Causing Unbond
CHAPTER 7 7 SUPPORTABILITY SUPPORTABILITY 7.1 INTRODUCTION
7.2 DESIGN DESIGN FOR FOR SUPPORTABILITY SUPPORTABILITY
7.2 DESIGN DESIGN FOR FOR SUPPORTABILITY SUPPORTABILITY
7.2.1
7.2.1 In-service In-service experienceexperience
Composite materials were introduced into the commercial aircraft industry during the early 1960's.
Composite materials were introduced into the commercial aircraft industry during the early 1960's.
Advanced fibers such as boron, aramid, and carbon offered the possibility of increased strength, reduced Advanced fibers such as boron, aramid, and carbon offered the possibility of increased strength, reduced weight, improved corrosion resistance, and greater fatigue resistance than aluminum.
weight, improved corrosion resistance, and greater fatigue resistance than aluminum.
The original composite parts, particularly thin-gage sandwich panels and secondary structures, The original composite parts, particularly thin-gage sandwich panels and secondary structures, ex-perienced durability problems such as low resistance t
perienced durability problems such as low resistance to impact, liquid ingression, and erosion. o impact, liquid ingression, and erosion. The faceThe face sheets of honeycomb sandwich parts were often only three plies or
sheets of honeycomb sandwich parts were often only three plies or less, which was adequate for less, which was adequate for stiffnessstiffness and strength, but did not
and strength, but did not consider the hostile service environment. consider the hostile service environment. Damage such as core crushing, im-Damage such as core crushing, im-pact damage, and disbonding, is often easily detected with
pact damage, and disbonding, is often easily detected with a visual inspection due a visual inspection due to the thin face sheets,to the thin face sheets, but can also be non-detectible without the aid of a relatively sophisticated nondestructive inspection but can also be non-detectible without the aid of a relatively sophisticated nondestructive inspection tech-niques.
niques. Occasionally damage is not detected initially and is, therefore, Occasionally damage is not detected initially and is, therefore, not repaired, resulting in growth not repaired, resulting in growth ofof the damaged area due to cyclic loading and other physical phenomena such as liquid ingression into the the damaged area due to cyclic loading and other physical phenomena such as liquid ingression into the core.
core. Metallic core materials can also experience Metallic core materials can also experience corrosion effects due corrosion effects due to moisture ingression and gal-to moisture ingression and gal-vanic effects associated with incompatible materials and/or inadequate corrosion protection.
vanic effects associated with incompatible materials and/or inadequate corrosion protection.
Repair processing can also further damage a sandwich structure if not conducted properly
Repair processing can also further damage a sandwich structure if not conducted properly. . Most re-Most re-pair materials cure at temperatures above the boiling point of water, which can cause concern for a pair materials cure at temperatures above the boiling point of water, which can cause concern for a dis-bond at the skin-to-core
bond at the skin-to-core interface during processing wherever interface during processing wherever trapped water resides. trapped water resides. Therefore, coreTherefore, core drying cycles are typically included prior to p
drying cycles are typically included prior to performing a repairerforming a repair. . In some cases, in-service fluids such asIn some cases, in-service fluids such as Skydrol can cause contamination to the area intended for rep
Skydrol can cause contamination to the area intended for repair. air. Complete removal of Skydrol from coreComplete removal of Skydrol from core materials is almost impossible, where the core continues to weep the liquid so that full bonding does not materials is almost impossible, where the core continues to weep the liquid so that full bonding does not occur.
occur.
Service experience data has been documented in reports from operators on parts involved in the Service experience data has been documented in reports from operators on parts involved in the NASA-sponsored Advanced Composites Energy Efficiency (ACEE) program, which supported the design NASA-sponsored Advanced Composites Energy Efficiency (ACEE) program, which supported the design and fabrication of sandwich composite parts such as the B727-200 elevators and the B737 spoilers and and fabrication of sandwich composite parts such as the B727-200 elevators and the B737 spoilers and
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Volume 6, Chapter 6, Chapter 7 7 SupportabilitySupportability horizontal stabilizers.
lems. Thirteen airbrakes are still on aircraft, aThirteen airbrakes are still on aircraft, and seven have been withdrawn from service for testnd seven have been withdrawn from service for testing toing to assess stiffness and residual
assess stiffness and residual strength.strength.
