AFML-TR-67-367

GRAPHITE FIBER/EPOXY RESINMATRIX COMPOSITES

"* tR. I. DAUKSYSN. N. PAGANO

R. C. SPAIN

TECHNICAL REPORT AFML-TR-67-3W7

APRIL IC68

This document has been approved for publicrelease and sale; its distribution is unlimited.

AIR FORCE MATERIALS LABORATORYAIR FORCE SYSTEMS COMMAND

WRIGHT-PATTERSON AIR FORCE BASE, OHIO

S~D D

JUN 2 5 1968

B

(t ! , H j' '

NOTICE

When Government drawings, specifications, or other data are used for any purposeother than In connection with a definitely related Government procurement operation,the United States Government thereby incurs no responsibility nor any obligationwhatsoever; and the fact that the Government may have formulated, furnished, or inany way supplied the said drawings, specifications, or other data, is not to be regardedby implication or otherwise as in any manner licensing the holder or any other personor corporation, or conveying any rights or permission to manufacture, use, or sell anypatented Invention that may in any way be related thereto.

This document has been approved for public release and sale; its distribution isunlimited.

W -F.-

Copies of this report should not be returned unless return is required by security

considerations, contractual obligations, or notice on a specific document.

6W - MWy 19 - C04SS - 36-780

AFML-TR-67-M67

GRAPHITE RBER/EPOXY RESINMATRIX COMPOSITES

R. I. DAUKSYS

N. N. PAGANO

tR. G. SPAIN

Tibs &dowu has bee approed for publ&relee and sa-, its distrbtlon is unkliitd

AFML-TR-67-367

FOREWORD

This report was prepared by the Plastics and Composites Branch, Nonmetallic MaterialsDivision, and was Initiated under Project Number 7340, "Nonmetallic and CompositeMaterials," Task Number 734003, "Structural Plastics and Composites." The work wasadministered under the direction of the Air Force Materials Laboratory, Research andTechnology Division with R. J. Daukays, N. J. Pagano, and R. G. Spain as Project Engineers.

The authors wish to gratefully acknowledge R. Kuhbander, C. Bradshaw, F. Von Seelan,and K. Meyer of the University of Dayton Research Institute for their contributions in thefabrication and testing of the composites described in this report; the Fibrous MaterialsBranch of the Nonmetallic Materials Division for the graphite single filament properties;and H. S. Schwartz, Nonmetallic Materials Division, for advice and arrangements of thepreliminary dynamic evaluations of graphite composites performed by M. Flelhsmann,General Electric Company allistic impact) and G. J. Petrak, University of Dayton Re-search Institute (fatigue).

This report covers work conducted during the period of June 1966 through September1967. The manuscript was released by the authors in December 1967.

i This technical report has been reviewed and is approved.

R. G. SPAIN, Acting ChiefPlastics and Composites Brancht. Nonmetallic Materials Division

I Air Force Materials Laboratory

f

AFML-TR-67-367

ABSTRACT

Fotw types of graphit fibers were utilized as reinforoements In a continuing experl-mental program of the fabrication and evaluation of epoxy resin matrix composites. Resultantcomposite data and analyses oontinua to demonstrate a high promise for graphite fiberreinforced resin oomposetes " high performance materials.

This abstract has beow aproved for public release and sale; its distribution Is unlimited.

£1

• • I I II

AFML-TR-67-367

TABLE OF CONTENTS

SECTION PAGE

I INTRODUCTION 1

Il DISCUSSION 2

Fiber Characteristics 2

Water Boil Exposure 2

Epoxy Coatings for Graphite Yarn 5

Fatigue, and Ballistic Impact 5

Mechanical Properties o Graphite/Epoxy Composites 8

Fundamental Mechanical Analyses of Composite Data 18

Fabrication 19

Testing 20

III CONCLUSIONS 21

REFERENCES 22

I

AFML-TR-67-367

TABLES

TABLE PAGE

I Graphite Fiber Properties 3

fl Yarn Handling Characteristics During Resin Impregnationand Winding (Fiber Sample No. B) 3

IM Average Flexural Properties of 8-Ply Balanced BidirectionalGraphite Composites After Prolonged Water Boil (FiberSample No. A) 4

IV Effect of 2-Hour Water Boil on Epoxy Yarn Coatings Balanced8-Ply Bidirectional Composites (Fiber Sample No. A) 4

