .
A A =0IM 2M
for M = 1,3,5.. .14M1/ 2 2_AI =-aPi(-l)(M-)/2 (2+i MR )/[-'m,2 (M -4) (R -RI)]lM 2 2 1
(Ni- )/2 2 2A2 M = aPi(-l) (2+i MR)/UIIiM (M -4) (R 2 -R 1 )]
These equations give th e complete elastic stress distribu-ti.on in an infinite, two-dimensional, anisotropic mater ia l with acircular hole. These solutions are valid only for homogeneousmedia, but are assumed valid a lso for mid-plane symmetric lamin-ates. Laminate strains are calculated using material complianceconstitutive relations. Laminate compliance coefficients S k arederived using classical lamination plate theory with uniAirec-tional material elastic constants, ply angular orientations, andply thicknesses. Assuming that laminate strain remains constantthrou]gh th e thickness, sLrains for individual plies along laminaprincipal material axes are calculated using coordinate transfor-mations. Stress distributions resulting from an arbitrary set ofin-plane loads (bearing z bypass) are obtained using th e prin-ciple of supcrposition (F 1re).
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To account fo r inelastic or nonlin,-ir material behavior atthe hole boundary, th e "characterist ic dimension" hypothesis ofWhitney and Nuismer was adapted (Reference 5). Their hypothesisstates that failure of. a composite material with a stress colncen-tration can be correlated with analytical predictio-s of pointstresses at a characteristic dimension from the edge of a stressconcentration (Figure 4).
THEORETICAL STRESS DISTRIBUTIONFAILURE TEST POINT
CHARACTERISTICDIMENSION
0- OR
Figuie 4. Characteristic Dimension Failure Hypothesis
The application of th e characteristic dimension failurehypothesis was extended to permit strength predictions for aniso-tropic laminates under general in-plane loadings, without requir-ing extensive laminate test data. To do this, laminate failureis predicted by comparing elastic stress distributions withmaterial failure criteria on a ply-b,,- ply basis at a characteris-tic dimension away from th e hole boundary.
Various material failure criteria can be used with the char-acteristic dimension failure hypot'Aesis. Interactive (Tsai-Hill,Hoffnman, Tsai-Wu) and noninteractive (maximum stress, maximumstrain) criteria were evaluated. Failure envelopes for each cri-teria fo r th e same set of graphite-epoxy (AS/3501-6) materialallowables ignoring matrix failure are illustrated in Figure 5.
Finite width effects have a significant influence on thecircumferential stress distribution around a loaded fastenerhole. A superposition of stress distributions from loaded andunloaded hole infinite plate solutions can be used to evaluatethe effects of finite width (Reference 6). In the loaded hole12
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analysis, the bolt load, P, is reacted (at infinity) by tensiland compressive loads oi P/2 (see Figure 6). By superimposingth e solution for an unloaded hole under a remote tensile loadingof P/2 (a stress of P/2Wt) the desired loading on th e bolt andoverall equilibrium is obtained. The resulting stress distri-but ion gives a good approximation of th e state of stress in aplate of finite width but differs from an exact solution in thatth e superimposed normal and shear stresses at the "edge" of th eplate are nonzero.
02- ksi
--0
- +20 -' ,-- +10
-300 -200 -100 +100 200S_,o
Tsai-Hil-
Tsai-Wu -20 00~ HoffmanI
S-Maximum re -40" I -- Maximum Strain
Figure 5. Failure Criteria Comparison
13
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Loaded Hole Unloaded HoleInfinite Plate Infinite Plate
P/2 ti0ltw - wPI I
P/2WP/W
SI I I wP/2Wt P/2Wt
P/2W P/2W P/wtFigure 6. Superposition of Solutions to Account for FInife Width
This methodology has been combined and incorporated into acomputer program entitled BJSFM. Capabilities are programmed tohandle material anisotropy, general in-plane loadin~gs (tension,compression, biaxiality, shear, bearing), multi-material (hybrid)laminates, and arbitrary hole sizes. Only mechanical propertiesfo r th e basic lamina (unidirectional ply) are required to obtainstrength predictions. A detailed description of the computerprogram is given in Volume 3.
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3. PREDICTT'N3 UISING BJSFM APPROACH - Predictions made using th eBJSkN -ce presented in this section to indicate th e scope andflexibility of th e developed methodology as well as indicate dataard information pertinent to analysis of bolted joints in compos-ile structures. The predictions shown are not intended to coverall iaminates or loading combinations.
a. Effects of Anisotropy on Stress Distributions - Unlikeisotropic materials, stress concentrations in composite materialsare affected by layup and load orientation. Predicted circumfer-ential Ltress plotted at an unloaded fastener hole boundary withuniaxial bypass loads indicates important differences betweenanisotropic composites and isotropic metals (Figure 7). Layupvariations which change laminate stiffness properties affect holeboundary stress distributions and stress concentration factors;however, metal distr ibutions are independent of their s tiffnessproperties. Also, if loading shifts away from principal materialaxes, shear-extensional coupling creates biaxial states of stressin the laminate. Peak stresses no longer occur 90" to load direc-tions and distr ibutions shift. Loaded holes also show th e samedependence on layup and load orientation (Figure 8). Thesecomplete stress distributions must be considered to determinefailure load, mode and location of failure initiation ofcomposite materials.
- ------ Isotropic
Composite YY
1_41
50/40/10 Lavup 70/20/10 Layup 70/20/10 Layup0o C =0o 0 = 450
Figure 7. Circumferential Stress Solutions at Unioadod Holes
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Isotropic materialComposite YY y
_x x/ /
/
50/40/10 Layup 70/20/10 Layup 70/20/10 Layup0 00 0 o = 450
GP1S.011544
Figure 8. Ciumfetrntlal Stress Solutions at Loaded Holes
b. Hole Size Effects - Hole size effects are also accountedfo r analytically. Stress gradients at the edge of the hole varywith hole size and laminate orientation. Smaller diameter holesproduce steeper stress gradients which decay rapidly as the dis-tance from the hole is increased (Figure 9). Applying a failure-riterion at a constant distance from the hole boundary, holesik.-. is accounted for due to th e varying stress gradient withhole z ze.
