GLASS-RESIN INTERACTION OF FILAMENT-WOUND COMPOSITES P
.
G
.
C0
N RA D A N D F
. J.
DA R MS
, Aerolet-General Corp., AzuJa. Calli.
A statistically designed program incorporating three types of glass and 1 1 variations in the resin was followed in fabricating
120 filament-wound pressure vessels.
Hydroburst data from these units were combined with
measured physical properties of the material and analyzed by multiple linear regression methods. Significant material properties and characteristics were determined and utilized in developing performancepredicting equations for the pressure vessels. These data are proposed as a set of criteria for the development of future resin systems of high performance internal pressure vessels.
the performance of a filament-wound pressure mostly by the strength of the glass fibers. the actual strength achieved is contingent on the orientation of the filaments, with respect to the membrane forces in the composite structure, and the physical continuity of the filaments. The resin is relegated the secondary structural role of augmenting the fiber efficiency by transferring the shear forces within the composite, maintaining the structural shape and filament orientation, and protecting the fibers from interfilament damage and from degrading environments. 'The efforts of this investigation were directed toward achieving higher performance and more reliable filament-wound structures by providing a better understanding of the role of each constituent in a filament-wound pressure vessel, determining the physical properties and characteristics of the materials that are most influential in establishing the performance of the composites, and assigning an index of performance to each significant material property for comparative studies in the development and selection of new materials. Careful consideration in the selection of matrix materials for filament-wound pressure vessels is necessary if the maximum performance is to be achieved. Resin? which is currently used by the filament winding industry in the fabrication of rocket motor cases, enables the glass roving to develop strength efficiencies of approximately 80% in the head areas and 90% in the cylinder as compared to the unidirectional strand THOCOH
A- vessel is influenced
strength. However, test data compiled during the initial phase of this investigation indicated that filament efficiencies varied concurrently with changes in the formulation and physical properties of the resins. T h e effect of these changes on the strength of filament-wound composite structures was noted early in the development of filament-wound 18-inch-diameter test chambers. Hydroburst data from these test units are illustrated in Figure 1 as longitudinal filament stress us. resin elongation. The 27Tc variation in filament strength, which is indicative of the degree to which the matrix material controls filament performance, demonstrated the need to understand the functions and interactions of the glass and resin in filamentwound composites. Materials
The types of glass fiber selected to provide variation in physical properties of the primary structural constituent of the composite were Owens-Corning's S-994 glass/HTS finish. E-glass/HTS finish, and YM-31 glass/HTS finish. Each glass was in the form of 20 end glass-filament roving. Four distinct resin systems (epoxy-fatty acid, epoxy-amine, polyester, and epoxy-anhydride) were selected and 11 formulations were developed. using ultimate elongation as the basic criterion for selrction of the resin formulation. This formulation criterion \\as arbitrarily based on the performance of the 18-inch-diameter chambers previously discussed. The resin formulation modifications necessary to produce changes in ultimate elongation also resulted in considerable changes in other physical properties. The characteristic profile of each formulation \\as defined by five physical properties : elongation, ultimate tensile strength, modulus of elasticity, toughness (area under the stress-strain curve), and notch toughness (ability of resin to sustain loads despite imperfections in the matrix), Material Property Development
A
7
%
ELONO
EP2OI/EYPOL IO22/T
9
0 25 ' 9 ELONO
O O W DER
322/ EPICURE 9 8 5
0 6 0 % LLONO
D O I DER
322/EPICURE
85s
I
10
20
I
I
30
40
RESIN ELONOATION
ao
!
I
J
60
70
1%)
Figure 1 . Comparison of filament stress and resin elongation for 1 8 -inch-diameter chambers
Glass Properties. E/HTS, S-994/HTS. and YM-31 glass rovings were selected to provide variation in the physical properties of the prime load-carrying member in the composite. T h e properties shown in Table I were obtained by unidirectional tensile tests developed a t Aerojet-General Corp. (AGC). Glass roving types were serialized: E for E/HTS, X for S-994/HTS, and Y for YM-31 glass rovings. The strand data for E and X fibers are based on tests from approximately 150 rolls of each material. The Y glass data represent the average of 20 sets of data. The strain rate sensitivity of the glass fibers must be considered in any research program involving filament-wound VOL. 5
NO. 1
MARCH
1966
9
Table 1.
