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UNCLASSIFIED
AD NUMBER
AD876197
NEW LIMITATION CHANGE
TOApproved for public release, distributionunlimited
FROMDistribution authorized to U.S. Gov't.agencies and their contractors;Administrative/Operational Use; Sep 1970.Other requests shall be referred to NavalAir Systems Command, Washington, DC.
AUTHORITY
Naval Air Systems Command ltr, 2 Apr 1971
THIS PAGE IS UNCLASSIFIED
Reproduced FromBest Available Copy
FINAL REPORT
THE EFFECTS 6F HIGH INTENSITY ELECTRICALCURRENTS ON ADVANCED COMPOSITE MATERIALS
N00019-70-C--073
15 SEPTEMBER 1970
N _DDC"
S.... • ., € .-- NOV 4 9B
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PH:Lf O
I'III
I THE EFFECTS OF HIGH INTENSITY ELECTRICAL
, CURRENTS ON ADVANCED COMPOSITE MATERIALS
IFINAL REPORT
I1 15 September 1970 ,'•""!•• ,• .,..
. , . . •. .-,.b • 2 -
by ~ f ';.~2:~
A. P. Penton and J. L. Perry of Philco-Ford Corporation
K. J. Lloyd of General Electric Company
I Prepared Under ContractN00019-70-C-0073 / D,
for D
Naval AiKSysterns Command -Department of the Tavy Nov 4 1970
I I�--•-ILCOI PHILCO-FORO CORPORATION
Aeronutr onic OviwsionNewport Beach, Calif. * S2663
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ACKNOWLEDGEMENTS
This program was performed :-nder the sponsorship of
the Naval Air Systems Commatid, N00019-70-C-0073,
with J. W. WilIis an the project engineer. The
authors also wish to acknowledge the contributions
of D--, W. M. Fassell and D. W. bridges,
Messrs. T. M. Place. D. R. Doner and C. G. Daniels
of PhitIco-rord' and Messrs. J, A. Plumer and F. A.
Fisher of General Electric Company.
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I CONTENTS
SECTION PAGE
S INTRODUCTION AND SUIMARY .......................... 1-1
2 INVESTIGATIONS OF CURRENT FLOW PROCESSES, DAMAGEMECHANISMS, AND INTERNAL PROTECTION TECHNIQUES ... ..... 2-1
2.1 Approach ............................. ... 2-12.2 Current Flaw Processes ...... .............. .. 2-132.3 Degradation of Filaments and Composites .... ...... 2-392.4 Development of Internal Protection Techniques
for Advanced Composites .... .............. ... 2-93
3 CONCLUSIONS ............................... ... 3-1
REFERENCES ................................ ... 3-3
APPENDIX A .................................. ... A-I
APPENDIX B. .. .. ... ... ................................... B-I
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ILLUSTRAT IONS
FIGURE PAGE
1-1 Block Diagram of Program Approach ..... ............. .. 1-3
2-1 Boron Filament, Graphite Yarn and Graphite TowElectrical and Breaking Strength Test Specimen ... ..... ... 2-5
2-22-2 Impulse Generator and Peripheral Measuring
Equipment ............... ......................... .. 2-6
2-3 Electrical Current Injection Test Set-Up .... .......... ... 2-7
2-4 Different Types of Current Probes Evaluated forFiber Electrical Current Injection Tests .... ........ ... 2-8
2-5 Typical Oscillograms .......... .................... ... 2-10
2-6 Waveshape Definitions ........... ................... .. 2-11
2-7 Test Set-Up for Higher Energy Current Injection Tests . . 2-12
2-8 Dielectric Strength of DEN 438 (2-6 mil thickness) ... ..... 2-15
2-9 Dielectric Strength of DEN 438/104 Glass Cloth(3-3.5 mil thickness) ............. .................. .. 2-16
2-10 Dielectric Strength of DEN 438/104 Glass Cloth(2-2.5 mil thickness) ........... ................... .. 2-17
2-11 Dielectric Strength of Whittaker 2387/104 Glass Cloth . 2-18
2-12 Set Up to Test Dielectric Strength of Boron Sheath ... ..... 2-19 12-13 Equivalent Model of Dielectric Test Specimen ........... ... 2-20
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IIII ILLUSTRATIONS (Continued)
FIGURE PACE
2-14 Dynamic Impedance of Boron Filament Specimen ...... ... 2-22
2-15 Dynamic Impedance of HMG-50 Graphite Yarn ........ ... 2-23
2-1.6 Dynamic Impedance of HM-S Graphite Tow .. ......... .. 2-25
2-17 Dynamic Impedance of Boron Unidirectional Laminates . 2-26
2-18 Test Set-Up for Current Flow Transverse to Boronj Filaments ................................... ... 2-27
2-19 Initial Model of Current Flow In a UnidirectionalI Composite ................................... ... 2-28
2-20 First Modification of Initial Current Flow ModelI of Figure 2-19 ............................. ... 2-29
2-21 Uni.t Length Current Flow Model for a Boron Filament , 2-30
2-22 Current Flow Model of Boron-Epoxy UnidirectionalLaminate .......................................... 2-31
2-23 Schematic of Two Filaments Within Laminate .... ....... 2-32
2-24 Resistance and Voltage vs. Length of Specimen .... ...... 2-34
2-25 Resistance and Voltage vs. Length of Specimen .... ...... 2-35
2-26 Resistance aid Voltage vs. Length of Specimen .... ...... 2-36
1 2-27 Resistance and Voltage vs. Length of Specimen .... ...... 2-37
2-28 Boron Filament Typical Strength Degradation vs.Crest Current Density (For Standard Waveform) .... ...... 2-41
2-29 Boron-Epoxy Typical Strength Degradation vs. Cresti Current Density (For Standard Waveform) ..... ..... .. 2-42
2-30 Degradation vs. Energy per Unit Resistance (BoronI Filaments) . . ................................. .. 2-44
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ILLUSTRATIONS (Continued)
FIGURE PAGE
2-31 Degradation vs. Energy per Unit Resistance(Boron Unidirectional Laminates-on a per filamentbasis) ................. .......................... .. 2-45
2-32 (800X) View of Unexposed Boron Filament Cross
Section ................. ......................... .. 2-46
2-33 (250X) View of Unexposed Boron Filament
Longitudinal Cross Section ....... ................ ... 2-46
2-34 (125X) View of Longitudinal Crzss Section of 2Boron Filament # 20 (4.93 x 10 Crest Amps per cm
of filament) ............... ....................... .. 2-47
2-35 (1000X) View of Cross Section2 of Boron Filament #20(4.93 x 10 Crest Amps per cm of filament) .. ....... .. 2-47
2-36 (2000X) View of Cross Section of Boron Filament #20(Core Region) ............. ....................... . 2-48
2-37 (800X) VieX of Boron Filament 2 #50 Cross Section(8.88 x 10 Crest A"ps per cm of filament) .... ....... 2-48
2-38 (80OX) View of Boron Filament #50 Cross Section(8.88 x 104 Crest Amps per cm2 of filament) .. ....... .. 2-49
2-39 (80OX) View of Boron Filament #50 Cross Section ...... 2-49
2-40 (4OOX) View of Unexposed Boron Filament Surface Replica. 2-51
2-41 (50OX) View of Boroi Filament #55 Surface Replica2(8.01 x 104 Crest Amps per cm of Filament) .... ....... 2-52
2-42 (250X) View of Boron Filament #50 Surface Replica(8.88 x 104 Crest Amps per cm2 of Filament) .. ........ 2-52
2-43 (lOOX) View of Unexposed Boron Epoxy Composite Surface . 2-53
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i ILLUSTRATIONS (Continued)
JFIGURE PAGE
2-44 (LOOX) View gf Exposed Boron Epoxy Composite Specimen#13 (,,,9 x 10 Crest Amps per cm2 of Filaments) ...... 2-53
2-45 (10OX) Cross Section View of Unexposed BoronEpoxy Composite .............................. ... 2-54
2-46 (10OX) Cross Section View of Exposed Boron2 EpoxySpecimen #26 (9.18 x 10 Crest Amps per cm ofFilaments) ......................................... 2-54
2-47 Polished Transverse View over Entire 1/2" Width ofBoron-Epoxy Tensile Specimens After ElectricalCurrent Injection ............................. .. 2-55
2-48 (80OX) View of Damaged Boron Filament in BU #26 Composite--at Point of Contact With Nickel Plating ........... ... 2-56
2-49 (80OX) View of Electrically Damaged Boron Filament inComposite Specimen BU#26 ..... ................ ... 2-56
2-50 (80OX) View of Electrically Damaged Boron Filament inComposite Specimen BU #26 ...... ................ ... 2-57
"2-51 (200OX) View of Core of Damaged Boron Filament (BU #26)That has been Etched to Show Evidence of Melting .... 2-57
-2-52 (2000X) View of Damaged Boron Filament (BU #26) ThatHas Been Etched but does not Show Evidence of Melting . 2-58
- 2-53 (2000X) View of Boron Filament in BU #26 CompositeWhere Major Portion of Core has been Lost Due to Meltingor Vaporization ............................. 2-58
2-54 (80OX) View of Electrically Damaged Boron Filament inComposite Specimen BU #13 .. ........... . .. 2-59
1 2-55 (80OX) View of Electrically Damaged Boron Filament inComposite Specimen BU #1.3 ........................... 2-59
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ILLUSTRATIONS (Continued) I '
FIGURE PAGE 12-56 (80OX) View of Electrically Damaged Boron Filament
in Composite Specimen BU #17 .... .............. ... 2-61 I2-57 (2000X) View of Boron Filament in BU #i7 Composite
Where the Core has Melted and Resolidified ....... ... 2-61 12-58 (lOOX) Boron-Epoxy BUN-2 Specimen in Which a 3220 Ampere
Crest Impulse was Injected into Outer Plies ...... .. 2-62 12-59 (IOOX) Boron-Epoxy BUN-I Specimen in Which a 4370 Ampere
Crest Impulse was Injected into Outer Plies ...... .. 2-62
2-60 (1700X) Electron Microscope Scan of Unexposed BoronFilament Survace (40 second scan) ... ........... .. 2-64 I
2-61 (1700X) Electron Microscope Scan of Boron Filament #13 ,
Surface (7.52 x 10 Crest Amps per cm2 of Filament)(40 second Scan) ........ .................... ... 2-64 1
2-62 (lOOX) Photomicrographs and X-ray Images of ControlSample of Non-exposed Boron Filament ............ ... 2-66
2-63 (1000X) Photomicrographs and Microprobe X-ray4 Images ofBoron Filampnt From Specimen BU-26 (9.18 x 10 CrestAmps per cm of Filaments) ..... ............... ... 2-67
2-64 (1000X) Photomicrographs and Microprobe X-ray4 Images ofBoron Filament from Specimen BU-26 (9.18 x 10 CrestAmps per cm2 of Filaments) ..... ........ ....... .2-68
2-65 Microprobe Analysis of Boron Filament Cores ...... .. 2-69
2-66 W-B: Melting Temperatures, DTA-Results and QualitativeEvaluation of Solid State Samples ............... ... 2-70 9
2-67 Resistivities of Tungsten, Tungsten Boride and Boronvs. Temperature ......... .................... .. 2-73
2-68 Calculated Boron Filament Stress ... ............ ... 2-76 12-69 Fraying of Graphite Fibers as a Result of Whipping
Action During Current Injection .... ............ .. 2-7S
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ILLUSTRATIONS (Continued)
FIGURE PAGE
2-70 HMG-50 Yarn Thermogravimetric Analysis .. ......... ... 2-80
2-71 HMG-50/Epoxy Specimens After 20.4 x 10 4 Amperes per cm2
of Filament Crest Current Injection--showing charring . 2-82
2-72 HMG-50/Epoxy Specimens After 24.4 x 104 Amperes per cm
og Filament Crest Current Injection--charred anddelaminated ........... ....................... ... 2-82
2-7' HMG-50/Epoxy Specimens After 55.8 x 104 Amperes per cm
of Filament Crest Currat.Injection--extensively charred
and delaminated ...... ..... ............ 2-83
4 22-74 HM-S/Epoxy Specimens After 25 x 10 Amperes per cm of
Filament Crest Current Injection--showing charring . . . 2-83
2-75 HM-S/Epoxy Specimens After 52 x 104 Amperes per cm CrestCurrent Injection--charred and exfoiiated .............. 2-84
44 2-76 (IOOX) HMG-50/Epoxy Specimen After 13.04 x 10 Amperes
per cm of Filament Crest Current Injection--no damage. 2-85
2-77 (IOOX) HMG-50/Epoxy Specimen After 20.4 x 104 Amperesper cm2 of Filament Crest Current Injection--burned
and delaminated ......... ...................... ... 2-85
2-78 (IOOX) 2 HG-50/Epoxy Specimens After 24.2 x 104 Amperesper cm of filament Crest turrent Injection--severelyburned and delaminated ..... ............... . . ..... 2-86
42-79 (IOOX) 2 HMG-50/Epoxy Specimen After 55.8 x 10 Amperes
per cm of Filament Crest Current Injection--severelySburned and delaminated ....... .................. ... 2-86
42-80 (.90X) HM-S/Epoxy Spec-nen after 21 x 10 Amperes per1 cm of Filament Crest Current Injection--no damage . . 2-87
2-8i (lOOX) iur-S/Epoxy Specimen After 25 x 104 Amperes percm2 of Filament Crest Current Injection--some burning3 and delamination .............................. .. 2-87
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ILLUSTRATIONS (Continued)
iFIGURE PAGE
2-81 (lQOX) HM-S/Epoxy Specimen After 25 x 10 Amperes per 1cmz of Filament Crest Current Injection--some burningand delamination .... ............................. 2-87
2-82 (O1OX) HM-S/Epoxy Specimen After 52 x 10o Amperes per cm2
of Filament Crest Current Injection--severely burnedand exfoliated ........... ...................... .. 2-88
2-83 (16,OOOX) Electron Microscope Scan of Unexposed HM-SGraphitc Filament Surface (40 sec scan) ........... ... 2-89
2-84 (16,OOOX) Electron Microscope Scan of gurface of GraphiteFilament From HM-S Tow #74 (98.71 x [0 Crest CurrentAmps per cm of Filament) (40 sec scan) .. ......... .. 2-89
2-85 Predicted Temperature Increase in H1MG-50 Yarn and 111S-Towas a Function of Peak Applied Current .. .......... ... 2-91
2-86 (lOOX) Hybrid-2 Specimen After 10.4 x 104 Amperes percm2 of Filament Average Crest Current Injection--no damage 2-95
2-87 (10OX) Hybrid-I Specimen after 9.84 x 104 Amperes per cm2
of Filament Average Crest Current Injection--no damage . 2-95
2-88 BUTCM 11 Specimen Subjected with 11.08 x 104 Crest Ampsper cm of Boron Filaments ...... ................ ... 2-97
2-89 BIC #2 Specimens Injected with 7.64 x 104 Crest Amps percm of Boron Filaments ........ ................ . . 2-99
2-90 BIC #3 Specimen Injected with 13.96 x 104 Crest Amps percm of Boron Filaments ........ .................. ... 2-99
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SECTION 1
j INTRODUCTION AND SUMMARY
'The objective of this program was to perform a research study to inves-tigate high intensity electrical current flow and resulting degradationin advanced composites and to investigate means of providing internalprotection from such damage. The approach followed in the the programessentially involved the selection of representative high modulus boronand graphite filaments and composites and the exposure of those filamentsand composites to precisely controlled electrical current flow. Necessaryelectrical, physical, chemical, microstructure, and mechanical analysesand tests were performed to permit modeling of electrical current flowprocesses and damage mechanisms. The laboratory analyses and testsperformed included photomicrographic analysis, electron microprobe,scanning electron microscope, X-ray diffraction, tensile tests, and physical?roperty measurements. The electrical properties measured included re-sistivity, dielectric strength, and impedance.
