Two engine components, fan blades and exhaust systems, were selected for composite materials development efforts in support of the supersonic cruise aircraft research (SCAR) engine program. The materials selected were boron/aluminum for fan blades and silicon carbide/superalloy sheet for the exhaust system. The current status of the research into applying these two composite materials to SCAR engines is reviewed

Signorelli, R. A.

English

NASA Technical Reports Server

COMPOSITE MATERIALS RESEARCH IN SUPPORT OF SUPERSONIC PROPULSION SYSTEMS
Robert A. Signorelli
NASA Lewis Research Center
SUMMARY
Two engine components, fan blades and exhaust systems, have been selected
for composite materials development efforts in support of the supersonic cruise
aircraft research (SCAR) engine program. The materials selected were boron/
aluminum for fan blades and silicon carbide/superalloy sheet for the exhaust
system. The current status of the research into applying these two composite
materials to SCAR engines is reviewed in this paper..
Significant progress has been made in improving the impact resistance of
boron/aluminum, and the improved material is being evaluated in prototype SCAR
fan-blade rig tests. Reaction at the silicon carbide fiber - superalloy matrix
interface has been identified as a problem area in fabricating composite sheet.
Diffusion barriers appear to control this interfacial reaction, permitting com-
posite development to proceed.
INTRODUCTION
Propulsion systems for supersonic cruise aircraft will require improved
materials to achieve low ground noise on takeoff as well as efficient subsonic
and supersonic flight. Some of the advanced-design engine components needed
for this purpose can be achieved through the use of advanced composite materials.
Composite materials offer the potential for better component performance than
conventional materials because they have higher modulus, lower density, and
greater strength. For example, using boron/aluminum fan blades for 275 0 to
3750 C (5000 to 7000 F) service can reduce fan-blade stage weight by 25 to 40
percent from that of standard titanium alloy blades. In addition, aerodynamic
performance is improved by the removal of midspan dampers, which is made pos-
sible by the greater stiffness of boron/aluminum. The lower blade weight also
permits lower disk, shaft, bearing, and containment ring weights.
Silicon carbide/superalloy composite sheet will permit higher material op-
erating temperatures than conventional superalloy sheet because the silicon
carbide reinforcement retains its strength to above 1100 0
 C (20000 F). Thus,
trade-offs are possible between higher operating temperatures and decreased
cooling-air requirements for the exhaust system. In addition, because the re-
inforcing filaments have less than half the density of conventional superalloys,
the density of the composite is 20 to 25 percent lower than that of a conven-
tional superalloy.
457
https://ntrs.nasa.gov/search.jsp?R=19770011074 2020-03-22T10:30:51+00:00Z
The current status of the research on boron/aluminum fan blades and sil-
icon carbide/superalloy sheet for exhaust systems for SCAR engine applications
is reviewed.
Values are given in both SI and U.S. customary units. The work was done
in U.S. customary units.
BORON/ALUMINUM FAN BLADES
Composite fan blades have been fabricated in related programs and have
shown excellent potential for fulfilling the requirements for advanced-engine
fan blades. Fabrication processing of such blades generally involves produc-
tion of monolayer composite tape by diffusion bonding two aluminum foils about
a uniaxially oriented boron fiber array. Plies cut from the composite mono-
layer are stacked to fill a die cavity and are hot pressed to a blade shape
such as that shown in figure 1. The desired blade properties are obtained by
suitable orientation of the plies and selection of the stacking sequence. The
relative advantages of boron/aluminum over titanium alloy, the standard fan-
blade material, are also shown in the figure. The composite has better than a
twofold strength/density and modulus/density advantage over titanium. Use
temperature of the boron/aluminum is also greater than that of aluminum alloys
(not shown in fig. 1). The boron fiber retains strength to well above 500 0 C
(9300 F) and increases the use temperature of boron/aluminum to 375 0 C (7000 F).
While many properties of composite materials are superior to those of con-
ventional materials, the impact resistance demonstrated has been inconsistent
and, in many cases, inadequate for fan blades. Notched Charpy pendulum impact
data for a titanium alloy and early boron/aluminum are compared in figure 2.
The composite material has less than half the impact resistance of a typical
titanium alloy used for fan blades.
Based on laboratory and rig tests of specimens and blades, it is probably
safe to say that the impact resistance of boron/aluminum composites to small
objects such as ice balls, gravel, rivets, and sand is acceptable. However,
resistance to large objects such as birds has been less than adequate. Bird
ingestion generally occurs at low altitudes because that is where most birds
fly. Therefore, bird impact damage is most likely to occur during takeoff or
landing of aircraft. Over half of the bird population is found at altitudes of
less than 150 meters (ref. 1). Figure 3 clearly illustrates the potential
danger of bird impact to aircraft engine fan blades. Even so, bird ingestion