V Average Flexural and Shear Properties of a 10-Ply UnidirectionalGraphite Yarn Composite After 2-Hour Water Boil (Fiber SampleNo. C) 6

VI Average Flexural Properties of 8-Ply Balanced BidirectionalGraphite Composites After 2 and 24 Hours' Water Boil (FiberSample No. B) 6

ViI Inltial Epoxy Coated Graphite Yarn Handling Characteristics(Yarns Subjected to Mechanical Handling During Resin Coating) 7

VIII Evaluation of Epoxy Yarn Coatings in Bidirectional Composites(Yarns Not Subjected to Mechanical Handling During ResinCoating) (Fiber Sample A) 7

IX Ultimate Tensile Strength of Beryllium, Scotchply, andGraphite/Epoxy Bidirectional Composites (Graphite FiberSample No. A) 10

X Ballistic Impact Resistance of Bidirectional Composites 10

XI Average Mechanical Property Data of a 10-Ply UnidirectionalGraphite/Epoxy Composite (Fiber Sample No. A) 12

XI Averae Mechanical Property Data of a Bidirectional Composite(Fiber Sample No. A) 12

XII Average Mechanical Property Data of Unidirectional GraphiteYarn/Epoxy Composites (Fiber Sample No. B) 13

XIV Bidirectional (00, 900) Composite Mechanical Property Data(Fiber Sample No. B) 14

XV Average Flexural and Sear Properties of Unidirectional andBidirectional Graphite Yarn/Epoxy Composites (FiberSample No. C) 15

XVI Average Mechanical Property Data of 10-Ply UnidirectionalGra e/ Epoxy Composites (Fiber Sample No. D) 16

- • o .

IAFML-TR-67-367

TABLES (CONTD)

TABLE PAGE

XVII Elastic Moduli of Unidirectional Graphite Yarn/ Epoxy Composites(Fiber Sample No. D) 16

XVIII Comparison of Unidirectional Composite Mechanical PropertyData Normalized to 60 Volume Percent Fiber 17

XIX Comparison of Bidirectional (00, 900) Composite MechanicalProperty Data Normal 'a to 60 Volume Pere-f" fber I

XX Material Properties Computed from Flexure Tests and ComparisonWith Experiment 20

t

vii

4. AFML-TR-67-367

S~ SECTION I

INTRODUCTION

The efforts reported here are an Integral continuation of the fabrication and evaluation ofpreviously reported data for graphite fiber reinforced epoxy resin matrix composites (Ref-erence 1). Several reinforcements are currently the subject of research directed towardimproved performance of resin matrix composites. Air Force requirements, in particular,emphasize the need for high mechanical-property-to-composite-density ratios. Althoughgraphite fibers of sufficient mechanical property levels to generate interest as to theirconsideration as potential reinforcements for high performance level composites are a com-paratively new material development, considerable progress has been realized in the prepara-tion of graphite fibers and subsequent composites with increased mechanical property levels.

Much of the effort discussed inthis report was concerned with the fabrication and evaluationof graphite fiber reinforced composites as to static mechanical properties, although theeffects of composite "water boll" exposures and some preliminary dynamic evaluation dataare also presented.

All composite evaluations in tension, flexure, shear, and compression were linear to failure.

AFML-TR-67-367

SECTION II

DISCUSSION

FIBER CHARACTERISTICS

Mechanical and physical property data were obtained for four types of graphite fibers.(Table 1). Samples A and B are Union Carbide products with the latter (Thornel 40) displayingincreased modulus and strength properties compared to sample A (Thornel 25). Samples Cand D were obtained from the Royal Aircraft Establishment (RAE) and consisted of 14-inchlong samples of tow, a loose ropelike fiber bundle without definite twist, consisting of manythousands of filaments.

Since sample B was recently developed as a continuous yarn with greater fiber strengthand higher modulus than those of other continuous graphitic yarns, considerable effort wasdevoted to yarn characterization and composite fabrication and evaluation.

Sample B (Thornel 40) handled as well as sample A (Thornel 25) during the winding ofprepreg tapes. A difficulty encountered during fabrication when working with sample B wasthe number of adhesive splices per pound. Table II lists the number of breaks and splicesencountered from two batches of Thornel 40 during impregnation and tape winding. Althoughthe number of breaks directly attributed to splices seems small, a generous portion of the"other breaks" were due to accidental contact of yarn frays with the adhesive used in yarnsplicing and with other parts of the yarn directly below the spliced section. As a result, thesplice would have a tendency to adhere to the graphite at the point of takeoff, causing yarnpull, damage to many of the filaments, and overall poor integrity during impregnation andtape winding.