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3.0
1:: xO2.6 00-00-0 0 0000 ply2.2 -. .dia
C.FiieWdhEfcs-T vlaiefcso iiego
1.85 di
0.125 dia 1.0-0 0,02 0.04 0.06 0.08 0.10 0.12
Disctance from Hole Boundary - in. OpIli.0 74
Figure 9. Effect of Hole Size on Stress Distributions
c. Finite Width Effects - To evaluate effects of finite geo-metry on stress solutions and establish limits on th e accuracy ofinfinite plate solutions, a comparison of BJSFM solutions withfinite element solutions was performed. Two example cases arei l lustrated: (1) a pure bearing specimen with a width-to-dia-meter ratio (W/D) of 8 and an edge-to-diameter ratio (e/D) of 9,and (2) a pure bearing specimen with a W/D of 6 and an e/D of 3.The results for th e first case are presented in Figure 10. Cir-cumferential stresses normalized to th e average bolt bearingstress are plotted about th e half-circle from directly in frontof th e neat-fit bolt (0Q) to directly behind th e bolt (1800).The dashed line indicates th e infinite plate BJSFM solution, th esolid line represents the BJSFM solution corrected using th eDeJong finite-width approximation method, and th e triangular sym-bols represent th e finite element solution. .- nults shown inFigures 10 and 11 indicate that for an e/D of 9, the BJSFM solu-tions, corrected fo r finite width, correlate extremely well withfinite element solutions where W/D ratios were greater than 4.
17
' , . . . . . . .
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1.2 - w/d =81id S
- 0.4 --- ]GBRG .~
A Finite ale;,ient model0 BJSFM - finite plate- BJSFM - infinite plato50/40/10 Lay pL
j ~~~~~-0 .4 .........0 20 40 60 80 101) 120 14 0 160 18 0
0 LOCATION ABOUT HOLE - DE GFigure 10. Correlation of Loaded Hole Analysis
1.4_ _.1 1, - BJSFM Prediction
1.2 ) Finite element (,/d = 9.0)n- d - 0.25-x E0/40/10 LayupE 1.0 NNW.
Z P.8-- _ _0.6 -0 4 8 12 16 20
Width/Diameter - w/d OP15-O1I46-Figure 11. Effects of Width on Stress Concentrations
Sensitivity of solutions to small e/D ratios is illustratedin Figure 12 by results obtained for the second case. At an e/Dvalue of 3, BJSFM infinite-plate solutions, approximately cor-rected for finite widths (long dash line), are significant~lyimproved over th e original BJSFM-infinite plate solution (shortdash line), but still differ from th e correct finite geometrysolution (triangle symbol line).
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'e/d = 3
1.2 - - w_
0.8
0.4. BJF fnt wit - -- ,- - - Finite element model0BJSFM - finite width : "BJSFM - infinite plate
50/40/10 Layup-0.4 - ,0 20 40 60 80 100 120 140 160 180
t) Location About Hole - Deg ON 3411 &WFigure 12. Correlation of Loaded Hole Analysis for Small Edge Distance Casesd. Characteristic Dimension Sensitivity Study - Various
laminate failure criteria can be used with th e BJSFM procedure.To fully evaluate th e characteristic dimension hypothesis, vari-ous correlative and parametric studies were performed. A sensi-tivity study was performed to determine th e effect of variouscharacteristic dimensions on laminate strength predictions.Loading configuration studies included unloaded and loaded fas-tener holes,
Results fo r a 30/60/10 layup of graphite/epoxy with unloadedhole sizes of .125, .25 and .50 inch are presented in Figure 13.The Tsai-Hill material failure criterion was used, with strengthspredicted when first ply fiber or shear failure occurred. Eachcurve initiates from a common predicted failure stress levelsince th e theoretical stress concentration at th e hole boundary(R c = 0.0) is independent of hole size. Due to stress gradientsaway from the hole boundary varying with hole size, predictedfailure stress levels change with the characterist ic dimension.The predicted failure stress curves each asymptotically approachthe predicted unnotched laminate strength with an increasing char-acterist ic dimension. These curves indicate that a 10% change inthe characteristic dimension yields a maximum 4% chang, n pre-dicted laminate failure stress for th e .125 inch diam, ,,r holeand a maximum 2% change fo r the .50 inch diameter hole. Also, ascharacteristic dimension changes occur, failure init iat ion anglepredictions change, as indicated in Figure 13 .
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70 100I,Unotchd LaminateS ro
"c0757590 0 Failunt__ 460 -. - Initiation Angle 90i ~ 0=.125--
,50 8
0 0.008 0.016 0.024 0.032 0 0.40 C.80 1.20Characteristic Dimension, Rc- in. 0pw14l547
Flgur; I&3 Effect of Charectedstic Dimension on Predicted Strength30/60110 Layup
e. Biaxial Loading Effects - To demonstrate the utility ofth e Bolted Joint Stress Field Model, a parametric analysis wasperformed to predict interactive effects of biaxiil loads on lam-inates with unloaded fastener holes. Laminate strength, failurelocation and critical plies were pi.edicted for a representativecomposite wing skin laminate.
Figure 14 presents predicted failure stress and indicateschanges in failure modes as biaxial load rat ics increase. Loadratios of increasing (NY/Nx) for this tension-tension case pro-vide relief at the fastener hole and laminate strength Increases.Increasing shear load ratios (Nxy/Nx), however, intensify stressconcentration effects at the fastener hole; laminate strengthdecreases and failure modes change from 0* Flies to 45 plies.Figure 15 presents th e predinted location at th e fastener holeboundary of first ply failure. Significant is th e indicationthat at certain biaxial load ratios, a wide arc of the hole bound-ary may become critical. Figure 16 presents strain concentrationfactors fo r critical plies within th e lamirate with respect tothe strain developed under Nx alone.