In the performance of this program specimen fabrication, physical testing,mechanism testing, specimen inspection and degradation mechanism analyseswere performed by Philco-Ford Corporation. Current injection tests,electrical property measurements and current flow process analyses wereperformed by the General Electric Company High Voltage Laboratory
In this program, f~ilamerts and epoxy resin composites of boron and graphite,filaments were studied. The boron filaments were of the 0.004 inch diametervariety that are manufactured by the chemical vapor deposition of boron ontoa 0.0005 inch diameter tungsten wire. Such filaments typically have a 55 x
6 tt wo106 psi modulus of elasticity. Two varieties of graphite filamentswere included. One was a 55 x 10 psi modulus graphite filament tow asmanufactured from"\a polyacrylonitrile precursor and the other was a
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50 x 106 psi modulus graphite filament yarn as manufactured from a rayonprecursor. The filaments included were manufactured by:
United Aircraft Corp.: Boron filament
Courtaulds, Ltd.: RH-S 55 x 1O6 psi modulus graphite tow
Hitco: HMG-50 50 x 106 psi modulus graphite yarn
In the composites evaluated, the epoxy resin matrices were WhittakerCorp. 2387 for the boron composites and DEN 438/MINA for the graphite compo-sites. Each composite was of 50-55% by volume filament content.
In the investigations performed to date by Philco-Ford Corporation andGeneral Electric Company, the approach used was shown in Figure 1-t. Thisfigure shows that a complete understanding of degradation processes andpossible m"thods for improving composite material response requires theinvestigatioWof the following interrelated areas:
(1) The resultant degradation mechanisms.
(2) The electric current flow process within the filamentsand composites.
To date it has been determined that boron epoxy composites degrade atcurrent levels where the boron filaments themselves begin to crackradially and transver3ely. For the standard waveform used in the degra-dation studies this occurred at crest current injection levels of above3.7 - 5.7 x 104 amps per cm2 of filament. Above these levels, degradatiouincreases in severity until total loss in filament integrity occurs, Themechanism of the filament degradation has been determined to be due to thefact that the tungsten boride core is much less resistive than boron and,as a result, carries the electric current that is injected into thefilaments. The energy dissipated in the core in the form of heat causesthe core to expand and stress the boron. This eventually causes theboron to crack.
The graphite filament epoxy compolites begin to degrade at crest currentlevels of 20-25 x 10 amps per cm of filament for the standard waveform.Their mechanism of degradation is based on the resin rather than thefilaments. Current flows in the filaments and is dissipated partiallyinto heat. At the damaging levels the heat initiates pyrolysis of theepoxy resin matrix. The pyrolysis gases cause a pressure build-upwithin the laminate until cracking and delamination occurs. At thispoint the air comes in contact with the hot filaments and gases and thecomposite bursts into flames.
1-2
DEFINE ELE(••CRiC CtRWENT DEFINE DR)RADATION
mFow PROCESSES
1EEMN DETERMINE_DETERM•INE Z DIELECTRIC ImwUamrsmS IN
OF F1-AME"IrSREAKDW11 TRENTH FLAMENTISO
F AND YARNS OF AL YARNS
ANALYZE RESULTTS
nETERMINF Z bTRSOOF MATRIX
(I2 t) O
II LEVELS, ETC.
CON ST RUCTEQUIVALENTr Z!I• iIM4ODEL OF
F W MECHANISMS IN
LABORTORYCOMPLETE
TESSI cOMPOSITES
I DETERMINEDIELECTRICBREAKDOWNSTRENGTH OF
j COMPOSITE PANEL
RESULT:
I ~UNDERSTANDI]NG OF
DEGRADATION PROCESSESAND PPfOTECTIVERE(Q.JIR124ENTS
I , FIGURE 1-1. BLOCK DIAGRAM OF PROGRAM APPROACIH
1-3
It is believed that the most viable approach for improving the resistanceof boron and graphite epoxy composites to degration due to current flowwill be improving the conductance of the composites through incorporationof secondary conductive paths. These paths must be less resistive thanthe filaments and not cause significant reduction of either the strengthor modulus of the composite or cause serious weight penalties. The utilityof this general approach was demonstrated by incorporating fine tungstenwires in one case, and graphite filaments in another case, within boron-epoxy laminates. Both graphite filaments and tungsten wire are lessresistive than the boron filaments. As the result, tests proved thatcurrent was diverted away from the boron filaments and the threshold ofdegradation of the composite increased.
Current flow processes within advanced composites are quite complex anddifficult to analyze. Essentially though, a composite acts as a complexarray of conductive paths (filaments) imbedded within and separated byfilms of electrically insulative resin. This model of current flow iscomplicated by the fact that the dynamic impedances of the compositespecimens are constantly changing during current flow. Also, the insula-tive resin films between filaments and layers or plies of filaments arenot continuous and uniform in thickness. Adjacent filaments vary in theirspacing and often are in contact with each other. As a result, currentflow is not restricted only to the filaments or plies of filaments intowhich the current was injected and degradation can occur in separate layers
or plies of filaments. jAll experimental results and analyses are discussed in Section 2.
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I SECTION 2
SINVESTIGATIONS OF CURRENT FLOW PROCESSES, DAMAGE MECHANISMS,AND INTERNAL PROTECTION TECHNIqUES
'" 2.1 APPROACH
The overall objective of this program was to perform research into the* effects of high intensity electric currents on advanced filament reinforced
plastic composites. Specifically, the work was to be directed toward- those current high strength and modulus filament types that have been
developed for use in structural composites for advanced aircraft. Also,the electric currents to be used in the study was to be representative ofthose that might occur as the result of a lightning strike to the surfaceof an aircraft. Following this overall approach, the program was orientedto study the processes involved in electric flow through filaments andcomposites and in the determination of the mechanisms of any damage resultingfrom the current flow. Introductory to this, however, was the materials
selections and characterizations that were made preparatory to the currentflow and damage mechanisms research. Also, during this program, certainapproaches toward minimizing damage due to current flow were conceived andevaluated.
S2.1.1 FILAMENTS AND COMPOSITES SELECTED
a. Filaments. In order to select materials for use in this efforta survey was made to determine those types of filaments and resins thatare in use for advanced composites so that materials that might be expectedto have unique electrical characteristics could be included for investigation.
Two types of graphite filaments and one type of boron filament were selectedas representative of commercially available advanced high modulus reinforce-
SDments. One type is Hitco's HMG-50, a continuous filament hig~i~modulus
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graphite yarn with a proprietary surface finish (E-8095) for the achievementof optimum composite properties. Yarn construction for HMG-.50 utilizes twopli~s of yarn with 720 filaments per ply with a filament diameter of 2.7 x10- inches, a twist of four turns per inch and a calculated yarn crosssectiongl area of 82.8 x to"6 square inch s. Typical yarn properties are n50 x 10 psi tensile modulus and 287 x 10 psi ultimate tensile strength. i
The second type of graphite filament selected is manufactured by Courtaulds(Type HM-S) and is a continuous twist-free high modulus roving tow with aproprietary surface treatment for the improvement of interlaminar shearstrength in the composite. The t w Gtrand of the HM-S contains 10,000filaments, each typically 3 x 10- inches in diameter. The calculated Icross-sectional area of the tow is 785 x 10-6 square inches. Typical
filament properties are 55 x 106 psi tensile modulus and 275 x 10' psiultimate tensile strength.
Boron filaments were obtained from the Hamilton Standard Division of UnitedAircraft Corporation. At the present time only one type of boron filamentis in current use (a pyrolytic boron coating deposited over a 0.0005 inchdiameter tungsten wire substrate). During the process of deposition ofboron.on the substrate, a chemical reaction occurs and portions of thesubstrate change from tungsten to tungsten boride.
A typical boron filament is 4 x 10-3 inches in diameter, with a 5 x 10-4
inch diameter tungsten or tungsten boride core. The filament typicallyhas one twist per tgn feet of length and has a calculated cross sectionalarea of 12.57 x 10" square inches.
b. Resin Matrices. Two epoxy resin systems were choseu as typical
of those presently in use for high modulus composites. Dow Chemical's DEN438/MNA resin system is typical of the unfilled epoxy resins used ingraphite composites for environments up to 350 0 F.
Whittaker Corporation's 2387 resin system is typical of those modifiedepoxy resins used with boron reinforcement for potential usage up to 350°F.
c. Composite Preparation. The filaments and resins to be used in the
program were fabricated into composite test specimens. For this program,the most simple filament orientation pattern was used; one in which allfilaments are unidirectionally oriented parallel to one another. Thecomposite laminates of each filament type were fabricated of precollimatedand preimpregnated filament layers using conventional 100 psi vacuum bagautoclave curing techniques with 3500F maximum cure temperatures. Thecompositions of the unidirectional test laminate aie summarized in Table2-I.
2-2
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TABLE 2-1
I CCOMPOSIE LAMINATE COMPOSrIION
Hitco Courtaulds HamiltonHMG-50 HMS Standard
Filament t Graphite Boron
SNumber of plies 2 2 5
Filament VolumeContent, 7. 50-55 50-55 48-54
Resin Matrix DEN438/MNA DEN438/MNA Whittaker2387
-I Resin Content,7% by weight 30-37 30-37 27-32
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"2.1.2 PHYSICAL, CHEMICAL AND MECHANICAL TESTS
\•our basic types of tests were performed on exposed as ýiell as non-exposedsOpecimens in order to investigate the degree as well as mechanisms ofdamqge in the filaments and composites through which current had been 3pass64. These were:
(1), Breaking strength of filaments and tensile strength"%,of composites.
(2) V'sual and photomicrographic inspection of filaments andco3posites. I
(3) EleCtron microprobes of selected filaments.
(4) Scanning electron microscope inspection of r,elected filaments.
As shown in Figure 2-I, individual specimens of grarnite yarn and tow aswell as boron filaiments were positioned on I" x 15" pieces of artboardand adhesively bo ded at points 1½ inches from eact, end. These filamentmounts were utili ed to prevent filament damage tc and from the electricalexposures and for purposes of breaking strength testing. For the breakingstrength tests, almainum tabs were bonded to the filament ends. Aftereach specimen was )ounted in the test machine, the artboard strip was cutwith a razor blade The spezimen was tested to failure at 0.05 in/sin ofmachine head trave;.
Each of the composite laminates fabricated were of 0.020-0.025 in. Ln
thickness. For the electrical exposure tests, as well as the tensile strengthtests, a specimen size of 0.5" x 7.0" (filaments in 7.0" direction) wasselected. Prior to the tensile tests, glass reinforced plastic tabs werebonded to the ends of each specimen. The tabbed specimen ends were thenclamped in the test machine and loaded to failure at 0.05 in/min. ofhand travel.
2..1.3 ELECTRICAL CURRENT INJECTION TEST PROCEDURE FOR MATERIAL DECRADATIONSTUDIES
Electrical testing was conducted at the General Electric High VoltageTest Laboratory. The impulse generator and peripheral measuring equipmentutilized are presented in Figure 2-2. Figure 2-3 illustrates an actualspecimen during testing and Figure 2-4 is a photograph of the differenttypes of current probes which were used.
End contacts for the graphite yarns and tow were achieved by painting theends of the filaments with a silver paint which penetrated and wet eachfiber thus allowing good electrical contact. The boron filament ends were
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placed between two aluminum foil pads and wetted with silver paint. Eachend of the filament along with its pool of conducting paint was then com-pressed between these two pads. Oacillograms were taken during the testsand included a voltage measurement across the test sample in addition toa measurement of the current through the sample. Typical oscillograms ofa boron filament, graphite yarn and graphite tow are represented byFigure 2-5, The top line represents voltage across the sample and thebottom line represents injected current. The sweep was 5 microsecondsper division.IFor these initial tests a standard waveform shape was established, asshowr in Figure 2-6. Natural lightning comes in many different configura-tions; therefore, when conducting electrical exposures that are intendedto simulate current flow as might result from lightning, it is necessaryto define and control waveform shape. One IEEE recognized standard(-) forlightning simulation recognizes the following impulse waves: an 8 x 20microsecond wave for lightning discharge currents and a 1.2 x 50 microsecondvoltage waveform. The meaning of this can be seen in Figure 2-6. Thevirtual value for the duration of the wavefront is as follows: for currentwaves, 1.25 times the time taken by the current to increase from 107. to90% of the crest value; for voltage waves, 1.67 times the time taken bythe voltage to increase from 304 to 90%/ of its crest value.The standard current wave applied in the exposure was somewhat of acompromise between the standard voltage wave and the standard currentwave as just defined.
The schematic set-up for the current injection tests at high current levelsis shown in Figure 2-7. The impulse or lightning current generator depictedis larger, in all respects, than the one shown in Figure 2-2. It not onlypossesses larger dimensions, but also higher voltage and energy capabilities.Since it is desirable to maintain a large resistance in series with the testspecimen in order to prevent fluctuations in the current flowing throughthe specimens, it became necessary to go to this unit for the higher energytests.
The primary difference to be noted in the set-up pictured in Figure 2-2and 2-7 is the technique used for measuring the injected current. Pre-viously these measurements were made by using a current pulse transformer.During testing these transformers were found to fall off slightly at lowfrequencies which required a small correction factor to be introducedwhen reading values out on the tail of the wave (a necessity when deter-mining the dynamic impedance of the specimen). The technique employedin measuring the current is to measure the voltage across a linear (non-inductive) resistance shunt in series with the test specimen. By usinga shunt resistance value which is much smaller than the test specimenresistance, there is an insignificant amount of error in the voltage signal
2-9
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jT tf = 1.25 (t2 - t 1 ) for current
tf = 1.67 (t 2 - ti) for voltage
I FIGURE 2-6. WAVESHAPE DEFINITIONS
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measured by the divider circuit. As the shunt resistance is a knownvalue, this error can, if desired, be eliminated entirety.
The unidirectional composite specimen electric current injections wereperformed in the same manner as were the injections of the filamentswith the exception of the electric contact. For the composites, each5 ~specimen was beve Ld on ea~ch end to 450* The specimen ends were thenvapor honed to ins re exposure of the filament ends. One-half inch oneach specimen end was then plated with a 0.001-0.0015 inch thick nickelcoating. In the case of the filaments, as previously noted, silver filledepoxy paint was used on the specimen ends. Both the filament and compositespecimens were clasped into the test fixture for current injection asshown in Figure 2-7.
The resistivity of each filament and composite specimen was measuredprior and subsequent to electric current injection. Also, AThermopapers"were used to provide a gross indication of the peak temperature of thecomposite specimens as the result of current injection.
Other special electric injection test procedures, such as dielectricbreakdown strength, were provided in the discussions of those results.
