By measuring the change in fracture stress of 203 micrometer diameter fibers of boron on tungsten (B/W) as a function of fiber diameter as reduced by chemical etching, it is shown that the flaws which limit B/W fiber strength are located at the surface and in the tungsten boride core. After etching to a diameter of 188 micrometers m virtually all fiber fractures were caused by core flaws, the average strength being 4.50 GN/sq m. If both the surface and core flaws are removed, the fracture strength, limited by flaws in the boron itself, is approximately 6.89 GN/sq m. This was measured on B/W fibers which were split longitudinally and had their cores removed by chemical etching. The longitudinal residual stress distribution was determined for 102 micrometer diameter B/W and B/C fibers

Behrendt, D. R.

English

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APR 1977
	
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(NASI-M-1-73t U) RESIDUAL STRESSES IN
BOROS/TUNGSTEN IND BOSON/CDRBCN FIBERS(NISI) 9 p K 1102/MF 1101
	 CSCL 11D
N77-21193
Unclas
G3/24 24348
RESIDUAL STRESSES IN BORONITUNGSTEN
AND BORONCARBON FIBERS
by D. R. Behrendt
Lewis Research Center
Cleveland, Ohio 44135
TECHNICA-L PAPER to be presented at the
One hundred and sixth Annual Meeting of the
Metallurgical Society of the American Institute of Mining,
Metallurgical and Petroleum Engineers
Atlanta, Georgia, March 6-10, 1977
1RESIDUAL STRESSES IN BORON/TUNGSTEN
AND BORON/CARBON FIBERS
by D. R. Behrendt
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
o	 INTRODUCTION
0
wAlthough composites made from boron fibers in metal matrices exhibit
high strengths compared to many metals, they have in general a much lower
ability to absorb impacts. One way that this impact energy absorption can be
increased is by increasing the fiber strength since the energy stored in each
fiber at fracture is proportional to the square of the fracture stress of the
fiber. By measuring the change in fracture stress of 203 µm (8 mils) diameter
fibers of boron on tungsten (B/W) as a function of fiber diameter as reduced by
chemical etching, Smith (ref. 1) has shown that the flaws which limit B/W fiber
strength are located at the surface and in the tungsten boride core. The average
fracture strength as limited by surface flaws is 3.59 GN/m 2 (521 ksi). After
etching to a diameter of 188 µm virtually all fiber fractures were caused by core
flaws, the average strength being 4. 50 GN/m 2 . If both the surface and core
flaws are removed, the fracture strength, limited by flaws in the boron itself,
is approximately 6.89 GN/m2 (ref. 2). This was measured on B/W fibers
which had been split longitudinally and had their cores removed by chemical
etching. These data indicate that it should be possible to increase the fiber
strength by a large amount before onr is limited by the bulk flaws.
Because the limiting flaws are located at the surface and in the core, it is
important to know what the residual stresses are in these fibers particularly
at the limiting flaw locations. Measurements have shown that the residual
stresses at these locations are compressive for B/W fibers (refs. 3, 4, 5).
More recent measurements (ref. 6) made on 203, 142, and 102 µm B/W fibers
have yielded the longitudinal residual stress distribution with radius, from the
surface to near the core. Also, in the case of the 102 µm fiber, the average
core stress was determined.
In this investigation the longitudinal residual stress distribution was deter-
mined for 102 pm diameter B/W and B/C fibers.
STAR Category 24
2EXPERIMENTAL
The fibers studied in this investigation were made by AVCO Systems
Division. The 102 pm diameter B/W $hers were deposited on a 12.7 µm dia-
meter tungsten wire resistively heated in a BCl3 
-H2 reactor. The 102µm
diameter B/C fibers were made by the deposition of boron on a pyrolytic gra-
phite coated carbon fiber (ref, 7). The deposition process produces an elonga-
tion of the growing fiber which, in the case of B/W, is closely matched by the
expansion of the tungsten lattice due to the boriding of the tungsten wire. If
boron is deposited directly on a carbon fiber, the elongation of the growing fiber
tends to break the carbon fiber into short sections. Hot spots are produced at