Production carbon-epoxy sandwich parts have demonstrated weight reduction, delamination Production carbon-epoxy sandwich parts have demonstrated weight reduction, delamination resis-tance, fatigue improvement,
tance, fatigue improvement, and corrosion prevention. and corrosion prevention. The poor service records of some partThe poor service records of some parts can bes can be attributed to fragility, the inclusion of non-durable design details, poor processing quality, porous face attributed to fragility, the inclusion of non-durable design details, poor processing quality, porous face sheets (insufficient thickness), and badly installed or poorly sealed fasteners.
sheets (insufficient thickness), and badly installed or poorly sealed fasteners.
7.2.2 Inspectability 7.2.2 Inspectability
Typical composite nondestructive inspection (NDI) methods available are: visual, Typical composite nondestructive inspection (NDI) methods available are: visual, through-transmission ultrasonics (TTU), pulse-echo ultrasonics,
transmission ultrasonics (TTU), pulse-echo ultrasonics, x-rayx-ray, enhanced optical , enhanced optical schemes, and thermogra-schemes, and thermogra-phy
phy. . Most airlines and military opeMost airlines and military operators use visual inspections supplemented with brators use visual inspections supplemented with both tap test anoth tap test andd pulse echo and low-frequency bond
pulse echo and low-frequency bond testing to locate damage. testing to locate damage. Because of the predoBecause of the predominance of visualminance of visual inspections, provisions should be made during the design phase for complete external and internal inspections, provisions should be made during the design phase for complete external and internal ac-cess for visual inspection of
cess for visual inspection of all components.all components.
With a sandwich configuration there are inspection difficulties associated with potted areas, detection With a sandwich configuration there are inspection difficulties associated with potted areas, detection of fluids that have leeched into the sandwich honeycomb core, disbonds of face sheets, foam core, and of fluids that have leeched into the sandwich honeycomb core, disbonds of face sheets, foam core, and damage within the core, as well as bond lines of stiffeners or frames that are bonded to internal face damage within the core, as well as bond lines of stiffeners or frames that are bonded to internal face sheets of sandwich components.
sheets of sandwich components.
7.2.3
7.2.3 Material Material selectionselection
It is important to select the appropriate constituent core, facing and adhesive m
It is important to select the appropriate constituent core, facing and adhesive m aterials for a sandwichaterials for a sandwich structure, considering how it is to be processed, its service environment, and its compatibility with structure, considering how it is to be processed, its service environment, and its compatibility with sur-rounding materials.
rounding materials. Ease of repair and the associated repair processes should also be taken intEase of repair and the associated repair processes should also be taken into consid-o consid-eration when selecting the material s
eration when selecting the material systems.ystems.
7.2.4
7.2.4 Damage Damage resistanceresistance
Components will be subject to mechanical damage from maintenance personnel, tools, runway Components will be subject to mechanical damage from maintenance personnel, tools, runway de-bris, service equipment, hail, lightning, etc.
bris, service equipment, hail, lightning, etc. Note that other types of damage may also be experienced byNote that other types of damage may also be experienced by sandwich composite structures,
sandwich composite structures, but are not addrebut are not addressed here. ssed here. Most composite Most composite components are designedcomponents are designed to specific damage resistance, damage to
to specific damage resistance, damage tolerance, and durability criteria. lerance, and durability criteria. A supportA supportable sandwich struc-able sandwich struc-ture must be able to sustain a reasonable level of damage without costly rework or downtime.
ture must be able to sustain a reasonable level of damage without costly rework or downtime.
Damage resistance is a measure of the relationship between the force or energy and impact Damage resistance is a measure of the relationship between the force or energy and impact foot-print/shape of an associa
print/shape of an associated damage event and tted damage event and the resulting damage characteristics. he resulting damage characteristics. A A structure withstructure with high damage resistan
high damage resistance will incur less damage from a ce will incur less damage from a given event. given event. Damage resistance levels should bDamage resistance levels should bee such that impacts that do not create visible damage will not degrade the strength of the structure below such that impacts that do not create visible damage will not degrade the strength of the structure below design requirements.
design requirements.
