NASA Technical Reports Server (NTRS) 19990032212: Development of Textile Reinforced Composites for Aircraft Structures

10 Pages·0.75 MB·English

by  NASA Technical Reports Server (NTRS)

Development of Textile Reinforced Composites
for Aircraft Structures
H. Benson Dexter
NASA Langley Research Center
Hampton, VA 23681 USA
To be presented at the
4th International Symposium for Textile Composites
Kyoto Institute of Technology
Kyoto, Japan
October 12-14, 1998

Development of Textile Reinforced Composites
for Aircraft Structures
H. Benson Dexter
NASA Langley Research Center
Hampton, Virginia 23681, USA
ABSTRACT 1.0 INTRODUCTION
NASA has been a leader in development of NASA initiated a research program in the early
composite materials for aircraft applications during 1980's to exploit the potential of textile reinforced
the past 25 years. In the early 1980's NASA and composites as a cost-effective method of producing
others conducted research to improve damage damage-tolerant primary aircraft structures [1].
tolerance of composite structures through the use of Several textile material forms, including 2-D and 3-D
toughened resins but these resins were not cost- weaving and braiding, multiaxial warp knitting, and
effective. The aircraft industry wanted affordable, through-the-thickness stitching were evaluated.
robust structures that could withstand the rigors of Structural performance, fabricability, scale-up
flight service with minimal damage. The cost and potential, and manufacturing cost were the primary
damage tolerance barriers of conventional laminated focus of this research. Some of the potential benefits
composites led NASA to focus on new concepts in of textile reinforced composites are as follows:
composites which would incorporate the automated reduced material and assembly labor costs through
manufacturing methods of the textiles industry and automated fabrication of multilayer multidirectional
which would incorporate through-the-thickness preforms; reduced machining and material scrap
reinforcements. The NASA Advanced Composites through use of near net shape preforms; elimination
Technology (ACT) Program provided the resources of cold storage requirements and limits on shelf life
to extensively investigate the application of textile for prepreg; reduced tooling costs for vacuum-
processes to next generation aircraft wing and assisted resin transfer molding compared to
fuselage structures. This paper discusses advanced conventional autoclave processes; and improved
textile material forms that have been developed, damage tolerance and out-of-plane strength as a
innovative machine concepts and key technology result of through-the-thickness stitching.
advancements required for future application of
textile reinforced composites in commercial transport In the late 1980's NASA researchers were looking to
aircraft. Multiaxial warp knitting, triaxial braiding develop breakthrough technologies that would
and through-the-thickness stitching are the three significantly change the way composite structures
textile processes that have surfaced as the most were being built. The NASA Advanced Composites
promising for further development. Technology (ACT) program [2], provided the impetus
and resources to pursue these goals. Working with
Textile reinforced composite structural elements that aircraft manufacturers, universities, and specialty
have been developed in the NASA ACT Program are textile fabricators, NASA began to develop a
discussed. Included are braided fuselage frames and preliminary design database on available material
window-belt reinforcements, woven/stitched lower forms and to focus research on key technology issues.
fuselage side panels, stitched multiaxial warp knit At that point in time, textile reinforced composites
wing skins, and braided wing stiffeners. In addition, moved from being a laboratory curiosity to large
low-cost processing concepts such as resin transfer scale aircraft hardware development. Initial
molding (RTM), resin film infusion (RFI), and emphasis was on mechanical properties and
vacuum-assisted resin transfer molding (VARTM) structural performance, but the need for cost-
are discussed. Process modeling concepts to predict effective fabrication concepts kept surfacing as the
resin flow and cure in textile preforms are also key technology need. With the new focus on
discussed. affordability, NASA awarded several contracts to
industry to begin development of new tooling
KEYWORDS: Textile Reinforced Composites, concepts that could produce aircraft-quality textile
Braiding, Knitting, Stitching, Resin Transfer reinforced composite structures[3]. Resin injection
Molding, Process Modeling methods such as resin transfer molding(RTM), resin
film infusion(RFI), andvacuum-assisteredsin
panels include strength, stiffness, and damage
transfemr olding
(VARTM) would be the keys to tolerance. Based on tests conducted previously by
successful fabrication of composite structures from NASA _nd Boeing (formerly McDonnell Douglas),
dry textile preforms. through-the-thickness stitching was chosen for the
wing t:over panels because a 100-percent
This paper summarizes the development of advanced improvenent in compression-after-impact strength
material forms, textile machine advancements, compared to laminated tape composites could be
analytical process models, fabrication of aircraft achieved [6].