these breaks due to the redistribution of the heating current through the higher
resistance of the boron. In the production of the B/C fibers at AVCO, this prob-
lem has been solved by coating the 33 pm diameter carbon fiber with a thin layer
of pyrolytic graphite L 3 pm thick. This layer has a sufficiently small shear
strength to decouple mechanically the elongating boron from the carbon core
and prevent breaking of the core.
The longitudinal residual stress distribution was calculated from measure-
ments of the change in length of the fiber produced by removal of the surface
by electropolishing. A complete description of this method is given in refer-
ence 6. Briefly a 1 cm long section of the fiber to be tested was mounted in a
gauge which allowed a continuous record of the fiber' s length vs time to be
made. The surface of the fiber was removed by electropolishing at a known
rate. From these data a plot of longitudinal strain, Ez, vs fiber radius, r,
was made. From the E  vs r data, the longitudinal residual stress, az, as a
function of r could be calculated from the equation (ref. 6)
r 2	
x 
r dEz(r) +
Q(ro, r) _ - ( Ec - Eb) 4 +	 ^2 - r Ez(r) (1)
where
E(ro, r) is the stress at a position r in the as - received fiber of radius ro.
Ec is the Young' s modulus of the core.
Eb is the Young ' s modulus of the boron sheath.
rc is the radius of the core with modulus Ec.
r is the radius of the electropolished fiber.
3CZ(r) is the axial strain when the fiber is electropolished to a radius, r, with
CZ(ro) = 0.
RESULTS AND DISCUSSION
Five or more separate runs were made on B/W and B/C fibers. in all the
runs using the B/C fibers, measurements were made up to the carbon core.
In the case of the B/W fibers, this was possible in only one of the runs because
of fiber breakage. Figure 1 is a plot of the longitudinal strain, E Z(r), vs the
electropolished radius, r, for typical 102 µm diameter B/W and B/C fibers.
Both fiber types showed an initial contraction. As the electropolishing continued,
they both ceased to contract and began to elongate near a fiber radius of 44 µm.
At the core surfaces the strains are positive for the B/W fibers and negative
for the B/C fibers.
With equation (1) the strains can be used to determine the residual longitu-
dinal stresses. In the case of B/W, DiCarlo (ref. 8) has shown that the core
consists of two tungsten borides; an inner part of W2B5, about 12.8µm in dia-
meter, and an outer part of WB 4, 17 µm in diameter. He reports average
Young's moduli of the two core borides and the boron sheath to be 669 GN/m2
for the W213 5, 407 GN/m2 for the WB4, and 393 GN/m 2 for the boron sheath.
In equation (1) we consider the outer core of WB 4 and the boron sheath to have
the same Young's moduli. Then
Ec = 669 GN/m2
Eb = 393 GN/m2
rc=6.4µm
for the B/W fibers. For the D/ C fibers the following values were used:
Ec = 41 GN/m2
Eb
 = 393 GN/m2
rc=19µm
Figure 2 is a plot of the longitudinal residual stresses calculated by using
equation 1 and the data shown in figure 1. Both the B/W and B/C curves show
the residual stresses are compressive at the surface and pass through zero
stress near r equal to 38µm. For the B/W fibers the stress increases in
tension to a maximum near 1020 MN/m 2 and then passes through zero stress
to an average compressive stress in the core of -1330 MN/m 2. The curve for
the B/C fibers shows quite 
s
 different behavior. After reaching a maximum
tension stress of 450 MN/m the stress drops rapidly with decreasing radius
to a compressive stress of -1160 MN/m 2 and then, changing sign, to an average
tension stress in the carbon core of 81 MN/m 2. This core stress reflects the
shear strength of the pyrolytic graphite coating.
In figure 3 is shown a reporduction of a scanning electron micrograph
(magnification of 1000X) of the B/W core after electropolishing has removed
the boron sheath. Two regions can be noted; the intact core which has a dia-
meter of 16. 3 µm, and a second region where the core surface has broken away,
probably due to handling after electropolishing. The outer diameter corres-
ponds roughly with the outer diameter of the WB4 tungsten ooride reported by
DiCarlo (ref. 7). Also in this figure the die drawing marks of the original
