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Room temperature creep and failure of borsic filaments
ROOM TEMPERATURE CREEP A N D FAILURE OF BORSIC FILAMENTS*
R. H. ERICKSEN
Composite Materials Development Division H, Sandia Laboratories, Albuquerque New Mexico 87115 (USA) (Received: 25 March, 1974)
SUMMARY Creep and time-dependent failure of Borsic (silicon carbide coated boron) filaments have been investigated at 25°C. Tests were carried out using individual filaments and specimens which consisted of a row of parallel filaments. The .filaments displayed anelastic behaviour as the creep strain recovered with time upon removal of the load. The amount of creep was small in relation to the strain obtained on loading. The creep failure strain o f parallel.filament specimens was mainly dependent on the applied stress, being lower than that for specimens which failed in tensile .tests. The slopes of the strain-log time curves, and the amounts of creep strain of parallel filament specimens tested at a given stress, did not correlate with the times to failure. There was, however, a relationship between the failure time and the homologous stress.
INTRODUCTION Creep and stress-rupture have been observed in several b o r o n or Borsic (silicon carbide coated boron) reinforced composites at both r o o m and elevated temperatures. 1 - 7 Relatively little is known, however, a b o u t creep o f the filaments themselves and no references have been found concerning time-dependent failure o f the filaments. Ellison and Boone s reported that at 816°C (0-43 Tin) secondary creep rates o f b o r o n filaments varied f r o m 5 x l0 - 4 to l × 1 0 - 2 h -1 for stresses between 1030 and 1900 M N / m 2. A t 538°C (0-35 Tin) Rose and Stokes 9 observed that the creep rate at 1 5 1 0 M N / m 2 decreased f r o m 2 x 1 0 - S h - I at l h to a nearly steady-state rate o f 1.4 × l0 - 5 h -1 after 15 h. A n t h o n y and C h a n g 1 obtained * This work was supported by the United States Atomic Energy Commission. 173 Fibre Science and Technology (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
174
R . H . ERICKSEN
creep data at 260°C and 690 M N / m 2 and reported creep rates between 2 and 7 x 10- 6 h - 1. In a previous study v it was found that Borsic filaments exhibited anelastic creep at room temperature and the strain increased linearly with the logarithm of time. The room temperature creep rate decreased linearly with time and typical rates at a stress of 1400 M N / m / were 3 × i0 -5 h -1 at 1 h and 3 x 10 -7 h - 1 at 100 h. The present investigation was undertaken to examine further the time-dependent behaviour of Borsic filaments at room temperature and to determine if failure under constant stress could be related to filament creep or to the filament tensile strength.
EXPERIMENTAL
Individual filaments and parallel filament specimens were prepared from sheets of Borsic-aluminium monolayer tape. The tape was fabricated by filament winding 0.11 m m Borsic on a cylindrical mandrel and then depositing 1100 aluminium by plasma spraying. Individual filament specimens having gauge lengths of 1.3, 5, 10 and 38 cm were obtained by dissolving the aluminium matrix from strips of monolayer tape using 5 per cent N a O H solution. Preparation of the parallel filament specimens has been described previously; 7 however, it is also summarised here. Strips of monolayer tape 25 cm long by 1.3 cm wide were cut with the filaments parallel to the length of the tape. The aluminium was then dissolved from a 1.3 cm long section across the centre of each strip using 5 per cent N a O H solution leaving a row of paraUel filaments (54 to 61 filaments). The specimens were placed on a teflon sheet and strain gauges bonded to the rows of filaments using conventional mounting techniques. The strain gauge epoxy did not adhere to the teflon and the specimens could be readily removed. Creep and tensile tests on individual filaments and parallel filament specimens were performed in air using an Instron load frame equipped with a feedback control system. Some creep tests on parallel filament specimens were also carried out using an Arcweld deadweight load frame. All specimens were held in wedge action grips with protective teflon inserts. The rate of loading in all tests was 10 MN/m2/sec. Some incremental stress tests were performed by increasing the stress after a given time to either a higher creep stress or until the specimen failed. Strain was not measured on individual filament tests except for the specimens having a 38 cm gauge length. In this case quartz whiskers were bonded to the filaments to define the gauge length and to serve as reference markers. The elongation was measured using a Gaertner optical extensometer having a resolution of about 2 x 10- 5. Strain was measured on all the parallel filament specimens using strain gauges and the resolution was about 2 × 10 - 6 . Steps taken to ensure the reliability of the strain gauge measurements were discussed previously.7 The test temperature was usually 25 + I°C and did not affect the creep data for
ROOM TEMPERATURE CREEP AND FAILURE OF BORSIC FILAMENTS
175
tests lasting less t h a n a b o u t 100 h. F o r longer tests the d a t a showed greater scatter with increasing time a n d this could be c o r r e l a t e d with r o o m t e m p e r a t u r e fluctuations; however, t e m p e r a t u r e v a r i a t i o n alone could n o t have caused the observed creep strains.
RESULTS AND DISCUSSION Tensile properties I n o r d e r to establish reference p a r a m e t e r s for c o m p a r i s o n with the t i m e - d e p e n d e n t data, tensile tests were p e r f o r m e d on b o t h individual filament a n d parallel filament specimens. The d a t a are s u m m a r i s e d in Tables 1 a n d 2. A s expected, the tensile failure o f i n d i v i d u a l filaments occurred over a range o f stress, and the m e a n strength decreased with increasing gauge length. T h e strength d a t a d i d n o t follow a n o r m a l d i s t r i b u t i o n as evidenced by a n o n - l i n e a r p l o t on n o r m a l p r o b a b i l i t y paper. A W e i b u l l d i s t r i b u t i o n p l o t I° ( t w o - p a r a m e t e r ) for 1-3 a n d 5 . 0 c m gauge length Borsic filaments is shown in Fig. 1. T h e points are r e a s o n a b l y linear at the higher
TABLE 1 TENSILE D A T A FOR BORSIC FILAMENTS
Gauge length (cm)
Mean failure stress (MN/m2)
Standard deviation (MN/m2)
Minimum failure stress .(MN/m2)
Maximum failure stress (MN/m2)
Number o f specimens
1"3 5"1 10"2 38.1
2831 2824 2804 2576
427 207 165 296
1040 2163 2225 1020
3272 3183 3011 2908
50 50 50 50
TABLE 2 TENSILE DATA FOR PARALLEL BORSIC FILAMENT SPECIMENS
Specimen number Number o f fibres 1 2 3 4 5 6 7 8 9 10 Mean Standard deviation Coefficient of variation
56 58 59 61 56 57 56 56 56 54
Failure stress (MN/m 2)
Failure strain x 106
Modulus (GN/m 2)
2306 2189 2115 2405 2363 1729 2170 2326 2150 2210
5860 5920 5070 6220 5870 4250 6370 6000 5470 5500
383 410 422 405 412 400 412 399 412 405
2196 191 0.087
5650 620 0"109
406 10 0.025
R. H. ERICKSEN
176
2
i
!