Although yarns C and D exhibited relatively high average tensile strength and modulus,fiber property data exhibited a large scatter, as illustrated by the maximum and minimumvalues shown in Table L Also, due to the short lengths c. loers, it is obvious that prepregfabrication would have been impossible by winding. Therefore, fiber alignment and im-pregnation were performed manually. This was time consuming and imparted poor controlover fiber volume, spacing, and alignment ir the resultant composites.

WATER BOIL EXPOSURE

As a means of simulating the effects of severe atmospheric conditions on compositeperformance, test specimens were subjected to a water boil environment prior to testing.A 2-hour water boil decreased the flexural strength of Thornel 25, epoxy matrix compositesby less than 6% (Reference 1). Work at present (Table IlI) illustrated an average decreasein flexural strength of only 10%, even after subjection to water boil for prolonged exposure(72 to 144 hours). Flexural modulus was less affected, showing a decrease of about 1.5%compared to the control specimens.

In another series of experiments designed to study the effects of various epoxy yarn coat-ings as yarn finishes for Thornel 25 (Fiber A) on composite properties, 2-hour water boildata were also obtained. Table IV exhibits the fact that no obvious deleterious effects couldbe attributed to the coatings. The increases in mechanical property values were probablydue to experimental error or variance in properties associated with the few fiber propertytests allowable with the small quantities of fiber available.

2

IAFML-TR-67-367

TABLE I

GRAPHITE FIBER PROPERTIES*

Tensf, StrengthPSI x 10-3 A B C D

Average* 173 205 231 296

Min. 158 172 59.9 163

Max. 195 256 355 370

Tensile modulusPSI x 10-6

Average* 25.7 43.0 39.1 47.3

Min. 24.1 34.8 30.0 33.3

Max. 26.8 49.0 45.2 54.9

Density g/cc 1.42 1.51 1.95 1.9 +.l1

Finish PVA PVA -- -

Construction

No. of plies 2 2 (tow) (tow)

Twist (turns/in.) 2.1 2.1 ..

Filaments/ply 720 720 ..

*Average property data are based upon five or more individual filament tests

TABLE U

YARN HANDLING CHARACTERISTICS DURINGRESIN IMPREGNATION AND WINDING

(FIBER SAMPLE NO. B)

Total Total Breaks DirectlyYarn Yarn Attributed Other

Yarn Length Breaks to Splices Breaks

Tube A 10,000 15 4 11

Tube B 16,000 14 4 10

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AFML-TR-67-367

Table V demonstrated that the properties of the yarn C composites were also not signifi-cantly altered by a 2-hour water boil. Short-beam shear stress and flexural strength andmodulus were virtually unchanged by the water boil.

Table VI shows that flexural modulus does not seem affected and strength parameterhave decreased 11 and 14% after 2- and 24-hour periods, respectively, for the B fibercomposites.

Thus, water boil tests on epoxy resin composites prepared with polyvinyl alcohol coatedgraphite fibers, epoxy resin coated graphite fibers, and uncoated graphite fibers indicaonly mildly deleterious effects. Based on these results it appears that graphite fiber rein-forced composites, unlih glass fiber reinforced composites, may not require flbie treat-ments to maintain composite mechanical properties during longperiods of humid atmosphericexposure.

EPOXY COATINGS FOR GRAPHITE YARN

Graphite fiber coatings were supplied by the Plastics and Composites Branch (MANC)for application (during fiber preparation by Union Carbide Corporation) to 5-ply yarns ofthe Thornel 25 type. Initially, a 2% solid solution in trichloroethane of various epoxy resinswas used as the coating medium. Most of the epoxy resins used In the coating Investigationwere of the eplohlorohydrin, blephenol A type and ranged in average molecular weioht fromabout 470 to 80,000. Epon 1031 dIffered In structure and was tetra-fumctional with an estimatedaverage molecular weight of about 1000. Union Carbide (U.C.C.) rated the handlng character-Isties during yarn fabrication and MANC rated them during tape winding. Table VII is In-dicaUve of handling characteristics as described by both organizations. Reeaminati ofthe yarn coating procedures showed that mechanical handling during the coating procedureexisted to at least some degree.

