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N
estled between the 767 and the
747 in terms of size, the Boeing
777 is the world’s largest twinengine airplane. It was initially conceived as an enlarged version of the
767, but it grew to 85% of the 747 in
actual size, and sports a wingspan
of nearly 200 feet and a fuselage approximately 11 feet in diameter. Its
passenger seating and range combination put it in a unique niche that
has allowed development of a generation of stretch and range variants.
To enable such a large twinengine airplane, Boeing had to
achieve significant reductions in
structural weight while maintaining
overall affordability. This was made
possible by the development of
breakthrough materials.
The 777 Program enabled the
maturation of a large number of materials that were under development
in the mid- to late-1980s. Materials
that were transitioned into production included new advanced 7000
and 2000 series aluminum alloys,
damage-tolerant composites, and
advanced titanium alloys. These
materials as well as non-structural
materials advances enabled a reduction in weight of over 5800 pounds.
The Boeing
777
The development of the
Boeing 777 was made possible
by the development
of breakthrough materials
that allowed reductions in
structural weight while
maintaining affordability.
used extensively in interface areas.
In addition, titanium replaced many
steel components in the landing gear
and engine strut area in an effort to
reduce weight and improve corrosion resistance.
Although structural materials receive the most attention, it is important to note that the 777 also paved
the way for a wide variety of nonstructural advanced materials. Significant material applications included the introduction of improved
passenger windows, and dust
covers more resistant to the environment and more able to withstand wear and tear. More-durable
materials were also developed and
implemented for insulation blankets,
interior paints, decorative inks, cargo
floors, and cargo liners. Furthermore, many of these improved materials also generated significant
weight savings.
Alloy developments
During the waning days of the mid1980s, a frustrated Boeing and its
aluminum suppliers shut down masBrian Smith
sive efforts to develop aluminumBoeing Aircraft Co.
lithium alloys. As a result of this exSeattle, Washington
perience, Boeing initiated a process
with these suppliers in which alloys
were first studied on paper. Suppliers were asked to proThe aluminum airplane
pose various ‘what if’ alloys for major structural applications.
From a structural-weight standpoint, the 777 is primarily
These ‘what if’ alloys were evaluated for benefit and afan aluminum airplane. Seventy percent of the overall strucfordability. This unique approach allowed promising alture is aluminum, including the wing box and fuselage. Of
loys to be identified early on and, unlike their aluminumcourse, the aluminum alloys are not the garden-variety
lithium counterparts, these alloys were robust to price and
aerospace materials of the past. These are engineered alproperty changes during
loys offering improved
development.
strength, toughness, and
The ‘what if’ process focorrosion resistance.
cused on advanced alDespite the predomiloys for wing and fusenance of aluminum, the
lage applications. For
777 does contain signifithe wing, Boeing identicantly more composite
fied a general need for
materials by weight than
higher-strength alloys
earlier Boeing aircraft.
with good toughness
The vertical fin, horiand improved corrosion
zontal stabilizers, and
resistance – relatively
passenger-floor beams
standard targets. Howutilize a Boeing/supplier
ever, in the case of the
developed toughened,
fuselage, Boeing had just
damage-resistant carbon
completed a rigorous refiber epoxy resin system.
view of fatigue and corTitanium alloy imrosion issues in its fleet
provements are critical
of aging airplanes. This
in combating the galvanic potential differThe Boeing 777-300ER’s new semi-levered landing gear system has performed effort brought into focus
ence between aluminum flawlessly during the flight-test program. The unique gear, which is manufactured by the need for advances
and Carbon Fiber Rein- Goodrich Corp., allows the airplane to rotate early by shifting the center of rotation in toughness, fatigue
forced Plastic (CFRP), from the main axle to aft axle of the three-axle landing gear truck. As the airplane crack growth resistance,
and corrosion resistance.
and titanium alloys are rotates, the nose is allowed to rise higher earlier.
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As a result of this innovative
process, Boeing and Alcoa were
able to generate and bring to
market a number of breakthrough alloys and heat treatments. The advanced fuselage
alloy 2524 yielded significant improvements in the design properties associated with fuselage
skin durability. To further address fleet corrosion issues, 777
designers worked diligently to
maintain the clad surface on the interior of the airplane, particularly in the
moisture-laden bilge area.
This material breakthrough was
married with advancements in 7000
series alloy heat treatment (T77511 –
retrogression re-age), which allowed
higher-strength 7150 materials for
fuselage extruded stringers. The result
was a structure that is tougher,
stronger, and more corrosion-resistant
than earlier designs.
The same technological breakthroughs that enabled application of
7150 alloys on the fuselage, were also
incorporated into wing alloy ‘what if’
studies. These studies identified a candidate alloy that had a particularly
unique combination of properties,
pricing, and corrosion resistance: 7055T7751. This alloy provides a nearly
10% gain in strength, with higher
toughness and significantly improved
corrosion resistance.