2.2 CURRENT FLOW PROCESSES
In studying the effects of high intensity electric current flow on advancedcomposite materials it is essential that more than just damage studies beperformed. In other words, the quantity of current flowing through adamaged element of a composite must be established. To do this, it is thenimperative that the processes whereby current flows into and through thecomposite be understood. This then results, hopefully, in an approachtoward mathematically modeling of and analytical techniques for predictingcurrent flow. Since a composite is composed of an array of conductivefilaments within a dielectric matrix of epoxy resin, certain electricalproperties such as dielectric strength and dynamic impedance must bemeasured as related to the current flow processes involved. Therefore, thefollowing is a summary of the electrical property characterization resultsand the understandkng of the electric current flow processes that has todate been developed.
2.2.1 DIELECTRIC STRENGTH TEST RESULTS--EPOXY RESIN MATRICES
Since epoxy resins are known to be non-conductive, it is essential thattheir dielectric breakdown strength be determined on films of resin thatare typical of the thickness of the resin film layers that exist betweenlayers or plies of filaments in composites. Such resin films act aselectrical insulation and tend to confine the current flow to the filaments
2-13
II
into which the current was injected. At a sufficient energy level theresin would breakdown thereby permitting current flow from one filament Uto another.
The resin films tested for dielectric strength were as follows: IFour sets of specimens were tested.
I. Pure DEN 438 film (2-6 mil thickness range)
2. DEN 438/104 glass cloth (3-3.5 mil thickness range)
3. DEN 438/104 glass cloth (2-2.5 mil thickness range)
4. Whittaker 2387/104 glass cloth (1-2.5 mil thickness range)
The resin films of interest were tested with 104 glass scrim cloth 1included because it is standard practice in industry that filaments becollimated onto the very thin (<0.0012 inch) glass scrim cloth. Thescrim then becomes an integral part of the interply resin film and occupiesa small fraction of the total volume of the composite.
The volt-time curves of Figure 2-8 through 2-11 were determined by exposing 1samples of each material, to three different voltage waveforms, one with afast rise time, a moderate rise time, and a slow rise time. The volt-ttmecurves then were constructed by plotting the voltage and times wherebreakdown occurred with each waveform shape.
2.2.2 DIELECTRIC STRENGTH TEST RESULTS--BORON SURROUNDING SUBSTRATE CORE
It had originally been suggested that the boron sheath which surrounds thetungsten core might act as a dielectric material because of-its high resis-tivity. Testing (conducted as shown in Figure 2-12) resulted in data whichindicates that measurable conduction occurs as low as 1.0 volt and that the
material behaves as a conductor and not as a dielectric as originallysuggested. From other data sources, it appears that the boron is a poorconductor.
During these tests, voltages were held for fixed periods of time andcurrent flow recorded. The resulting data did show considerable variation,iand for this reason additional tests were conducted under more controlled
conditions. The results of these tests made on the boron, show it to havea high value of resistance as would be expected from calculations. Ofinterest, however, is the fact that with a voltage (V) of 35 to 50 voltsan avalanche phenomenon occurs and the boron does, in fact, breakdown.
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IInitially, at low voltages, the resistance of the approximarely 5-inch Ilong filamenL is measured at 1 wegohm (as determined from voltage andcurrent measurements). If the voltage is increased, however, a value isreached (somewhere between 35 to 50 volts) where the resistance dricpsinstantaneously to approximately 500 ohms (again determined from thevoltage and current measurements).
Consider now what one would expect, as determined by calculations. From 5earlier current injection tests it was found that mercury (because ofsurface tension) made poor contact with the substrates of the filaments.It may, therefore, be assumed that the test piece under consideration hereis depicted by Figure 2.13. That is to say, electrical contact is notmade by the mercury, with the substrate, at either end.
Boron in contact Substrate Jwith H9
lZL.(thickness of boron) Jx__ X
FIGURE 2-13. EQUIVALENT MODEL OF DIELECTRIC TEST SPECIM
SFor boron surrounding substrate (one end only):
mean radius of boron - 1.125 x 10-3 inch
A - 21 (1.125 x 10 3)(2.54)(0.635) - lL.4 x 10-3 cm2
= 0.25 (2.54) = 0.635 cm 1- (2 - 0.25)(10 3)(2.54) - 4.45 x 10 3cm
.B = 1.2 x 10 A - Cm -: (1.2 x 106)(4.45 x 10o-) 6RB - OB A 11.4 x10- 0515 x 10
II I2-0 3
B- |
iI
S~or for both ends
Ra W 1.03 megohms"IThus, agreement is obtained between the calculated and measured.
If the boron does, in fact, breakdown completely, then the final valueI of resistance should be that of the substrate.
For an approximately 5 inch length of spccimen (pure tungsten substrate):
-6 2
0 O- 5. 5 x 1O06A - cm
3 A - 1.265 x 10t6 cm2
t= 5 (2.54) - 12.7 cm
SRw - =w Lfi (5.5 x 10 6)(12.7) - 55.2 •2A 1.265 x 10"6
I It is known that the substrates are not pure tungsten, but are borides oftungsten. Measurements at test and dynnmic impedance plots have shownthese substrates to have an impedance value approximately 10 times thatof pure tungsten. Therefore, this 5 inch length of substrate shouldmeasure about 550 ohms. Since the measured values were in the order of500 ohms, close agreement between measured, and antic4 pated, is observed.
- 2.2.3 DYNAMIC IMPEDANCE OF FILAMENTS
The dynamic impedance of boron filaments subjected'to 2 amp, 5 amp and7 amp curtent levels (4.3 x 2 2 Mts wave) have been obtained from the oscillo-grams taken during current irjection for the degradation studies. Themedian values are plotted in Figujre 2-14. Also shown are the maximum andminimum value measured at each time increment. It will be noted that ifeach of the curves obtained is extended back to t = 0, they all convergeat 1300 - 1350 ohms;,indicating that the median value of this specimentype before test is in order of 1300 ohms. The lowest value measured atthe time of test was 1396 ohms, but it has been established that goodelectrical contacts have not been achieved. A first look at these curvesraises the question that perhaps the negative temperature coefficient ofresistivity for boron is more significant than first thought. Also,preliminary analysis indicates that the substrate is not pure tungstlh,
as previously known, so the magnitude of the spread between resistivitiesof the boron and substrate core becomes a question of prime importance.
A similar set of dynamic impedance curves for the HMG-50 graphite yarnfor exposures of 13 amp, 132 amp, and 163 amp (3.12 x 2 2 Msec); and 30U amp
* (3.5 -33 1Lsec); and of 500amp ( 2 . 4 - 2 5sec) are given in Figure 2-15. The
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Imedian values are plotted, but also shown are the maximum and minimumvalues measured at each time increment. S
I
The dynamic impedance curves for graphite tows subjected to 625 amp, 3000amp, and 5000 amp current levels is'given on Figure 2-16. Dynamic impedancesfor boron unidirectional laminates subjected to 2600 amM 3900 amp, and 5200 Iamp current levels are plotted on Figure 2-17. At the 3900 amp level halfof the samples exhibited a partial breakdown in impedance. Therefore,Figure 2-17 shows three plots at the 3900 amp current level: the upper Iplot is of those specimens showing no breakdown effect, the lower plot isof those specimens showing breakdown effects, while the center plot is a
median of all specimens at this level. I2.2.4 CURRENT FLOW TRANSVERSE TO BORON FILAMENT DIRECTION
Some testing has been done on boron unidirectional ,transverse specimens(fiber direction 900 to that of specimen, length). The specimens weresupported in a test jig in the same manner as all previously testedspecimens.
The 90°0 boron filament orientation will not provide a conductive pathuntil a resin breakdown or a flashover occurs. This breakdown voltage,therefore, becomes a determining factor.
The test circuit used is shown in Figure 2-18. This circuit allows acontrolled voltage wave to be applied to the specimen. In order to .establish a conductive path through one of these 6-inch long specimen inthis configuration, it 'is estimated that approximately 1.5 inches of resinmust be broken down.
From the dielectric strength curves just presented, it can be seen that300V/mil would be a reasonable figure to work with for resin dielectricstrength.
The required voltage for breakdown would, therefore, be (1500 mils) xc300V/mil) = 450 kV. A voltage of this magnitude could be expected tobreakdown a 6-inch air gap. Therefore, with the specimen present in thegap, flashover could be expected at voltages considerably below this 450 kVvalue. This expectation became reality when flashover of the test pieceoccurred a' 100 kV.
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LightningCurrentGenerator Vo~tage
Divider a
L -- Test Specimen
To Dual--- •Current Beam
Wave scilloscope
Measurement
Shunt
SK7 Voltage
i FIGURE 2-1!. TEST SET-UP FOR CURRENT FLOWTRANSVERSE TO BORON FILAMENTS.
I2-27!1
I
2.2.5 BASIC ELECTRICAL Cu1RRENT FLOW I•DOS
As reported in Reference 2 by Philce-Ford and General Etectrtc, a model was
envisioned for the case of an electrical current impulse introduced into Ione jide of a composite, or into one ply. This model is reported below:
I,current flow
RI
Slightnfi strike
= ------ - current out
RF " resistance of Rr 3resin msatrix on
RI.2,3 - resi~stances of filament*in successive layers I
FIGURE 2-19. INITIAL UODSL OF CURRX.(f Fi.AM INA UNIDIRECTIOMAL COQW)'HSE. I
Results of this program to date now allow for mcdifications to this model 1and for the introduction of ochers.
Observation of current flow during impulse terts have shown that thefilaments do not fail at the current crest. 'Chat is to say, the current -waveform is centinuous.
As discussed in Section 2.3, boron filament ftilure probably results fromthe differences in thermal expansion between the substrate and the surroun-
ding boron. Furthermore, the severe transverse and axial cracking of boronfilaments has been observed to occur sometime after the current impulse hasceased.
It has also been observed dur ng this program that the resin matricesexhibit a definite voltage breakdown level. W.ith this much informationthe initial model may be modi~fied as, Figure 2-dO.
The zener diodes represent the breakdown strength of the resin matrix andRI, R2, and R3 are again the filament substrate resistances. This model,however, only applies te the case of an electrical current impulse intro-duced into one side of the outermost ply of a boron/epoxy unidirecti.onal
2-28 1!I
I
Point of current flow __
'i•• ,•300v/rail
2R•: , " /VV•"= - "' •current out
S300v/mail
FIUR 2-20. FIS OIIAINOF IKILCRErF(IMDEL1OF FTUE2-19.
I laminate., ,It assumes that the first stroke or pulse of cur~e~t is carried
by the outer ply of filaments. When the threshold current of degradation ofthat ply has been exceeded, the filaments fail by transvers~e or axial
-, cracking, which interrup~ts the current flow path by increasing the resis-I ~ tance of that ply. As t~he resistance increases so does the voltage drop
" ~along the ply until the breakdown level of the resin film 'between plies ofS~filaments is reached. After this interply resin punctures, the current
S~goes to the next, or deeper, filament or layer of filaments. This process.. is assumed to be progressive if multiple strokes are applied.
'" More basic than this composite current flow model is that of a singlei " boron filament. Shown in Figure 2-21 is a unit length model of such afilament.
If contact is made directly with the filament substrate, then predominately-- all of the current flows in the substrate ',Rs). This results from the fact
that the conductivity of the substrate is much greater than that of the' ~boron (R. << %R). If contact it•, made with the boron only, then current
flows in the b~oron (RB) until a sufficient voltage appears at some pointte cause the avalanche effect noted and reported in Section 2.2.2. Thisavalanche breakdown effect is represented, in the model, by the zenerdiode and shunt resistor. This model is incorporated in the initial modeipresented, but is reduced simply to R.. The reduction is valid since the20-25 volts required to breakdown the boron would surely, be present oncethe interply resin punictured.
If this simplified model of the filament is again employed, a general• model of a boron/epoxy unidirectional laminate can now be developed. This
II modei considers the larainate on a per unit length basis, and allows currentI "" entry from any point. (Figure 2-22).
SIn this general model all filaments are represented simply by k, form. reasons just explained;, all interply resin layers are represented as
/ zener diodes, which effectively model their breakdown strength; and all
.2-I
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FIGRE2-0. IRT )DIICT ON F NIIALCURET FOW~(DE
[.I~ - L
Boron lament
"A =R Ii
AR ARB ARe
RS ARS~A B B
AL
Equivalent Model Of Boron Filament
FIGURE 2-21. UNIT LENGTH CURRENT FLOW 1VDEL FORA BORON FILAMENT.
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outer resin layers are modeled completely in order to allow current entry
from any point on the surface. A current flowing at the specimen surface
would cause a voltage rise across ARr (resin resistance), in parallel
with the zener diode. Once the resin breakdown voltage is reached, as Irepresented by the diode, a direct low resistance current path to the
filament is formed.
It should be noted that the model delineated here is that of a finite Iportion of the whole specimen. Although construction of a model of the
entire specimen is possible, it would be extremely complex.
2.2.6 INITIAL CONS IDERAT ION OF LVRRENT FLOW PARAMETERS THAT AFFECT
BORON F ILAMENT DEGRADAT lO0
For further discussion it will be assumed that current is injected directly
Into the filament substrate core and that the current is carried uniformly
by all filaments within the specimen. Consider now any two filamentswithin a laminate (based on the general model preseAted in Figure 2-22), Ias shown in Figure 2-23.
AQ-Rsm ~-I
FIGURE 2-23. SCHEMATIC OF XWO FILAMENTSWITHIN LAMINATE.
it will be noted that the filament is made up of incremental sections.
That is the total filament resistances are: JR = L LR (The subscripts m and n are used only to'
facilitate the discussion, by generatingR = L.R a means of differentiating between the two I
S• S substrates.) Recall also that the zener
diodes represent the breakdown strength of
the epoxy resin separating the filaments. I
2-32 1
I
i The current flow process for the particular case when current was injectedirto the outer ply has already been presented. In addition to that singularcondition, it now becomes desirable tu explain the current flow processes ingeneral. An explanation of the reduction in impedance seen when higherlevel current pulses are injected is also desired.
Until now all discussions have considered the substrates of boron filamentsto be homogeneous, and thus behaving as uniformly distributed resistors asshown in Figure 2-24. Most investigations, however, have shown that thesubstrates are not pure tungfnten, but bor4.dea of tungsten. In fact innearly all cases there is no free tungsten at all within the substrate. Itmay, therefore, be assumed chat the resistance per unit length af eachfilament will be inconsistent resulting in a non-uniform resistor, and theoverall effect will vary from filament to filament.
Return now to filaments m and n of Figure 2-23. Since these are twotypical filaments, it is entirely possible and probable that their makeupsare different with respect to substrate composition and that neither one isuniform. This situation is shown io Figure 2-25.
There are two distinct possibilities as to how the specimen initially fails.The first of which is depicted in Figure 2-26. Here it can be seen thatbetween points A and B of the curves for filament n, 25% of the total voltageis supported by approximately 7%o of the specimen length. This amount ofstress could possibly cause a breakdown (flashover) within the filament andthus initiate a "snowball" effect. Just what results after this initialfailure is unclear, mainly becý.use of the complexity of the situation. Thebreakdown would result in a 25% reduction in resistance, with a simultaneousincrease in current through the filament and voltage rise across it. Atthe same time, only 93% of the initial specimen length is supporting thevoltage.