structural components, and lessons learned working
with various textile material forms, machines and Blade sliffeners and integral spar caps were chosen
processes. by Boeing as stiffening elements for the upper and
lower v_ing cover panels. The upper cover skin
2.0 TEXTILE MATERIAL FORMS AND stiffeners, and spar caps are fabricated with Saertex
STRUCTURAL ELEMENTS multiaxial warp knit fabric. However, due to the
curvaturt, of the lower wing cover panel, contoured
As preliminary design databases were generated, triaxiall) braided stiffeners were selected. The
NASA and the U.S. aircraft manufacturers began to stiffener_ and spar caps were stitched onto the wing
focus on the textile processes that offered the most skins to form integral wing cover panels with no
promise. The textile processes listed in figure 1 were mechanical fasteners.
evaluated. The advantages and limitations of each
process were established and decisions on further Before proceeding to design and fabrication of a full-
development were made. The textile material forms scale wing box, Boeing fabricated a 2.4 m by 3.7 m
shown in figure 2 have shown the most promise for stub box with the stitched/RFI process as shown in
application to aircraft structures. Multiaxial warp figure 7. An interior view of a stub box cover panel
knitting is a highly tailorable automated process that is shown in figure 8. This box was stitched with a
produces multidirectional broadgoods for large area compute., controlled heavy duty single needle
coverage. Two-dimensional and three-dimensional machine similar to those used in the quilting
braids are used to create stiffeners, frames and industry. The stub box was successfully tested at
beams with complex cross-sections. Through-the- NASA Langley and met the design requirements set
thickness stitching is an effective way to debulk forth by Boeing for commercial transport wing
preforms and to achieve improved out-of-plane structure: ;.
strength and damage tolerance of composite
structures. 3.0 ADVANCED TEXTILE MACHINE
DEVELOPMENT
The structural elements shown in figure 3 were
selected by NASA and the aircraft manufacturers to The development of a high speed multi-needle
demonstrate the applicability of textiles to fuselage stitching machine and improvements in the
structures. The combination of weaving and stitching multiaxi_l warp knitting process were required to
was used to fabricate lower fuselage side panel achieve iffordable full-scale wing structures. The
preforms[4]. The fuselage panel shown in figure 4 stitching machine had to be capable of stitching
has four circumferential frames and four longitudinal cover pas,el preforms that were 3.0 m wide by 15.2 m
stiffeners. The intersections of the woven stiffeners long by 38.1 mm thick at speeds up to 800 stitches
and frames have continuous fibers through the per minu:e. The multiaxial warp knitting machine
intersection to provide structural continuity. The had to b4: capable of producing 2.5 m wide carbon
flanges of the woven frames are stitched to the fabric wth an areal weight of 1425 g/m 2. The
woven skin material. Braided frames were used to
advanced stitching machine shown in figure 9 was
fabricate curved fuselage keel panels [5], a typical
develop_l by Ingersoll Milling Machine Company to
panel is shown in figure 5. The fuselage parts were
meet the requirements of a full-scale wing box [7].
fabricated with either matched metal heated RTM
The high speed stitching heads were developed by
tooling or RFI autoclave processes.