is rare, and usually does not cause severe operational changes in modern turbo-
fan engines. However, as shown in figure 4, even standard titanium alloy fan
blades can be damaged as a result of bird impact. The damage shown in this
figure for a titanium alloy blade is representative of a severe impact condi-
tion. Also shown is an early boron/aluminum blade subjected to severe impact
in a whirling-arm rig test. However, the poor impact resistance that led to
the multiple failure of this early boron/aluminum blade does not represent the
potential of this type of composite.
458
Studies to understand and improve boron/aluminum impact resistance have
been undertaken at several laboratories (refs. 2 to 8). The major variables
that were investigated in a program undertaken by the Lewis Research Center to
improve boron/aluminum fan-blade impact resistance were fiber diameter, matrix
alloy ductility, fabrication processing, and fiber ply orientation. The choice
of variables was influenced by efforts to minimize the embrittling effects of
the low strain-to-failure boron fibers in an otherwise relatively ductile alum-
inum alloy. Large-diameter fibers (< 0.2 mm) were selected to increase the
interfiber distance in the composite and thereby reduce the volume of the mat-
rix constrained by the high-modulus, low-strain boron fibers and permit greater
strain in the composite upon impact. Fabrication processing was improved to
obtain low-porosity, well-consolidated composites and a high degree of bonding
between the fiber and the matrix and between matrix fiber plies. However, the
bonding temperature was kept low to minimize reaction at the fiber/matrix in-
terface since such reaction generally degrades composite properties. In ad-
dition, the fiber ply orientation was selected to provide a viable compromise
between the properties required in the various directions and the impact re-
sistance of the composite. Significant increases in impact resistance were ob-
tained through these approaches (refs. 8 to 10). Only some of the results are
jincluded herein for brevity.
Increasing fiber diameter from 0.1 millimeter to 0.2 millimeter increased
notched Charpy impact strength from less than 40 joules to 96 joules (fig. 5).
These larger diameter fibers were incorporated in specimens that had the best
combination of the other beneficial effects (such as better processing and a
more ductile matrix) identified in the program. Increasing the ductility of
the matrix by using commercially pure, 1100 alloy aluminum increased Charpy im-
pact strength from less than 20 joules to 96 joules (fig. 6).
The marked improvement in impact resistance is illustrated in figure 7,
which compares impact values of a typical titanium alloy fan-blade material,
an early boron/aluminum composite, and an improved boron/aluminum composite. A
tenfold increase in laboratory pendulum impact values, as shown in figure 7,
is very encouraging but not necessarily indicative of satisfactory foreign-
object-damage resistance at the high-velocity impact conditions that fan blades
encounter in normal service. Fan and compressor blades of the improved boron/
aluminum are being fabricated and tested, and preliminary results are encour-
aging. Composite J-79 blades containing improved boron/aluminum material are
shown in figure 8 after bird impact testing in single-blade, rotating-rig tests.
These blades were included in a joint Air Force - NASA program. Each of the
three blades was impacted by a real bird under conditions simulating aircraft
takeoff conditions. These blades showed evidence of some deformation and de-
lamination at the tip of the airfoil near the impact point. However, no portion
of the blade was broken off by the impact.
Improved impact-resistant boron/aluminum is currently being used in a
prototype SCAR blade. Figure 9 shows one of the preliminary blades fabricated
from boron/aluminum for evaluation, which includes rotating-rig impact tests.
Additional blades are being fabricated for evaluation in the near future. The
encouraging results obtained in this impact testing suggest that foreign-
object-damage resistance, the remaining major technical impediment to success-
459
J
ful flight demonstration of boron/aluminum blades, may have been overcome.
SILICON CARBIDE/SUPERALLOY COMPOSITE SHEET FOR EXHAUST SYSTEMS
Exhaust system components can be a significant portion of SCAR engine
weight. The high-temperature portions of a SCAR engine, which operate at tem-
peratures to 980 0 C (18000 F), have been estimated to weigh 550 kilograms
(1200 lb) (private communication from R. A. Howlett, Pratt & Whitney Aircraft,
East Hartford, Conn.). Substituting silicon carbide/superalloy composite sheet
for the conventional superalloy could reduce that weight by about 20 to 25 per- 
1cent or by about 120 kilograms (264 lb). This weight reduction is due to the
lower density of the silicon carbide fiber. In addition to the lower density
advantage, the silicon carbide fiber retains greater strength and stiffness at
the operating temperature, 9800
 C (18000 F). The silicon carbide/superalloy
composite sheet can be expected to have four times the tensile strength, twice j
the elastic modulus, and five times the 1000-hour rupture strength of conven-
tional superalloy sheet at 9800
 C. These superior mechanical properties can be
used to design more efficient systems that use less cooling air and to reduce
weight by using thinner sections.
