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8/13/2019 Damage Tolerance and the Composite Airframe
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Damage Tolerance and the Composite Airframe
Allen J. Fawcett (ATF/AR)Boeing Commercial Airplanes
787 Program
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OutlineOutline
Deriving Composite Primary Structure Design
Requirements and Criteria
Testing in the Support of Damage Tolerance
Service History and the Effect on Damage Tolerance
Criteria
Proper Maintenance Reporting in the Field
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Levels of Damage Tolerance AssessmentLevels of Damage Tolerance Assessment
Is damage a
result of in-
flight discretesource
event?
Is damage
immediately
obvious?
Is damage
detectable by
plannedinspections?
Does
undetectable
damagegrow?
NO NO NO NO
YES YES YES YES
Residual strengthwith get-home loads
per 25.571 (e)
Immediaterepair
Residual strengthloads per 25.571
(b)
Becomesdetectable?
NO
Residual
strength
withultimate
loads
YES
Damage growth assessment forinspection planning to ensure that
detectable damage does not drop below
CDT during an inspection interval
Safe life assessmentper FAR 25.571 (c)
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Basic RequirementsInterpreting the FARs for CFRP Structure
Basic RequirementsInterpreting the FARs for CFRP Structure
Ultimate Load - AC20.107aSmall Damages, Damage not Expected to be found
AC20-107a covers no detrimental damage growth philosophy
Limit Load - 25.571bVisible Damage
Disbonded Stringer Between Arrestment FeaturesDisbonded Facesheet Between Arrestment Features
Fail Safety
Continued Safe Flight and Landing 25.571e &AC25.571CUnknown Source - Large Damage Applicable to all PSEs
Rotating MachineryBird, Hail and Tire Strike
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Residual Strength CurveResidual Strength Curve
Damage Size or Notch Length
R
esidualstrength
Ultimate Strength
Limit Strength
70% Limit Strength
BVIDDiscrete Source Damage
~ Failed or completely severed frame,
stiffener, or chord with failed or
completely severed attached skin or
web. Ref. AC 25.571-1C (8c)
Residual Strength vs Damage Size or Notch Length
Detectable Damage ~ Failed, or severely damaged,
frame or stiffener with associated damage to attached
skin or web (or) Failed, or severely damaged, skin panel
or web with associated damage to attached stringers,
stiffeners, frame or rib. Ref. FAR 25.571 (b)
Rev. L
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Understand Small DamagesUnderstand Small Damages
Understand what small damages, sometimes known
as BVID, can do to the structure as related to:Ultimate Load
No Detrimental Damage Growth
Satisfying AC 20-107a
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Barely Visible Impact Damage DefinedBarely Visible Impact Damage Defined
BVID
Small damages which may not be found during heavy
maintenance general visual inspections using typical lighting
conditions from a distance of five (5) feet
Typical dent depth 0.01 to 0.02 inches (OML)
Dent depth relaxation must be accounted for
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Wing Panel Residual Strength TestWith BVID
Wing Panel Residual Strength TestWith BVID
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Light Skin Stringer - BVID ImpactsLight Skin Stringer - BVID Impacts
Stringer Cap Impact
Stringer Web Heel Impact
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Criteria Requirements for Visible DamageCriteria Requirements for Visible Damage
Airframe must support design limit loads without failure.
No detrimental damage growth during fatigue cyclingrepresentative of the structures inspection interval.
One missed inspection is assumed (two interval requirement)
Validated by testing
Airframe must be able to support residual strength loads
until the damage is found and repaired.
Damage state contains both visibly detectable and associated non-visiblydetectable damage. Impact
Location
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Wing Skin Visible Impact DamageWing Skin Visible Impact Damage
OML Impact, 1 Diameter impactor
Impact Energy: Greater than 8000 in-lbs
Residual Limit Load
No Growth for a missed inspection interval
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Fuselage Skin Visible Impact DamageFuselage Skin Visible Impact Damage
Residual Limit Load
No Growth for a missed inspection interval
Inside damage associated with OML VID
OML VID Impact
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Criteria Requirements for Disbonded Discrete ElementsCriteria Requirements for Disbonded Discrete Elements
Check is for a disbonded stringer or facesheet
Airframe must support design limit loads and residual
strength flutter requirements without failure.