I
.T *
o 1.3 cm gage * 5.1 cm gage • Ref. I 0
*
o
o
* o ~o *o *o *o *o
0 -1 I
Ln L.n _'~-p"(~ i /
o* -2 0
-3
0
-4
-5
Fig. 1.
I
I
7J6
I
i
78
i
i
80 Ln
i
82
8.4
Weibull distribution plot of Borsic filament strengths.
stress levels but deviate from linearity at lower stresses. The use of a three-parameter Weibull function did not significantly improve the linearity of the plot. The tensile strengths of 0,11 mm boron filaments (5 cm gauge length) were reported by HerringlX and are also plotted in Fig. 2. All three sets of data show similar behaviour, with the boron data having slightly greater curvature and being displaced from the Borsic data by an amount corresponding to a strength difference of about 690 M N / m z. This difference is in good agreement with the relative strength of 0.10 mm boron and 0.11 mm Borsic filaments reported by the manufacturer. 12 The strength of a bundle of filaments was calculated from the Weibull distribution parameters for the 1-3 mm gauge length individual filament data, using the method outlined by Corten.13 The calculated bundle strength was 2380 M N / m 2, which is higher than the mean strength of 2200 M N / m 2 determined experimentally.
0.41
: " .. 0.40
~ gOQQQ
PERCENT STRAIN
O
O D
OOQQ
OO
OIDIQD
@~
Q O
I
Ow
0.39
.01
I
•
I
L
I
I
IO
I00
TIME
Fig. 2.
1000
(hr)
Creep curve for a parallel Borsic filament specimen.
ROOM TEMPERATURE CREEP AND FAILURE OF BORSIC FILAMENTS
177
Although the failure modes of the parallel filament specimens were not studied in detail, acoustic emission data indicated that few, if any, filament breaks occurred prior to failure. The failure process is probably dominated by the breakage of filaments in the weaker part of the strength distribution. The poorer fit of the data to a Weibull distribution in this region may account for some of the disagreement between the calculated and the experimental bundle strengths. Prevo and Kreider ~* have made similar comparisons between calculated and measured bundle strengths for 0.14 m m Borsic filaments and also found the calculated values to be higher than those obtained experimentally. Failure of the parallel filament specimens also occurred over a range of stresses; however, not enough tests were performed to enable a comparison between the normal and Weibull distributions. Stress-strain curves for these specimens were linear to failure or to the point where fibre breakage began. The mean modulus was 406 G N / m 2 from the data shown in Table 2. This value is in good agreement with the average value of413 G N / m z reported by Line and Henderson. 15 The coefficient of variation for the modulus data is lower than that for either the failure stress or strain--an observation also reported by Herring~6--and is consistent with the observation ~ that the elastic modulus of boron filament is almost completely independent of process conditions.
Creep Because of the small strains involved in Borsic filament creep the optical extensometer was of limited accuracy for individual filament tests. Nevertheless, the data obtained from the individual filaments did verify that the creep results from parallel filament specimens were representative of individual filament creep and not due to the mechanics of the test technique. Since precise strain measurements were obtained from the parallel filament specimens, they were utilised to obtain the creep data reported below. A typical creep curve is shown in Fig. 2. The creep strain can be described by a simple logarithmic time law and for this investigation the creep behaviour will be represented as e = ~ log t + c where e is strain, and ~ and c are independent of time, t. The creep rate ~ is ct/t and ~ is the slope of the strain versus log time plot or the strain per decade in time. The creep behaviour is anelastic and nearly all of the creep strain recovered upon removal of the load, as reported previously, v The recovery curve is also linear with the logarithm of time. When a specimen was crept, unloaded and held until most of the time-dependent strain was recovered, then reloaded to the original creep stress, the new curve closely followed the same strain-time path as the one obtained on the first loading cycle. Any influence of stress on the creep rate is reflected in the value of ct. Data obtained at stresses between 690 and 2070 M N / m 2 resulted in values of ~t that generally increased with stress; however, the variation in ~ between tests at a given
178
R. H . ERICKSEN
TABLE 3 CREEP COEFFICIENTS OBTAINED ON INCREMENTAL LOADING OF A PARALLEL BORSIC FILAMENT SPECIMEN (e=~logt +c)
Stress ( M N / m 2)
689 1034 1378 1723 2067
o~ × 10 6
7"2 15'1 9'4 18'4 14"1
c 3( 10 3
1'54 2'32 3"15 4"00 4'92
stress was about the same as the range of values obtained in all the tests. As a result, it was not possible to determine a stress dependence for the creep rate from these data. In order to reduce the scatter by eliminating specimen-to-specimen variation, the stress was increased stepwise on a single specimen. Creep data obtained at incremental stresses between 690 and 2070 MN/m 2 are shown in Table 3. At each stress increase the time was taken as zero to determine ~. The values of :t did not show a progressive increase with increasing stress, although the creep at each stress increment still followed a logarithmic time law. Additional work will be required to resolve the effect of stress on the creep behaviour as this result shows that the variation is not solely due to differences between specimens.