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55 1-7 00Nxy Ply Fails-oo,,Op~ x
Fails
05015
S = 0.250
ICONN,
45
-496'5011 LayuIJ
o0 0.10 0.20 0.30 0.40 0.50Ny/Nx
FIgure 14 . Effect of Biaxial L*)adIng on Predicted Strength
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130 I46/50/4 Lavup
120 0
- 110LA. > ~~~~0.25 -. 0 / .1.10/ - o1
4-.ol '50 PlyFails
0C 1001-9
00 PlyV. ,Fails
0 0.1 0.2 0.3 0.4 0.5N y
Figure 15. Effect of Blaxial Loading on Failure Location
22
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39 ( 03S-0 N
3.7 1!
UL JxV
Ic ________46/50/4 Layup
v 35
0
00LO
+45ly Faiils
'21 z-
0 0.1 0.2 0.3 0.4 0.5Ny VNx
Figure 16. Effect of Blaxial Loading on Ply Stiain ConcentrationsExperimental and analytic studies have indicated that fail-
ure locations at fastener hole boundaries shift depending onlayup and biaxial stress states. The BJSFM methodology accountsfor this phenomenon and, further, reveals thc critical nly at thefailure location. Data shown in Figures 14, 15 and 16 show thatfailure analysis must account fo r a ll locations around the fasten-er hole, since th e critical area is layup and loading dependent.This is best indicated by Figure 15 which shows not only continu-o'is shifts with biaxial loading, but also indicates wide arcs ofthe hole perimeter may become equally sensitive to failure. Thisin(licates the potential errors which can arise by using methodswhich predict laminate failure by referencing unnotched laminatestrength data at "preselected" or "representative" loc-ations onthe hole boundary.
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f. Pure Bearing Strength Study - Strength envelopes are pre-dicted based on ply-by-ply analysis to determine critical pliesand failure initiation location. The results of a ply-by--plyevaluation of strength based on first ply fiber rupture fo r th e50/40/10 layup under pure bearing loads is presented in Figure17. The ply strength envelopes predict initial failure in 90*pl.ies f. all load directions with relatively constant laminatestrength up to a load angle of 25 and a gradual decrease from25' to 90*. This predicts that while unloaded holes show a pro-nounced sensitivity to off-axis bypass loading, bearing strengthsfo r this laminate are relatively unaffected by off-axis loadings.Analytically, this behavior is a direct result of the differencesin local stress distributions resulting from th e two loadingi ~ cond it ions.
! ~ ~32o,0
28050/40/10 LAYUP
STRESS 20 ,---__5PLE
KSI " .. /-O L p--1240
80 ---
0 20 40 60 80 120STRS EG OFF PRINCIPAL MATERIAL AXIS
OPI).1PLY)
Figure 17. Effect of Bearing Load Direction on Predicted Strength
24DOFF
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g. Effects of Environment - Effects of environment on lami-nate strength can also be accounted fo r with th e BJSFM analysis.Unidirectional material properties (stiffness and strength), asaffected by an environment, are used to redefine laminate beha-vior. Bearing versus bypass failure envelopes were predicted fora 50/40/10 layup at room temperature dry and at elevated tempera-ture with moisture. For this layup, results indicate that littlechange in predicted strength occurs fo r high bypass loads associ-ated with fiber dominated modes of failure which are usually notadversely affected by temperature and moisture (Figure 18). How-ever, elevated temperature and moisture do affect matrix proper-ties resulting primarily in lower shear modulus, shear strengthand lower compression strength. The lower shear st iffness causesa redistribution of high bearing stresses to the stiffer 45*plies in compression. This, coupled with decreased compressionstrength, results in fiber compression failures at lower bearingstresses than corresponding RTD properties (Figure 18). Althoughthe failures fo r this layup initiate in the same vicinity aroundthe hole, different phenomena cause the failures due to changesin material mechanical properties with temperature and moisture.
80RT D and ETW00 Plies -OCR = 900 PB ---- BypassFiber Critical/ ___
60 - -c~40
> /--ETW F-RTD+450 Plies 00/900 PliesOC =c0OCR 450Fiber Critical Shear Critical
20- _ __ _
0 20 40 60 80 100Bearing Stress - ksi 0p1S-OI 1B-M
Figure 18. Effect of Moisture and Temperature on Laminate Strength
25
' ~ ~~~~_.Mi.0 'l "-
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SECTION IVTEST DATA EVALUATION
1. METHODOLOGY VERIFICATION: EVALUATION OF JOINT DESIGN VARI-ABLES - TASK 2 - The primary objective of th e Task 2 expe~rimentalprogram was to verity developed methodology through strengthtests over a range of bolted composite joint design variables.Test data were also used to provide direction fo r development offurther improvements in th e methodology. The Task 2 Test Matrixis presented in Figure 19.The selection of test variables was guided by informationgathered under the Task 1 - Literature Survey. Data obtainedfrom earlier MCAIR in-house test programs were identified underthis survey (Reference 1). MCAIR test programs outlined inFigures 20 and 21 were used to supplement the Task 2 experimentaldata. Ten different layups oi th e 0, +45' , and 90 family ofply orientations were tested under tension and compression load-ings. Effects of hole size, thickness, countersinking, edge andwidth distances, and joint eccentricity (single versus double
shear) on th e strength of laminates with a single loaded orunloaded fastener hole were evaluated. Specimen configurationsfor unloaded fastener hole and puce bearing evaluations aredetailed in Figures 22 and 23.
These MCAIR data provided an initial basis for the effectsof layup variations on laminate strength fo r th e two bounds ofbolted composite joint load-transfer, i.e., zero bolt bearing(100 percent4 bypass+ strs an tota load ;rn fir t-hrt'iuh asingle fastener (pure bearing). The availabili.ty c these datapermitted early verification of developed methodology, reducedthe scope of layup variations to be tested, and permitted Task 2experimental efforts to include a more comprehensive range ofother bolted composite joint design variables.
a. Task 2 Test Plan - The effects of 12 design variables onjoint strength were evaluated. Tests were performed at threeenvironmental conditions: room temperature dry (RTD), roomtemperature wet (RTW), and elevated temperature wet (ETW). Envi-ronmental conditions were selected to be realistic with respectto deployment profiles for multi-mission, high-performance super-sonic aircraft. Tests were conducted at an elevated temperatureof 250F and, when applicale, specimens were conditioned to anequilibrium moisture content of approximately 0.8 percent byweight. All specimens tested at 250F were held at temperature10 minutes before testing. Specimens were loaded in tension orcompression to failure. In general, a replication of four testswere performed at each test condition.Hercules AS/3501-6 Type II (ten mil thick) graphite-epoxywas used as the principal material for fabrication of testspecimens. Limited testing was performed using NARMCO T300/5208graphite-epoxy.