Pathe Industries, Inc. The lower cover preform for
the full-scale wing box is shown on the stitching
Because of the high load intensity and the propensity machine Oed in figure 9.
for foreign object damage, NASA focused most of its
textiles research on damage tolerant wing structures, Multiaxi_l warp knitting is a highly automated
figure 6. Since most of the wing weight is in the process for producing multilayer broadgoods.
upper and lower cover panels of the wing, they are a Compared to woven broadgoods, the knitted fabrics
logical choice for pursuit of cost and weight savings. have less crimp since the individual tows are not
Some of the global design considerations for wing interlaceC. Early machine concepts lacked proper
tension control to maintain fiber alignment. Saertex conventional autoclave processes are the lower cost
and Liba teamed up to upgrade tension control of tooling, reduced cost of energy to cure composite
mechanisms and to improve overall quality of carbon parts, and almost unlimited part size(i.e., no size
fabrics. A schematic of the Saertex/Liba machine is constraints based on the size of the autoclave). Until
shown in figure 10. This machine can produce 5-ply recently, VARTM was primarily used to fabricate
carbon fabrics in one pass through the machine. A glass reinforced polyester and vinyl ester composites.
two-step process is required to produce a 7-ply fabric However, due to recent developments in resin and
for the Boeing full-scale wing cover panels. Splicing preform technologies, aircraft manufacturers are
concepts have been developed to produce fabrics up beginning to show significant interest in VARTM
to 2.5 m wide. processes for graphite-epoxy and graphite-
bismaleimide composite systems. One drawback to
4.0 FABRICATION OF TEXTILE VARTM processes has been low fiber volume
REINFORCED COMPOSITES fraction compared to the higher fractions achievable
with autoclave pressure. However, stitching and
Three resin transfer processes are commonly used to debulking methods have been developed to achieve
produce composites from dry textile preforms: (1) preforms that are near net shape with little or no
resin transfer molding (RTM), (2) resin film infusion further compaction required during processing.
(RFI), and (3) vacuum assisted resin transfer
molding (VARTM). RTM is a good process to NASA has conducted contractual research with
achieve high fiber volume fraction for complex Seemann Composites, Inc. to establish the feasibility
shapes such as the curved fuselage frames shown in of their VARTM process to produce aircraft quality
figure 5. RTM requires the use of expensive composite structures. Their proprietary process,
matched metal heated tools, such as Invar, and high called SCRIMP (Seemann Composites Resin
pressure to pump liquid resin into net shape tools. Injection Molding Process) utilizes a resin
High quality parts can be achieved but the cost is distribution media to achieve full wet-out of the
prohibitive for large parts such as wing skins. preform, figure 13. In addition, Seemann has
developed a reusable bagging concept that
RFI is a process being pursed by NASA and The eliminates most of the costs associated with
Boeing Company to develop cost-effective wing conventional bagging procedures. Seemann
structures for commercial transport aircraft [8]. The Composites has also demonstrated SCRIMP for
RFI process developed by Boeing consists of an lightly-loaded general aviation aircraft structures.
outer mold line tool, an epoxy resin film, a near net Figure 14 shows the one-sided tooling concept and
shape textile preform, an inner mold line tool and a the graphite preform for a small aircraft fuselage
reusable vacuum bag. Resin slabs are placed on the section and figure 15 shows the completed
outer mold line tool and the preform and inner mold graphite/epoxy part after resin injection and cure.
line tools are placed on top of the resin. The entire Current and future tooling developments for integral
assembly is covered with a reusable vacuum bag and heating will eliminate the need for oven cure and
the part is placed inside an autoclave. After the postcure of composite parts fabricated with VARTM
resin is melted, vacuum pressure is used to infuse processes.