The yarn coatings were repeated with the elimination of mechanical handing prior to andduring the application of epoxy resin coatings. This second set of yarns was of 2-ply oon-struotion and a 1% resin solution was used as the oaoting medlum. Resin pickup was sp-proximately 0.6 to 0.9% by weight. Accordingly, tapes using these yarns were wound and epoxymatrix oomposites were prepared and evaluated (Table VIII). All yarns coated by te secondprocedure handled satisfactorily during tape winding. Inspection of the data (Table VIMindicates generally similar initial property levels for all the yarns costed with the epoxresins as well as properties not significantly different from composites fabricated usingsimila yarns coated with 0.1% polyvinyl alcohol.

Although the choice of graphite yarn coatings would Involve considerations of the coatingcompatibility with composite resin matrioes, the data seem to Indicate that materials of eavvery low moleoular weights are adequate as coatin and Impart good handling charactissto graphite yar Hf the coatings are applied to the graphite yarns prior to smbjectioa of theyarns to mechanical contact..

FATIGUE. AND BALLISTIC IMPACT

Most of our effort to date has conoerne the characterisatim of gabt/euy oompoesieas to static properties. Hih performance composite structural olements are obvboislysubjected to dynamic and repeated lood. Thu. Invsaons were intated to charatmaeegraphite/epoxy omposites umder alternating loadings in toeslebtigue and ads. an induedhigh rate of strain as in ballistic impact tests. Tbese evalustlans are, as yet, In a my prme-liminary stags and have uoW only Fiber A (hrmnel 25) as the omosite reooe'mee.

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AFKL-TR-67-367

Fatigue data are graphically IUlustratedl in Figure 1, where curves are displayed whichcoqmpar continuous beryllium and glass composttes to a continuous graphite/epoxy composite.The graphite and beryllium composites used the same matrix and were fabricated as describedIn the febrication section of this paper. The beryllium, glass (Sootchply). and graphite corn-posites were of 12-ply, bidirectional (0' 900) balanced construction.

Althoug nown of th specimens failed In the grip area. a good portion of the samples didfail outside the pp length, which would account for some of the data scatter. AUl stresseswere calculated using the gap section area.

Table EC lists the ultimate static tensile strength of the beryllium, glass. and graphite!epozy composite.

Throughout the repetitive tensile-tenaile cycles, the graphite/epoxy comnposites were ableto sustain a considerably greater percentage of their ultimate stress than either the berylliumor glass c mposites at simlar numbers of cyclic loadings.

Both *a beryllium and glass composites failed at about 104 cycles at 70% of the ultimatestrngSih, but the graphite composite specimens resisted the same cycling while stressed to

90% of ultimate strength.

For the hige number of alternating cyclic loadings, it was observed that the graphite!

epoxy composite were abe to withstand 167 cycles when stressed to 70% ultimate strength.conversely, tho berylliumi and glass composites maintained loes than 45% of their ultimatestrengSths for the increased cycling.

Two of the continuous graphite/epoxy specimens which were cycled at 50 to 70% ultimate

tensile strength for 26 x 10 6 and 17xz10 cycles. respectively, without failure, were subjeoteto static tensile tests to determine the retention of ultimate strength. The tests indicatedthat the samples possessed over 92% of their original tensile strengths.

Very preliminary data Is presented showing the - cthA of high-speed projectiles strikinggrahit/epxypanels (Tabl x). All paneds wore bidirectional and wes of various thicknesses.

The critarion, for failure Is the formation of a crack which can be observed optically at loxto sox mapifoation on the reverse aide or tension side of the Impression. Impact resistanceIs d@BMFan AS the maximumA ensrg lnoarted to the specimens without Inducing failure.Althov data is sparse and no afttemp has been made to oarrc the results to a normalizedthickness. th dibrenoe, in the thiolmsesas of th 7-ply --h-eepx composite and theberylliii sad glass Sooftaly pandes is relatively small. Therefore based on initial trials,it Oppea" doat the grahte composites are superior to the sootcholy composites, yet in-briam to the bidirectional beryllium epoxy panel for the- configuatio tetd.

MECHANCAL PKCPUCRTERS OF GRAPHNrE/RPOri CCKPSIPTES

Tdabe Xl hog XDC are r epresentative of the meohancial, properties of epoxy matrixmofit@ ftriooled withi graphite fiber comnsdescribed in Table L Bach table alsoceamfiber property data, as sampled from the actual spool usned in winding or on a batch

basin ase no casee with the RAE tow.