-- It must be pointed out that the failure need not occur at the filament end,the stressed region of other filaments might be internal from the ends.Thus, simultaneously and continuously the current magnitudes and voltagestresses are changing within a laminate, resulting in an extremely compli-cated situation.
The second possibility for initiation of failure is depicted in Figure2-27. Here it is seen that the two filaments under consideration have adifference in potential between them, at one point, which is equal to 45%of the voltage across the entire specimen. Once this potential differenceexceeds the breakdown strength of the epoxy resin (approximately 300V/mil)the resin will puncture.
2-31"'I
I,
I
"t of . 80 OfFilament I Yoltaqe AcrossResistance 60 - - .0 peci
40 -- ~ ~-- 40J
2020
o._ 100 io 20 30 40 50 60 70 80 90 1.00
% of Specimen Length
FIGURE 2-24
Resistance and Voltage vs. Length of Speci~men
2Il
2-34I
IS .... I
III
1.00 " I - I100
80%- of---- 3-_ 0 %QOf,i 1aaent Voltage Across.:s6an• 60 -- r re t• "60 Specimen
40 40
20
2,0, 00 10 20 30 40 50 60 70 80 90 100
% Of Specimen Length
FIGURE 2-25
Resistance and Voltage vs. Length of Specimen
2-35
I
LI
I'• I
I,, I
--- -- -- -• -I 8100 B100
II 25%
Saf eFiltment m - %F-i Ia men t 0 1 i i>0ý • ... , , I 1 6o Veitage
Resistance iI Acro••Across
40- "100'00040 Specimen
2A ZO 20
I e tInI I00 10 20 30 4 5 60 0 70 6. M T
% Of Specimen Length
FIGURE 2-26 JResistance and Voltage vs. Length of Specimen
III
2-36 1
II
!I
/I¸
I
S~Filament
-0 - 80% Of I %Of
Filament -0 60 45% Voltage
Res istanceL Across
"" 26•' 20
20• / iFilament n0 0
0 10 1020 30 40 50 60 70 80 90 .100
SOf Specimen Length
-- F•OPRE 2-27
Resistance and Voltage vs. Length of Specimen
I2-37
IA
I
Once again, the sequence of events after initial failure are complexand unclear. The two filaments at the point of puncture seek the samepotential which is determined by the parallel resistance combinationsof their two halves, and again the currents through the filaments andthe voltage stresses across them change.
It also becomes evident that combinations of these two nichavisms canoccur simultaneously, and to varying degrees, adding further complicationsafter failure initiation. Clearly these initiation factors are of primeimportance for this study.
2.2.7 FORCES ON SPECIMENS CAUSED BY INJECTED CURRENTS
High currents, injected into some graphite fiber composite specimensduring this investigation have caused a separation of fibers. This hasbeen witnessed as varying degrees of "ballooning" and exfoliation offibers. A question which now arises is how the "ballooning" relates tothe current.
Inside of a conductor itself is a mechanical force exerted between themagnetic flux and the current-carrying conductor. The force between the0.0003 inch diameter graphite filaments of the conductor is an atttaction,so that current in a conductor tends to contract or "pinch" the conductor.This being the case, the resulting "ballooning" which is observed wouldnot result directly from the current flow, but is due perhaps to a secondaryeffect such as gas vapors escaping from the pyrolyzing resin materialsat an extremely rapid rate, as discussed in Section 2.3.
2-38 1
I
I
In2.3 DEGRADATION OF FILAMENTS AND COMPOSITES
In accordance with the test procedures discussed in Section 2.1, severalseries of tests were performed whereby electric current of varying crest
amplitudes and waveforms was injected into filament and composite specimens.
The filament specimens included boron monofilaments, HMG-50 graphite yarn
and HM-S graphite tow. Also, as previously discussed, unidirectional epoxy
resin matrix specimens of each filament type were similarly exposed. Crest
amplitude and waveform as well as resistivity before and after test wererecorded. Visual observations during and after current injection were
noted. Also, current applied per unit cross-seitional area of the specimen
as well as per unit cross-sectional area of filament in the composites
were calculated. From each set of typically six specimens, two were used
for photomicrographic inspection and four were tested for strength sub-
sequent to the current injections. All such data is tabulated in the
appendix of this document. Following is a discustion of the degradationobserved and interpretation of the degradation mechanisms involved.
2.3.1 BORON FILAMENT AND COMPOSITE DEGRADATION
In order to determine the limiting factors relative to boron-epoxy compositedegradation, both the composites and filaments were subjected to electric
current injection. The tests and inspections to determine degree of
degradation, causes of degradation, and mechanics of degradation as well
as degradation models developed are summarized as follows:
a. Strength Degradation of Boron-Filaments and Composites. The
tensile strength of exposed boron filaments was determined subsequent to
electric current injection. The results were compared to the tensile
strength of non-exposed filaments from the same lot and roll of filament.
The typical results for the standard waveform are illustrated in Figure 2-28.
Typically the tensile strength of the filaments was unaffected until a
current crest amplitude in excess of approximately 3.7 x 104 amperes per
cm of filament cross-section was injected. The strength degradation then
was rapig mith tot4l-dts~ntegration occurring when crest currents of 7.5 -8.0 x 10 amperesrper cm were injected. These filaments disintegrated
into varying lengihs of segments that were often split axially.
Similarly, the typical degradation of boron-epoxy composites as the result
of current flow per the standard waveform is illustrated in Figure 2-29. The
threshold for degradation of the composites appears to be in excess of a
crest current injection level of 5.7 x 10 amperes per cm 2 of filament cross-
section. The current densities were computed by dividing the crest current
level, by the total cross-sectional area of the filaments within the composite.
As noted, the strengths dropped rapidly until a crest current density of
7.85 x 104 amperes ner cm2 was achieved. Attempts to 2 test at current injec-
tion levels of greater than 12.3 x 10 amperes per cm (where the composite
was degraded to 39% of its original strength) resulted in flash-over
2-39I
L
across the 9.)ecimen. it is noted, however, that the compositesdid not degy.ade as severly at the same average current densities as wasnoted in tVe results of the siogle filament tests. This can be explainedby conside ation of two points. First, even though the filaments withinthe compoijite may be split and cracked, they still will reinforce the epoxyresin to a degree and the compotite will have some (but reduced) structuralintegrity. Secondly, the current is not distributed eqnally among eachof the boron filaments within the composite. The filaments do carry thecurrert because of the highly non-conductive dielectric nature of the epoxyresiro itrix. The boron filaments typically have a resistivity of 4-|8 x 10 ohm-cm. As till be noted later, degraded boron-epoxy compositesshrow evidence that oniy a portion of the filaments were degraded. It ispfesently thought that the degraded fraction carries the majority of thecurrent and that some of the filaments carry little or no current. The (1degraded filaments over the 0.025 inch x 0.50 inch cross-section of thecomposite specimens occlored at random. This occurred even though thecomposite ends were scarf'64,e vapor honed to expose all, filament ends and Iover plated with nickel for a1ectrtc. I contact.
xi
In addition to the fact that the\electric current is not uniformly dis- jpersed throughout all filaments within a composite, it also appears thatthe waveform may be a more important factor in degree uf damage. than isthe crest amplitude of current injected. In the majority of the tests jperformed, a typical injected current waveform shape was utilized and theexposure levels were changed by varying the crest amplitude of the current.T-his can be a misleading approach. The degree of degradation is not solelydependent on the crest amplitude. instead, it is dependent an the total !energy dissipated within the specimen, which in turn is proportional tothe total energy injected and the particular wave shape or wave form.The total energy injected is varied by lengthening or shortening thecurrent waveform in addition to varying the crest amplitude. To demon-strate this, current injections of boron filaments were accomplished atthe same crest but with different waveshapes. To accomplish this, thefront time and tail times of the waveshapes were varied. When the waveformwas varied from the standard tf = 2.5 - 4.3Usec and t = 22 - 25usec, thedegree of degradation varied from the typical valuesof Figures 2-28 and2-29. Typical examples of the influence of waveform on degree of degrada-tion are as follows:
PercentCurrent Cress, Strength
A.nperes per cm Front Time/Tail Time Degradation
(1) 8.88 x 104 1.2/13.5 71
(2) 7.40 x 104 4.0/24.0 100
(3) 7.40 x 104 '"r€0/12.0 13
2-40
I
l
I
100 It 1/3.7
1 95-.90
1 851 80
75-E-4
70
"67/4.44S65
'60-
55
~50-
45 45/5
• 40
35-0
o~ 35
2522/6.17
20
15
10
5
0 1 1 1 1 0/-8 -1 2 3 4 5 6 7 8
CREST CURRENT PER UNIT CROSS-SECTION AREA OF FILAMENT(104 AMPERES/CM2 )
j FIGURE 2-28. BORON FILAMENT TYPICAL STRENGTH DEGRADATION VERSUSCREST CURREr'T DENSITY (FOR STANDARD WAVEFORM)
2-41
I
_ _O_________._ _ ____ I '100 ______ 100/5.7 _____________
' - .
90 -1
' 70
z
S60 60/6.35 1
50 48/10.8S50/7.851 1- 40
S~39/12.3
o 30
r,
Io- I120k
10
,•I 1 I 1 i 1 I 1
5 6 7 8 9 10 11 12 13
CREST CURRENT PER UNIT CROSS-SLCTION AREA OF FILAMENT CONTENT I(1O4 AMPERESACM,2)
FIGURE 2-29. BORON/EPOXY TYPICAL STRENGTH DEGRADATION VERSUS CRESTCURRENT DENSITY (FOR STANDARD WAVEFORM)
2-42 3
'I
As can 2 be'seen in (1) and (2), a crest current density of 7.4 x 104 amperes
per cm produced total disintegration (100% degrgdation) while a highercrest current density (8.88 x 1O0 amperes per cm ) produced only 717 lossin strength because a shorter waveqhape was used, which represented a lowerenergy ipput. Also, as can be seen in (3), the same current density of7.4 x 10 amperee per cw2 produced only a 13% loss in strength with ashorter waveform. Because of these aad similar experimental results, itappears that the most meaningful relationship between ielectric current flowand boron degradation is one which is based on energy input and resultingenergy dissipation, and not solely on crest current amplitude. Furthermore,it was noted during the current injection of boron filaments that distinte-gration did not occur at crest but an interval of time, later.
To further illustrate the dependance of degree of degradation of boronfilaments and composites or total energy, Figures 2-30 and 2-31 weredrawn. In these, percent degradation versus energy dis•ipatiori per unitresistance was plotted for all waveforms and current amplitudes. On thisenergy basis, all data points fall within bounded regions regardless ofwaveshape.
b. Photomicrographic Inspection of Degraded Boron Filaments andComposites. A series of photomicrographs were taken olf boron filamentsand composites that showed a marked dectease in strength. The photo-micrographs were taken in order to better understand the changes and/ordamage that occurred within the filaments as the result of electric currentitJection. Standard techniques were used in taking these photomicrographs.
For reference purposes in examining the photomicrographs of damaged fila-ments, Figures 2-32 and 2-33 are cross-sections of f!laments that have notbeen exposed to current injection. In both of these pictures the 0.0005inch diameter substrate core is readily distinguishable within the 0.004inch diameter boron filament.
Figure 2-34 (Specimen B-20) is a longitudinal view of an actual boronfilament which represents a six specimen set of boron filaments which lost547. of their tensile strength, after exposure to 4 amperes current injec-tion. Note the longitudinal and axial cracks and compare with the unde-graded control specimen in Figure 2-33. Figure 2-35 and 2-36 show aburned spot or hole between the core and boron sheath, corresponding tothe line in Figure 2-34. Also note the irregular cross-section inFigure 2-35.
Figures 2-37, 2-38 and 2-39 illustrate three types of damage occurring inboron filament #50, which had a current injection level of 7.2 amperes.The specimen from which the piece was taken was mechanically tested andyielded a breaking strength 707. lower than unexposed filaments. Surfacespallation from the filament is indicated because of its irregular outer
2-43
I
C '.I r4i_________ ________ (-J .- 4 1-4
x x x x
tn .-4 in en
o±to
04
~0
U)
41
U)
4 -4J
U,
__K~% ______D I "-ý4I
1.4
-C) 0 0I-- C" -
T44 u aI I!; aT!SUOL -U014UPP16-)G % f2-441
I4
______ 4.I ___ _______- - - __ M
-r-
CN 14.0
_ _ _ _ _ _ _ _ _ _ _U_.
0T
-4
4Jj
cc M
2-45
FIGURE 2-.2. (80OX) VIEW OF UNEXPOSED BORON FILAMENT
CROSS SECTION.
Ii
I
FIGURE 2-33. (250X) VIEW OF UNEXPOSED BORON FILAMErT ILONGITUDINAL CROSS SECTION.
2-46 1
IIII
FIGURE 2-34. (125X) VIEW OF LONGITUDINAL CROSS SECTION (F BORONFILAMENT #20 (4.93 x 10 Crest Amps per cm of Filament.)
i. FIGURE 2-35. (10OOX) VIEW OF CROSS SECTION OF BORON FILAMENT #20(4.93 x 104 Crest Amps per cm2 of Filament.)
2-47
I
I.
FIGURE 2-36. (2000X) VIEW OF CROSS SECTON OF BORON FILAME~r #20(Core Region)
FIGURE 2-37. (800X) VIEW OF BORON FILAMEN`T !1z50-CROSS SECTION(8.88 x 104 Crest Amps per cm2 of Filament)
2-48
FIGURE 2-39. (800X) VIEW OF BORON FILAMENT~ #50 CROSS
SECT ION
1 2-49
I
I
I
surface. The internal damage ranges from burned holes to cracks, as can 3be seen.
Figure 2-40 is a photomicrograph of the surface condition of an unexposed 1filament using the replica technique. Figure 2-41 and 2-42 are, respectively,replicas of the surface :ondition of two exposed boron filaments, B-55and B-50. As noted above, B-50 had been exposed to a crest current injec-tion of 7.2 amperes. Both show evidence of surface cracks. The B-55 Ifilament shattered into many >0.08 inch Long pieces as a result of a6.5 ampere current injection. The replica is of one of the pieces.
Figure 2-43 is a 1OOX picture of the surface of an unexposed unidirectionalboron filament composite. Please note that only a portion of each boronfilament is visible because of the surrounding epoxy resin. Figure 2-44is a IOOX view of a boron epoxy composite that has been exposed to adamaging level of current injection. As can be seen, considerable trans-verse and longitudinal cracking is present in the surface boron filaments.Baced on the total number of filaments, the current injection level averaged Iapproximately 7 amperes per filament, a level previously determined to causedamage in single filament tests.
Figures 2-45 and 2-46 are cross sections, respectively of one unexposedand one exposed boron filament composite. The complete 1/2 inch cross-sectional width of three exposed composite specimens is shown in Figure2-47. Examination of these figures yields the observation that specimen IBU 1,26 has the greatest number of damaged filaments and BU #17 has theleast. This is in proportion to their strength degradation as reportedin Table A-5 of the appendix. The tensile strengths of BU #26, #13, and I1:17 after current injection were degraded by 75%, 677, and 10%, respectively.Since the number of filaments degraded in each specimen varies, and causeda variation in strength retention, it is probably safe to assume thatfewer numbers of filaments carried more current in the core of the specimensshowing less degradation. This phenomena is no doubt caused by the electri-cal contact to the specimen ends, wherein the nickel coating is probably Inot in uniform contact with all filament ends.