Glass Serial N o .
x E Y
Physical Properties of 20 End Glass Filament Roving Strand D a t a Ultimate tensile Standard Modulus of strength, k.s.i. deviation, k.s.i. elasticity, p.s.i. 440 30 12 x 106 352 27 10.8 X lo6 314 1 6 . 0 X 106
Glass T y p e S-994/HTS E/HTS YM-31 /HTS
Table II.
Nomenclature
Density, Lb.ICu. Inch 0.088 0.092 0.101
Abbreviations and Designations for Resins and Curing Agents T y f e of Chemical Manufacturer
EP-201 DOWDER-332 Paraplex P-43 EPON 828 EPON 1031
RESINS Cyclic, aliphatic-based epoxide Diglycidyl ether of bisphenol .4 Polyester, orthophthalate-based Diglycidyl ether of bisphenol A High-functionality epoxide
Empol 1022 T-9
CURING AGENTS Dimer acid Stannous octoate
Epicure 855 MEKP Paraplex P-1 3 MNA BDMA HHPA
Aliphatic amine Methyl ethyl ketone peroxide Polyester, containing styrene Methyl nadic anhydride Benzyldimethylamine Hexahydrophthalic anhydride
Union Carbide Corp. Dow Chemical Co. Rohm & Haas Co. Shell Chemical Co. Shell Chemical Co. Emery Industries Metal & Thermil Chemical Corp. Jones-Dabney Co. Lucidol Division Rohm & Haas Co. National Aniline Division Maumee Chemical Co. National Aniline Division
SOD
500
400 Y)
300 W
!!
d 200
/’ IO0
/ /
-’ /
0
~
1.0
2.0
3 .O
composite structures. T h e ultimate tensile strength of the glass fibers, obtained a t various rates of elongation, is plotted in Figure 2. This figure shows that increases in filament strength are to be expected when rapid rates of loading are employed. A strain rate for testing all material specimens and composite structures (4-inch-diameter pressure vessels) was set a t 1% per minute. This value positions the intercept of the 1% strain rate curve with the strand failure curves where small differences in rate of load will not appreciably affect the test results. Resin Formulations and Properties. Three of the four resin systems (epoxy-fatty acid, epoxy-amine, and polyester, designated resin systems 1, 2, and 3, respectively) were selected to provide the program with large ranges in matrix properties. 10
-i
NOTE: I. TESTING WAS CONDUCTED I N ACCORDANCE W I T H AGC STRAND T E S T ( V I N Y L COATED ROVING1 2 A L L MATERIAL W A S 2 0 END ROVINQ
/
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
4.0
5.0
6.0
7.0
8.0
A list of the resins and curing agents is included in Table I1 for reference. Each resin formulation was tailored to have a n elongation within certain limits by altering the ratio of resin and curing agent in the matrix system. The limits of elongation were selected as 0 to lo%, 10 to 20%, 20 to 357,, and above 35y0 and are referred to as elongations 1, 2, 3, and 4, respectively. The testing methods used in determining physical properties of the resins were generally in compliance with the American Society for Testing Materials (ASTM) procedures (7), except for properties not covered by ASTM. The standard ASTM conditioning procedure D 618-61 was followed in preparing each set of specimens for test. The elongation, ultimate tensile strength, and modulus of elasticity were obtained by ASTM
D 638-61T-Type I.