747
777
767
757
707/720
• Regional and intercontinental market flexibility
• State-of-the-art, service-ready features and
technology
• Industry-leading performance and economics
• Range and capacity growth ensure future family
commonality
727
737
1950
1960
1970
1980
1990
2000
Weight
saved
Durability
improved
Boeing aircraft and the years they were introduced into service.
Alloys:
Ti 10-2-3
Al 2XXX-T3, -T42, -T36
Al 7055-T77
Al 7150-T77
Ti- 6-4 ELI
Ti 15 -3 -3 3
Ti B21S
Ti 6-2-4-2
Composites:
Toughened CFRP
Pitch core
Perforated CFRP/Nomex
9: Fin and stabilizer
3: Upper skin
and stringers
4: Upper spar
2: Aft
chord
bulkhead
4: Seat tracks
9: Floor
beams
5: Stabilizer
attach
fittings
4: Crown stringers
4: Keel beam
4: Belly stringers
Uncolored
2: Fuselage skin
1: Truck beam
and braces
Glare bulk cargo floor
6013 Al alloy
Lightweight sealants
Al mesh
AV-30 corrosion inhibiting compound
Dense core potting
CFRP comp. cascade
Al-Li 8090 sound damping angles
RTM CFRP chine
Breakout of advanced materials on the 777.
42
8: Aft heat shield
8: Engine
mounts
6: ECS ducting
7: Tail cone outer sleeve
7: Tail cone plug
7 & 8: Aft core cowl
10 & 11: Thrust reverser cowl
11: Inlet cowl inner barrel
Toughened carbon fiber epoxy
Efforts to develop an improved
carbon fiber epoxy resin system date
back to the early- to mid-1980s. These
efforts also originated with Boeing’s
in-service fleet experience. Since the
production of the 757 and 767, airline
customers have had to contend with
thin-gage composite structures in a
wide number of applications. Complaints about this material’s sensitivity
to impact damage and the difficulty
of repair were many.
In response to these complaints,
Boeing initiated and led a significant
effort to develop a toughened epoxy
matrix that would be more resistant
to damage. Supplier efforts were repeatedly thwarted by the negative impact of toughening agents on hot/wet
compression strength.
Fortunately for Boeing, Toray had
been working diligently on a resin
system that involved a toughening interlayer. The resulting system set a
new standard for toughness and
strength in composite material technology. Impact test results demonstrated to the airlines that this new
system also suffered significantly less
damage, and that such damage could
be repaired in a manner similar to repair of existing aluminum structures.
This breakthrough in CFRP toughness was optimized to enable Continuous Tape Laying Machines (CTLM)
to fabricate structures, resulting in reduced manufacturing costs. The new
toughened matrix CFRP is used for
the main box cover panels and the
main box spars. The main torque box
cover panel consists of an integrally
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1%
11%
2XXX
7%
11%
Misc.
Steel
Titanium
Composites
Aluminum
Aluminum alloys and other advanced
materials by weight on the Boeing 777.
2024
150
60% slower
fatigue crack growth
100
2024
2XXX
10
10
20
30
40
Stress intensity factor Kmax, ksi-in.1/2
40
50
60
70
Typical tensile yield strength, ksi
Toughened 2000 series aluminum alloy properties. This alloy is for the body skin.
7055-T77
7150-T77
Pitting
Corrosion performance, exfoliation rating
Titanium alloys
Titanium applications have increased with each major commercial
airplane introduction. In the case of
the 777, the use of titanium was expanded into previous CFRP structure
areas to minimize the risk of galvanic
corrosion that is present with aluminum. For this application, betaannealed Ti-6Al-4V ELI (Extra Low
Interstitial) was introduced into the
commercial fleet, and it provides the
maximum damage tolerance properties for titanium alloys.
Titanium was also selected for
landing gear components. The single
largest titanium application, and perhaps the biggest challenge, was applying Ti 10-2-3 to the main landing
gear truck beam. This application
challenged Boeing’s metallurgists to
develop tight process controls for
welding the three pieces that made up
this component. (Note: As part of a
subsequent cost reduction effort,
Boeing ultimately converted the three
forgings to a single forging.) The resulting truck beam saved substantial
weight and also resulted in a design
without the typical corrosion and
paint damage risks associated with
high-strength steel landing gear components.
Titanium alloy developments in the
early- to mid-1980s were pushed into
new product forms and applications
for the 777 as well. While earlier
Boeing airplanes included titanium
for landing-gear springs and high-
2XXX
17% improvement in
toughness
100
A
7150T77
7050T76
7075T73
7055T77
B
Goodness
C
7075-T6
Traditional
strength/
corrosion
behavior
7150-T6
D
Severe
60
65
70
75
80
85
Compresssion yield strength, ksi
90
95
Advances in 7000 series corrosion-resistant aluminum alloys.