Figure 2-48 shows a cracked boron filament just under the nickel platedcontact on boron epoxy specimen -:.26. Figures 2-49 and 2-50 of damagedboron filaments in the same composite show tl, typical radial cracking asa result of current injection, as previously reported by Philco-Ford andGeneral Electric (Ref. 2). Again within damaged composite AU #26,Figure 2-51 shows a damaged boron filament that was etched with Murikamis Ireagent to indicate that the core was melted and resolidified. Figure 2-52shows another damaged filament that was etched in the same manner but doesnot show evidence of having been melted due to current flow. Figure 2-53 Iis a view of the core of a boron filament in BU #26. The core has beenlost either due to melting or vaporization as a result of the currentinjection. 3
2-50 3I
IIII
II
FIGURE 2-40. (40OX) VIEW CF UNEXPOSED BORONFILAMENT SURFACE REPLICA.
2S~2-51
FIGURE 2-41. (5OOX) VIEW OF BOR N FILAMENT #55 SUR ACEREPLICA (8.01 x 10i"Crest Amps per cm5
of Filament)
FIGURE 2-42. (250X) VTEW OF BORQN FILAMENT 450 SURFACEREPLICA (8.88 x 10'4 Crest Amps per cm2
.of Filament)j
2-52
L
II
FIGURE 2-43. (lOOX) VIEW OF UNEXPOSED BORON EPOXY COMPOSITE SURFACE
II, _____!
FIGURE 2-44. (100X) V-V OF EXPOSED BOR05 EPOXY COMPOSITE SPECIMEN #13(ý x 10q Crest Amps per co, of Filaments)
2-53
L
I.
Ii~I
I
I
FIGURE 2-45. (10OX) CROSS SECTION VIEW OP UNEXPOSED BORON EPOXY
COMPOS ITE.I
FIGURE 2-46 . (100X)CROSS SECTION VIEW OF EXPOSED BORON EPOXYSPECIMEN ':26 (9.18 x 1.0 Crest Amps per cm1 ofFilaments) 3
2-54.-
IOP
I , 0 OI 00gq 0c0000 A. 0000100
0. 0,004
I4ý910 6000go 000
00000 0000
0000 00 0.4,900,**4qbI
(86 0 CRET A*S ER M2 F FLAMNT
BORONi UNID4RECTIONAL LAMINATE SPECIMEN # BU-13(8.64 10 CREST AMPS PER CM2 OF FILAMENTS)
17
1 .1
2266
a * 0
FIGURE 2-47. POLISHED TRANSVERS IWOE-13ENTIRE 1/2" WIDTH OF BORON/EPOXY TENSILE
SPECIMENS AFTrER ELECTRICAL CURREINT INJEC-boN.
2-55
FIUE24.I0X IW FDMGDBRNFLMETI U#6CMOIEATPITOIOTC WT IKLPAIG
FIUR _ _ __8O)VEWO LCRCLL AAE BRO IAETI
FIGURE2-4. COMPOS ITEW OFEDAMAED BORO FI#E26NBU#6.OP~f~
I ATPOIN OFCONTCT WTH ICKE PLAING
WAo
3FIGURE 2-51. (2000X) VIEW OF CORE OF DAMAGED BORON FILAMENT (BU #26)THAT RAS BEEN ETCHED TO SHOW EVIDENCE OF MELTING.
1 2-57
I
II
S~I
FIGURE 2-52 . (200OX) VIEW OF DAMAGED BORON FILAMENT (BU #26) THAT IHAS BEEN ETCHED BUT DOES NOT SHOW EVIDENCE OF MELTING I
/
\1
MAJOR PORTION OF THtE CORE HAS BEE','. OST DUE TO ME•LTINGOR VAPOR IZAT ION.
2-58
1
II
Figures 2-54 and 2-55 show 80OX views of the cross sections of electri-cally damaged boron filaments in composite specimen BU #13. Figure 2-55iu of special interest because it shows evidence of diffusion of someof the core material into the surrounding boron.
Figures 2-56 and 2-57 are of electrically damaged boron filaments in com-posite BU #17. Figure 2-57 has been etched to show that the portion ofthe 1/2 miL core remaining was melted and has resolidified. As can beseen, the remainder of the core ran into the radial cracks where itresolidified.
For one set of unidirectional boron ,poxy composites, current was purposelyinjected into the outer plies only in order to determine whether damagewould be restricted to the filaments in those plies. Instead of nickelplating the specimen ends, aluminum foil tabs were bonded to 1/2 inch ofthe ends of each specimen to the surface plies only. Conductive silverfilled epoxy paint was used for bonding the tabs to the specimen. Theelectrical contacts for current injection were made to the tabs. Onespecimen each then was injected with the standard waveform at crestcurrent amplitudes of 2500, 3220 and 4370 amperea as tabulated in Appendix A.In the previous tests of specimemwith nickel plated ends, it had beendemmonstrated that the 2500 amp crest was at the threshold above whichdegradation initiated. In this case it had been speculated that ifcurrent were restricted to the outer plies that the much higher currentload per filament would result in severe degradation in those plies.Instead, however, the 2500 ampere test resulted in no apparent filamentdegradation thereby indicating that the current must have been distributedthroughout the filaments in the specimen and not restricted to the surfaceplies. Photomicrographs of the specimens injected with the 3220 and 4370ampere waveforms are shown in Figures 2-58 and 2-59. As can be seen, thespecimen injected at the 4370 ampere crest perhaps has a higher incidenceof filament degradation in the outer plies, however, both specimens havefilament damage in the interior two plies. This migration of current,as evidenced by the damage, from the exterior plies to the interior pliesis probably because of close proximity and random contact points of fila-ments. In other words, the dielectric resin barrier between plies offilaments does not appear to be uniform to a degree that will result incurrent being restricted to the plies in which it was injected.
c. Scanniin Electron Microscope Inspection of Degraded BoronFilament. Boron filament B-13 was selected for scanning electron micro-scope (SEN) inspection because it was one of a group of filaments thatexhibited 50-75 percent degradation as the result of current injection.The filaments had not segmented or split even though their strengths had beenreduced. It was thought that the SEI might show some micro-crack or faultinitiation that at higher current levels results in splitting and segmen-j tation of #-he filaments. In a similar manner, the replica technique
2-59
L A
FIGUE 254. 80O) VEU O EL~rRIALL DAAGEDBORN FIAMET'TCOMPSITESPECMEN U #1
..... ...
FIGURE 2-54. (800X) VIEW OF ELEcrRICAL.LY DAMAGED BORON FILAMENT INJCOMPOSITE SPECIMEN BVJ #13.
2-601
FIGURE 2-56. (800X) VIEW OF ELECTRIC.ALLY DAMAGED BORON FILAMENT INCOMPOSITE S'PECIMEN BU #17.
Vd-
FIGURE 2-57 (2000X) VIEW OF BORON FILAMENT IN BU ;;17 COM'-POSITE WHERETHE CORE HAS MELTED AND RESOLIDIFIED.
1 ~2-61
'I
Vii
In ~I .
I
I
FIGURE 2-5 9 (10OX) BORON/EPOXY BUN-2 SPECIMEN IN WHICH A 3220AMPERE CREST IMPULSE WAS INJECTED INTO OUTER PLIES.
2-62
FIGURE 2-59. (LOOX) BOP.ON/EPOXY BUN-i SPECIMEN IN WHICH A 4370 1AMPERE CREST IMPULSE WAS INJECTED INTO OUTER PLIES.
2-62
\I
I photomicrographs of totally degraded filaments, Figures 2-41 and 2-42,had shown surface cracks. Figure 2-60 shows a non-degraded non-injectedboron filament surface. at 1700X by the SEN. Figure 2-61 is a boronfilament that has been exposed to a 6.1 ampere crest waveform. Four otheridentically exposed specimens from the same group exhibited a 50-75A degra-dation in tensile strength. If this strength decrease is caused by theinitiate.on of faults they are apparently not in the form of surface cracks.The identity of the protrusious (white spots) is not known, however, theyare in evidence in both the non-injerted and injected filaments. FurtherSEN scans of filaments exposed to higher current injection levels shouldbe performed.
d. Electron Nicroprobe X-ray Analysis of Degraded Boron Filament. Anelectron microprobe X-ray analysis was performed on two boron filamentsfrom unidirectiou..l boron composite specimen BU-26. Both of these filamentsexhibited evidence of having had a molten phase irr their core as theresult of current injection. This was manifest in photomLcrographs ofcross sections of the filaments where the presence of the substrate corematerial in adjoining cracks in the boron phase was suspected. Electronbeam traverses were performed across the 0.0005 inch diameter substratecores and permeated cracks of two filaments to confirm location and con-centration of boron and tungsten. The core composition and profile werecompared with similar traveyses on a control--unexpose'd boron filament.
'Electron mdcroprobe X-ray images were also made of the respective crosssections to give a visual rresentation of the location of boron andtungsten in Lhe filament cross section.
Electron bean traverses were performed with'an electron beam energy of15 KV and 250 nano-amps. Beam diameter was 1 micron witb a steppinginterval of I micron. That is, the beam was stepped across the sample inintervals of 1 micron steps. At each step the relative intensity, of% and Bgradiation were simultaneously integrated and printed out on ac rt recorder. Through the use of standards of tungsten and boron andradiation absorption corrections, the recorded intensity data were reducedand calculated as concentrations of the respective elements. Theytraverseacross a sample then making a concentration profile of the desired elements.
Electron microprobe X-ray images were made by scanning an electron beamacross an area of the filament and displaying the relative intensity of theSand B% radiation on an oscilloscope. Pictures of the oscilloscopedisplay, then depict, in a qualitative manner, the location of W and Bin the filament cross section. There X-ray images were made using a1 micron diameter beam operating at 15 KV and 500 na. A beam sweep rateof 25 sec/cm was used to generate a picture approximately IOOOX magnifica-tion.
2I 2-63
I
FIGUE 2-0. 170O), ,-LETRaMICRSCOP SCN OFUNEXOSEBORONFILAENT SRFAI(40 scondscan
FIUR 26L (70X EECRO MCISCP
FIGUREN-I (-60 second) scan)R
MICROCOPE CAN F U2EP-SE
II
Shown in Figures 2-62, 2-63 and 2-64 are photomicrographs and X-ray imagesof the control and the two degraded filaments from BU-26, respectively.The photomicrograph in Figure 2-63 clearly shows the core material in aboron crack adjacent to the core. A similar condition, although not asclear, was seen in Figure 2-64 in t:he crack designated by the arrow.
The sample current images shown in Figures 2-63 and 2-64 confirm thefilament orientation in relation to the cracks. The respective ' X-rayimages in Figure 2-63 d and e and Figure 2-64 d arnd e indicate t epresence of tungsten in the cracks shown in photonticrographs.
The path and direction of the electron beaw traver3Cs are also shown in* Figures 2-62, 2-63 and 2-64 on photo-icrographs taken after the electron
microprobe study. In both traverses across the cra:ck, s't.•r spikes oftungsten were recorded with corresponding decreases in boron concentration.The tungsten in the crack had a composition similar to that of the corenear the boron interface, e.g., WB12 or WBI1 2 + B. No differences in con-centration gradients or composition were detected in traverses across thethree cores. or 72-45. The diffusion zone associated with the tungstencore was 18 to 20 microns in diameter. The outer 2 to 3 Acrons of thecore having a high boron composition in excesa of WB12 . A narrow region ofcomposition corresponding to WBI, adjoins this outer area. The majorcomposition of the cores was contined to a diameter of 10 to 12 micronsand corresponded to W2 B5 . No pure tungsten nor compositions correspondingto UB or W2B were detected in or near the center of the core. A schematicof the core atialysis is shown in Figure 2-65.
Shown in FL-•ure 2-66 is the phase diagram for W-B recently reported byRudy and co-workers (Ref. 3). lit this diagram the boron filament corecomposition is designated to exist from W2B5 to WBA2 + B. The melting pointrange of this compositional zone lies between 2150 and 19700 C.
The'tungstei, boride composition of substrate core material located inadjoining cracks of boron was found to be equivalent to WBE 2 or WB12 + B.This compcsition closely corresponds to the outer composition of the coreadjacent to the boron phase. The major composition of the core correspondsto UW •B The melting point of the core compositions are between 21500and I9)0oC.
2-65
I
, ,
a. PHOTOMICROGRAPH OF ELECTRON b. SAMPLE CURR~ENT IMAGEBEAM PATH
IU1NGSTEN MX-RAY IMAGE d. BORON K cvX-RAY IMAGESEC-1
FIGURE 2-62. (10OX) PHOTOMICROGRAPHS AND X-RAY IMAGES OFCONTROL SAMPLE OF NON-EXPOSED BORON FILAMENTfl.
2-66. I
a.P110OTOMCROGRAiPH, 100OX b. PI{OTONICROGRAPII OF TRAVKRSE
SAMPLE CURRENT IMAGE d. TUNG~CSTEN M
FIGUJRE 2-63. (iQOOX) PHOTOM~ICROGRAPHSAND MICROPROBE X-RAY IMAGES OFBORON FILAMENT FROM SPECIMENBU-26 (9.18 x t0 CREST AMPS PER1 CM2 OF FILAMENTS)
I / 2-67
BORON KaY
FIGURE. 2-4 10X HOOIRGA
a. ~ ~ ~ ~ ~ ~ ~ 91 xiYOICORP 10 IOO .PO~ CRESTMPS OFTAERSE O
SAOPLE CKa E IMAGE d.T-6T8 ~IMG
I
I BORONk'B12 + B
W kB12 W2 B5 CR
II
BORON VSBECRBOUNDMRY
O0
so-I
08 -40 ,
20 B
016 12 10 8 6 4 2 0 2 4 6 8 10 12 14 16
MICRONS FROM CENTER OF CORE
"- FIGURE 2-65. MICROvROBE ANALYSIS OF BORON FILAMENT CORES
2i
; l 2-69
rU
A INCIPIENT MELTINGO SPECIMEN COLLAPSED* SHARP MELTING (TINC = TCOLL.)
3400 -M BY DTA0 TWO-PHASE (SOLID STATE)i SINGLE PHASE
!ME MoB PARTIALLY DECOMPOSED
3200
3000
2800 ICOMPOSITION10 0
2600 A, 0Ao\ '
ra1 0
SII 10Z --"'2400 a 0.',o ,
2200 I0.L------N-iII \,
il • • II o I
w .wB I"tWB " "2000 2 12 II CLWB 01+ aWB 1 . &R
BI II W+,a +WB g+W2 B :1w 2 B5 It~ -~WB B-I -- 124 12
II I I I . I I I I I ,
0 20 40 60 80 100W - ATOMIC % BORON-- B
IFIGURE 2-66. W-B: MELTING TEMPERATURES, DTA-RESULTS AND QUALITATIVE
EVALUATION OF SOLID STATE SAMPLES. QUENCHED ALLOYS
2-70 1II
II
e. Deradati2 o Nodel for Electric Current Impulsed Boron FilamentIEjox Composites. The degradation of the boron filament epoxy compositedue to electric current impulses is controlled by the response of theboron filaments to current flow. It now appears that the concept involvedin the initial boron filament degradation model is correct. That model, asdiscussed in Reference 2, is based on the fact that the core of the fila-ment (tungsten boride) has a much lower resistivity than does the boronsurrounding it. The electric current then ftows predominately through thecore. Some electric energy then is dissipated within the core as heat.The heat build-up causes a thermal expansion of the core both radially andaxially. The outer boron sheath then acts to restrain the expansion ofthe core, which results in a build-up in hoop and axial tensile stresseswithin the boron. These tensile stresses are additive to the residualstresses that are, in all probability, present as the result of the manu-facture of the filament. At such a point when the internal stressesexceed the strength of the boron, the boron sheath cracks either radiallyor transversely, whichever the case may be. Efforts have been made toimprove mathematical interpretation of the degradation model. The evidenceof melting, vaporization, and diffuslon in the core region appears to be insupport of the probability of intense thermal energy build-up within thecore.