Time-dependent failure The time to failure of 10 cm gauge length individual filament specimens increased markedly as the stress was decreased below the range where tensile failure occurred. Table 4 shows data obtained at three stress levels. At 3010 MN/m 2 most specimens failed on loading and those which were loaded failed within one hour. Decreasing the stress increased the specimen life and fewer specimens failed on loading. At 2380 MN/m 2 the specimens which survived 3 h were loaded to failure and the data are shown in Table 5. There is no indication of any tensile strength degradation in the specimens pulled to failure as a result of the short creep exposure. The results of creep tests on parallel filament specimens at 1850 and 2010 M N / m z are summarised in Tables 6 and 7. Qualitatively the specimens exhibited the same stress-rupture trends as observed for the individual filaments, although the range of stress where stress-rupture occurred in times less than 100 h appeared to be slightly larger for the parallel filament specimens. At 1630 M N / m z a parallel filament specimen was unbroken after 850 h. Because of the wide variation in creep rates observed at a given stress, the creep tests at 1850 and 2010 MN/m 2 were carried to failure to see if any correlation existed between the creep parameters and the failure times. The amount of creep strain and the slopes of the creep curves obtained on the parallel filament specimens
ROOM TEMPERATURE CREEP AND FAILURE OF BORSIC FILAMENTS
179
TABLE 4 STRESS RUPTURE D A T A FOR INDIVIDUAL BOP.SIC FILAMENTS
3010 M N / m 2
Specimen number
2960 M N / m 2
Failure Failed on Failure time loading time (h) ( M N / m 2) (h) 2889
0.03
2990 2749
80
0"9
Failed on loading ( M N / m 2)
Pulled to failure at time t
72
3087 at t = 192 h 2990 at t=264h
2825 3141 at t = 234 h
1950
2735 2914 2735
0'1 2081 2976 2935
I1 12 13 14 15 16 17 18 19 20
Failure time (h)
2756
0"2 6 7 8 9 10
2910 M N / m 2
Loaded to failure at time t (MN/m2)
33 3066 at t = 48h
2997 2997 2942 2811 2921 2660
2170 2625 2770 2811 2900
0.7 2735 2398 2790
TABLE 5 STRENGTH OF INDIVIDUAL BORSIC FILAMENTS AFTER 3 HOURS AT 2740 MN/M 2
Specimen number
Failure stress (MN/mZ)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
3032 2846
Failure time at
2740 M N / m 2 (h)
Failed on initial loading ( M N / m 2)
0"2 2999 2935 2701 2859 2976 2956 2880 2914 0"01 2542 2935
180
R. H. E R I C K S E N
TABLE 6 CREEP D A T A FOR PARALLEL FILAMENT SPECIMENS TESTED AT 2 0 1 0 MN/M 2
Specimen number
Failure time (h)
1 2 3 4 5 6 7 8 9 10
37"0 4'4 3"5 0'9 4.3
Initial strain × 106
~x x 106
Modulus (GN/m2)
Failure strain x 106
4940 16.8 408 4980 13.1 404 4860 11-3 414 5020 2.6 401 5130 nil 392 Failed on loading at 1564MN/m2 5260 2.3 383 5040 0.14 398 4790 nil 420 4820 2-5 414
5-9 9'3 0'4 64.0
5090 5070 4970 -5110 5270 5080 -4830
TABLE 7 CREEP DATAFOR PARALLELFILAMENTSPECIMENSTESTEDAT 1850 MN/M2
Specimen number 1 2 3 4 5 6 7 8 9 10
Failure time (h) 40 26 2.3 5.1 2.5 158 1 4.2 0.3 >450
Initial strain x 106
c~ x 106
4610 4530 4790 4470 4570 4820 4560 4500 4380 4710
nil 2.0 6.5 4.3 nil 9.1 4.0 0.9 2.8 8.6
Modulus (GN/m 2) 400 407 385 412 403 382 404 410 421 391
Failure strain x 106 4670 4580 4820 4530 4590 5030 -4570 -4750
Loaded to failure at t = 450 h
w e r e c o m p a r e d w i t h the failure t i m e s a n d n o c o r r e l a t i o n c o u l d be f o u n d . T h e t o t a l strain at failure (creep strain plus i n s t a n t a n e o u s strain o n l o a d i n g ) was also c o m p a r e d w i t h the failure t i m e a n d the results are s h o w n in Fig. 3, a l o n g w i t h the failure strains o b s e r v e d f r o m the tensile tests a r b i t r a r i l y p l o t t e d at t = 0.01 h. A l t h o u g h t h e r e is s o m e t e n d e n c y for the failure strain to increase w i t h f a i l u r e t i m e at a g i v e n c r e e p stress, the a m o u n t o f c r e e p strain is s m a l l relative to the l o a d - o n strain, so t h e t o t a l strai~ at failure is largely d e t e r m i n e d by t h e c r e e p stress. A s m a y be seen f r o m Fig. 3, t h e r a n g e o f strain o v e r w h i c h c r e e p failure o c c u r r e d is l o w e r t h a n t h a t for the case w h e r e failure t o o k p l a c e u n d e r d i r e c t tensile l o a d i n g . It is well k n o w n ~~ , l s t h a t b o r o n o r Borsic f i l a m e n t s c o n t a i n flaws w h i c h l i m i t the f i l a m e n t strength. W a w n e r a n d Satterfield x9 h a v e c o r r e l a t e d the s t r e n g t h o f a f i l a m e n t w i t h the t y p e o f flaw o b s e r v e d at t h e f r a c t u r e surface. A l t h o u g h t h e r e h a v e b e e n n o direct o b s e r v a t i o n s o f slow c r a c k g r o w t h in b o r o n filaments, L i n e a n d H e n d e r s o n 1 s a n d G u n y a e v et al.,2° using the r a m i f i c a t i o n s o f f r a c t u r e m e c h a n i c s , h a v e i n d i c a t e d t h a t t h e relatively s m o o t h surface o b s e r v e d o n the f i l a m e n t f r a c t u r e
R O O M T E M P E R A T U R E C R E E P A N D F A I L U R E OF BORSIC FILAMENTS
181
face corresponds to a region of slow crack extension. Slow crack extension can explain the observed time-dependent failure of this material, and could arise through an environmental cracking or a stress corrosion mechanism, or by the same process which results in the anelastic creep and internal friction observed in the filaments. Tests to determine the effect of ageing at room temperature and the effect of humidity reported by Wawner ~8 did not indicate a degradation in strength, as would be expected if environmental cracking were taking place. It is not clear, however, that environmental factors may not be playing a role. Smith and Shahinian zl have suggested that the large effect of water vapour on the fatigue properties of boron-aluminium composites m a y be attributed to the sensitivity of the fracture strength of boron filaments to water vapour. Insufficient data exist about the anelastic behaviour of boron to enable one to judge if such behaviour could cause slow crack extension. Firle 1~ concluded from his internal friction measurements that the overall anelastic behaviour of boron filaments is similar to that observed in metals where dislocation pinning occurs.
l
• TENSILE o CREEP 2735 MN/m 2 • CREEP 1837 MN/m 2 0,60 FAILURE STRAIN (%)
o o 0.50
oO
o
o
o o
o •
0.40
0.1
f I
•
=.