Stress-strain curves were developed for each resin formulation and then the toughness (area under the stress-strain curve) was calculated. Each of the stress-strain curves is present for comparison and to illustrate the characteristic profile of the resins in Figures 3 through 6. IO
1 I
The notch toughness is a property adopted by metallurgical technology to evaluate the effects of cracks, notches, and imperfections in metallic specimens. As related for use in resins, notch toughness evaluates the capacity of the resin system to sustain loads despite imperfections in the matrix. The specimen, like the tensile test specimen, is cast of resin but is rectangular (1 x 8 x l / 8 ) instead of in the conventional dumbbell shape. A center notch is machined in the casting by drilling a 0,187-inch-diameter hole in the center of the specimen, cutting V-grooves on each side of the hole perpendicular to the principal load axis 0.080 inch deep, and initiating cracks by a sharp instrument at the extremities of the grooves 0.060 inch long; therefore, the total design length of the center notch is 0.467 inch. The specimen is then stressed in tension until fracture. The notch toughness is calculated by the equation
where
K = notch toughness T = tensile load divided by cross-sectional area 0
5
IO
LO
I5
STRAIN
Figure 3. system
Stress-strain curves for epoxy-fatty
Figure 4.
30
25
U N . / I N . I X IO-'
W'
=
a
=
acid resin
specimen width ' j p total crack width
Stress-strain curves for epoxy-amine resin system
m
$TRAIN IIU./IU.)
0
5
IO
I5
STRAlU I l U . / I N . I
Figure 5.
25
20 X
30
35
16'
Stress-strain curves for polyester resin system
40
x'id'
Figure 6. Stress-strain curve for epoxy-anhydride resin system VOL. 5
NO. 1
MARCH 1966
11
Table 111.
Formulations and Physical Properties of Resin Systems
Ultimate Tensile Strength, P.S.I.
Modulus of Elasticity, K.S.I.
Toughness, P.S.I.
h'otch Toughness, P.S.I. .\/In.
6.2 15.3 25.2
6594 3366 3405
267.0 187.0 163.4
295.1 400,5 664.6
442 401 354
6.6 10.5 24.9 58.4
8225 6520 4955 2150
334.2 260.3 207.6 86.9
420.3 543,7 958,O 982.9
345 1497 921 41 8
6.2
5387
250.4
240.4
361
18.6 30.5
2003 1757
57.4 38,2
276,5 380.1
348 355
3.0
11,831
479.7
198.3
377
Resin Serial
-Yo. 1 11 21 31 2
12 22 32 42 3 13 23 33 4 14
Chemical Formulation, Parts by Weight Epoxy-fatty acid (EP 201/Empol 1022/T-9) 100/35/1,5 100/39/1.5 100/41 / l . 5 Epoxy-amine (DOW DER 332/Epicure 855) 100/30 100/60 100/85 100/100 Polyester (P43/MEKP) 100/2 (P43/P13/MEKP) 50/50/4 40/60 /4 Epoxy anhydride (Epon 828/Epon 1031/MNA/BDMA) 50/50/90/0.5 Table IV.
Elongation,
5%
Hydroburst Data of 4-Inch-Diameter Composite Specimens
GlasJ Comt)osite Strand Burst SpeEimen Strength, pressure, Std. deu., Sertal N o . K.S.I. p.s.1. p.s.1. 329 3120 El 1 93 329 3070 E21 121 329 2920 E31 136 329 3822 El2 245 329 4088 E22 35 329 3919 E32 207 329 3910 E42 242 329 3525 El 3 130 E23 344 3470 98 E33 329 3425 115 468 3878 x11 137 420 3810 X2 1 102 468 3494 X31 151 420 4800 x12 65 420 4930 x22 21 1 420 4790 X32 124 420 4638 X42 21 464 4825 X13 100 420 4319 X2 3 27 464 4035 x33 43 469 4721 X14 56 315 3400 Y12 84 315 3020 236 Y22 315 3370 Y32 87 Each set of data represents average of 5 test specimens.
The resin formulations and properties are presented in Table 111. As noted by the serializing system, the first number indicates the resin elongation group, whereas the second number indicates the particular resin system. This system was combined with the glass designations E, X, or Y , to serialize composite specimens-e.g., E-42 indicates a composite specimen fabricated with E-glass and an epoxy-amine resin system with over 35% elongation. Composite Properties. The effect of variations in physical properties of the glass and resins on the strength of filamentwound composites was obtained by hydrobursting 4-inch-diameter pressure vessels fabricated with the materials defined above. The 4-inch-diameter test specimen as shown in Figure 7 was selected because of its excellent reproducibility and low cost. Major design features include a bidirectional filament winding 12
IBEC PRODUCT RESEARCH A N D DEVELOPMENT
Hydroburst Data Ult. lone. U t . lone. jifilarnent compos ite stress, k.s.i. stress, k.s.i.