AFML-TR-6 7-367

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AFML-TR-67-367

TABLE IX

ULTIMATE TENSILE STRENGTH OF BERYLLIUM, SCOTCHPLY, AND

GRAPH1TEa/ EPOXY BIDIRECTIONAL COMPOSITES

(G te Fiber Sample No. A a

No. o AverageGage Tested Ultimate Strength

Composite Width Specimens PSI x 10-3

Scotchply 1/ 2 in. 2 56.8

Beryllium 1/2 in. 2 43.0

Graphite 1/2 in. 2 38.9

aAverage single filament properties: 158 x 103 PSI tensile strength

26.8 x 106 tensile modulus

TABLE X

BALLISTIC IMPACT RESISTANCE OF BIDIRECTIONAL COMPOSITES

Thickness Volume % Impact ResistanceMaterial (inches) Fiber (foot-pounds)

Graphite/epoxy 0.063 - 0.060 63.3 0.157(7 plies)

Graphite/epoxy 0.085 - 0.090 62.2 0.417(10 plies)

Graphite/epoxy 0.098 - 0.101 63.9 0.499(12 plies)

Scotchply 0.068 - 0.072 65.0 0.072

Be wire/epoxy 0.071 - 0.074 50.0 0.339

10

AFML-TR-67-367

Fiber volume calculations were based on densities of 1.42, 1.50, 1.90, and 1.90 gn/ml forFibers A, B, C, and D, respectively. A density of 1.2 g/ml was used for the epoxy resinmatrix.

Shear data were low for all composites and as true shear breaks were not clearly evidentfor the majority of the 00 unidirectional and bidirectional specimens, actual shear strengthvalues can be expected to be somewhat higher than the reported shear stress values. Themode of failure appeared to be one of crushing or a combination of crushing on the face of thebeam under the loading nose and tension on the opposite face.

Flexure specimens typically failed in a manner similar to that encountered in the sheartests.

Ultimate tensile strengths were extremely difficult to obtain as none of the breaks, exceptthose reported for composites of Fiber B, occurred in the gage lengths.

Tables XI and XH are indicative ofproperties obtained from unidirectional and bidirectionalcomposites using Fiber A (Thornel 25) as the reinforcement.

The data presented in Tables XIII and XIV consist of results obtained with the Fiber B(Thornel 40) reinforced composites. Both the unidirectional and bidirectional properties arehigher than those obtained with the continuous Fiber A composites. It has been observed thatas the longitudinal modulus of graphite filaments is increased, there is a noticeable decreasefor short beam shear values. This is especially noticeable when comparing the Fiber A andB composites to the Fiber C and D composites, of which the latter have moduli that aresignificantly higher. The near circular cross-section of the C and D fibers, an outcome of theparticular precursor employed, reduces the surface area and perhaps the possibility ofmechanical interlock with the matrix. The cross-section of the B fibers are still highlyirregular and although shear values are not extremely pronounced over the C and D fibers,they are greater. It seems obvious that different fiber morphologies and fiber finishes arealso factors which may affect the shear values; however, coupling must be improved torealize high translation efficiencies of current and future graphite fiber reinforcements.

The only unidirectional tensile strength reported is for the B fiber composites. Specialpreparation of the specimens consisted of "dog-boning" the faces as well as the sides inorder to facilitate a gage break. Attempts to obtain tensile strengths for the A, C, and Dunidirectional composites did not use this method of specimen preparation and resulted ingrip and shoulder breaks and low values.

Table XV depicts the properties of the Fiber C reinforced composites. As describedearlier in this report and as apparent from the unidirectional data, control of fiber volumeand, in turn, mechanical properties Is a problem when fabricating with short lengths of tow.Short-beam shear stress values were particularly low for the bidirectional composites.

Although somewhat better control of fiber volume was obtained with the Fiber D composites(Tables XVI and XVn1), the fabrication technique with graphite tow does not seem to lend itselfto the fabrication of composite panels of significant size. The tensile and flexural moduli arehigher than those of the Fiber A, B, and C composites. Notably, however, fiber strengthtranslation as determined from flexural strength data, is low (about 40%) compared to the Bcomposites (greater than 95%).

Tables XVIII and XDC are composed of unidirectional and bidirectional data, respectively,taken from the previous mechanical property tables and normalized to 60 volume percent tosimplify comparison of the various composites.