-->
J
i10 FAILURE TIME
I I00
I I000
[hr)
Fig. 3. Failure strain for tensile and creep testing of parallel Borsic filament specimens. Another approach which can be used to examine stress rupture data involves statistical analysis. Leichter and Robinson 22 have developed an analysis of fatigue life for graphite, and Wilkins 23 has investigated the probability of failure of brittle materials subjected to static or dynamic fatigue. Wilkins' analysis is based upon the establishment of a relationship between failure time and homologous stress (the ratio of the applied stress to the instantaneous fracture stress) and this relationship has been shown in the case of bulk graphite. 24 The parallel filament data of this investigation were examined to see if a similar relationship between homologous stress and failure times could be obtained. The homologous stress, ~c/~i, was calculated for the specimens tested at a given creep stress, ~c, by assuming that the
182
R. H. ERICKSEN
I,O
i
•
•e
•
~c= 2735 MN/rn 2
o
O'c= 1837 MN/m 2
0.9 <:TC
••
~f 0.8
0,1
i
t
I
tO FAILURE
Fig. 4.
I
I
tO0 TIME
tO00
(hr)
Relation between homologous stress and failure time for parallel Borsic filament specimens.
weakest specimen in the series of tensile tests, failing at a tensile stress, o I, could be paired up with the creep specimen which failed in the shortest time, and so forth. This gives a different ~s for each specimen tested at a given a c. The results are shown in Fig. 4 for two stress levels. Although the data are not extensive, the results indicate a linear relationship between the homologous stress and the rupture time, as was the case for graphite. Wilkins 23 has shown how this relationship can be used to estimate the failure probability and to apply proof testing to decrease it. Since filament failure is an important mechanism of composite failure, it is possible that a similar relationship between the homologous stress and the failure time might also exist for Borsic filament reinforced composites. CONCLUSIONS
(I) Borsic filaments exhibited creep and time-dependent failure at room temperature. The creep strain recovered with time after the load was removed (anelastic creep). (2) The slopes of the strain-log time curves, and the amounts of creep strain of parallel filament specimens tested at a given stress, did not correlate with the times to failure. (3) The failure strain of parallel filament specimens held at a constant stress was primarily a function of the stress level. Failure during creep occurred at strains that were lower than those observed on specimens loaded directly to failure in tensile tests. (4) A linear relationship existed between the homologous stress (creep stress divided by the instantaneous failure stress) and the failure time of the parallel filament specimens.
ROOM TEMPERATURECREEP AND FAILURE OF BORSIC FILAMENTS
] 83
ACKNOWLEDGMENT T h e writer wishes to a c k n o w l e d g e the assistance o f M r L. E. H e r m e s m e y e r in carrying o u t the e x p e r i m e n t a l aspects o f this work.
REFERENCES 1. K. C. ANTHONYand W. H. CHANG, Trans. ASM, 61 (1968) p. 550. 2. I. J. TOTH, Composite Materials: Testing and Design, STP 460, American Society for Testing and Materials, Philadelphia, Pa. (1969) p. 236. 3. W. H. SHAEFERand J. L. CRISTIAN, Evaluation of the Structural Behavior of Filament Reinforced Metal Matrix Composites, Air Force Materials Laboratory Report AFML-TR69-36, Vol. II (1969) p. 63. 4. E. M. BRE~NANand K. G. KREIDER,Met. Trans., 1 (1970) p. 93. 5. D. J. CnWIRUT, Tensile Creep of Boron/Epoxy and Boron/Epoxy-Reinforced 7075-T6 Aluminum Alloy, National Bureau of Standards Tech. Note 722 (1972) p. 1. 6. P. D. SHOCKEr, K. E. HOFER and D. W. WmGHT, Structural Airframe Applications of Advanced Composite Materials, Vol. IV: Mechanical Properties--Static, Air Force Materials Laboratory Report AFML-TR-69-101, Vol. IV (1969). 7. R. H. ERICKSEN,Met. Trans., 4 (1969) p. 1687. 8. E. G. ELLISONand D. H. BOONE,J. Less Common Metals, 13 (1967) p. 103. 9. F. K. Rose and J. L. STOKES,Advanced Methods to Test Thin Gage Materials, Air Force Materials Laboratory Report AFML-TR-68-64 (1968) p. 88. 10. See, for example, G. T. HM-INand S. S. SHAPmO, Statistical Models in Engineering, Wiley, New York, 1968, p. 108. 11. H. W. HERRXNG,Fundamental Mechanisms of Tensile Fracture in Aluminum Sheet Unidirectionally Reinforced with Boron Filament, National Aeronautics and Space Agency Report TR-R-383 (1972) p. 34. 12. Hamilton Standard Division of United Aircraft Corp., Windsor Locks, Conn., Technical Data Sheets HS CM-2-K (1971). 13. H. CORTEN, Micromechanics and fracture behavior of composites, in: Modern Composite Materials, L. Broutman and R. Krock (eds), Addison-Wesley, Reading, Mass., 1967, p. 53. 14. K. M. PREVOand K. G. KREIDER,J. Comp. Materials, 6 (1972) p. 338. 15. L. E. LINE and U. V. HENDERSON,Boron filament and other reinforcements produced by chemical vapor plating, in: Handbook of Fiberglass and Advanced Plastics Composites, G. Lubin (ed.), Van Nostrand, New York, 1969, p. 201. 16. H. W. HERRING, Selected Mechanical and Physical Properties of Boron Filaments, National Aeronautics and Space Agency Report TN-D-3202 (1966) p. 9. 17. R. E. FmLE, J. Appl. Phys., 39 (1968) p. 2839. 18. F. E. WAWNER,Boron filaments, in: Modern Composite Materials, L. Broutman and R. Krock (eds), Addison-Wesley, Reading, Mass., 1967, p. 244. 19. F. E. WAWNERand D. B. SATrERFIELD,Sampe Journal (April-May, 1967) p. 32. 20. G. M. GUNYAEV,A. F. ZmGACH, B. V. PEROV, E. A. MITROFANOVAand V. A. YARTSEV, Polymer Mech., 6 (1970) p. 1126. 21. H. H. SMITHand P. SH~INIAN, Effect of water vapor on fatigue behavior of an aluminumboron composite, in: Corrosion Fatigue: Chemistry, Mechanics, and Microstructure, O. F. Devereux (ed), National Association of Corrosion Engineers, Houston, Texas, 1972, p. 499. 22. H. C. LEICHTERand E. ROBINSON,J. Am. Ceramic Soc., 53 (1970) p. 197. 23. B. J. S. WILKINS,J. of Mat., 7 (1972) p. 251. 24. B. J. S. WILKINS,J. Am. Ceramic Soc., 54 (1971) p. 593.