To make the silicon carbide/superalloy composite sheet a viable material
system, certain problems must be addressed. The property advantages can be
achieved only if fiber-degrading chemical interaction at the fiber/matrix in-
terface can be minimized. Reaction rates at the 980 0 C (18000 F) service tem-
perature are expected to be tolerable; however, reactions occurring during the
higher temperature fabrication processing can be a problem. Typically, the
composite is made by first hot pressing superalloy foils together with silicon
carbide fiber arrays to form monotapes. A secondary diffusion bonding of
stacked layers of the monolayer tapes forms the composite sheet. Limited fab-
rication processing techniques have been evolved to bend, cut, and join the
composite sheet into exhaust system components.
Research has been underway to overcome the interfacial reaction through
the use of diffusion barrier coatings on the silicon carbide fibers (ref. 11).
Figure 10 shows the marked improvement provided by diffusion barrier coatings
in reducing reaction during composite processing. The uncoated fiber has
formed a thick reaction zone, but the tungsten-coated fiber has no visible re-
action. Although tungsten is an effective coating, it requires a greater thick-
ness than desired and incurs a weight penalty, being twice as dense as super-
alloys. Alternative carbide coatings are currently being deposited on silicon
carbide fibers, and preliminary compatibility results suggest that the fiber/
matrix reaction can be controlled. Composite test specimens also are being
fabricated to assess the properties of the composite in high-temperature ten-
sile and long-time rupture tests.
The properties of silicon carbide/superalloy and conventional superalloys
are compared in figure 11. The data shown are projections expected from the
composite currently being processed and evaluated.
460
In summary, considerable progress has been made, but much work remains be-
fore composite sheet can be applied to exhaust systems.
CONCLUDING REMARKS
Significant progress has been made in developing boron/aluminum and
silicon carbide/superalloy sheet composites for application in SCAR propulsion
systems. A marked improvement in boron/aluminum impact resistance has been ob-
tained in laboratory tests, and this technology is currently being applied to
fan blades. The progress made suggests that foreign-object-damage impact re-
sistance, the remaining major technical impediment to the use of boron/aluminum
blades in SCAR propulsion systems, may have been overcome.
Excellent progress has been made in studies of silicon carbide/superalloy
sheet. Diffusion barrier coatings have been identified and applied to the
silicon carbide fibers, and preliminary data suggest that these barriers can
control interfacial reactions. While much work remains before silicon carbide/
superalloy sheet can be used in SCAR engine exhaust systems, the potential ap-
pears promising.
461
REFERENCES
1. DeJong, A. P.: Their Airspace or Ours? Shell Aviation News, No. 390,
1970, pp. 2-7.
2. Kreider, K. G.; and Prewo, K. M.: Boron-Reinforced Aluminum. Composite
Materials, Vol. 4, Metallic Matrix Composites. Kenneth G. Kreider, ed.,
Academic Press, 1974, pp. 400-480.
3. Hancock, J. R.; and Swanson, G. D.: Toughness of Filamentary Boron/Aluminum
Composites. Composite Materials: Testing and Design, Second Conference.
STP-497, Am. Sac. Testing Mater., 1972, pp. 299-310.
4. Prewo, Karl M.: The Charpy Impact Energy of Boron-Aluminum. J. Compos.
Mater., vol. 6, Oct. 1972, pp. 442-455.
5. Olster, E. F.; and Jones, R. C.: Toughening Mechanisms in Continuous
Filament Unidirectionally Reinforced Composites. Composite Materials:
Testing and Design, Second Conference. STP-497, Am. Sac. Testing Mater.,
1972, pp. 189-205.
6. Hoover, W. R.; and Allred, R. E.: Toughness of Borsic-A1 Composites with
Weak Fiber-Matrix Bonds. SC-DC-71-4467, Sandia Labs., 1972.
7. Melnyk, P.; and Toth, I. J.: Development of Impact Resistant Boron/
Aluminum Composites for Turbojet Engine Fan Blades. (ER-7806, TRW Equipment
Labs.; NAS3-17763), NASA CR-134770, 1975.
8. Signorelli, R. A.: Metal Matrix Composites for Aircraft Propulsion Sys-
tems. Proceedings of International Conference on Composite Materials.
Am. Inst. Mining Eng., 1976, pp. 411-432.
9. McDanels, David L.; and Signorelli, Robert A.: Effect of Fiber Diameter
and Matrix Alloys on Impact-Resistant Boron/Aluminum Composites. NASA
TN D-8204, 1976.
10. McDanels, David L.; and Signorelli, Robert A.: Effect of Angleply and
Matrix Enhancement on Impact-Resistant Boron/Aluminum Composites, NASA
TN D-8205, 1976.
11. Cornie, J. A.; Cook, C. S.; and Anderson, C. A.: Fabrication Process De-
velopment of SiC/Superalloy Composite Sheet for Exhaust System Components.
(Westinghouse Electric Corp.; NAS3-18921), NASA CR-134958, 1976.
462
STRENGTH
DENSITY
Ti	 BIAI
Ti	 BIAI
0
RELATIVE NOTCHED
CHARPY IMPACT
Figure l.- Relative advantages of boron/aluminum
composite blades over titanium alloy blades.
TITANIUM	 EARLY
ALLOY	 BIAI
Figure 2.- Relative notched Charpy pendulum impact
strength of titanium alloy blades and early
boron/aluminum composites.
463
rw
•t..:, Sri
,r
461. , 4^r^
^^.
w" .-	y	 4
V ,	
% ,
41^
	 w ,
Figure 3.- Example of bird density near an airport.
TITANIUM
	 B/AI
Figure 4.- Impact damage to representative fan blades.
464
60 ^—	 80
NOTCHED CHARPY	 60
IMPACT,	 40
ft-lb	 J
40
20
20
'	 80
100
60	 80
NOTCHED CHARPY
IMPACT,	 40
ft-lb
20
0
.01
	