resin into the preform. Once infiltrated, the part is
cured under pressure and temperature in an NASA is also investigating the feasibility of
autoclave. The keys to producing aircraft quality SCRIMP to produce aircraft quality heavily-loaded
parts with the RFI process are understanding the primary structures. Additional technology
compaction and permeability characteristics of the development is required to achieve dimensional
preform and understanding kinetics and viscosity control and acceptable fiber volume fractions for
profiles for the resin as a function of temperature. thick structural elements. Innovative tooling
Figure ll shows a full-scale 13m long stitched/RFI concepts will be required to meet typical assembly
wing cover panel fabricated by Boeing under tolerances for aircraft structures. Stitching will be
contract to NASA. The one-piece reusable vacuum required to achieve near net shape prior to resin
bag used to cure the cover panel is shown in injection. The reusable bagging concept for a three
figure 12. stringer panel representative of wing structure is
shown in figure 16. The ease of removing this bag
VARTM processes have been used for many years to from the stiffened panel is illustrated in figure 17,
fabricate fiberglass reinforced composite structures. and the finished panel (after resin injection and cure)
The U.S. Naval Surface Warfare Center in Bethesda, is shown in figure 18.
MD has been the major promoter of this technology
for composite marine applications [9]. The major To eliminate trial and error processes, analytical
advantages of VARTM processes compared to models are required to predict resin flow into textile
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preforms. The models must be verified through concepts to restore damaged structure to original
precise experiments to demonstrate the modeling strength must also be developed.
accuracy. Three-dimensional models are required to
capture response adequately for complex preforms Analytical models are required to eliminate costly
such as wing cover panels that contain trial and error processes that are frequently used in
stitched/knitted fabric skins and stitched/braided tool design and development of processing cycles.
stiffeners. The objectives of the analytical model are Resin viscosity and cure kinetics must be
to predict flow front position, resin viscosity and characterized to consistently achieve high quality
degree of cure as a function of temperature and time. composite parts. Since compaction and permeability
A 3-D RFI process simulation model is under behavior are different for each fiber architecture and
development by Virginia Polytechnic Institute and preform configuration, empirical relationships must
State University [10]. The RFI simulation includes be developed for input to analytical models.
resin flow, heat transfer, and thermochemical
elements. A schematic of the 3-D finite element Tooling concepts that can accommodate variability
model for infusion of a stitched blade stiffener is in dry preform bulk and permeability must be
shown in Figure 19. Experiments are currently being developed to achieve uniform resin flow and fiber
conducted to verify accuracy of the 3-D finite wet-out. Dimensional tolerances on tooling is
element model. For a two-stringer stitched panel, the critical to avoid racetracking or short circuiting of
predicted temperature distribution was within 6- resin during the infusion process. Further
percent of measured temperature and the predicted development of compaction methods and soft
resin wet-out times were within 4 to 12-percent of tooling concepts such as VARTM will lead to
measured times. reduced ._crap and lower manufacturing costs.
5.0 LESSONS LEARNED FROM TEXTILE 6.0 CONCLUDING REMARKS
DEVELOPMENT
Major advancements in the development of textile
Early attempts to develop complex equipment to preforms in concert with resin transfer tooling
fabricate near net shape multidirectional multilayer concepts makes it feasible to fabricate high quality
fabrics were unsuccessful. This result was primarily damage tolerant aircraft structures with these
caused by the fact that various textile technologies material forms and processes. Recent developments
were being stretched beyond their technical in improved tension control of tows in the multiaxial
boundaries. Two- and three-dimensional braiding warp knilting process have made this the material of
showed a lot of potential but available machine choice for large area multidirectional broadgoods.
capacity limited the architecture and the size of the Innovative tooling concepts have been developed to
preforms that could be achieved. Folding or produce curved braided preforms for application to
postforming of triaxial braided preforms offered the fuselage frames and wing stiffeners for commercial
most flexibility in achieving small cross-section transport aircraft. NASA and Boeing have
complex shapes. Multiaxial warp knitting proved to demonstrated that through-the-thickness stitching of
be the best process for large area multiaxial dry textde preforms can provide a 100-percent
multilayer broadgoods but structural shapes had to be increase in damage tolerance of composite wing
achieved through postforming and stitching. structures compared to laminated tape construction
Through-the-thickness stitching proved to be the best techniques.