252 248 236 31 1 332 317 317 285 281 277 324 319 292 400 412 400 388 403 360 339 395 284 251 281
161 159 150 199
213 205 203 182 179 177 209 205 189 258 266 258 250 260 232 217 254 182 162 181
Hoop ,filamknt stress, k.s.i. 243 239 227 297 318 305 304 274 269 266 317 31 1 285 390 401 390 377 392 351 330 384 277 247 274
Cylinder composite, stress, k.s.i. 109 107 102 132 143 137 137 123 121 120 140 138 127 176 178 172 167 175 156 145 171 123 110 122
pattern, balanced-in-plane head contours, and re-usable fabricating mandrels. The balanced-in-plane head contour, developed primarily for rocket motor cases ( 3 ) :was employed to maintain a balance between the forces in the shell of the pressure vessel and the directional strength developed by the filaments. The performance of this vessel configuration when fabricated with S-994 glass filament and Shell 58-68R epoxy resin (resin system 14) is shown in Figure 8. The standard filament winding pattern was adjusted so that the strength of the cylinder was slightly redundant; therefore, all failures occurred in the head sections. This facilitated a study of the composite crazing phenomenon, which is believed to have a significant effect on pressure vessel performance. Because all specimens failed in the head sections. the ultimate longitudinal filament stress could be calculated for each of the
7
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_I.a:-j Figure
7.
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OL.3,
Cross section of composite test specimen
E, .""
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Figure 9. Effect of resin system variation on performance of composite test specimen
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~spsc~menrFigure 8.
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-%
Performance and reproducibility of test vehicle
pressure vessels. All the effects of material variations on performance of the composite specimens have been based on this parameter. For statistical purposes, fivc specimens incorporating the same combination of glass filament and resin matrix were fabricated and hydroburst a t the same time. A total population of 120 pressure vessels was required to complete the study.
Analysis of Experimental Dala
Test Results. Hydroburst data from vessels fabricated with the different glass filament and resin systems are presented in Table IV and represented by bar charts in Figure 9. T h e range of data variation within composite specimens using resins with the same constituents but various formulations VOL 5
NO. 1 M A R C H 1 9 6 6
13
is shown as R values. These data substantiate the original hypothesis that the resin has significant influence on the stress attained by the filaments. A review of these data also indicates that pressure vessel performance can be attributed to :
1. 2. 3.
Changes in type of glass Changes in type of resin Changes within the resin systems
37% 23% 6 to 17y0
The effect of resin elongation on longitudinal filament stress of the 4-inch-diameter vessels is shown in Figure 10 for comparison with data initially developed from the 18-inch-diameter vessels (Figure 1 ) . The similarity of the curve configuration indicates that material scale-up factors exist for larger filament-wound pressure vessels. A concise analysis for geometric scale-up factors was performed for the Air Force ( 3 ) through interpretation of data obtained from the same type of subscale specimens used in this program. The validity of the geometrical analysis was demonstrated by the designing, fabrication, and hydroburst of two 44.3-inch-diameter filament-wound chambers, both of which burst a t exactly the design burst pressure. These full scale units were fabricated with X-glass and resin system 14. The phenomenon of resin craze was studied through the medium of sequence photography. Pictures were taken of each vessel a t 5-second intervals during the pressurization cycle. The craze threshold pressure, together with the corresponding longitudinal filament stress a t that pressure, was determined for each vessel. The craze thresholds of units incorporating polyester and epoxy-fatty acid resin systems were difficult to discern and therefore (because of the uncertainty of the data) were not included in the crazing analysis. Table V summarizes all the test data. Definition of Significant Characteristics. Multiple linear regression analyses were performed utilizing the benefits of a high speed digital computer program to provide a statistical evaluation of the 4-inch-diameter specimen test data and
Head Crazing Threshold Clnch-Diameter Composite Specimen Longitudinal Specimen Craze Pressure, Filament Stress, Serial N o . P.S.I. K.S.I. 52.7 653 El 1 32.3 399 E21 20.3 252 E31 52.2 640 E12 E22 804 56.2 E32 Not discernible E42 2148 174.0 El 3 489 39.4 E23 579 46.8 E33 781 93.0 x11 321 26.8 x 21 544 45.4 X31 242 20.2 x12 685 55.8 x22 875 73.0 X32 1208 101 . o X42 2213 185.0 XI 3 194 15.3 X23 372 30.9 x33 289 24.2 X14 158 13.2 Y12 1070 89.2 Y22 1053 87.4 Y32 1249 104.0
Table V.