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The shear value expressed in parenthesis (Table XVII) for the A composite is probablya more realistic average since it is basedon a large number of tested samples from differentpopulations (Reference 1).

FUNDAh2UNTAL MECHANICAL ANALYSES OF COMPOSITE DATA

The maecanioal properties presented In each of the preceding tables have all been calculatedfrom conventional equations for homogeneous materials; ie., the difference in moduli of theindividual plies and the geometry of the stacking sequence are not considered in theseequations. Although the properties defined in this way are useful for quality control and com-parison of materials, they do not represent intrinsic material properties of the compositematerial being tested. It the mechanical properties of the individual plies of a laminated beamare known, the response of the number can be predicted. Therefore, to obtain a measure ofthe accuracy of the experiments and the quality of the fabrication processes, It Is Importantto correlate the data in a particular table. Furthermore, since it is difficult to measure thelongitdinal strength of a composite containing high strength filaments in a tension test, onecan estimate this property from the results of a flexural test. In view of this discussion, theflexure results will be analyzed by the equations presented in Reference 2.

The longitudinal and transverse moduli of a unidirectional ply, EL and ETs respectively,

can be determied from the flexural moduli (moduli computed by the use of the homogeneousbeam expression of the 00 and 90 bidirectional beams. However, ET Is very low, and the cal-

oulation for this quantity Is subject to a large ipercentage) error due to small variations inthe measured data. Hence the value of ET measured in the tension test was utilized, and the

average value of EL calculated from the flexural modull of the 0" and 90 bidirectional beams

was employed for subsequent calculations. The value EL is determined by the expression

F

where 9i is Youg~s modulus of the Ith ply in the direction of the beam axis,

nF :8 " El (3Me -3i +1) (2)

i:1and 3' is the flexural modus, N - 2n Is the number of plies, the top (and bottom) ply of thebalanced laminate being represented by I = n.

The longitudinal tensile strength of a unidirectional composite can be estimated from therep ed strength of the V bidreotional beam. Let gr'represent the maximum stress given byth homogeneou flexure formua, i.e., the value given in the preceding tables. The longitudinalstrength oc a -1d recticnal compositeaIn can be aprby

"N5 Efn (3)m" F

L9tt10 r' represet the conventional calculation for Interlaminar shear stress and re,

the corrected value as disoussed In Equation 2. then r' and rm and related by

l* NMr r (4)

m F

18

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AFML-TR-67-3"7

where nH 4 1" Ei(2i -1} (5)

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Applying Equations 1 through 5 to the reported data yields results shown In Table XX.

The quantities in parentheses in Table XX represent experimental values reported for theunidirectional flexure tests In the present paper. Good agreement is observed here betweentheory and experiment. Poor correlation, however, is noted between the C composite tMi-directional and bidirectional beams containing approximately 56% filaments (Table XV).This is evidently an example of the difficulties in fabrication and data scatter of filamentproperties, which were noted earlier in this report.

The flexural moduli of bidirectional laminates may be used as a means of comparingmaterial performance; however, a stacking sequence of fixed identity should be used In thecomparison. To compare the A composite flex properties to the analogous data for the othertwo materials, the former were recalculated through Equations 1, 2, and 3 on the basis of an8-ply bidirectional laminate. The moduli of the Thornel 25 epoxy 0 and 90" bidirectional

66beams and the reported strength of the 0" beam are thus found to be 7.8 x 10 , 3.8 x 10.,

3and 79.7 x 103, respectively, which are more realistic figures for comparative purposes

than the respective values of 7.2 x 10,6 4.4 x 106 , and 73.4 x 10 3 as reported In Table XU.The differences are not large in the present case but they would be more pronounced as thenumber of plies decrease.

Reasonable correlation seems to exist between the elastic moduli measured In the flexureand axial experiments. Typically, for the span-to-depth ratio of 20 employed, th longitudinalmodulus measured in the flex tests is lower than that determined from the axial t if theeffect of shear deflection is considered (Reference 2). these values are brought into closeragreement.

FABRICATION

The epoxy resin matrix system utilized throughout this studly oonsited of 100 parts byweight ERL 2256 resin, and 27 parts by weight ZZL 0820 hardener, dissolved In 80 parts byweight methylethyl ketone (ME1K.