tungsten wire can be seen in the borided core as the striations running longitu-
dinally along the core.
Figure 4 is a reproduction of a scanning electron micrograph (magnification
1000X) of the carbon core after removal of the boron by electropolishing.
Three distinct regions can be seen. The region with the smallest diameter of
33 µm corresponds to the original carbon fiber. The next region has a dia-
meter of 36 µm which corresponds to the diameter of the original carbon fiber
after coating with the pyrolytic graphite. The third region which made up most
of the length of the specimen had a diameter of 38 µm. This layer is most
likely produced by a slight interdiffusion of boron and the pyrolytic graphite.
These findings are of interest not only because they contribute to an under-
standing of the factors which limit the strength of boron fibers, but also because
of the possibility that this knowledge can be used to significantly increase the
strength and impact energy absorption of these fibers. in the case of B/W
fibers, the strength limiting flaws are located at the surface and in the coi
The surface flaws can be removed by chemical etching as shown by Smith
(ref. 1). The removal of the core flaws is a somewhat harder task; however,
the strength could be increased if the residual compressive stress in the core
could be increased. This might be possible through changes in the fiber pro-
duction process. Such a possibility is currently being studied In the case of
the B/C fibers, the situation is not quite as clear due mainly to the lack of in-
formation concerning flaw location.
5CTTTA?AA1:Z V
Residual stresses are one of the factors which influence the fracture strength
of brittle materials such as boron deposited on tungsten or carbon by chemical
vapor deposition. We have measured the longitudinal residual stress distribu-
tion for both 102 µm (4 mil) diameter boron on tungsten, B/W, and boron on
carbon, B/C, fibers. The stress distributions were calculated from measure-
ments of changes in the lengths of the fibers produced by surface removal by an
electropolishing method For both types of fibers, the residual stress varied
from a compressive stress of approximately -1300 MN/m 2 (188 ksi) at the sur-
face to tensile stress in the boron near the core. This stress for the B/W was
approximately 1020 MN/m 2 (148 ksi) and for the B/C it was approximately
450 MN/m 2 (65 ksi). Closer to the core and in the core significant differences
in the residual stresses were observed for the B/W and B/C fibers.
6
..-- -,--	 '.
REFERENCES
1. R. J. Smith, NASA Tech Note D8219, Washington, D. C., 1976.
2 F. E. Wawner, Jr. and D. B. Satterfield, Appl Phys. Lett., 1967, vol. 11,
pp. 192 - 194.
3. R. M. Witucki, Air Force Materials Laboratory Technical Report A FML-TR-
66-187, 1967.
4. K. A. Faughnan, Proceedings of the Twenty-Ninth Annual Conference of the
Wide World of Reinforced Plastics, (pp. 4-B, 1 to 4-B, 10) Society of the
Plastics Industry, Inc., New York, 1974.
5. R. J. Diefendorf, private communication.
6. D. R. Behrendt, NASA Tech. Memo X-73402, Washington, D.C., 1976.
7. It E. DeBolt, et al., private communication. 1971
8. J: A. D1Ca,rlo, . private communication.
EN
W
2
4
OC
r
4n
I/
r,
RADIUS, r, µm
0	 .5	 1.0	 1.5	 2
RADIUS, r, mils
Figure 1. - Longitudinal strain e ? for B1W and BIC
fibers produced by electropollshing the fiber to a radius
of r.
1200
150
100 	 800 ^-	 BIWv	 L EE
N	
I 
c 
e	 50	 400 F
	
0 N	 0 ----- ----	 BIC
N	 K
J	 y
Q	 -50 r v+ -400Z	 JQ
-100	 o
o -150
J	 c -1200
-200
-1600
	
0	 10	 20	 30	 40	 50	 60
RADIUS, r, Pm
	
0	 .5	 1.0	 1.5	 2
RADIUS, r, mils
Figure 2. - Longitudinal residual stress distribution a 	 r) at a
position r from the fiber center in the as-received fiber.
0
0
T
L4
7EJ -
Figure 3. - Scanning electron micrograph of tungslen boride core alter
complete removal of boron sheath by elettropolishing of BMl 102 ym
i4 mih diameter liter. X1000.
•,ure 4. - Scanrc,, 	 t • u micrograph of (anion Core after complete
removal of boron sheath In electropolishing of 8/C 102 ym 14 mill
diameter filer, x1000.
NASA-Lewis


Residual stresses in boron/tungsten and boron/carbon fibers

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