R. H. ERICKSEN
Composite Materials Development Division H, Sandia Laboratories, Albuquerque New Mexico 87115 (USA) (Received: 25 March, 1974)
SUMMARY Creep and time-dependent failure of Borsic (silicon carbide coated boron) filaments have been investigated at 25°C. Tests were carried out using individual filaments and specimens which consisted of a row of parallel filaments. The .filaments displayed anelastic behaviour as the creep strain recovered with time upon removal of the load. The amount of creep was small in relation to the strain obtained on loading. The creep failure strain o f parallel.filament specimens was mainly dependent on the applied stress, being lower than that for specimens which failed in tensile .tests. The slopes of the strain-log time curves, and the amounts of creep strain of parallel filament specimens tested at a given stress, did not correlate with the times to failure. There was, however, a relationship between the failure time and the homologous stress.
INTRODUCTION Creep and stress-rupture have been observed in several b o r o n or Borsic (silicon carbide coated boron) reinforced composites at both r o o m and elevated temperatures. 1 - 7 Relatively little is known, however, a b o u t creep o f the filaments themselves and no references have been found concerning time-dependent failure o f the filaments. Ellison and Boone s reported that at 816°C (0-43 Tin) secondary creep rates o f b o r o n filaments varied f r o m 5 x l0 - 4 to l × 1 0 - 2 h -1 for stresses between 1030 and 1900 M N / m 2. A t 538°C (0-35 Tin) Rose and Stokes 9 observed that the creep rate at 1 5 1 0 M N / m 2 decreased f r o m 2 x 1 0 - S h - I at l h to a nearly steady-state rate o f 1.4 × l0 - 5 h -1 after 15 h. A n t h o n y and C h a n g 1 obtained * This work was supported by the United States Atomic Energy Commission. 173 Fibre Science and Technology (7) (1974)--© Applied Science Publishers Ltd, England, 1974 Printed in Great Britain
174
R . H . ERICKSEN
creep data at 260°C and 690 M N / m 2 and reported creep rates between 2 and 7 x 10- 6 h - 1. In a previous study v it was found that Borsic filaments exhibited anelastic creep at room temperature and the strain increased linearly with the logarithm of time. The room temperature creep rate decreased linearly with time and typical rates at a stress of 1400 M N / m / were 3 × i0 -5 h -1 at 1 h and 3 x 10 -7 h - 1 at 100 h. The present investigation was undertaken to examine further the time-dependent behaviour of Borsic filaments at room temperature and to determine if failure under constant stress could be related to filament creep or to the filament tensile strength.
EXPERIMENTAL
Individual filaments and parallel filament specimens were prepared from sheets of Borsic-aluminium monolayer tape. The tape was fabricated by filament winding 0.11 m m Borsic on a cylindrical mandrel and then depositing 1100 aluminium by plasma spraying. Individual filament specimens having gauge lengths of 1.3, 5, 10 and 38 cm were obtained by dissolving the aluminium matrix from strips of monolayer tape using 5 per cent N a O H solution. Preparation of the parallel filament specimens has been described previously; 7 however, it is also summarised here. Strips of monolayer tape 25 cm long by 1.3 cm wide were cut with the filaments parallel to the length of the tape. The aluminium was then dissolved from a 1.3 cm long section across the centre of each strip using 5 per cent N a O H solution leaving a row of paraUel filaments (54 to 61 filaments). The specimens were placed on a teflon sheet and strain gauges bonded to the rows of filaments using conventional mounting techniques. The strain gauge epoxy did not adhere to the teflon and the specimens could be readily removed. Creep and tensile tests on individual filaments and parallel filament specimens were performed in air using an Instron load frame equipped with a feedback control system. Some creep tests on parallel filament specimens were also carried out using an Arcweld deadweight load frame. All specimens were held in wedge action grips with protective teflon inserts. The rate of loading in all tests was 10 MN/m2/sec. Some incremental stress tests were performed by increasing the stress after a given time to either a higher creep stress or until the specimen failed. Strain was not measured on individual filament tests except for the specimens having a 38 cm gauge length. In this case quartz whiskers were bonded to the filaments to define the gauge length and to serve as reference markers. The elongation was measured using a Gaertner optical extensometer having a resolution of about 2 x 10- 5. Strain was measured on all the parallel filament specimens using strain gauges and the resolution was about 2 × 10 - 6 . Steps taken to ensure the reliability of the strain gauge measurements were discussed previously.7 The test temperature was usually 25 + I°C and did not affect the creep data for
ROOM TEMPERATURE CREEP AND FAILURE OF BORSIC FILAMENTS
175
tests lasting less t h a n a b o u t 100 h. F o r longer tests the d a t a showed greater scatter with increasing time a n d this could be c o r r e l a t e d with r o o m t e m p e r a t u r e fluctuations; however, t e m p e r a t u r e v a r i a t i o n alone could n o t have caused the observed creep strains.
RESULTS AND DISCUSSION Tensile properties I n o r d e r to establish reference p a r a m e t e r s for c o m p a r i s o n with the t i m e - d e p e n d e n t data, tensile tests were p e r f o r m e d on b o t h individual filament a n d parallel filament specimens. The d a t a are s u m m a r i s e d in Tables 1 a n d 2. A s expected, the tensile failure o f i n d i v i d u a l filaments occurred over a range o f stress, and the m e a n strength decreased with increasing gauge length. T h e strength d a t a d i d n o t follow a n o r m a l d i s t r i b u t i o n as evidenced by a n o n - l i n e a r p l o t on n o r m a l p r o b a b i l i t y paper. A W e i b u l l d i s t r i b u t i o n p l o t I° ( t w o - p a r a m e t e r ) for 1-3 a n d 5 . 0 c m gauge length Borsic filaments is shown in Fig. 1. T h e points are r e a s o n a b l y linear at the higher
TABLE 1 TENSILE D A T A FOR BORSIC FILAMENTS
Gauge length (cm)
Mean failure stress (MN/m2)
Standard deviation (MN/m2)
Minimum failure stress .(MN/m2)
Maximum failure stress (MN/m2)
Number o f specimens
1"3 5"1 10"2 38.1
2831 2824 2804 2576
427 207 165 296
1040 2163 2225 1020
3272 3183 3011 2908
50 50 50 50
TABLE 2 TENSILE DATA FOR PARALLEL BORSIC FILAMENT SPECIMENS
Specimen number Number o f fibres 1 2 3 4 5 6 7 8 9 10 Mean Standard deviation Coefficient of variation
56 58 59 61 56 57 56 56 56 54
Failure stress (MN/m 2)
Failure strain x 106
Modulus (GN/m 2)
2306 2189 2115 2405 2363 1729 2170 2326 2150 2210
5860 5920 5070 6220 5870 4250 6370 6000 5470 5500
383 410 422 405 412 400 412 399 412 405
2196 191 0.087
5650 620 0"109
406 10 0.025
R. H. ERICKSEN
176
2
i
!