"Assuming this failure model, the degree of correlation with failures noted
in experimental tests can be determined by calculating failure stresses atdifferent current levels. In calculating the relationships of current flowand failure stress in the boron using this analytical model, the followingassumptions were made:
(I) That virtually all of the current flows through the core,
(2) That the time duration is sufficiently short to precludeheat transfer between the core and the boron,
(3) That all energy dissipated in the core is in the form of heat,
(4) That both the boron and the core are homogeneous.
As the current passes through the core, resistance heating causes thecore to expand against the sheath material creating excessive radial,tangential and axial stresses. (There are already present in the filamentresidual stresses that formed upon cooling the filament to room temperatureafter its growth at elevated temperature. These stresses are discussed inthe Appendix.
The following is an estimate of the maximum temperature rise which can beexpected in the core when subjected to current input in the form of a pulsewith a front time, tf 3.121A sec. and a tail time tt =221 sec.
2-7i
!
II
"An expression for the rate of development of heat in conductor requiresan expression for the powec input to the circuit. The energy given by
the charge is:
dw - dq (voltage drop) = 1,dtAV
and the rate at which energh is given up, or the power input P, is
doew Is~dt
If the current ts in amperes and the potential difference is in volts thenthe pow'er is in joules/sec or watts.
In the spk•cial case in which the conductor under consideration is a pureresistance'\R, all of the energy supplied is converted to heat, and then
AV LR, an'
2ýiAV =i M~(1
For most materials including tungsten and tungsten boride, (Ref. 5) thevalue of elecrtrical resistance, and therefore the rate of heat production (increases with increasin temperature. For some materials, notably boron,(6)carbon and fusled silica, it is negative. The temperature dependency of
the various constituents of the boron filament are presented in Figure 2-67.As an initial approximation we shall assume R is constant for the core andequal to
pj/A where 0 = 21.0 x 10-6 ohm-cm. IEquation (I) is derived for a constant current level. For a varying Icurrent, such as a pulse, the heat dH developed in time dt is
dH = Ri 2 dt
and in a finite time interval from 0 to t the heat is
2H = j Ri 2dt
01
2-72
FI
I
IIS...400T ....T.-- "...
1 ~~~3500 HP
IIM. p.w(W
""00-- jp... . .T - -250t
H .P. (B)
200 -p, TUNGSTENBORIDE I
1500 I __-_ _-_ ___-----_
1000 1',B
500 --- 1--
- LI
0 10 101 100 10 101
p (OHM-C?)
FIGURE 2-67. RESISTIVITIES O& TUNGSTEN, TLUNGSTEN BORIDE AND BORONVERSUS T7MPERATURE
1 ,2-73
I/
or since R is assumed constant H = R I dt0
This integration was performed graphically for a unit peak ampere currentpulse of Table 1, Reference 6 with the result:
H 45P 2ax2 2
. f i 2dt = 14.8 (1 amp 2 - sec0
Consider a single filament specLnen 15 in. in length (38.1 cm) as used Indegradation testing, then P. = 632 ohms.
The specific heat of the tungsten-boride is given in Reference 2 as0.06-0.09 cal/gm/°C and so using a value of 0 4075 the maximum temperaturerise of the core material weighing 6.27 x 10- grams canbe calculatedfrom
H = R (14.8) (I max) 9359 (I max)
and from 2 -6R ~9539 (Ima) x I0"
AT weight x specific heat (6.27 X 104 ) (0.075)
AT= 198.9 (I max) 2 = 358 (1 m 2 oF
This temperature chan e in the core alone will give rise to axial stressin the boron sheath of
7B = (Acore /Asheath) Ecore core ATcore
or
08 -8 6 -62SB = (19.6 x 10"/1256.6 x 10-) 60.0 x 10 (4.1 x 10-) (358) 12max
ýB = 1376 (1max) 2
22-74
II
I
This temperature induced axial stress is additive to the residual axialI stress of 1686 psi (calculated in the Appendix). Therefore,
OB(acial) = 1374 (1 ) + 1686 (2)
Stmilarly, the radial and tangential stresses at the filament-core interfacewhich result from the electrical current may be calculated from:
a - 55,900 + 6R core
r core
oe (I - Ucore)core
and
6 core R core Ccore ATcore
giving
55,900 + 125,800 (Imax) 2 (3)
while the tangential stress at the filament-core interface is expressed by
at = 57,670 + 129,800 (Imax) 2 (4)
The stresses expressed by equations 2, 3 and 4 are plotted in Figure 2-68for values of maximum current.
In sumary then, from Figure 2-68, it is predicted that for the standardwaveshape, hcron filaments would crack radially due to the high tangentialstresses at a current flow of 1.5 to 2 amperes. Al so, the predicted currentflow where axial stresses would cause segmentation is 15-20 amperes crest.In other words, radial cracking shouP.d occur at lower levels of currentflow than would axial segmentation due to transverse cracks. The predictedcurrent levels where these cracks will occur does not correspond directlyto the experimental evidence reported fo7 single boron filament tests;however, this is probably due to the following:
(I) Uncertainty in the values of resistivity used, particularlyunder conditons of high current loading rates.
2-75
LI
!II
1,000,000--- RADIAL
! ,'-'TANGENTIALSTRESSTANGENTIA-LXIA
S SSTRESS
100,000 - .
CALCULATED STRESS PER_ REFERENCE 1
10,000
10 ,,100
PEAK CURRENT, IMAX (AMPERES)
FIGURE 2-68. CALCULATED BORON FILA)MENT STRESS
2-76
(2) Uncertainty as to the strengths of the boron and core, underthe conditions of heat rie noted.
(3) Thermal. shock effects not accounted for in mathematical model.
It has beent reported previously that filament splitting (due to axialcracks) and filament segmentation both began to occur at approximately thesame crest current amplitudes,. It has been observed though that segmentedfilament:s always also have radial cracks, but there have been some instancesof radially cracked or split boron filaments that are not segmented orotherwise cracked transversely. This then is a probable indication thatradial cracks occur at lower impulse levels than do transverse cracks.The differences in the impulse levels, however, is probably not as signifi-cant as calculated and reported in Figure 2-68.TFurther details of the mathematics of the boron failure model developmentare given in Appendix B.
2.3.2 GRAPHITE FILAMENT AND COMPOSITES DEGRADATION
In the same manner as for the boron filaments and composites and accordingto the previously discussed test procedures, specimens of HKG-50 graphiteyarn, HB-S graphite tow and epoxy matrix unidirectional composites of eachreinforcement were injected with different levels of crest current amplitudeusing the standard waveform.. These current injection tests are tabulatedin the appendix. Following are discussions of the respective tests andanalyses performed to determine degree of degradation and mechanismsinvo lved.
a. Strength Degradation of Graphite Filaments and Composites. As theresult of current injection it was found that the breaking strength ofHMG-50 yar s and HH-S t~w bundles was reduced at crest curr~nt amplitudesof 19 x 10 amps per cm of HMG-50 and 86 x 104 amps per cm of HK-S. Twofactors are believed to contribute to this strength reduction. The first,and probably the most important is the considerable excitation and movementthat occurred at the higher current injection levels. This excitation andmovement is believed to have caused physical abrasion and damage to thefilaments. In the case of the IHIG.-50 yarn, the excitation and movementwas obvious at the 19 x 104 amp per cm level, which also corresponded tothe point at which strength degradation occurred. The excitation becamemore violent at higher current injection levels. At crest current injec-tion levels of 45 -. 56 x 104 amps per cm2 the breaking strength wasreduced by approximately 40%. In the case of the HK-S tow bundles excitationand movement was obvious at crest current injection levels of 29 x 104 ampsper cm2 of filament and higher; however, a reduction in breaking strengthdid not occur until crest current injection levels of 86 x 10 amps per"cm2 were used. Thts is thought to be due to the loosely bound nature ofthe tow bundles within which the filaments are free to move. The filaments
2-77
11
in the HNMG-.50 yarn are, however, twisted and plied together tightly. Assuch, then, very little movwuenrt could caune flamnent damage in the yarn. 3The filament damage in the RMG-.50 yarn as the result of current Injectionis obvious in Figure 2-69.
The other contributing factor to the I{WG-50 yarn and HM--S tow degradation Icould be filament oxidation and heat build-up within the filaments due tothe dissipation of electric current. Example of weight loss in the HMG-50yarn tests are shown in Table 2-2. Since graphite yarns often have aplastic sizing (and such sizings can burn off to create a weight loss), thethennogravimetric analysis (TGA) curve of Figure 2-70 was measured. Aweight loss of only 0.8 - 0.9%. was noted at temperatures to lO000°C. Undersuch inert atmosphere heating, plastic coatings would have reduced to aresidual carbon coating on the filaments with typically a 50% weight lossin the form of effluent Il H2 0 and CO. The 7.5% weight loss of the yarn
2' 2 i<ispecimens then exceeds the amount that could result from loss of sizing,thereby indicating a true loss in weight of the graphite filaments.
TABLE 2-2 jWEIGHT LOSS OF GRAPHITE YAR.N AFTER ELECTRI(AL
CURRENT INJECTIONI
SPECIMEN CREST CVUU24T 0gECFED, WEIGHT CF 8" SECTION, WEIrTNO. 10' AMPS IC GRAM M.O
HMG-L Control (none) 0.0146 I-2 Control (none) 0.0145-3 Control (none) 0.0146-4 Control (none) 0.0149 I-5 Control (none) 0.0146-6 Control (none) 0.0146-7 Control (none) 0.0146 I-8 Control (none) 0.0145-9 Control (none) 0.0148
-10 Control (none) 0.01460.0146 AVG 0
HlMG-80 45 0.0136-83 46.7 0.0134
0.0135 AVG 7.5
IMG-70 59.9 0.0134-78 54.2 0.0136
0.0135 AVG 7.5
2-78 I
41
1*7
FIUR 2-9 RYNIFGAHT IESA EUTOWHPIGATO UIGCRETIJCIN
2-7
0
ý-4 0 F. >
z -4
Vcz~
00
tNz
00
2-80c
I
In the same manner as for the boron composites, unidirectional HMG-50/epoxyand HI4-S/epoxy graphite filament composites were exposed to series ofelectric current injection at increasing crest amplitudes, but with thestandard waveform shape. In the case of the HMG-50 composite specimens,no strength degyadation was observed at crest current density levels ashigh as 13 x 1 , amperes per cm of filament cross-section. At aninjected crest current density level of 20.4 x 104 amperes per Cm2 offilament the specimens burst into flames. The resulting specimensexhibited a 43% reduction ýn tensile strength. The specimens immediatelyafter exposure are shown in Figure 2-71. 4The MG-50/epo~y unidirectionalcomposite specimens exposed to 24.2 x 10 amperes per cm of filament
delaminated in addition to burningas-shown in Figure 2-72. The HMG-50epoxy specimen exposed to 55.8 x I 4 amperes per cm2 of filament crestcurrent burned even more violently and delaminated more extensively, asshown in Figure 2-73.
Graphite HM-S tow/epoxy unidirectional composite specimens 2 exposed atcrest current densities as high as 21 x 10 amperes per cm4 were notdefraded. However, identical specimens exposed at 25 x 10 amperes percm burned and charred, as shown in Figure 2-74. The specimens exhibiteda 46% redu~tion in tensile strength. The HM-S epoxy specimens exposedto 52 x 10+ amperes per cm2 burned and exfoliated severely, as shown inFigure 2-75.
b. Photomicrographic Inspection of Degraded Filament Composites,The Figure 2-79 photomicrograph shows an HMG-50 specimen after 13 x 0'ampere per cm of filament injection. No damage occurred. Figures 2-77,
2-78 and 2-79 are, respectively, photomicrographs of cross-sections ofspecimens shown in Figures 2-71, 2-72 and 2-73. Th increasing, severityof damage4for current injection levels of 20.4 x 10 n 24.2 x 10 and55.8 x 10 amperes per cm of filament cross-section is obvious.
Figure 2-80 shows a non-damaged HM-S/epoxy 5pecimen cross-section thathas been exposed to 21 x l10 amperes per cm of filament crest current.Figures 2-81 and 2-82, respectively, show the damaged cross-section ofspecimens from Figure 2-74 and 2-75.
c. Scanning Electron Microscope Inspection of Degraded Graphite
Filaments. In an attempt to determine if the surface of graphite filamentswas changed in any way due to current injection, some scanning electionmicroscope scans were made. Figure 2-83 and 2-84 are scans of the surfaceof an unexposed HM-S filament and a filament from an H04-S tow that hadbeen injected with 98.71 crest amps per cm2 of filament. Considerabledecrease in breaking strength had been measured for tow specimens from the
same group as the filament in Figure 2-84. As can be seen, it appearsthat the surface of the filament in Figure 2-84 has been altered. Thetwo scans, which were taken under identical conditions, show much less
2-81
L
I____ I
"" I
I
FIGURE 2-71. 1114G-50/EPOXY SCECIMENS AFTER 20.4 x 104
AMPERES PER CM- OF FILAMENT CREST CURRM
INJECTION--showing charring.
/I
FIGURE 2-72. HMG-50ýEPOXY SPECIMENS AFTER 24.4 x 104AMPERES
PER CM- OF FILAMENT CREST CURRE~r INJECTION-- icharred and de•laminated. J
2-87
LI
i
I
t[I
III
I
FIGURE 2-73. HMG-5O/EPOXY SgEC1MENS A•F'ER 55.8 x 10 4
AMPERES PER, CH' OF FILAMENT (:RESTCURRENT INJECTION--extensively charredand delaminated.
F
FIGURE 2-74. HM-S/E OXY SPECIffEINSAFTER 25 x 10 AMPERESPER C- OF FILAMENT CREST CURRENT INJECTION--
showitg charring.
I2
I0 I
• II
I
IFIGRE2-5 H-SEPXY PEIMN3 FTR 2-84 ~ 1fERSPE
FIGURE 2-76. (IOOX) HMGQO0/EPOXY SPECI N AFTER13.04 x 10 AMPERES PER CM OF FILAMENT CRESTCURRENT INJECTION--NO DAMAGE.
FIGUIRE 2-77. (lOOX) HMG-50/EPOXY SPECIMEN AFTER
20.4 x 104 AMPERES PER CM 2 OF' FILAMENT CRLSTCURRENT INJECTION--BURNED AND) IELAMINATEi).