The continuous graphite yarns were passed through a resin pot and wound about a marelcovered with a removable Teflon-coated tape. A tension of a o ately 1/4 lb was madn-tained from takeoff to mandrel take-up. The MEK was allowed to evaporate and Ow resinwas B-staged at room temperature (usually 24 hours) which imparted ood handling harao-teristice to the resultant prepreg tapes.

Since only short lengths of RAE tow were available, the tows were ladUvidually aliped ana Teflon-coated fabrio, affixed at the two ends by means of maskin toep, Impeat byspraying of the catalysed resin solution, and B-staged to a hadln consistemoy similar tothose preprep that were filament wound.

Prepreg tapes of both the continuous and discontinuous filaments w cut to fit variousmolds and laid-up in two confguratin, Le., asdlreotlal ad b-d1reotiomal. The mastityof the bidirectional panels were symmetrically laid-up with a V" and 90 alteruation of plieswith respect to de central axis of tho panel.

The owning cycle oonslsted of 2 hours at 1so'F and 4 hours at S0"F. A pressure of 100 PSIwas maintained throughout the cycle.

19

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A T IM L -T R -6 7 - 36 7 T B E X

MATERIAL PROPERTIES COMPUTED FROM FLEXURE TESTAND COMPARISON VVTH EXPERIMNT

ZL' 106 PSI i' 103 PSI Tins do,0 PS ~r . goo. 1O03 PSI

hhber A Composite 10.5 (13.1) 111 (111) 2.89 3.22

V/ F 63

Fiber BComposite 17.16 6.1) 127(121) 2.80 3.45

V/vF 62 56

Fiber C Composite 22.1 (21.6) 81.7 (80.2) -- --

V/ F 66

TxWTDO

Mechbanical property data me obtained usinlg procedures of Federal Test SpecificationL-P406b an a general guide. Flezural tests were conducted usinlg the t~hree-point loading

* method with a span-t-eh ratio of 20 to 1. Tensile modulus values were determined by* testing rectangular coupons. whereas "dog-boed" specimens wer employed for the de-

temination of tensile sbwegi. Short-beam sheer valvss ware obtained by the three-point* loadig method with samples of a 6-to-i span-to-dept~h ratio. Sbar modulus tests were

pr 9 orzume on as-prepared panls according to the plate twist methiod disc~ussed by Tual(Hssrewe 3). Comupression moduli were obtained frmrectangular coomPons and strengtvalues haom "dog-boned" samplges. Composite denisities were m2easured by water Immersion.Volum percent fiber calculatious were acquired assmeing a vodidlsse composite, via a resindissolution technique developed by Kdanm~lr (Reference 4). Data not reported herein has

indcaedthat the gaaphite composites describe have void ontents of only a few tenohe ofa;, peren.or less.

Unless; ofrwtse speolfied. tests performed on barectional composites were with t00

The fatiguel were comnuctIedI using a fhenck fatigue machine whichki equl~ed wtautomatict mean and alturation load maintainers. The fatigue cycling consisted of tensileloading to a particular stress level and returning to 1/10 of the original tensile stress Im-posed. All toste were lperazormed at room temperature and at a frequency of aroimftely2200 cycles per minuts.

Generally. on ballistic impact test consistedl of imparting a 0.174-hicbdiamete. opper-coated steel B-B projectile at a 2 In. x 2 in. Panel clamped In a vise. A Denjaina air rifewhich has baee modified to atilize a compressed nitrogmn gas sipply was mued. A pressureregulator was used to control the as pressures whlich in turn, determined the velocityOf the projectile.

20

AFAL-TR-67-367

SECTION m11

CONCLUSIONS

The previously reported oontention that graphite fibers appear of high promise for thedevelopment of high performance plastic composites is felt to have been further substantiatedby this more recent effort.

Fundamental mechanical analyses of current experimental graphite composite data appearsto reinforce this assessment.

Prolonged water boil of Thornel 25 and Thornel 40 graphite fiber reinforced epoxy resinmatrix oomposites did not result in substantial decreases in flexmral properties.

Several uncatalyzed epoxy resins were applied as finishes to Thornel 25 graphite yamsdaring yarn preparation. The epoxy finishes varied both in molecular weight and epoxideequivalents and were applied In amounts of about 0.6 to 0.9 weWgh peroent. Subsequentmechnical property dete of epoxy composites yielded results equivalent to thoseobtained with yarns cmoventionally coated with 0.1 weight pereont of polyvinyl alcoho Allthe coated yarns were apoitely equivalent in handl characteristics when mechanicalhandling was avoided prior or daring resin coating application to the yarns.