No detrimental damage growth during fatigue cycling
representative of the structures inspection interval. One missed inspection is assumed (two interval requirement)
Validated by testing
Airframe must be able to support residual strength loads
until the damage is found and repaired.
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Criteria Requirements for Discrete Source DamageCriteria Requirements for Discrete Source Damage
This check is for threats from Unknown Damage Sources,
Rotating Machinery, Bird, Tire and In-Flight Hail
Airframe must support continued safe flight loads and
residual strength flutter requirements without failure.
No repeat cycle loading required
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Criteria Requirements for Unknown Source DiscreteSource Damage
Criteria Requirements for Unknown Source DiscreteSource Damage
The airframe shall be capable of completing a flight during
which complete failure of a structural segment, such as a frame
or stiffener, with associated skin or web, occurs due to anundefined source.
Analysis, supported by component tests, shall demonstraterequired residual strength loads (get home loads) without
failure.
Typical basis, appropriate environmental residual strength
allowables or design values.
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Compression Panel Unknown Damage Source
C it i R i t f T t f Bi d Ti &
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Criteria Requirements for Treats from Bird, Tire &In-Flight Hail
Criteria Requirements for Treats from Bird, Tire &In-Flight Hail
Bird
Continued safe flight and landing following impact of a 4 lb bird; 8 lb for empennage
Bird impact tests on test articles or components representative of A/C design.
Tire
No single tire / wheel threat may prevent continued safe flight and landing.
No single tire / wheel threat may cause leakage of hazardous fuel quantities.
Analyses supported by test evidence
In-Flight Hail
Large dia hail @ cruise for continued safe flight and landing.
For typical dia hail @ cruise: no moisture intrusion, maintain ultimate strength for full
DSO, include effects of environments.
Applies to all airframe primary structure with exposed frontal area in level flight Tests representative of A/C design details of surface panels and supporting
structure.
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Bird StrikeBird Strike
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Simulated Bird Gel-Pack Impact VideoSimulated Bird Gel-Pack Impact Video
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Stiffener damage at impact sites
Impact Site
Impact Site
Damage from 4# Gel Pack Impact
NO spar web damage
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Tire ImpactsTire Impacts
Runway Debris
Tread debrisdefined by FAA
AC 25.963-1
Ground Plane
235 mph23 to SideOf Body
-15 +15-15 +15
Ground Plane
3030
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Tire Impact Test SetupTire Impact Test Setup
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Tire Fragment Impact TestTire Fragment Impact Test
Criteria Requirements for Threats from RotatingCriteria Requirements for Threats from Rotating
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Criteria Requirements for Threats from RotatingMachinery
Criteria Requirements for Threats from RotatingMachinery
The airplane should be able to complete a flight during which damageoccurs due to uncontained:
- Fan blade impact or engine failure.
- Failure of rotating machinery
Analyses, supported by large component testing, shall demonstrate ability to
predict containment of dynamically imposed penetration damage to thepressurized fuselage, and will be used to show compliance with residualstatic strength requirements for design.
Within the zones the airframe is subject to damage from these events,required residual strength loads are considered as part of aircraft levelfailure analyses that produce an aircraft probability of loss equal to, or lessthan, 1 in 20.
Use typical basis strength accounting for appropriate environmental effects.
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RotorburstRotorburst
Bl d I tBl d I t
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Blade ImpactLarge Damage Arrestment
Blade ImpactLarge Damage Arrestment
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Engine DebrisEngine Debris
Runway Debris
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Engine Debris TestingEngine Debris Testing
I Fli h H il
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In-Flight HailIn-Flight Hail
Criteria Realized Through Pre-Production Testing
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Load
introduction
structure
Strongback
attachment
near BL 0
MLG
fittings (not
shown)
Nacelle
fittings on
front spar
and lower
panel (not
shown)
Pre-production Wing Test Boxo Major component level testing to provide validation of the analysis tools for certification
credit and risk reduction.