I
.T *
o 1.3 cm gage * 5.1 cm gage • Ref. I 0
*
o
o
* o ~o *o *o *o *o
0 -1 I
Ln L.n _'~-p"(~ i /
o* -2 0
-3
0
-4
-5
Fig. 1.
I
I
7J6
I
i
78
i
i
80 Ln
i
82
8.4
Weibull distribution plot of Borsic filament strengths.
stress levels but deviate from linearity at lower stresses. The use of a three-parameter Weibull function did not significantly improve the linearity of the plot. The tensile strengths of 0,11 mm boron filaments (5 cm gauge length) were reported by HerringlX and are also plotted in Fig. 2. All three sets of data show similar behaviour, with the boron data having slightly greater curvature and being displaced from the Borsic data by an amount corresponding to a strength difference of about 690 M N / m z. This difference is in good agreement with the relative strength of 0.10 mm boron and 0.11 mm Borsic filaments reported by the manufacturer. 12 The strength of a bundle of filaments was calculated from the Weibull distribution parameters for the 1-3 mm gauge length individual filament data, using the method outlined by Corten.13 The calculated bundle strength was 2380 M N / m 2, which is higher than the mean strength of 2200 M N / m 2 determined experimentally.
0.41
: " .. 0.40
~ gOQQQ
PERCENT STRAIN
O
O D
OOQQ
OO
OIDIQD
@~
Q O
I
Ow
0.39
.01
I
•
I
L
I
I
IO
I00
TIME
Fig. 2.
1000
(hr)
Creep curve for a parallel Borsic filament specimen.
ROOM TEMPERATURE CREEP AND FAILURE OF BORSIC FILAMENTS
177
Although the failure modes of the parallel filament specimens were not studied in detail, acoustic emission data indicated that few, if any, filament breaks occurred prior to failure. The failure process is probably dominated by the breakage of filaments in the weaker part of the strength distribution. The poorer fit of the data to a Weibull distribution in this region may account for some of the disagreement between the calculated and the experimental bundle strengths. Prevo and Kreider ~* have made similar comparisons between calculated and measured bundle strengths for 0.14 m m Borsic filaments and also found the calculated values to be higher than those obtained experimentally. Failure of the parallel filament specimens also occurred over a range of stresses; however, not enough tests were performed to enable a comparison between the normal and Weibull distributions. Stress-strain curves for these specimens were linear to failure or to the point where fibre breakage began. The mean modulus was 406 G N / m 2 from the data shown in Table 2. This value is in good agreement with the average value of413 G N / m z reported by Line and Henderson. 15 The coefficient of variation for the modulus data is lower than that for either the failure stress or strain--an observation also reported by Herring~6--and is consistent with the observation ~ that the elastic modulus of boron filament is almost completely independent of process conditions.
Creep Because of the small strains involved in Borsic filament creep the optical extensometer was of limited accuracy for individual filament tests. Nevertheless, the data obtained from the individual filaments did verify that the creep results from parallel filament specimens were representative of individual filament creep and not due to the mechanics of the test technique. Since precise strain measurements were obtained from the parallel filament specimens, they were utilised to obtain the creep data reported below. A typical creep curve is shown in Fig. 2. The creep strain can be described by a simple logarithmic time law and for this investigation the creep behaviour will be represented as e = ~ log t + c where e is strain, and ~ and c are independent of time, t. The creep rate ~ is ct/t and ~ is the slope of the strain versus log time plot or the strain per decade in time. The creep behaviour is anelastic and nearly all of the creep strain recovered upon removal of the load, as reported previously, v The recovery curve is also linear with the logarithm of time. When a specimen was crept, unloaded and held until most of the time-dependent strain was recovered, then reloaded to the original creep stress, the new curve closely followed the same strain-time path as the one obtained on the first loading cycle. Any influence of stress on the creep rate is reflected in the value of ct. Data obtained at stresses between 690 and 2070 M N / m 2 resulted in values of ~t that generally increased with stress; however, the variation in ~ between tests at a given
178
R. H . ERICKSEN
TABLE 3 CREEP COEFFICIENTS OBTAINED ON INCREMENTAL LOADING OF A PARALLEL BORSIC FILAMENT SPECIMEN (e=~logt +c)
Stress ( M N / m 2)
689 1034 1378 1723 2067
o~ × 10 6
7"2 15'1 9'4 18'4 14"1
c 3( 10 3
1'54 2'32 3"15 4"00 4'92
stress was about the same as the range of values obtained in all the tests. As a result, it was not possible to determine a stress dependence for the creep rate from these data. In order to reduce the scatter by eliminating specimen-to-specimen variation, the stress was increased stepwise on a single specimen. Creep data obtained at incremental stresses between 690 and 2070 MN/m 2 are shown in Table 3. At each stress increase the time was taken as zero to determine ~. The values of :t did not show a progressive increase with increasing stress, although the creep at each stress increment still followed a logarithmic time law. Additional work will be required to resolve the effect of stress on the creep behaviour as this result shows that the variation is not solely due to differences between specimens.