250
Fracture toughness Kapp, ksi/in.1/2
stiffened skin with I-section stiffeners
at a constant spacing. The basic skin
ply lay-ups are quite simple, with doublers inserted as pre-kitted units. This
approach permits the panels to be laid
up by the CTLM, resulting in significant cost reductions. To achieve accurate part control, the stiffeners are
pre-cured and co-bonded to the skin
panel during the panel cure cycle.
1000
Fatigue crack growth rate da/dN,
m-in./cycle
Fracture toughness Kapp,
ksi-in.1/2
70%
200
Fuselage
Upper wing
surface
(increased strength)
Lower wing
surface
(increased durability)
200
2324-T39 Type II
150
2024-T351
2324-T39
193
0s
-
100
197
0s -
Ch
alle
nge
199
0s
196 2224-T3511
0s
7150-T651
7055-T7751
7075-T651
50
7178-T651
40
60
80
Typical yield strength, ksi
100
Advanced aluminum alloys with higher toughness and improved corrosion resistance.
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temperature environmental control ducting, these alloys had
several performance and inservice shortcomings. During
the design of the 777, Boeing’s
metallurgists worked closely
with parts manufacturers to upgrade to Ti 15-3-3-3 for both
clock-type springs and ducting.
Another major step forward
was the selection of Beta-21S titanium for the engine plug and
nozzle hot structure, normally fabricated of nickel-base alloys. Beta-21S,
developed for its high resistance to oxidation, resulted in significant weight
reduction for this exhaust component.
Non-structural materials
In the interest of creating a preferred
airplane, Boeing’s materials engineers
concentrated on every detail, and
identified the potential for breakthroughs in some less obvious areas,
for example:
By filling traditional
sealants with microballoons, over 300 pounds
of weight was eliminated
while keeping the same
basic properties.
Through detailed analyses and tests, Boeing confirmed that an entire coat
of paint could be eliminated from the lower portion of the fuselage interior. Amazingly, while
this change eliminated
3.6 square inches
0.5 square inch
over 250 pounds of
Comparative composite material damage resistance. After a weight, the primary dri270 in.-lb impact, the conventional composite material on the ving force behind its inleft shows much more damage than the new 777 advanced com- corporation was imposite material on the right.
proved paint adhesion
Ti-6Al-4V,
beta annealed:
Fracture toughness KIC, ksi-in.1/2
100
Goodness
1980s
damage
tolerant
structure
90
80
VT-22STA:
2000s, damage
tolerant forgings
70
60
Ti-6Al-4V,
mill annealed:
50
1960s
general
structure
40
30
120
140
Ti-10V-2Fe-3Al STA:
1980s, high
Ti-6Al-6Vstrength forgings
2Sn/Ti-6Al-4V
VT-22 STA:
STA:
1960s general
2000s, high
structure
strength
160
Ultimate tensile strength, ksi
180
200
Titanium alloy development has progressed in both strength and toughness.
44
and better corrosion resistance.
These changes typify the innovative
thinking that enabled the development of a preferred airplane in terms
of cost, weight, and affordability.
Boeing 777 to 7E7
The 777 represented a breakthrough
in materials applications for commercial aircraft. The introduction of this
airplane was well-timed to drive a
number of critical advances in materials technologies to maturity, with
the end result being implementation.
The rate of incorporation for these advances onto the 777 is remarkable, and
reflects the high degree of alignment
in research work over the five years
preceding the design effort. This research was clearly focused on fleet
concerns raised by airlines, and the deliberate development of enhanced performance materials that were costeffective.
Just as the 777 was a breakthrough
in terms of materials applications, the
7E7 promises to provide an even
greater opportunity for innovation,
both in technical advances and in the
creation of the cooperative process
needed to develop these technologies
with our global partners.
To compete against products that
are based on many of the same material technologies found on the 777,
the 7E7 engineers must consider further technology breakthroughs and
expand the application of advanced
technologies beyond the current
norm.
Fortunately, materials development
in the last five years has been
promising. Today, confidence has increased in composites as a primary
structure, based on 777 successes. Encouraging progress has been made in
aluminum, steel and titanium technologies. Finally, understanding the
need for environmentally responsible
processes has also grown. Many technologies are now maturing in this area
and offer an opportunity to design
and produce an airplane that is not
only cost and performance preferred,
but more environmentally friendly
■
than airplanes of the past.
For more information: Brian Smith is the
Chief Engineer of Commercial Airplane
Boeing Materials Technology Organization at the Boeing Airplane Co., Seattle,
Washington; tel: 425/237-3516; e-mail:
brian.w.smith@boeing.com.
ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2003