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[ _ _ _ _C lU
+ >'>Ifs , 0m ot~~&F>> NOocB- i ; c
C2L L2hI!0_j - __ "0I
Kl Lz caOO@~ -cow0L-C~
CL
LU LU i lU0
CL C. CL CL CL I.,- >-CIL
B-B f - -B
ca c
LU B-jUZVZf LUu0j LU LUIE ca cc -M Uot C3 0 ~-C ,
LL cn n Ln L
27 0
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o~11
LU
- ci 4.
z ~ n-ma,>RRC_:- C44"44F , f 5
4.- 6
,, I [
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Laminate % Layers of No. _ Hole Dia. Loadinr N ofGroup 00 +45 900 Plies 0.01 0.125 0.250 0.500 Ten. Com. TestsA 70 20 10 40 V V V 24B so 40 10 40 V V V V V 24
120 V V 18C 30 60 10 40 / V V V V V 24
10 8S50 10 40 40 V V V 12E 40, 20 40 40 V VI V/V 12F 40 50 10 40 V V V V 12G 30 30 40 40 IV' IV/V 12H 20 40 40 40 V V V V 121 20 70 10 40 V V 12J 10 80 10 40 V VV 12
84A 70 20 10 40 X X X 6B 50 40 10 40 x X X 6
120 X X X 630 60 10 40 X X X 630
Total No. Tests 222V Noncountersunk SpecimensX Countersunk Specimens 0Pl13111"iI
Figure 20. MCAIR Evaluation of Unloaded Hole Specimens Test Matrix
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Phase Layup %Layers No. Hole Die eld wid Single Double TotalNo. No. 0O +450 0o Plies 0 0.250 0.375 0.500 0.250- 1.5 2.0 2.5 3.0 4.0 4 6 8 Sheer Shoat Specinons1 170 20 10 20 0 V V/ V V V/ - 87TI 2 50 10 40 20 0 V .-- -V- V- V - 9I 3 50 40 10 20 V V V*N/ V V A 861[ 4 40 20 40 20 - V.-V-V- - V - 6U 5 40 40 20 20 - V . . . .- V-- - V - 6R 6 30 20 50 20 - V - - - - - V - 6I 7 30 60 10 20 0 V V V VVVVVVV - 87R 8 20 40 40 20 -- V - . . -V-V -- V-V - 61 9 20 60 2020- - V/ -V V - -V -- 6HI 1010 80 10 2P - V - . . .- V-- -V V - 611 11 55 40 5 20 0 V - - - V V V - -V / - 15
---- 15 92 103 72 36----Total 323Each test replication of 3 0P13-0115.121Countersinke Baseline
specimens in double shear
Figure 21. MCAIR Evaluation of Loaded Hole Specimens Test Matrix
11.000 ind ' J.500T
0-/ 3.000 in.900 _.1i
_ _d = 0.250F 00 -/ 1.500 in.1900 _j_
d =0.12510- 00 O_____--__ 0.75 in.
40 Plies 2000 T0.375 in.&---0Strain Gage
Figure 22. Unloaded Hole Specimen Geometry30
ip- t9 -' - i4'
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eI90 -- \--
450
Strain Gage
PTest Specimen Bload
Figure 23 . Loaded Hole Specimen Configuration
Four test specimen c:onfigurations were used: (1) a single-bolt pure bearing specimen, (2) a baseline two-bolt-in-tandembearing-bypass specimen, (3) a two-bolt load interaction specimenand (4) a four-bolt fastener pattern specimen. Both single andtwo-bolt-in-tandem specimens were loaded in either a single-shearor double-shear configuration (Figure 24). All specimens werestrain gaged. Recorded data included: failure load and strain,continuous load versus strain plots to failure, selected loadversus deflection plots to failure, thickness, width, hole diam-eter measurements, weigh- gain of humidity exposure specimens,and selective photographic documentation of failed specimens.31
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(a) Double Shear Face Configu ration0.025 Ma x Ga p
Strain GageSte
P I Load --- PkClevis JGraphite/EpoxySpecimen 0.025 Max Ga p
(b) Single Shear Fac. Configuration-Graphite/Epo'y Specimen
fSteel Load Block
OF13-OlI1-4t
Figure 24 . Specimen Loading Configura\)Ions
Prior to specification of load-clL. ge. metry (thickness,width, and material), a parametric study was pe4rformed to evalu-ate th e effect of clevis-to-laminate stiffness ratios on loaddistr ibutions for the baseline two-bolt-in-tanlem joint. Tbeobjective was to quantify the errors in load dio-tribution whichcould arise due to material st iffness variabil i ty. This wou.Id,in turn, provide a theoretical data base to aid in selection oiload-clevis geometries and material. The re,3ults of this studyare presented in Figure 25. Based on theke results, a loadclevis-to-laminate st iffness ratio of 10 or more was selected.This choice results in unequal bolt loads, but c'uarantees that nodistribution error greater than 0.5 percent sh-uld o.cur due tolaminate stiffness variations expected from specimen to specimen.