Damage resistance of sandwich structures may be improved by increasing facing thicknesses and/or Damage resistance of sandwich structures may be improved by increasing facing thicknesses and/or by using denser core.
by using denser core. However, thHowever, thicker facings that decrease damage visibility may actually lead to anicker facings that decrease damage visibility may actually lead to an increased risk of not det
increased risk of not detecting internal damage. ecting internal damage. For example, thicker facings are For example, thicker facings are more likely to “springmore likely to “spring back” to their original contour or shape after an impact event that fractures or locally crushes the core.
back” to their original contour or shape after an impact event that fractures or locally crushes the core.
This may also result
This may also result in a disbond between tin a disbond between the facing and core. he facing and core. Reinforcement fibers having Reinforcement fibers having high strainhigh strain capability can also have a positive effect on impact resistance, and the selection of toughened matrix capability can also have a positive effect on impact resistance, and the selection of toughened matrix ma-terial can greatly enhance damage
terial can greatly enhance damage resistance. resistance. As discussed above, water ingress into a As discussed above, water ingress into a sandwich panelsandwich panel after impact damage is another supportability concern associated with sandwich construction.
after impact damage is another supportability concern associated with sandwich construction.
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Volume 6, Chapter 6, Chapter 7 7 SupportabilitySupportability 7.2.5
7.2.5 Environmental Environmental compliancecompliance
Many aspects of the design, repair and maintenance of sandwich structures are impacted by Many aspects of the design, repair and maintenance of sandwich structures are impacted by envi-ronmental rules an
ronmental rules and regulations. d regulations. Environmental compliance is primarily Environmental compliance is primarily concerned with the concerned with the correct dis-correct dis-posal of hazardous wastes.
posal of hazardous wastes. A significant portioA significant portion of a waste stream is made up of materials thn of a waste stream is made up of materials that cannot beat cannot be used within their useful life.
used within their useful life. Typical heavy Typical heavy metal culprits include cadmium pmetal culprits include cadmium plated fasteners and lated fasteners and chro- chro-mated sealants and primers.
mated sealants and primers.
Consideration must be given to removal o
Consideration must be given to removal of coatings. f coatings. Chemical paint removers are not aChemical paint removers are not acceptable forcceptable for most polymer matrix comp
most polymer matrix composites because the active ingredientosites because the active ingredients will attack the matrix. s will attack the matrix. Abrasive paint re-Abrasive paint re-moval techniques, such as plastic media blasting, have proven successful on polymer matrix composites.
moval techniques, such as plastic media blasting, have proven successful on polymer matrix composites.
Cleaning is one of the pr
Cleaning is one of the primary maintenance processes that imary maintenance processes that create hazardous waste. create hazardous waste. Many of the clean-Many of the clean-ing processes that previously utilized ozone depletclean-ing solvents and other hazardous chemicals are beclean-ing ing processes that previously utilized ozone depleting solvents and other hazardous chemicals are being replaced with aqueous cleaning processes.
replaced with aqueous cleaning processes. If a component is constructIf a component is constructed such that water intrusion is aed such that water intrusion is a concern, as in the case of
concern, as in the case of sandwich structures, then aqueous cleaning of sandwich structures, then aqueous cleaning of the part may also be a problemthe part may also be a problem..
Machining of carbon fiber laminates
Machining of carbon fiber laminates during cutting and trimming operations produces during cutting and trimming operations produces particulates thatparticulates that are considered a nuisance dust by bio-environmenta
are considered a nuisance dust by bio-environmental engineers. l engineers. TLV (TLV (Threshold Limit ValuThreshold Limit Values) limits werees) limits were updated in 1997 by the American Conference of Government and Industrial Hygienists (ACGIH) to define updated in 1997 by the American Conference of Government and Industrial Hygienists (ACGIH) to define loose composite fiber/dust exposure
loose composite fiber/dust exposure limits for composite workers. limits for composite workers. Excessive exposure may require tExcessive exposure may require thehe use of NIOSH-certified respirato
use of NIOSH-certified respirators with HEPA rs with HEPA filters. filters. Resins and adhesives used in sandwich structuResins and adhesives used in sandwich structuresres may cause dermal sensitization in some workers, so that silicon-free/lint-free gloves should be mandated may cause dermal sensitization in some workers, so that silicon-free/lint-free gloves should be mandated for use.
for use. This will also help to ensure that a This will also help to ensure that a contaminant-free bonded sandwich assembly contaminant-free bonded sandwich assembly is achieved.is achieved.