o Testing will validate finite element models, and provide certification data for wing box
fatigue (no growth with LEF), damage tolerance (VID and large cuts) and static strength(after repair) certification.
g gWing Structural Development Test Plans
P P d ti T t A ti lP P d ti T t A ti l
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Pre-Production Test ArticlePre-Production Test Article
Lower Panel in Trim
Wing Box Assembly
P P d ti Wi T tiP P d ti Wi T ti
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Pre-Production Wing TestingPre-Production Wing Testing
Overall Test Article
& Load Reaction
Structure
Outboard, Looking In
Preproduction ComponentsPreproduction Components
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Preproduction ComponentsTypical Test Load Sequence
p pTypical Test Load Sequence
Apply BVIDs1) Initial Strain Survey
2) Fatigue Spectrum 1 DSOincluding Load EnhancementFactor
3) Strain Survey
4) Fatigue Spectrum 1 DSOincluding Load EnhancementFactor
5) Strain Survey6) Fatigue Spectrum 1 DSO
including Load EnhancementFactor
7) Strain Survey
8) Design Limit Load
Verif ication, MultipleConditions
Apply Visible Impact Damages
9) Fatigue Spectrum, TwoInspection Intervals including LoadEnhancement Factor
10) Fai l Safe (Limit) Loads
Apply Element Damages11) Continued Safe Fl ight,
70% DLL
Repair Element Damagesand Visible Damages
13) Design Ultimate LoadValidation
14) Destruction Test
1
BVID &
Repair
2 3
5
VisibleDamages
Element
DamageRepair All Damages
1 2 3 4 5 6 7 8 9 10 11 1113
14
1
2
3
5
Service HistoryService History
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Service History777 In-Service Experience-CFRP Empennage
y777 In-Service Experience-CFRP Empennage
Some reported service-induced damage incidents
associated with the main torque boxes
FOD damage due to engine run-up Area of skin/stringer disbond repaired with blind fasteners
Hailstorm damage
No damage to main torque box structure
Damage due to impact with maintenance stand
Damage to front spar, main torque box skins, aux spar and leading edges
Bolted titanium sheet metal repair on front spar, skin, other parts replaced
Damage due to impact with service truck Damage to front spar and main torque box skin
Bolted ti tanium sheet metal repair on front spar and skin
Runway debris
Damage to main torque box skin and rib
Bolted ti tanium sheet metal repair on skin and rib shear tie
777 In Ser ice E perience Details777 In Service Experience Details
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777 In-Service Experience-Details777 In-Service Experience-Details
Empennage Stringer Disbond - Engine Thrown Debris
AOG Damage Description
Straight-forward, effective repair
777 In-Service Experience777 In-Service Experience
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777 In Service ExperienceSydney Hailstorm
777 In Service ExperienceSydney Hailstorm
777 In-Service Experience777 In-Service Experience
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pSydney Hailstorm
pSydney Hailstorm
2.5 to 3.0 hail dented the fixed 5 ply honeycomb structure
shown here but did no damage to the CFRP main torque box
777 In Service Experience Details777 In Service Experience Details
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777 In-Service Experience-Details777 In-Service Experience-Details
Skin/Spar DamageGround
Handling Equipment Impact
Conventional Bolted Repair
777 In Service Experience Details777 In-Service Experience-Details
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777 In-Service Experience-Details777 In-Service Experience-Details
Ground Equipment Impact
777 In Service Experience Details777 In-Service Experience-Details
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777 In-Service Experience-Details777 In-Service Experience-Details
Runway Debris
Proper Maintenance Reporting in the FieldProper Maintenance Reporting in the Field
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Proper Maintenance Reporting in the FieldProper Maintenance Reporting in the Field
Accidental events happen and the aircraft design must account
for these.
CFRP structure needs to designed robust to account for those
events which are not known at the time of design.
Most high energy damage events are easily seen and reported
but some high energy but low speed blunt events may require
reporting at the time of incidence.
Training for ground handling personnel working around
composite and aluminum airframes is an important part of
damage detection.