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P1 P2
.TotaI _,__ Tota,S~ClevisLaminate
Load Clevis Stiffness Bnlline - 0 - 10Specimen Stiffness Fraction of Total Load
0.6 Layup P1 P230/60/10 0.442 0.55850/40/10 0.457 0.54370/20/10 0.465 0.535
0.5
S"--10 (Baseline)I- 0.40..0
0.3
30/60/10 50/40/10 70/20/100.2 f0 4 8 12 16 20
Ex - Laminate Stiffness Specimen - (106 psi) I14k116.MFigure 25. Edfect of Laminate Stiffness on Specimen Boll-Load Distributions
Individual test specimens were fabricated from 19 panels.Stacking sequences are detailed in Figure 26. Panels were cut,collated, and cured per standard MCAIR process specification.Al!. specimens were tested to static failure according to Figure19 under tensile or compressive loadings. Complete details ofall procedures, test results and support requirements arecontained in Volume 2 of this report.33
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a0 + Io00c0+
,, + I ISO0 Q ICCzS20
zCc + 1 00+ 10coo
2C.2+0 la~oI +0 1080+0 IC +01080+0 lgo V
2- zz ooooo~ i ~ ~ ~ a0
4L_ _ _ _ _ 06___ -j
~0_ _ _ _ __ _ _ _ _ _ - touJr
~ -NI~w~a~-0>9L ~ .E
34j
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b. correlation of Ik7SFM Predictions With Ex erimentalResults lu u G..Ctlit UumnhIpres experimentalresults ol-tained under Task 2 and predictions using the BJSFM.Where IRJSFMl/test correlation was not possible, data trends are
?or aninitMiXlveriicaionofe JSF i-rccedur fo th cae o anunloadedfastener hole. Using the IV.SFM procedure, strength predictionswere made for all test layups and teati conditions. Lamrinate elas-tic properties were calculated using unidirectional ply laminaelastic constants. Laminate failure was assumed to occur whenfirst ply fiber or shear failure w'as predicted, based on the Tsai-Hill or maximum strain material failure criterion applied at acharacteristic dimension away from the hole. Only laminajstrength and stiffness data were required for this analysis;these data are shown in Figure 27. RTD data are average valuesobtained fromi MCA1R tests performed on unnotched unidirectionallaminate sandwich beams, arid 0*/9Q@ rail-shear specimens; RTWand ETW data are estimatod.
Poete ;ROOM RT Wet 250OF WetTemperature IMstimsaed)*11 (Estimafted)**Elastic Constantsfj
is Eilt(106 psi) 18.85 18.85 18.54"* Elc (106 psi) 18.20 18.20 17.80
"E2 (10'P psi) 1.90 1.72 1.27* 12 (06 psi) 0.85 0.77 0.60
120.30 0.30 0.30" f 1t (pin./in.) 12,206 12,206 12.735"telu (pin./in.) 176012,6948, 6
"a~t (pin/in.) 5,380* 712 (pin./inj* 20,352 22,470 18,461is F 1tu(ksi) 230 230 236e Ficu (ksi) 321 231 151is F2tu (ksi) 9.50 -0 F2cu (ksi) 38.90 35.20 26.00to F1 2 (ksi) 17.30 17.30 11.00
'Based on linear-elastic behavior GPSSIIS1-ttS-Approximately 0.86% moisture
Figure 27 . A5/3501-6 Lamina Mechanical Properties
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Correlation of predicted tensile strerg9th with test datifrom ten layups is shown in Figure 28. Currelation of predicte.ieffects of hole size on laminate tensile strength with *-eat datais shown in Figure 29. For the AS/3501-6 material system, a char-acterietic dimension (Rc) of 0.02 inch was used fo r all tensilefailure analysis. This value was obtAineti b, comuparing .250 inchhole-sire strength data from the 50/40/10 laminate with predic-tions, as illustrated in Figure 30.
-- Theoretical * Test data A Predicted120 I~~~Rc 0.02 in.100 -- %0 0 Plies
90/
080
0D 20 60 60 8 10
b
S6040'U.I
3x 400I10
20
0!
% 450 Plies OpIzt..
Figure 28 . Effect of Layup Variation on Unloaded Hole Tensile Strength
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180II ' -- Ifstu4Uiross d160 1
140g "__I I-~120 -- Predicted
S 100J .x 80 7-2 0/ 0 10 )
4 XLLI 60 --SI-- Wi/40/1 0 fA
40 J! ~30160/10 10)
20 "14,000
12.000
.~10,000 -__0.250 CS K
I AverageM ' Hole Size(n 8.000
6,000 --- 70/60/1
4,000
2,000-0 0.1 0.2 0.3 0.4 0.5 0.6Hole Diameter - in . Qp Ml.5Iw
Figure 29. Effect of Hole Size on UnIo6ded Hole Tensile Strength
37
i! . ...... ... ~ ~~j ,. .....- ,.
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72 , IRc -d 0.250
FtU-
p66./- Predicte0~ I
S64 - -U-
62 --0.02 Value of R. Usedin All Tension Analysis
58|0.015 0.017 0 .019 0.021 0 .0 2 3 0.025 0.027
Characteristic Dimension, Rc - in.
Figure 30. Effect of Variation of Rc on Predicted Tensile Strength
Location of failure initiation depends on layup and holesize. Predicted failure location occurred on the hole boundaryand at 90* to th e applied load direction for all but two cases.The exceptions are th e 70/20/10 layup with 0.250 or 0.500 in.diameter holes. Predicted origins of failure were at 65* fo r the0.250 in. diameter hole and 70 for th e 0.500 in. diameter hole.in these two cases, failure was predicted in th e +45* plies. Allpredicted locations of failure correlated visually with test data(e.g., Figure 31).
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(30/60/10, 0.500 in. Dia Hole)
700/j
(70/20/10, 0.500 in. Dia Hole)Piedicled f-jilure Ore,)jIjl~jl GPIe-Ot.117
Figure 31. Predicted Failure Orientations Were Verified
!trength predictions fo r a biaxially loaded laminat., with anopen fastener hole were also verified by test . This loading con-dition was achieved with uniaxial. loading at various angles rela-tive to the laminate principal material axis. Due to orthotropicmaterial hehavior, off-axis uniaxial loading causes shear-exten-sional coupling which produces a biaxial stress state in the testspeciiilen gage area. Using th e 0.02 in. characteristic dimension,correlation of tes;t data wit]h predictions is shown in Figure 32.