Time-dependent failure The time to failure of 10 cm gauge length individual filament specimens increased markedly as the stress was decreased below the range where tensile failure occurred. Table 4 shows data obtained at three stress levels. At 3010 MN/m 2 most specimens failed on loading and those which were loaded failed within one hour. Decreasing the stress increased the specimen life and fewer specimens failed on loading. At 2380 MN/m 2 the specimens which survived 3 h were loaded to failure and the data are shown in Table 5. There is no indication of any tensile strength degradation in the specimens pulled to failure as a result of the short creep exposure. The results of creep tests on parallel filament specimens at 1850 and 2010 M N / m z are summarised in Tables 6 and 7. Qualitatively the specimens exhibited the same stress-rupture trends as observed for the individual filaments, although the range of stress where stress-rupture occurred in times less than 100 h appeared to be slightly larger for the parallel filament specimens. At 1630 M N / m z a parallel filament specimen was unbroken after 850 h. Because of the wide variation in creep rates observed at a given stress, the creep tests at 1850 and 2010 MN/m 2 were carried to failure to see if any correlation existed between the creep parameters and the failure times. The amount of creep strain and the slopes of the creep curves obtained on the parallel filament specimens
ROOM TEMPERATURE CREEP AND FAILURE OF BORSIC FILAMENTS
179
TABLE 4 STRESS RUPTURE D A T A FOR INDIVIDUAL BOP.SIC FILAMENTS
3010 M N / m 2
Specimen number
2960 M N / m 2
Failure Failed on Failure time loading time (h) ( M N / m 2) (h) 2889
0.03
2990 2749
80
0"9
Failed on loading ( M N / m 2)
Pulled to failure at time t
72
3087 at t = 192 h 2990 at t=264h
2825 3141 at t = 234 h
1950
2735 2914 2735
0'1 2081 2976 2935
I1 12 13 14 15 16 17 18 19 20
Failure time (h)
2756
0"2 6 7 8 9 10
2910 M N / m 2
Loaded to failure at time t (MN/m2)
33 3066 at t = 48h
2997 2997 2942 2811 2921 2660
2170 2625 2770 2811 2900
0.7 2735 2398 2790
TABLE 5 STRENGTH OF INDIVIDUAL BORSIC FILAMENTS AFTER 3 HOURS AT 2740 MN/M 2
Specimen number
Failure stress (MN/mZ)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
3032 2846
Failure time at
2740 M N / m 2 (h)
Failed on initial loading ( M N / m 2)
0"2 2999 2935 2701 2859 2976 2956 2880 2914 0"01 2542 2935
180
R. H. E R I C K S E N
TABLE 6 CREEP D A T A FOR PARALLEL FILAMENT SPECIMENS TESTED AT 2 0 1 0 MN/M 2
Specimen number
Failure time (h)
1 2 3 4 5 6 7 8 9 10
37"0 4'4 3"5 0'9 4.3
Initial strain × 106
~x x 106
Modulus (GN/m2)
Failure strain x 106
4940 16.8 408 4980 13.1 404 4860 11-3 414 5020 2.6 401 5130 nil 392 Failed on loading at 1564MN/m2 5260 2.3 383 5040 0.14 398 4790 nil 420 4820 2-5 414
5-9 9'3 0'4 64.0
5090 5070 4970 -5110 5270 5080 -4830
TABLE 7 CREEP DATAFOR PARALLELFILAMENTSPECIMENSTESTEDAT 1850 MN/M2
Specimen number 1 2 3 4 5 6 7 8 9 10
Failure time (h) 40 26 2.3 5.1 2.5 158 1 4.2 0.3 >450
Initial strain x 106
c~ x 106
4610 4530 4790 4470 4570 4820 4560 4500 4380 4710
nil 2.0 6.5 4.3 nil 9.1 4.0 0.9 2.8 8.6
Modulus (GN/m 2) 400 407 385 412 403 382 404 410 421 391
Failure strain x 106 4670 4580 4820 4530 4590 5030 -4570 -4750
Loaded to failure at t = 450 h
w e r e c o m p a r e d w i t h the failure t i m e s a n d n o c o r r e l a t i o n c o u l d be f o u n d . T h e t o t a l strain at failure (creep strain plus i n s t a n t a n e o u s strain o n l o a d i n g ) was also c o m p a r e d w i t h the failure t i m e a n d the results are s h o w n in Fig. 3, a l o n g w i t h the failure strains o b s e r v e d f r o m the tensile tests a r b i t r a r i l y p l o t t e d at t = 0.01 h. A l t h o u g h t h e r e is s o m e t e n d e n c y for the failure strain to increase w i t h f a i l u r e t i m e at a g i v e n c r e e p stress, the a m o u n t o f c r e e p strain is s m a l l relative to the l o a d - o n strain, so t h e t o t a l strai~ at failure is largely d e t e r m i n e d by t h e c r e e p stress. A s m a y be seen f r o m Fig. 3, t h e r a n g e o f strain o v e r w h i c h c r e e p failure o c c u r r e d is l o w e r t h a n t h a t for the case w h e r e failure t o o k p l a c e u n d e r d i r e c t tensile l o a d i n g . It is well k n o w n ~~ , l s t h a t b o r o n o r Borsic f i l a m e n t s c o n t a i n flaws w h i c h l i m i t the f i l a m e n t strength. W a w n e r a n d Satterfield x9 h a v e c o r r e l a t e d the s t r e n g t h o f a f i l a m e n t w i t h the t y p e o f flaw o b s e r v e d at t h e f r a c t u r e surface. A l t h o u g h t h e r e h a v e b e e n n o direct o b s e r v a t i o n s o f slow c r a c k g r o w t h in b o r o n filaments, L i n e a n d H e n d e r s o n 1 s a n d G u n y a e v et al.,2° using the r a m i f i c a t i o n s o f f r a c t u r e m e c h a n i c s , h a v e i n d i c a t e d t h a t t h e relatively s m o o t h surface o b s e r v e d o n the f i l a m e n t f r a c t u r e
R O O M T E M P E R A T U R E C R E E P A N D F A I L U R E OF BORSIC FILAMENTS
181
face corresponds to a region of slow crack extension. Slow crack extension can explain the observed time-dependent failure of this material, and could arise through an environmental cracking or a stress corrosion mechanism, or by the same process which results in the anelastic creep and internal friction observed in the filaments. Tests to determine the effect of ageing at room temperature and the effect of humidity reported by Wawner ~8 did not indicate a degradation in strength, as would be expected if environmental cracking were taking place. It is not clear, however, that environmental factors may not be playing a role. Smith and Shahinian zl have suggested that the large effect of water vapour on the fatigue properties of boron-aluminium composites m a y be attributed to the sensitivity of the fracture strength of boron filaments to water vapour. Insufficient data exist about the anelastic behaviour of boron to enable one to judge if such behaviour could cause slow crack extension. Firle 1~ concluded from his internal friction measurements that the overall anelastic behaviour of boron filaments is similar to that observed in metals where dislocation pinning occurs.
l
• TENSILE o CREEP 2735 MN/m 2 • CREEP 1837 MN/m 2 0,60 FAILURE STRAIN (%)
o o 0.50
oO
o
o
o o
o •
0.40
0.1
f I
•
=.