.014	 .02
BORON FILAMENT DIAM, Cm
Figure 5.- Effect of filament diameter on impact strength of
boron/aluminum composites. 14atrix, 50-vol % 1100 aluminum.
80 F	 100 r__
I^^I
60
J
40
20
0
2024	 6061	 1100
MATRIX ALLOY
INCREASING DUCTILITY)
------------ 	 l
Figure 6.- Effect of matrix ductility on impact strength of boron/aluminum
composites. Boron fiber content, 50 vol %; fiber diameter, 0.02 cm.
465
75 f—
	 100
60 ^—
	 80
45 ^-
	 60
NOTCHED CHARPY
IMPACT,
ft-lb	 J
30 F—	 40
15 I-	 20
Dom_	 0 1 	 1	 1	 1	 1	 1	 1
TITANIUM	 UNIDIRECTIONAL	 IMPROVED
ALLOY	 BIAI	 UNIDIRECTIONAL
BIAI
Figure 7.- Relative impact resistance of titanium alloy, unidirectional
boron/aluminum, and improved unidirectional boron/aluminum.
J 79 BORONIALUMINUM BLADES 	 w
7% SPAN STARLING IMPACT
AT AIRCRAFT TAKEOFF CONDITIONS
Figure 8.- J-79 boron/aluminum blades after bird impact testing at
takeoff conditions. (Starling impact at 70 percent of span.)
466
6,e
REACTION
ZONE
Figure 9.- Prototype SCAR boron/aluminum blade.
UNCOATED
	
COATED
Figure 10.- Relative interfacial reactions of coated and uncoated
silicon carbide fibers in a superalloy matrix.
467
	9800
 C	
5x106
I , 4
I
I
	
I	 i	 3
	
I	 ^
	
I	 ^
	
!	 2
1
_i	 i	 i	 i 0
SUPER- SiC/SA
A LLOY
2x10
ELASTIC
1	 MODULUS/
DENSITY,
m
	
in.
0
6X105
	1.5x 104
F
	
[11000 HR, 9800
 C
STRESS-	 4	 1.0 I
RUPTURE
STRENGTH/	 j
DENSITY,	 I	 II
in.	 2	 m	 .5	 I	 I
	
I	 I
	
I	
I
	
I	 I
	
I	 II
0	 0-
SUPER- SiC/SA
ALLOY
Figure 11.- Properties of silicon carbide/superalloy and conventional
superalloys.
4 6 8	 NASA- Langley, 1977


Composite materials research in support of supersonic propulsion systems

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