Uncured prepregs, adhesives an
Uncured prepregs, adhesives and resins are treated as d resins are treated as hazardous materials. hazardous materials. Scrap materials shouldScrap materials should be cured prior to disposal in order
be cured prior to disposal in order to reduce the HazMat dispoto reduce the HazMat disposal costs. sal costs. It is important to eIt is important to ensure thatnsure that scrap materials containing carbon fibers are sent to non-burning landfills; pyrolized carbon fibers freed by scrap materials containing carbon fibers are sent to non-burning landfills; pyrolized carbon fibers freed by resin burn-off can represent a respiratory and electrical hazard.
resin burn-off can represent a respiratory and electrical hazard.
7.2.6
7.2.6 Reliability Reliability and and maintainabilitymaintainability
Maintainability of a structure is achieved by developing appropriate customer-focused methods of Maintainability of a structure is achieved by developing appropriate customer-focused methods of inspection and maintenance during the d
inspection and maintenance during the design phase. esign phase. Accessibility is an important Accessibility is an important factor when designingfactor when designing structures for repair
structures for repair. . Sufficient access should be provided to properly inspect, prepare the damage area,Sufficient access should be provided to properly inspect, prepare the damage area, fit and install repair parts and use repair tools and
fit and install repair parts and use repair tools and bonding equipment.bonding equipment.
Designing for repairability is an
Designing for repairability is an essential element in the effective use of comessential element in the effective use of composite materials in aircraftposite materials in aircraft structures.
structures. Repair considerations shoRepair considerations should influence the choice of lay-up pattuld influence the choice of lay-up patterns and design strain levels.erns and design strain levels.
The repair philosophy should be set during the conceptual design stage and the repair designs should be The repair philosophy should be set during the conceptual design stage and the repair designs should be developed along with the component
developed along with the component design. design. Candidate repair designs shouCandidate repair designs should be tested as pld be tested as part of theart of the development test program.
development test program. Repair concepts and materRepair concepts and materials should be standardized to the maximum ex-ials should be standardized to the maximum ex-tent possible.
tent possible.
7.2.7 Repairability 7.2.7 Repairability
A component’s “repairability” needs to be considered as part of the design process, starting with a A component’s “repairability” needs to be considered as part of the design process, starting with a reasonable/sufficient margin of safety, and should include provisions for reasonable repair size limits.
reasonable/sufficient margin of safety, and should include provisions for reasonable repair size limits.
Figure 7.2.7 shows the maintenance development philosophy established during the Boeing/NASA Figure 7.2.7 shows the maintenance development philosophy established during the Boeing/NASA Ad-vanced T
vanced Technology Composite Aircraft Sechnology Composite Aircraft Structures (Atructures (ATCAS) composite fuselage program. TCAS) composite fuselage program. Inspection andInspection and repair procedures must
repair procedures must be considered during be considered during design selection. design selection. Any lightning strike protectioAny lightning strike protection systemsn systems that are needed on specific
that are needed on specific components should also be designed to be components should also be designed to be repairable.repairable.
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FIGURE 7.2.7
FIGURE 7.2.7 Rules for maintainable sandwich structures (Reference 7.2.7).Rules for maintainable sandwich structures (Reference 7.2.7).
Design concept developments should include parallel efforts to establish maintenance procedures.
Design concept developments should include parallel efforts to establish maintenance procedures.
Maintenance procedures established
Maintenance procedures established afterafter design features are set will typically result in unnecessarilydesign features are set will typically result in unnecessarily complex repairs.
complex repairs.
Inspection techniques are integrally tied with potential damage levels, so the inspection plan must Inspection techniques are integrally tied with potential damage levels, so the inspection plan must mature along with the design.
mature along with the design. Cost-effective methods to addrCost-effective methods to address large areas of structure are needed toess large areas of structure are needed to initially detect damaged regions.
initially detect damaged regions. Visual methods are most Visual methods are most frequently used, but more sophfrequently used, but more sophisticated meth-isticated meth-ods that can detect less severe damage are also possible.
ods that can detect less severe damage are also possible. Damage that may not be found over the life ofDamage that may not be found over the life of the structure with the selected inspection method must sustain Ultimate loads, while damage levels that
ods that can detect less severe damage are also possible. Damage that may not be found over the life ofDamage that may not be found over the life of the structure with the selected inspection method must sustain Ultimate loads, while damage levels that