-->
J
i10 FAILURE TIME
I I00
I I000
[hr)
Fig. 3. Failure strain for tensile and creep testing of parallel Borsic filament specimens. Another approach which can be used to examine stress rupture data involves statistical analysis. Leichter and Robinson 22 have developed an analysis of fatigue life for graphite, and Wilkins 23 has investigated the probability of failure of brittle materials subjected to static or dynamic fatigue. Wilkins' analysis is based upon the establishment of a relationship between failure time and homologous stress (the ratio of the applied stress to the instantaneous fracture stress) and this relationship has been shown in the case of bulk graphite. 24 The parallel filament data of this investigation were examined to see if a similar relationship between homologous stress and failure times could be obtained. The homologous stress, ~c/~i, was calculated for the specimens tested at a given creep stress, ~c, by assuming that the
182
R. H. ERICKSEN
I,O
i
•
•e
•
~c= 2735 MN/rn 2
o
O'c= 1837 MN/m 2
0.9 <:TC
••
~f 0.8
0,1
i
t
I
tO FAILURE
Fig. 4.
I
I
tO0 TIME
tO00
(hr)
Relation between homologous stress and failure time for parallel Borsic filament specimens.
weakest specimen in the series of tensile tests, failing at a tensile stress, o I, could be paired up with the creep specimen which failed in the shortest time, and so forth. This gives a different ~s for each specimen tested at a given a c. The results are shown in Fig. 4 for two stress levels. Although the data are not extensive, the results indicate a linear relationship between the homologous stress and the rupture time, as was the case for graphite. Wilkins 23 has shown how this relationship can be used to estimate the failure probability and to apply proof testing to decrease it. Since filament failure is an important mechanism of composite failure, it is possible that a similar relationship between the homologous stress and the failure time might also exist for Borsic filament reinforced composites. CONCLUSIONS
(I) Borsic filaments exhibited creep and time-dependent failure at room temperature. The creep strain recovered with time after the load was removed (anelastic creep). (2) The slopes of the strain-log time curves, and the amounts of creep strain of parallel filament specimens tested at a given stress, did not correlate with the times to failure. (3) The failure strain of parallel filament specimens held at a constant stress was primarily a function of the stress level. Failure during creep occurred at strains that were lower than those observed on specimens loaded directly to failure in tensile tests. (4) A linear relationship existed between the homologous stress (creep stress divided by the instantaneous failure stress) and the failure time of the parallel filament specimens.
ROOM TEMPERATURECREEP AND FAILURE OF BORSIC FILAMENTS
] 83
ACKNOWLEDGMENT T h e writer wishes to a c k n o w l e d g e the assistance o f M r L. E. H e r m e s m e y e r in carrying o u t the e x p e r i m e n t a l aspects o f this work.
REFERENCES 1. K. C. ANTHONYand W. H. CHANG, Trans. ASM, 61 (1968) p. 550. 2. I. J. TOTH, Composite Materials: Testing and Design, STP 460, American Society for Testing and Materials, Philadelphia, Pa. (1969) p. 236. 3. W. H. SHAEFERand J. L. CRISTIAN, Evaluation of the Structural Behavior of Filament Reinforced Metal Matrix Composites, Air Force Materials Laboratory Report AFML-TR69-36, Vol. II (1969) p. 63. 4. E. M. BRE~NANand K. G. KREIDER,Met. Trans., 1 (1970) p. 93. 5. D. J. CnWIRUT, Tensile Creep of Boron/Epoxy and Boron/Epoxy-Reinforced 7075-T6 Aluminum Alloy, National Bureau of Standards Tech. Note 722 (1972) p. 1. 6. P. D. SHOCKEr, K. E. HOFER and D. W. WmGHT, Structural Airframe Applications of Advanced Composite Materials, Vol. IV: Mechanical Properties--Static, Air Force Materials Laboratory Report AFML-TR-69-101, Vol. IV (1969). 7. R. H. ERICKSEN,Met. Trans., 4 (1969) p. 1687. 8. E. G. ELLISONand D. H. BOONE,J. Less Common Metals, 13 (1967) p. 103. 9. F. K. Rose and J. L. STOKES,Advanced Methods to Test Thin Gage Materials, Air Force Materials Laboratory Report AFML-TR-68-64 (1968) p. 88. 10. See, for example, G. T. HM-INand S. S. SHAPmO, Statistical Models in Engineering, Wiley, New York, 1968, p. 108. 11. H. W. HERRXNG,Fundamental Mechanisms of Tensile Fracture in Aluminum Sheet Unidirectionally Reinforced with Boron Filament, National Aeronautics and Space Agency Report TR-R-383 (1972) p. 34. 12. Hamilton Standard Division of United Aircraft Corp., Windsor Locks, Conn., Technical Data Sheets HS CM-2-K (1971). 13. H. CORTEN, Micromechanics and fracture behavior of composites, in: Modern Composite Materials, L. Broutman and R. Krock (eds), Addison-Wesley, Reading, Mass., 1967, p. 53. 14. K. M. PREVOand K. G. KREIDER,J. Comp. Materials, 6 (1972) p. 338. 15. L. E. LINE and U. V. HENDERSON,Boron filament and other reinforcements produced by chemical vapor plating, in: Handbook of Fiberglass and Advanced Plastics Composites, G. Lubin (ed.), Van Nostrand, New York, 1969, p. 201. 16. H. W. HERRING, Selected Mechanical and Physical Properties of Boron Filaments, National Aeronautics and Space Agency Report TN-D-3202 (1966) p. 9. 17. R. E. FmLE, J. Appl. Phys., 39 (1968) p. 2839. 18. F. E. WAWNER,Boron filaments, in: Modern Composite Materials, L. Broutman and R. Krock (eds), Addison-Wesley, Reading, Mass., 1967, p. 244. 19. F. E. WAWNERand D. B. SATrERFIELD,Sampe Journal (April-May, 1967) p. 32. 20. G. M. GUNYAEV,A. F. ZmGACH, B. V. PEROV, E. A. MITROFANOVAand V. A. YARTSEV, Polymer Mech., 6 (1970) p. 1126. 21. H. H. SMITHand P. SH~INIAN, Effect of water vapor on fatigue behavior of an aluminumboron composite, in: Corrosion Fatigue: Chemistry, Mechanics, and Microstructure, O. F. Devereux (ed), National Association of Corrosion Engineers, Houston, Texas, 1972, p. 499. 22. H. C. LEICHTERand E. ROBINSON,J. Am. Ceramic Soc., 53 (1970) p. 197. 23. B. J. S. WILKINS,J. of Mat., 7 (1972) p. 251. 24. B. J. S. WILKINS,J. Am. Ceramic Soc., 54 (1971) p. 593.