2-85
FIGUE 278. 10O) HG-50EPOY -PECIEN FTE24.2x I4 AMERE PE CW F FILAMNT RES
FIGURE 2-78. (100X) flMG-50/EPOXY SPECIMEAFR
24.2 x 1o AMPERES PER CM2 OF FILAMEINrF CRESTI:RE7 NJECTION--SEVERELY BURNED AND DELAMINATED. i
2-86
.....I....3r
FIGURE 2-80.. (J.OOX) HM-S/EP9XY SPECIMEN AFTER 21 x 104
AMPERES PER CH- OF FILMN CREST CURRENT INJECTION--NO DAMAGE.
-- FIGURE 2-81. (100X) HM-S/EPXYS3PFCIMEN AFTER 25 x 10O4
a. ~AMPERES PER~ CM OF FILAMIENT CREST CURRENT INJECTION- -
SOME BURNING AND DELAMINATION.
2-87
FIGUE 282.(tOX) H-S/PQX SPCIME AFER 2 1AMPEES ER M4 F FLAMNT RES CUREIINJCTIN--EVEELYBUNEDANDEXF~fAEI
2-88
FIUE2-3U16OO)EbTO
*FGURAHE .83. r SURFACEX (4 eL e sRO n )
j FIGURE 2-84. (1.6,OOOX) ELECTRON MICROSCOPESCAN OF SURFACE OF GRAPHITE F:lAMENT FROM
HM-S TOW q#74 (98.71 x 10' CREST CURRENT
AMPS PER CM2 OF FILAMENT) (40 sec. scati)
* 2-89
idg I ng li 91 4 (I SV tf h4' eXO e fp set I1 ilmet I,(. - IT I ýi e~otild po5ssit) Iy be (lie t k)
loqs ot mater i II fr, ýI herl iice Of the Hi Iaitent ýas thle resltt of oxida-F. i or FLAr ther workt w ic it I e scu geIert.roi n it r(Thcip# wotild be reqt Iredbe fore tLae:e observat ions catuld be tatkeri as concins ive. Al so, -icatig, (i
f i I AWe IItts ft'0A (iIf r i ( 'rV t f.'XpIO Sr It e(veI:, stiou I d kidd (I frt. Ihi, I e videnrce i f,Iiti f act. , s ur face i oss (Id tit I, ox i dat: i on Ihas oce uIr red.
d,. erda o Model1 for Elect.r ic; Curr-ent: Imtls~~~ed G~raphaite FL laiif-mt1
~Xy Cm'si The degradatioai mientanti sus ot ' elect.r ical ly i mpulIsed1114C- SO graphite yarn or IIM-'. graph it.etow re) at .Jforced r-orr5i t-jses appearti tobe qu i.te simple .in c ompar i. soi to the~ failure riode I of boronl Miilaent:
recinaforced czompos it es. N tnh e case ,) f t he graph ifte f i~ex rc-inra ferdcomposi~tes, the degrada i oil is riot due to a breakdown of t hii fil.ame'nt".I ustoad , tlhe degradat ion of thte compos it es, when impul.sed at danagixig l evelIs,
is due Fio pyrolIyzat ion ol tLe es in avid irechanicai separation or de lamina-t ion of the fibers one from another. 'Fie sequence for tliis is presentlybeli. eved to be as foll ows:
1I) [flect rc energy f lows t~nrovght the f i laments and crietrgy is1dissipated it. the form of all almost instantaneous hea,.t uui! Jiswithin the gcaphite filarnei~ts.j
(2) At damaging current levelIs the aim'ost instantaneous build-upof hreat caus~ theurral decomposition (pyrolysis) of the epoxyresin that is in f.-ttimate contact with tire Fitlam'entzý. This(lCcoiposkt ~on forms ganeous species.
:1) The rapid decomposition of thec resin~ surroundinF the filamentsinto gase~ous species causcs a very rartid incecas e in pte,.s-ireIwithiei the composite.
(4) At a poiilL where thre internial pressure exceeds the dynamic ten-jsite strength of the remaini~ng resini matrix, the compositeexfoliates or delaminates.
5) As air then corneE in contact with the -very hot filaments and1decom'-Ioslrng rz~sinl, the whole mass bursts into flames. Thieflames continue until the temperature of the mass is reduced
to be low the point where the epoxy res in stops burning.
An estimate of tie temperature rise which can be 'exm'eted i.n a graphite
yarn and a graohite tow material subjected to ar ele!ctrical current pulseis showni ic Figure 2-85. A current pulse shape with a front time of tf
3. l~se an a t~.ltin, t 2 24sec wars ased. An analytical expressionfor the curreint as a fuinctjoi cof t ime was derived for this puilse shape.
This expressfo %,r: as squared and integrateOd to provide an expression forthei~e~alOf i 2dt. Assuming the r.arni and tow resistance remained
2-901
YARN TOW
CROSS SECTION 534.2 x 10-6 %064.5 x 10-6 ;2
U DENSITY 1.68 1.. (M/3
A 35.6 35.6 0:
P0RESiSTIVITY 1.35 x 10-- 0.83 x 1(pQ- 3 jM.'.(,
SPECIFIC HEATf0. 0. 3 CAGI, iG/C
10,000
1,00 - A
410
loo
10° ' ! -. . .. . . . ....... .......... ... .... ... .......... ...
10 100 1000 10,000
PEAK CURRENT (AMPS) PER YARN OR Tow
FTGTjRE 2-85. PREEDICIM TEMPERATURE INCREASE IN HMG-50 YARN AND HMS-TOW
I AS, A FUNCTION O PEAK APPLIED CURRENT
.3 2-91
I!
constant and that all energy was converted to heat, the temperature
itcrease Twas cal.culated from JRr I 2 dt
o W(5)WC
where R - test sample resistance IW - test sample weightC - material specific heat
An actual carrent-time pulse from an oscillograph record was plotted andintegrated graphically as a check of the analytical expression. Thisanalysis also assumes no heat energy is lost by ra'diation or conductionto the surroundings and assumes the entire energy of the pulse is 1deposited within 45Usec.
From Figure 2-85 it can be seen that the HMG-50 yarn calculated tempera-ttire rise at 300 amperes injection was 30000C. For the HM-S tow at a3000 amp current injection, the calculated temperature rise was 80000 C.
Because of hleat losses that would have occurred, these temperature riseswould not have' been realized.
From Figure 2-85 it can be seen th.at the build-up in heat within the graphitefilaments is rapid with increasing levels of current injection. Also, the 1rate of heat loss or dissipation is expected to be low because of the highlythermal insulative characteristics of the epoxy resin matricels. TheWhittaker 2387 resin and the DEN 438/KNA resin were tested and exhibitedthermal. conductivities of 1.99 and 1.66 Btu-in/hr-ft - F. Because of thelow thermal condictivity of the epoxy resin, heat is dissipated into theresin slowly and the resin in immediate contact with the graphite filaments
is exposed to a temperature that causes pyrolysis.
As previousiy discussed, burning of 1MG-50 graphite unidk.rectional coy-posites initiated at a crest current level of 19-20 x 10 amps per cm of IMG-50 yarn. That current level corresponds to a crest amplitude of
approximately 100 amps per 1MG-50 yarn. From Figure 2-85 it can be seenthat the resulting peak temperature would be 330 C for HKG-50 filaments.
This is typical of the temperature where epoxy resins begin to pyrolyze.
2-9213,
I
'3 2.4 DEVELOPMENT OF INTERNAL PROTECTION T'ECH•TQuES FOR ADVANCED COMPOSITES
A direct approach to improving the resistance of boron epoxy and graphiteepoxy composites to damage from electric currents would be to improve theelectrical conductivity of the composites. This then would result inless energy dissipation and heat build-up. In the case of the boron epoxycomposites the heat build-up within the filaments causes filament degrada-tion and in the case of gr4phite epoxy composites the heat build-up causesresin pyrolysis. Since the basic electric conductivity of the filamentscannot be improved without altering this make-up, and probably theirstructural properties, a viable ajprpoach would involve establishingalternate and more conductive paths witkn the composites. Obviously,"these alternate paths must occupy a minini -volume so that the filamentvolume content and structural properties of thee composite are not com-promised. Two such approaches have been investigai-ed-anddare discussed asfollows:
a. Hybrid Boron-Graphite Filament Epoxy Composites. As previousl..discussed, the impulsive electric current degradation threshold forunidirectional graphite epoxy composites is apparently 4-5 times greaterthan for unidirectional boron epoxy composites. Therefore, it was con-ceived that dispersions pf graphite filaments between the plies of boronfilaments could well serve to improve the resistance of the boron com-posite to damage. Furthermore, the resistivity of the HM-S tow graphitefilaments is typically 0.825 x 10-3 ohW-cm, whereas the resistivity of theboron filaments is typically 4-8 x 10 ohm-cm. The order of magnitudelower resistivity of the graphite filaments should result in primarycurrent flow in the interply graphite filaments instead of in the boroifilaments. Also, the diameter of the graphite filaments is typically0.0003 inches or less as compared to 0 .004 inches for the boron filaments.This should make possible the incorporation of very thin layers ofgraphite filaments without significantly decreasing the boron filamentvolume.
The specimens described in Table 2-3 were provided to this program froman internal devilopment project in progress. As noted in Table A-6 ofthe Appendix, no severe strength degradations were measured for eitherHybrid-I or Hybrid72 specimens at Injected crest current densities of9.84 or 10.40 x lO amperes per cm of filament cross-section,respectively. One each of the two Hybrid-I and Hybrid-2 specimens showeda somewhat lower tensile strength than did their respective unexposedcontrol specimens (114:143 and 140:176, respectively); however, the otherspecimens of each Hybrid had virtually the same strength as the control.Apparently, this approach toward improving the electrical current damageresistance of boron composites has some merit because the crest currentdensity (based on total filament cross-sectional area) was approximatelytwice the level that would have produced significant strength reductionin boron epoxy composites.
2-93
L
iI -.
TABLE 2-3
COMPOSITION OF HYBRID #1 AND HYBRID #2 BORON- IGRAPHITE FILAMENT UNIDIRECTIONAL COMPOSITE
SPECIMENSi
COMPOS IT ION VOLUME %
Hybrid-I:Boron filaments 34lH•-S Tow Graphite
Filaments 15
Epoxy resin 51 IHybrid-2:
Boron Filaments 49HM-S Graphite Tow II 1Epoxy Resin 40
Photomicrographs of one each of the exposed hybrid specimens are shownin Figures 2-86 and 2-87. The incorporation of graphite filamentsbetween layers of boron filaments is shown. There is no evidence of anyof the star shaped boron filament cracks that are produced by currentinjection. The few cracks shown are of the type sometimes produced when 4the specimens are polished in order to take the photomicrographs. I
b. Boron Composites With Imbedded Conductive Wires. This approachwas conceived based on a conicept of imbedding an array of fine conductivewires within a composite to direct electric current flow away from thefilaments. To be a practical approach to improving the resistance ofcomposites to damage from high intensity electric currents, certainobjectives must be met:
(1) The wires must be of a metal with good electric conductat|ce 1and, as such, be of superior conductance to the boron andgrphite filaments. 1
(2) The wires must be of fine diameter so that they can be incor-porated to a sufficient degree as to direct current flow withoutdisplacing a significant volume of filaments or adding severeweight penalties. If either happened the advantage ot using a Icomposite material could be reduced or eliminated. I
2 -94' I
L -
Ih
FIGURE 2-86. (lOOX) HYBRID-4 SPECIMEN AFTER 10.4 10 L 4
AMPERES PER CM' OF FILAMENT AVERAGE GRESTCURRENT INJECT ION--NO DAMAGE.
FIGURE 2-87. (10OX) '4YBRID-~ SPECIMEN AFTER 9.84 x 1.04
AM1PERE.) PER CM OF FILAMENT AVERAGE CREST
CJRRENT INJECrION--N0 DAMAC'ý.
II 2-95
" ~I[
(3) The conductive wires must not cause any adverse effectson the composite, either thermally or mechanically.
A survey was made of the availabtil.ty of fine wires or thin woven fabricsof conductive mt:als. The finest wire and thinnest fabric available was oftungsten. The material chosen war as supplied by the Newark Wire ClothCompany of Newark, New Jersey. It is a 140 x 140 mesh cloth that is woven Ifrom 0.0012-0.0013 inch diameter tungsten wire. Tungsten has a resistivityat room temperature of 5.5 x 10- ohm-cm, considerably lower than for
either the boron or graphite filaments. 3Two unidirectional laminates weze fabricated of boron filaments and 2387
epoxy resin with the tungsten cloth incorporated. The fabrication processwas identical to that used for all other boron epoxy laminates. In one
laminate (BUTcA), one layer of the tungsten cloth was incorporated betweenthe center two plies of the four ply laminate. In the other laminate(BUMC), one layer of the tungsten cloth was laminated onto both faces of Jthe four ply laminate. 'in both laminates one of the two wire directionsin the tungsten cloth was parallel to the boron filament direction. Inexactly the same manner as for the boron test specimens, 1/2 inch widespecimens of both the BETC and BUTCX laminates were prepared for current15injection by nickel plating of the speclmen ends.
The electric exposure test data and mechanical. property data for the BUTCand BUTC specimens is in Table A-7 of the Appendix. As can be seen, the
non-exposed BUTC and BUTCH specimens, respectively, had tensile strengthsOf 144,000 psi and 155,860 psi. Both were as strong or stronger than1
unexposed boron control specimens tested in the program, which indicatedthat the addition of the tungsten cloth did not significantly reduce theboron filament content or otherwise decrease the strength of the boroncomposite. As also noted in Table A-7, the BUTC specimens 4that were 2
exposed to an average crest current injection of 7.88 x 10 amps per cm2
of filament Vad no decreare in strength. At a current injection level
of 13.5 x to amps per ca; only a 24% strength reduction occurred. Forboron epoxy composites with no tungten cloth incorporated, a 50% strengthreduction resulted from a 7.85 x 10 amps per cm injection.
I2The BU7CH specimens exposed to 8.5 x 104 crest amps per cm2 injectionexhibited < 9o average decrease in strength. The BUTCH specimens exposed
to 13.5 x 10 amps per co of current injection delaminated down the
center adjacent to the layer of tungsten cloth. This resulted in a 46%
decrease in tensile strength.
The bilk densities of the BUTC and BUTrCY laminates were 1.93 and 1.82gm/cm , respectively. The tyqical densities of boron-epoxy composites
rangc from 1.83 - 1.915 gm/cm . Therefore, the incorporation of the
tungsten wire on both sides of a four ply boron laminate (BUTC) risulted
2-96
II
Iin a >100", increase in resistance to gradation with only 0.8% increasein bulk density (weight penalty). The incorporation of the tungsten wirebetween the center two plies of a four ply boron epoxy laminate resultedin > 507. in resistance to degradation with no measurable weight penalty.
Figure 2-88 is a photamicrograph of one of the B1 CM specimen that wasexposed to 9toss current inJection of 13.08 x LO amps per cm2 of filament.
4 The fact tihat the tungsten wire, between the two center boron plies,carried__a high currei.t load is evidenced by the fact that some of thewires h~ve melted. Also, none of the typical star shaped cracks in boronfilament'*, as typicaliy caused by current flow in the boron filaments, arepresent, except for the one filament in contact with tungsten that hasbeen melted. This further indicates that the tungstcn wire carriedsufficient current to keep current flow in the boron filaments to belowa damaging level. All specimens but B1UTCM #1 in this group delaminated.