-ompo s pp from continuous Thronel 25 and Thornel 40 yarns by pe windingexhibited better general trnslafton of fiber properties than did omposite prepared fromthe short lengths RA graphite tow. In part, at least, ts soms attributable to the generallypoorer composite quality associated with the hand lay-up technique which was used in fabrica-tion of oomposites with RAE tow.

Shortý-bea shear tests of grapite composites typically rsults in failure modes other thntaterlaminar shear and were reported as shear stress values. However, a clear tre-n ofdecreasing oomposite shear values with increasing graphite fiber moduli seems evident.Aocordingly, efforts are now underway to improve the shear values at graphite compositesas well as other mechanical properties at composites, by fiber treatents to promote fiber-resin oadting.

Initial, fatigue data Indicate epoxy resin graphite composites to be superior to comparableberyllium and glass filament reinforced composites.

preliminary ballistic tmpact data sh graphite composites to sustain higher impact load-nsp than glass composites ad lower impact loadings than berylium omposites.

21

t

AFUL-TR-67-367

REFERENCES

1. R G. S.ao, R. T. Sohwartz,, .p t FbM Reinformd ComposiMte, presented at the10th National SAMPE Symposium and Exhdbit, San Diego. California. VoL 10. November1967.

2. N. J. Pagano, Analysri of the Flexure Test of Bidirectional Compoejts. Journal of Com-posite Materials, Vol. 1. No. 4. Ootober 1967.

3. Stephen W. Tsai. pgeimnetal DetermiMnaio of the ElastiO Behavior of Orthotropj•?te Journal of Engineering for bdustry. August. 1965.

4. Ronald Kzibnder. Detmning Fiber Content of Grphite Yarm-Epoxy Resin Compit,University of Dayton Tecbnioal Memorandum No. UDRI-TM-66-103, January. 1966.

22

UNCLASSIFIEDSecurity clessificatan

DOCUMENT CONTROL DATA.- R & D(Sseuity Classfiationjat,. tilft.. body of abstract and Irmfoxind ann.otation. Iust be entered oleo Of, oeraall ot a classified,

IORIGINATING ACTIVITY (Corporate awthor) s5. REPORT SECURITY CLASSIFICATION

Air Force Materials Laboratory UNCLASSIFIEDWright-Patterson Air Force Base, Ohio 26 GROUP

S. REPORT TITLE

GRAPHITE FIBER! EPOXY RESIN MATRIX COMPOSITES

4. DESCRIPTIVE NOTES (T4'P. . rofport and intIowive dotes)

S. AU THOR(SI (Pilot rulaw widda Initial, last trne)

Dauksys, R. J.. Pagano, N. N., Wpin, R. G.

a. REPORT DATE 7a. TOTAL NO. OF PAGES bG. No. or mrEs

February 1968 31 1h.CONTRACT OR GRANT NO. 94. ORIGINATOWS REPORtT NUMSERISI

AL PROJECT NO. 7340 AFML-TR-67-367

C.Task No. 734003 Ob. OjT'HEsRw EPORT NMOt (Any 0001' lINss I St MW~ be avelsdd

IC. DISTRIOUTION STATEMENT

This document has been approved for public release and sale; its distribution is unlimited.In DDC. Aval frm, CFSTL

$I. SUPPLEMENTARY NO0TES 12. SPONSORING MILITARY ACTIVITY

Air Force Materials LaboratoryWright-Patterson Air Force Base, Obio

IS. AUISTRACT

Four types of graphite fiber were utilied as reinforcements In a continuing experi-mental program of the fabrication andevaluation of epoxy resin matrix compoefsifte.sultantcomposite data and analyses continus to demonstrate a high promise for graphite fiberreinforced resin composites as high performance materials.

This abstract has been approved for public release and sale; it. distribution Is unlimited.

DD~..1473 UNCLASSIIE8"Multy Classifictiaon

UNCLASFIEDhmwlty ClaeainlctI.

Kay LINK A LINK S LINK CKEY WORld S

ROLE WSI ROLE WT ROLE WT

Graphite fiber/epoxy composites

Royal Aircraft Establishmet

Water boll

Mechanical properties

Epoxy costivg.

Ballistic Impact Test

Patigue

".--I I- -

UNLMFE

- cbe • u.