A characteristic dimension of Rc = .025 inch was used forall compressive failure analysis. As in the tension case, Rc wasinitially determined using only .250 inch hole size strength dataobtained from tests performed on the 30/60/10 laminate (Figure33); however the final value of .025 inch was based on a best fitof all hole-size strength data fo r the 30/60/10 layup. Predic--tions and test data on th e effects of hole size on compressionstrength fo r this layup as well as the 50/40/1) and 70/20/10layup are presented in Figure 34. A carpet plot of compressivestrength predictions fo r all ten layups of Figure 20 is corre-lated with test data in Figure 35. Test data and predictions arefor the open hole, .250 inch diameter, specimen configuration.
7\ preliminary evaluation of the effects of temperature oncoMpressive strength of laminates with .250 inch diameterunfilled fastener holes was also performed. Three differentlaninates were tested at room temperature and 250F. Using theBJSFM procedure, strength predictions fo r both room temperatureand 250F wJere based on a constant characteristic dimension of0.025 inch and the Tsai-Ilill Failure Criterion. Correlation of
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60- . __Rc = 0.0250 -48/48/4 ,AYUP-w/d = 6
Fu 40FAILURE __REDICTEDSTRESS 30
KSI 00 PLY REF2 0 F __ __ _
10 d 0.250 F0
0 10 20 30 40 50 50 70 80 90 100S- LU AD ORIENTATION - DEGFigure 32. Effect of Off-Axis Loading on Failure Stress
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-66_0.025 Value Used for IAll Compression Analysis
-642, I
d = 0.250 Data I- Predictedx -60LL
2 -58 _= e" 30/60/10 LayupKY
u. .0Rc 0. 0d7 0 0.250
Chrateisi Dies, R-inIN54 1S~00 Ply Ref-521 1i
0.019 0.021 0.023 0.025 0.027 0.029 0.031Characteristic Dimension, Rc - in.
Figure 33. Effect of Variation of Rc on Predicted Compressive Strength
41
___ __
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-180 I I
Sso ICu u
-140 x 6d
LL 120-100
S10/20/1
-80 __ 04/bA
Hole Diameter in.20,000 T
18,000n Prediction
16,000 .c!14,000 -
70/20/10 WeightedFgr 12,000 or Hole Size tx for 0.250 Csk
r..... .,- 50 ...10g - -'-- 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Hole Diameter - in. PIM
Figure 34 . Effect of Hole Size on Unloaded Hole CompreSSIve Strength
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Theoretical 0 Te%t Data A Predicted120 %o o 'ie ,
90 r = 0.025
'ioo ~80 _______007 1 8 7060"*80
050
0 60
U 40U_
01
0 20 40 60 80 100%+450 Plies
GPI"11t -101
Figure 35. Effect of Layup Variation on Unloaded Hole Compressive Strength
test data and predictions is i l lustrated in Figure 36. Resultsindicate that effects of enemperature on unloaded hole laminatestrength fo r a wide range of layups can be satisfactorilypredicted by using th e BJSFM procedure with appropriatetemperature-corrected values of lamina elastic constants andlawnina strengths.
For all test conditions, compressive failure modes of openhole specimens were similar. Laminate material in th e net-section area "broomed-out" symmetrically about th e thicknesscenterline, typical of compressive failures in unnotchedlaminates.Failure initiation points were predicted to occur on th ehole boundary 85-90' frow the specimen length axis in all but th e
highly orthotropic layups. For these latter layups (70/20/10 and50/10/40), failure initiation points were predicted to occur at1250. However, due to extensive laminate compressive damage, vis-ua l verification of failure ini t iat ion points was not possible.43
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SwSxCU d =0.25xCU
180
i 80 50/4b/10
Rc = 0.025 in."-60 Test data.42/5 0 Predict3d
LL w/d =640f-m=20/60/20
T0 50 100 150 200 250 300
Temperature - OF
Figure 36. Effect of Temperature on Unloaded Hole Compression Strength
(2) Strength of Laminates With Loaded Fastener Holes -In th e Task 2 test matrix (Figure 19), evaluations of boltedcomposite joint strength can be sub-grouped under four studies:(1) effects of orthotropic mechanical properties, (2) effects ofjoint geometry, (3) effects of loading and fastener patterninteractions, and (4) effects of through-the-thickness variables.A baseline 50/40/10 layup and joint geometry (single or twofastener-in-tandem confiquration) detailed in Fiqure 37 is usedthroughout th e Task 2 test program as a reference point. Detailsof specific specimen geometries are shown in Fiv,res 38, 39, 40and 41.
Evaluation of effects of orthotropic mechanical propertiesincluded layup and load orientation variables. Effects of lami-nate variations (70/20/10 and 30/60/10 layups) on joint strengthwere compared to th e 50/40/10 baseline strength data for th e loadcondition of bearing and bypass loads aligned with a principalmaterial axis. Effects of bearing load, off th e principalmaterial axis, on ultimate bearing strength were evaluated forth e 50/40/10 layup under pure bearing loadings. As-manufacturedand moisture conditioned specimens were tesCed in tension andcompression at room temperature and 250 0 F.44
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15.00 -1.000---- - - 0.750
;" , I,,,, ,, p o 1.500I I /SSG0 2 +0.0022 _/0 Strain Gage -0.0000 20 Plies
Notes: TLaminate: 50/40/10 LayupStacking Sequence: [+450, 00, -450, 00, 900, 00, +450, 00, -450, 00]sThickness (t): 0.208 in. Nominal (20 Plies)Fastener Type: ST3M 453-4 (0.2495 + 0.0000/-0.0005 in. dia)Torque Value: 50 in.-Ib (1/4 in. Fastener)Load Configuration: Double-Shear OPI"-116-47
Figure 37. Two-Bolt In.Tandem Specimen Baseline Configuration
45
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15.00-3.50-wi
S--I ---Cut Off and d0GStrain Re-Drill for t
Next Test-II 1 .