The delamination was probably the result of the hot tungsten wire causingresin pyrolysis and gaseous blowing.
4FIGURE 2-88. BUTCM #1 SPECIMEN SUBJECTED WITH 12.08 x 10
CREST AMPS PER CM2 OF BORON FILAMENTS.
II2-97
I
I
Figure 2-89 and 2-90 are, respectively, B 9TC specimens. whch have beenw injectld with current levels of 7.64 x,10 and L3.96 x 10 crest amps
"per cm of boron filament cross-section. Specimens of the sane group asBUTC #2, Figure 2-89, showed no lose in tensile strength. Specimens ofthe same group as BLUTC #3, Figure 2-90, showed an average loss in tensilestrength of 24%. There are indications of the star shaped boron filamentcrack , as typically caused by current injection, in specimen BUTC #3. Inboth BUYTC #2 and #3 it appears that some of the surface tungsten wireshave shattered. This type of tungsten wire damage probably occurs atlower current level's than would cause the tungsten melting evidenced inFigure 2-89.
2 -9 1
II
2-98II •, | I F• I I ~
"tI
0.N
74
I 0
FIGURE 2-89 BUIC f2 SPECIMENS nijEarEl WITH 1 .64 x 104'
CREST AMPS PER CM 2 OF BORON FILAMENTS.
FUUnJRE 2-90. BIJTC #3 SPECIMEN INJECTED WITH 13.96 x I0 4
CREST AMPS PER CM2 OF BORON F1LAMEICS.
1 2-99
II
SECTION 3
CONCLUSIONS
(I) Electric current flow processes within epoxy resin composites ofboth boron and graphite filaments is analogous to a complex array ofresistors (filaments) with the epoxy resin serving as a dielectricbarrier between filaments. The processes, however, that determinethe proportions of current flowing in each filament or layers offilaments are very complicated and difficult to mathematically model.Many factors such as why current flow tends to concentrate at randomin a fraction of the total bokon filaments in a composite are not fullyunderstood at present.
(2) The limiting fa,-rms hi the degradation of boron/epoxy composites aretne boron filaments themselves. The filamentbegin to crack and breakup at crest current dehsities of 3.7-5.7 x l0*amperes per cm2 for thestandard waveform used in the tests reported. Above that point, lossin strength is rapid with increasing current levels.
(3) The limiting factor in the degradation of graphite/epoxy filamentsis the resin matrix. For the HMG-50/epoxy and HM-S/epoxy compositestested with the standard waveform, degradation by burning of the resii-was initiated at crest current densities of 20-25 x 10 amperes per cm offilament cross-section, for the standard waveform used. At that pointthe temperature of the filaments was sufficiently high (600-800 0 F) toccause pyrolyzation of the epoxy resin. At increasing current levelsthe build-up of pyrolysis gases within the composite is so'rapid as tocause delamination.
I
3-1
p
(4) The control, 1ing factors in the degree of degradation of the f. lamrentsand cowposites tefited are the total electric energy input and thefraction thereof (di[ssipatted in the form of heat. To demon, rait' thiscrest current amplit:ude as well as waveform duration was varied. Assuch, crest current amplitude is less important then the waveform or Uor total energy.
(5) It appears feasible that the resistance of boron-epoxy conIFosit,.s todegradation by electric currents can be Inaproved through the lncorpor-ation of alternate and more conductive paths. Fine wires of conduc-tive metals can be ixsed to provide >5O-1O00%/ improvement withoutdecreasing the strength of the composite. i
3
ISI
3-2 1
I
I,
I
IRFERENCES
I. USA Standard 062.1, 1967, (IEEE Standard No. 28).
2. "Lightning and Static Electricity Conference, 3-5 Dec. 1968, Part !i,"AFAL-TR-68-290 Part II, May 1969.
3. E. Rudy and St. Windisch, "Related Binary Systems Mo-B and W-B,"Ternary Phase Equilibria In Transition Metal-Boron-Carbon-SiliconSystems, Part !, Vol. III, AFML-TR-65-2, July 1965.
4. F. W. Sears, "Electricity and Magnetism, "Addison-Wesley PublishingCo., Inc., Cambridge, Mass., 1953.
5. J'. T. Milet and S. J. Welles, "Boron" Electronic Properties InformationCenter Data Sheet, DS-151, Feb. 1967.
6 H. S. Carslaw and J. C. Jaeger, "Conduction of Heat in Solids," 2ndEd., Oxford at the Clarendon Press, 1959.
7. F. E. Wawner, Jr., "Boron Filaments," Chapter 10 of Modern CompositeMaterials, edited by L. J. Broutman and R. H. Krock, Addison-WesleyPublishing Co., Reading, Mass., 1967.
8. R. J. Roark, "Formulas for Stress and Strain," 3rd Ed., McGraw-HillBook Co., Inc., New York, 1954.
9. j. Selsing, "Internal Stresses in Ceramics," Journa, of the AmericanCeramic Society, Vol. 44, Aug. 1961, p. 419.
1 3-3
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I / APPENDIX B
RESIDUAl. ANI) INDUCED STRESSSPRESET IN BORON FILAMENTS
Boron monofilarnents are produced commuercially by chemical. vapor- deposition(CVD) process in which boron forms on a hot (-7i0 0 F) tungsten wire byhydrogen reduction of boron trichioride vapor.M-
F. E. Wawner, Jr., on pages 254-256 of Reference 7 states that. during thedeposition of the boron onto the tungstero substrate the core increases indiameter from its original diameter of 0.0005 in. to 0.00065 in. placingthe deposited boron layer in tenision. We can calculate an equivalentinternal pressure to replace the core that will produce the same expansionof the internal diameter. !~sing the material properties of Reference 7 andEquation (27) of Reference 8 (uniform internal radial pressure p lb.
per sq. in. with longitudinal pressure zero or externally balanced)
2 2R R 2 + R I +
2 '
where p =internal, pressure (psi)
E B modulus of elasticity for boron (psi)
R inner radius (in.)
R . outer radius (in.)
i oVB Poisson's ratto
Supersnripted sumbers in parertheserefer to referenced materiam Listed on
page 79.
ofteitrnldaee. B-teiaeta rpete fRfeec n
B qain(7 f eeec uifr nenlrdilpesr b
I
which for LR 0.00015/2 0.000075 7.5 x 10- 5 in. becomes 37.5 x 10- 5 = .6xI7ps
P 1 6psi
2.5 x 10 4.0 X 10-6 + 6.25 x 10- 8
60.0 x 106 4.0 x 10-6 - 6.23 x L0-8
UBy using Equation 29 of Reference 8 (unifono internal pressure p lb.per sq. in. in all directions) which is probably the most reasonableapproximation, we obtain: 3
2 2 2
a R2 1 I (2)EP "P -B [R22 Rl ( 2 R2 _ R 2 [ 2
2 1 2 11
or
7.5 x lo-5jp 2.5 x 10-4 4.0 x 10.6 + 6.25 x 108- 6.25 x 10-8-
60.0 x 10 4.0 x 10 - 6.25 x tO8 4.0 x 10 = 6.25 x 10
and p - 2.16 x 10-7 psi. i
An internal pressure of this magnitude would result in a circumferentialor tangential stress, at, at the inner surface of the boron core of j
R2R + R1 7 7
S2 2 ] 2 2.16 x 10" (1.0317) = 2.23 x 10 psi
2 L
Now the boron case is-obviously incapable of supporting tensile stresses of 1this magnitude and therefore core growth must take place at the expenseof sheath material. This conclusion is supported by statements on page 254of Reference 7 which states, "Small voids have been observed along the inter-face and are Lentatively attributed to rapid diffusion of boron into tungstenand subsequent vacancy condensation along the interface between the twomaterials (Kirkendall effect)."
B-2
I't
II
For the moment assume that the boron sheath deposition and the coretrans formation from tungsten to tungsten.-boride with its accompanyingcore growth can all be accomplished without the creation of residualstresses existing at the deposition temperature of 2000° F. Residualstresses existi.ng at room temperature wi1 then be the result of coolingthe composite filament from the stress free deposition temperature to roomtemperature.
SUsing a cc're diameter of 0.0005 in. (using the larger value of 0.00065 in.changes the final result by only 1000 psi.), a filament diameter of 0.004 in.,a coefficient of thermal. expansion for the boron sheath of 5.0 x 10-6
in./in./ 0 F for the core we can calculate the resultant longitudinal thermalstress. Upon cooling 20000F the boron sheath by itself would shrink anamount
CB AT (5.0 x lo-6) (2000°F) = 0.010 i,,./in.
while the core would shrink only
W Mi C~wAT "- (4.1 x 10-6 )(20000F) = 0.0082 in./in.
a difference of 0.0018 in./in. But sitice the core and the shcath areintimately bunded together
CB must = Cw
and so the boron sheath must be strefchedst- nme amount by the core (resultingin a residual tensile stress in the sheath) and the core must be compressedby the outer boron sheath. From static considerations the total fcrce actingon a unit length of the sheath must be balanced by an equal force acting overthe area of the core, or
PB = Pw ,or aBAB A owA (3)
where aB is the stress level in the boron sheath and a is the stress levelin the core. AB and A are the respective cross sectional areas where
"A = (DB)2 1256.6 x 10-8 in. 2 andB 4 B
Sii (Dw)2 -
A (D 2 19.6 x 10-8 in.2
B-3
IA
F|I
Now assuming that the coefficients of thermal expansion and the modulus Iof elasticity of both the boron sheath and the tungsten core are constantthroughout the 20000 F temperature range', then (3) becomes
EB BAB = E ww Aw
or solving for IE A
B BAX O (4)I
And also since the total amount of strain from a zero stress room tempera- Iture condition must be 0.0018 in/in, because the sheath and core are inintimate contact, we can also write,
IEB + (w = .0018 in/in.
Substituting (4) into (5) yields,
EAEA w C + tw -. 0018
E BA B w
or Ew = .001772 in/in.
and the average compressive stress in the tungsten core is
7= Ewfw = 60.0 x 106(0.001.722) = 106,310 psi
and the average tensile stress in the boron sheath is
IB E BfB = 60.0 x 10 6(0.0018 - 0.001772) - 1686 psi.
This value for the compressive stress in the cors is somewhat lower than theexperimentally me sured value of 150 to 200 x 10 psi and the theoretical
value of 211 x 10 psi cited on page 254 of Reference 7. However, this
IB-4I
I
II
analysis assumes linear material properties over a large temperature range,I uses for the coefficient of thermal expansion of the boron sheath thecoefficient measured in the Aeronutronic Research Laboratories for a singlefilament and uses a coefficient of thermal expansion of tungsten-boride forthe core. Any, or all, of these assumptions or measurements are subjectto some adjustment. Ir any event, the resultant longitudinal tensile stressin the boron sheath is quitd small (i686 psi) while theresultant longitudinalcompressive stress in the corle (100 to 200 x 103 psi) is large. These-tresses are oriented along the axis of the filament and are not directlyadditive to the stresses calculated and reported.We can now, however, consider the temperature-induced radial stresses.
Again, assuming that the deposition process results in a stress free com-posite filament formed at 2000F, residual stresses are developed as theboron sheath shrinks around the slower, shrinking core. These stresses Lanbe calculated in a manner similar to that of the Philco-Ford paper inReference 2. Except here there is a thermal contraction in both the sheathand the core, rather than a thermal expansion of the core alone as assumedby Phiico-Ford in Reference 2.
Proceeding as in the longitudinal analysis, the free thermal contraction ofthe sheath inner radius alone would be:
-6 -6SRB = R, tB AT = (.00025)(5.0 x 10 )(2000) - 2.5 x 10 in.
while that of core alone would be:
6Rw = RacYAT = (.00025) (4.1X 10- 6) (2000) - 2.05 x 10-6 in.
a difference in radial deflection of .45 x 10-6 in. Eut since the coreand the sheath must each shrink the same amount
must = 6Rw (shrinkage from depasition temperature)
or
6RB + RW - .45 x 10-6 in. (6)
(deflection from free thermal contraction of zero stress condition.)
B-5I
I
j B-S
II;
,°
Iand so the boron sheath must be expanded some amount from the free or zerostress condition, resulting in a tensile radial stress and the sheath actingon the core will compress the core. Again from a consideration of a staticforce balance, the pressure at the interface between the sheath and thecore must act equally on the sheath and core. For the core:
6R p -0( - V~,w Ew
or the internal pressure, p, equals
5Rw
R (Ii 0I - Vw)
w
and for the sheath
2 2R I R 2 + R 1
6R B E1 (2 2 2 + B)
B R 2-R1R2 - 1
or
P BRI R2 2
1+ + VB 2 2
R2 -RI
Equating the vwo pressures gives
6R w 6R B5K - = RB2(7)
1. 2 2F VW- ) _(+ V)w EB _2 2
R2 -1R
Substituting equation (6) into (7) gives
.45 x 10 -6 RB 6B
R2 21 R R + RI +
w EB 2 2 R
2 1R
B-6
and using Vw .3 along with the previous conatants we get,
6 RB M 2.869 x 10 in.,
or the inner radius of the boron sheath is forced by the slower shrinkingcore to be this amount larger than the stress free condition, resulting ina residual radial streas ait the inner surface of the sheath, equivalentto an intenvzal pressure of:
2.86- 5 .597 4
1• (2 RIj + B----
' R2 - R2
This internal pressure gives rise to tangential and radial stresses in theboron sheath. The maximum values ofar and ct occur at the inner surface
and are expressed by
2 + R2
a P, ai 2 2 = 1.0317 Pr R2 - R
2 1
or
"Cyr = 55,900 psi, (t = 57,670 psi
These stresses are in the planeof the filament and are directly additiveto the stresses produced by an electrical current passing through the core.
The tensile radial stress C r is a maximum at the inner surface of the sheath
and decreases to zero at the outer surface, while the tangential stress
a is a tensile maximum at the inner surface and decreases, but not to zero,with increasing radius. A plot of the radial and tangential stresses is"shown in Figure A-I. Within the core the radial stress is equal to the tan-"gential stress and this stress is a constant compressive stress equal to Por 55,900 psi.
F. E. Wawner, on page 256 of Reference 7 attibutes the often observed radialcracks in the boron sheath to the stresses calculated from the radius ofcurvature assumed by split and longitudinally polished filaments. These
j B-7
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(300
1:21'
$4-.4
00
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lia
st:resses, however, are the axial or long&itudfinal. stresses and act In tlheplane of the r.adial. cracks and are riot t:he cauae of' radial cracking. Radialcracks are the result of th.c tangentialI (or hoop) sit:resses calculat:ed above,and this stress does not go from tension to compression wlth increasing3 radial position as does the axial. streoas, and therefore, the only reasoncracks might be observed to terminate ht. -ome radial. distance is because thetangential stress level drops below the level requi-irod to propaIgate tile
g crack.
The actual amplitude of the stresses calculated in this study are subjectto the sheath and core siles used and the physical pioperzy data used. Theshape of the stress-location curves,, however, is fundamental to tile assijmp-tion that the boron sheath tends to s3hrink onto the core material an thecomposite filanent cools from the high t'ewper-ature A the depositionprocess.
The equation developed in Reference 9 apd referred to in Reference 7 wasdeveloped for internal stresses in a spherical thick walled medium and maynot be directly applicable-Wo the cylindrical case of the boron filament.
1B
S~B-9