textile process to achieve improved damage
tolerance. Compared to processing with glass fibers, Recent it vestments in a second generation stitching
all textile machines investigated had to be slowed machine _vith multiple heads are expected to pay-off
considerably when carbon fibers were used. Lessons in terms _)fimproved quality, higher speed, and lower
learned to-date indicate that no one machine can cost. Low cost resin transfer molding processes are
produce the desired fiber architectures for all now being applied to fabrication of aircraft quality,
complex shape aircraft structural preforms. In heavily-loaded primary structures. Modeling studies
addition, more stringent in-process controls and of resin transfer molding will focus on prediction of
inspection techniques are needed to minimize scrap resin flow into complex textile preforms to insure
and to reduce costs. Invariably, processing defects high quality, high speed fabrication at lower costs.
and handling damage will occur. Since scrap of Additioni.l development of out-of-autoclave vacuum-
large expensive parts is not an option, repair assisted resin transfer molding (VARTM) processes
concepts must be developed. Methods to reinfuse could lead to significantly lower tooling and
resin starved areas must be developed and repair fabrication costs for large area composite structures.
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7.0 REFERENCES 10. Loos, A. C., et al, "Resin Film Infusion(RFI)
Process Simulation of Complex Wing
1. Dexter, H. B.; Camponeschi, E. T. and Peebles, Structures", Proceedings of the Fifth NASA/DoD
L., ed., "Proceedings of Working Group Meeting Advanced Composites Technology Conference,
on 3-D Composite Materials", NASA CP 2420, NASA CP 3294, Vol 1, Part 2, 1995, pp. 811-
1986. 833.
2. Dexter, H. B. and Stein, B. A.: "Advanced
Composite Materials for Airframe Structures",
TextileProcess Advantages Umltatlons
Fiber-TEX 1987- First Conference on Advanced
High in-plane properties Lowtransverse and
Engineering Fibers and Textile Structures for LowCrimp Good tailorsbility auf-of-plane prooer_ies
Uniweave Highly automated preform Poor fabric stability
Composites, NASA CP 3001, Part 1, 1988, pp. 1- fabrication process Labor intenswe
29. plylay-up
Good in-plane properties Limited tailorsbility
Good drapebility foroff-axis
2-D Woven H_hly automated preform properties
3. Davis, J. G., Jr. and Bohon, H. L., ed.: "First Fabric Fabhcstion process Lowout-of-plane
NASA Advanced Composites Technology InSteugrrfaeldlyfowrolvaergne sahreaapescovpeorasgsiebka properties
Conference", NASA CP 3104, Part 1, 1991. Extensive data base
Mo0erste in-plane and out-of-plane Limited tailorability
properties foroff-axis
3-D Woven Fabric Automated prstorm fabrication process properties
4. Suarez, J. A. and Buttitta, C., et al.: "Novel Limited woven shapes possible Poor drepabllity
Composites for Wing and Fuselage Applications- Good balanca inoff-axis properties Size limitation due to
Automated preform fabrication process machine availability
Textile Reinforced Composites and Design 2-O Braided Preform Wellsuited forcornplax curved shapes Low out-of-plane
Good drapebility properties
Guidelines", NASA Contractor Report 201612,
Slow preform
September 1996. Good balanne inin-plane and fabrication process
3-D Braided Preform out-of-plane properties Sizehrn_ation clueto
Well suited forcomplax shapes machine availability
5. Jackson, A. C.; Barrie, R. C. and Chu, R.L.: Good tstlorability for I_lancad
"PTroecxetieldeings Compoosfite theFuseTlahgierd DNeAveSlAopment"A,CT WMaurlptiaxKlanlit MHuigtthif-liatnyeb-yprielaacrunatetoiomhnaigptehrdopptrheorrcotpeiuersegssfhoprumt opurto-opLfeo-prwtliaense
Conference, NASA CP 3178, Vol 1, Part 1, material suited forlarge area coverage
1992, pp. 79-96. Good in-plane properties Small reOuctlon in
Highly automated process in-plane properties
Stitching provides excellent damage Pooraccassibilily
tolerance and out-of-plane strength tocomplex
. Hawley, A. V.: "Detail Design Development of a Excellent assembly aid curved shapes
Transport Aircraft Composite Wing",
Proceedings of the Sixth NASA/DoD ACT
Conference, NASA CP 3326, Vol 1, Part 1, Figure !. Application potential of textile reinforced
1996, pp. 131-154. composite materials for aircraft structures.