Each set of data represents average of 5 test specimens.
material variations. The objectives of these analyses were to determine which glass and resin properties had significant effects upon the performance of the pressure vessels. determine which material characteristics controlled the crazing threshold phenomena. and provide predicting equations which would relate pressure vessel performance to glass and resin properties that contribute significantly to vessel efficiency. A regression analysis does not define cause and effect relationships between variables, and therefore does not provide a physical interpretation of the phenomena analyzed. However, it demonstrates whether particular variables are highly correlated and it progressively orders the independent variables in decreasing order of importance. In addition, knowing these significant factors allows the investigator to establish predicting equations relating the independent variables to the dependent variable (in the present analysis the significant resin properties are related to the performance of the pressure vessel). The addition of each variable to the predicting equation is based upon the residual variation remaining from the total variation explained by all the preceding independent variables in the equation. .4 more complete and precise 'explanation of regression analysis methods has been published ( 2 , d , 5). The initial analysis indicated that the strength of the specimens could be predicted to within 707, when the ultimate tensile strength of the glass fibers was the only physical property considered. When specimens incorporating Y-glass were eliminated because of questionable material quality, the ultimate tensile strength of the fibers accounted for 627, of the variation. The physical properties of the resin influencing the strength of the composite structure were ranked in order of importance by the amount of variation as follows: 1. 2. 3. 4. 5.
Notch toughness Square root of resin content by volume Ultimate tensile strength Elongation Toughness (area under stress-strain curve)
I&EC PRODUCT RESEARCH AND DEVELOPMENT
6% 2% 2% 1%
When the cross products of resin properties were included in the regression analysis and the resin content was neglected, the rank was:
1. Notch toughness X toughness 2. Ultimate tensile strength 3. 4. 5. 6. 7. 8. 9. 10.
Elongation X notch toughness Toughness toughness Ultimate tensile strength Elongation X toughness Notch toughness notch toughness Ultimate tensile strength Elongation X ultimate tensile strength Elongation
x
x
In addition to ranking the physical properties and their cross products, a regression equation was established. The equation which predicts the filament efficiency (developed longitudinal filament strength divided by measured strand strength) for composite structures with the same approximate resin content is: F.E.
=
-15.9861
+ 0.8567 ( N T X
4.3326 ( T ) X 3.7727 (To) X lop4
To)
+ -
- 3.7269 (EL X AVT)X
+ 0.1484 ( T X
(EL X To) X 1 . 1 8 8 ( T X d V T )XlO-5-
14
15%
To) X lo-' 4- 0.3859 X
+ 5.6719 (.YT) X
-
1.4993 (EL X T)10-4+1.3865(EL)
UOTE :
~
I.-
0 REPRESENTS
2-
I
I 0
E H T S Q L A S S ROVINQ S 994 QLASS R O V I N Q
I
I
1
1
20
40
80
00
hESlN
Figure 10.
REPRESENTS
ELONQATION
(%I
Effect of resin elongation on composite performance
f I L A M E N T EFFICIENCY
where
F.E. = filament efficiency
* LONPITUDINAL f I L A M E N T
STRESS AQC STRAND T E S T STRLNQTW
1.10 7 1 ,
- filament strength in composite filament strand strength -
~
>YT
TO ?'
EL
d&