Design w e dVaribleVariations w __________ariable (in.) (in.) (in.) Plies in.Load Oreintation All 1.50 0.750 0.250 20 0.208Countersink (h/t) 1 2.25 1.125 0.375 20 0., j8(h/t) 2 2.25 1.125 0.375 40 0.416
(h/t) 3 2.25 1.125 0.375 60 0.624Single Shear Baseline 1.50 0.750 0250 20 0.208(Protruding Hd) e/d 1.50 0.500 U.250 20 0.208
w/d 1.00 0.750I 0.250 20 0.208d 2.25 1.125 0.375 20 0.208
Stacking Sequence A:: 1.50 0.750 0.250 20 0.208Fastener Torque All 1.50 0.750 0.250 2C 0.208Thickness t1 2.25 1.125 0.375 40 0.416t 2 2.25 1.125 0.375 60 0,624
OP1lC-11546
Figure 38. Variations of Single Fastener Specimen Configurations
4.6
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- 15.00-,----3.50 S-" e
-400 Strain Cut Off and / -- dGage Re-Drill for tNext Test r i
Disgn Vaitos w & d S tVariable Variations (in.) (in.) (in.) (_r.) (in.)_
Layup All 1.500 0.7500 0.2500 1.500 0.208Edge Distance (e/d) 1 1.500 0.3750 0.2500 1.000 0.208
(e/d) 2 1.500 0.5000 0.2500 1,000 0.208(e/d) 3 1.500 1.0000 0.2500 1.000 0.208
S1 1.500 0.7500 0.2500 0.500 0.208S2 1.500 0.7500 0.2500 0.750 0.208
Widxh (w/d) 1 1.000 0.7500 0.2500 1.000 0.208(w/d) 2 1.250 0.7500 0.2500 1.000 0.208(w/d) 3 2.000 0.7500 0.2500 1.000 0.208
Hole Size D1 1.137 0.5685 0.1895 0.758 0.208D2 2.250 1.1250 0.3750 1,500 0.208D3 3.000 1.5000 0.5000 2.000 0.208
Single Shear All 1.500 0.7b00 0.2500 1.000 0.208Fastener To'que All 1.500 0.7500 0.25U0 1 1.000 1 0.208
OP13-0115.45
Figure 39 . Variations of Two-Bolt-IWn.Tandem Specimen Configurations
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15.00
0.750/T
V ~0.7501s00 Strain Gages
0.208
[I_ ;; -VlDesign d W S
Variaale (in.) Variation (i.) (in.) (in.)Load Intaraction 0.250 01 (100) 2.00 2.836 2.880
02 (22.50) 2.50 2.414 2,6130 (450) 3.50 2.000 2.828
0.375 0 (00) 2.25 3.800 3.800
P0 P
Loading Configuration 0P151W5Figure 40. Variations of Load-Interaction Specimen Configurations
48,=8 p.
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n o 1.000 2.500
3.50 - 1.000 0.750-- 7.50
____ ___ ____ _____ ____ __ _ __ _ ____ ___ji 0 208
GPIS.IO11-6
Figure 41. Fastener Potterm Specimen Geometry
In th e second group, effects of joint geometry, joint vari-"ables evaluated include edge distance (e), width (w), and hole-size (d) fo r th e 50/40/10 laminate. Specimens were tested intension at room temperature dry and selectively at 250*F aftermoisture conditioning.
Studies of loading and fastener pattern interactioninvolved: (1) evaluation of th e effect of bearing stress onlaminate bypass strength and (2) evaluation of stress concentra-tion interactions in multi-fastener patterns. Bearing-bypassinteractions were evaluated fo r two layups (50/40/10 and 30/60/10) with bearing and bypass 2oads aligned with th e 0* principalmaterial axis and, fo r th e 50/40/10 layup, when bearing loadswere oriented at 100, 22.50, and 450 relative to bypass loadswhich were parallel to a principal material axis. Four-boltspecimens were used to evaluate th e interaction of fastenerstress concentrations fo r two layups (50/40/10 and 30/60/10) andwere tested selectively in tension and compression at room temper-ature and 250F after moisture preconditioning.
49
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Five through-the-thickness joint variables - (1) torque-up,(2) single shear (joint load eccentricity), (3V laminate thick-ness, (4) countersinking fasteners, and (5) stacking sequence -were evaluated to determine th e possible three-dimensional inter-actions at individual fastener holes. Single-fastener purebearing and two-fastener bearing plus bypass specimens weretested in tension and compression at room temperature dry condi-tions. Selective tests were performed at room temperature and250F with specimens moisture preconditioned. Strength data werecompared with the baseline joint configuration.
(a) Layup Variation - Effects of layup variation onlaminate bearing strength were evaluated using the two-fastenerin-tandem specimen. Three layups (70/20/10, 50/40/10 and30/60/10) were tested in tension and compression. Tests wereperformed at RTD, RTW, and ETW. Results of all tension tests arepresented in Figure 42. Results of all compression tests are pre-sented in Figure 43. In both sets of data increases in grossfailure strain were linear with increases in percent of +450plies contained in the layup. Parallel trends were oserved formoisture conditioned specimens tested at 250F (ETW), withstrains genezzlly lowered by 800-1000 win/in fo r all layups bothin tension and compression. Specimens of th e 50/40/10 layup,which were moisture preconditioned and tested at room temperature(RTW) exhibited little strength changes. Associated ultimatebearing strengths reflected similar trends between RTD and ETWdata with approximately a 30-40 KSI loss in bearing strength forall layups tested at ETW.
Associated joint load-deflection data indicated thepresence of nonlinear mechanical behavior. Predictions, usingth e BJSFM procedure, and joint loads at which initial nonlinear-ity occurred are also presented in Figure 42 and 43. Predictionswere based on th e previously determined Rc values of 0.02 inchand 0.025 inch for tension and compression respectively.(b) Load Orientation - Effect of material ortho-tropy on pure bearing strength was evaluated using a single-fastener specimen fabricated at various angles off the 0principal material axis. Specimens were tested selectively atroom temperature dry and at 250'F after moisture conditioning.