7. Thrash, P. J., and Miller, J. I.: "Design of a lltlli/i
Stitching Machine for Transport Aircraft
Composite Wings", Proceedings of the Sixth
NASA/DoD ACT Conference, NASA CP 3326,
Vol 1, Part 1, 1996, pp. 191-208.
8. Markus, A. M.: "Resin Transfer Molding for
Composite Primary Wing and Fuselage MulUaxlal warp knit 2-D trlaxlal braid
(stitched &unstitched) (stitched &unstitched)
Structures", Proceedings of the Ninth
DoD/NASA/FAA Conference on Fibrous
Composites in Structural Design, NASA CP If J:_ [ JI
3154, 1992, pp. 141-167. _il_l
z m zl m xx _I
I/_Jl l/ !//JIIIr 111
9. Critchfield. M.O., et al, "Low-Cost Fabrication
of Composite Ship Structures Using Vacuum-
Assisted Resin Transfer Molding", Proceedings 3-D braid Knitted/stitched
of the Composites in Marine Applications
Figure 2. Textile material forms.
Symposium, November, 1992, Seattle, WA.
Skin/Stiffened Panels Window Belt UpperAndLower StiffenersAnd
Cover Panels Spar Caps
..._.Lx_
Circumferential Frames Keel Beam Frames
Intercostal Clips MainLanding
Gear Attachment
Figure 3. Application of textile reinforced
composites in fuselage structures.
Figure 6. Application of textile reinforced
composites in wing structures.
Figure 4. Woven/stitched lower fuselage side panel
preform.
Figure 7. Stitched/resin film infused wing stub
box.
Figure 8. Interior view of stub box cover panel.
Figure 5. Curved braided frames for fuselage keel
structure.
6
Figure 9. Advanced stitching machine for wing Figure 12. Reusable vacuum bag for wing cover
cover panels. panel.
Advantages of low-cost RTM process:
• Resin and fiber used inlowest cost form
• Prepreg process eliminated
• Freezer storage & shelf lifeproblems
eliminated
• Low cost, one-sided tooling
• Low energy, low pressure out-of-autoclave
processing
• Utilizes net-shape, damage tolerant textile
preforms
• Large integral structure minimizes secondary
bonding and fastening
Challenges for aircraft applications:
Figure 10. Schematic of Saertex/Liba multiaxial •Out-of-autoclave cure resins with adequate
warp knitting machine. properties
• Dimensional tolerances with low-cost tooling
v=xl_t_
\
sRecsin_.DPis_tri-buPti.o.nn,.dMedia
Figure ]3. Low-cost vacuum-assisted resin transfer
molding process.
Figure 11. Stitched/Resin film infused composite
wing cover panel.
Figure 14. Vacuum-assisted resin transfer molding
tooling for fuselage section.
Figure15.Vacuum-assistreedsintransfemr olded
fuselagseection. Figurt1,,8.StiffenepdaneflabricatebdyVARTM.
Program Structure
• Flow
• HeatTransfer
• ResinKinetics
• ResinViscosity
• PreformCompaction
• ResidualStressandWarpage
Figure16.ReusablveacuumbagforVARTMof Ply Drop Off Single Blade Stiffener
3-stringepranel.
Preform/Tooling Assembly
Au,mi.u=m.o,..o=
Aluminum blocks ....
Stitched
graphite
preform
Aluminum baseplate
Figure 17. Removal of reusable vacuum bag from
Figure 19. Three-dimensional resin film fusion
VARTM stiffened panel.
model.
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