Strengths at room temperature dry conditions are presentedin Figure 44. Included are bearing stress values at which ini-tial nonilinearities or abrupt changes were observed in jointload-deflection data. All off-axis test specimens were loaded indouble-shear with fastener torque-up of 50 inch-pounds. BJSFMpredictions indicate first ply fiber rupture or shear failure,based on a maximum strain failure criterion applied at a charac-teristic dimension of 0.02 inch away from the hole boundary.Correlation of this l inear-elastic analysis with initial pointsof nonlinear joint deflection behavior, indicates that laminashear or fiber failures are likely before ultimate bearingfailure occurs.
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I I Double-shear"Gr3~oss 4d 3d Ultimate failureF 50 in.-lb Torque-Up 0 Inlitial nonlinearity
160
120
.0 xPred.cted
40 _________ ____________6,000 30/60/10
'000
Ci
* 2,000
702/0 20 4Predicted
60 80toPercent of 1450 Plies
OP13-O 115-340
Figure 42. Effect of Layup Variation on Joint Tensile Strength51
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d -,0.250 --II
PTotal 6dI P2
XGross 4d 3d50 in.-lb Torque-Up U 0 A Ultimate strengthDouble-Shear 0 A Initial non-linearity170
130- , f ETW
U-
90 /
50"- 7 ,000, ra I
C -J__-__--5,000 PredictedS-5,000 -
IIl.0.0,
--1,000,0 20 40 60 80 100
Percent of 450 Plies CIP13-0115,44
Figure 43. Effect of Layup Variation on Joint Compressive Strength
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d = 0.250-\ Double-shear0 Vo\N _T 50 in,-Ib torque-upP 6d 0 in.-lb torque'up0 [0 Initial nonlinearity
50/40/10 layup3d160,
40
Figu ein.Shenr Faitinr hte re80
ditioned ~ reicedpcmesat20e.Thoe ictlped:cin tbt T0 20 40 60 80 100Off-Axis Loading Angle, 0i - deg a P13-0115i-Figure 44. Effect of Off-Axis Loading on RTD Pure Bearing Strength
Strength of moisture conditioned specimens is presented inFigure 45. Trends appear to be similar to RTD results with joint.gross failure strains reduced by approximately 700-1000 kimi/ inand bearing strength by approximately 40 KSI in the moisture con-ditioned specimens at 250*F. Theoretical predictions at both RTDarid ETW are indicated in Figure 45, versus strain data for ini-tial. points of nonlinearity. Both ETW and RTD prediLctions werebased on the .02 inch v~ilue of Rc. Results from off-axis lcidingat 100 under compression loading ara presented in Figure 46.
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P p1 6d_
bl, Initial cuTest Condition x Nonlinearity _ _ X50/40/10Layup ks i Prcent A ksi Percent A an./in. Percent Aof IRTD of RTD of RT DRT D 153 - - -2,640 -RT W 157 +2.7 122 +10.4
-.2,830 +7.3ETIW 113 -26.3 76 -3.8 -1,980 -25.11J
OP130115.42
Figure 46. Effect of Temperature and Moisture on Compressive Strength
(c) Edge Distance - Effects of edge distance onjoint: strength were evaluated by testing two-fastener-in-tandemdouble shear specimens . Tests were performed on dry specimens atroom temperature and on moisture conditioned specimens at 250F.3pecimen.-, were loaded to failure in tension. Additional RTD dataon the effects of edge distance on pure bearing (single-fastener)streorgth were available from an earlier WCAIR in-house program.
Results of RTD an d ET W tests are presented in Figure 47 .Specimen failure modes at all e/d ratios exhiLited shearout tobearing-shearout failures. Joint bearing strenoth an d grossfailure strain data exhibit a marked bilinear behavior. Reducedjoint strength with earlier insensitivity to e/d ratios wasexhibited in the ET W tests.
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d 0.250\P1 + P2 Id
50 in.-Ib Torque-Up x 4d e160i RT D
120 .TV
C,,80
405,000
RTD4,000
S' __ ETW -3,000
2,000
1,000 0 1 2 3 4 5Edge Distance/Diameter , e/d OP134-11E-40
Figure 47. Effect of Edge Distance on Joint Tensile Strength
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Effects of fastener spacing (S) on joir.t strength wereevaluated at RTD test conditions. Results (Figure 48) indicatethat fo r high-bearing to bypass joint loadings, strength reduc-tins occur as S/d ratios decrease from 4.0 to 2.0. Whilefastener spacing creates a beneficial "shadowing effect" at th ern '.-section on stress concentrations, this is not th e case in1, gh bearing-to-bypass multi-fastener joints where bearing orshearout failures prevail. The proximity of adjacent fastenerholes acts like an edge-of-part to internal fasterner holes,reducing bearing strength capability and overall laminate straincapability.
Single fastener (pure bearing) joints were tested in single-shear at RTD fo r edge distance/diameter values ranging from 1.5to 3. Effects of torque-up (30.0 increased to 50.0 inch-pounds)are reflected in a 25 KSI increase in bearing strength (Figure49) fo r the e/d ratio of 3. The stacking sequence used in thesetests was identical to th e number 6 stacking sequences of Task 2(Figure 26). The increased strength of the more homogeneousstacking sequence (number 1) is also indicated in Figure 49.(d) Width - Effects of width on joint strength wereevaluated using the 50/40/10 laminate in a two-fastener-in-tandem double-shear configuration. Specimens were tested tofailure in tension at RTD and ETW. Results are presented inFigures 50 and 51. For RTD tests, specimens failed in th e netsection at the lower w/d ratios of 3 and 4, and failed in bearing-shearout at the w/d ratios of 6 and 8. Effects of temperature(250 0 F) and moisture content reduced strengths markedly at th e
higher w/d ratios but not at th e lower w/d ratio of 4. Thisdecreased sensitivity reflects a transition to fiber dominant net-section failures which are insensitive to effects of moisture.Failures fo r RTD specimens occurred in bearing-shearout at w/dratios of 6 and 8 and transitioned to net-section at w/d ratiosof 4 and 5. Decreased shear and co
