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Damage Tolerance Methodology - ESAC - Ref. X029PR0608046 - Issue
1
Damage Tolerance Methodology
Chicago, IL
Prepared by Emilie MORTEAU, Chantal FUALDES
Presented by
Chantal FUALDESAirbus Head of Composite stress analysis
Composite Senior Expert
FAA Workshop for Composite Damage Tolerance and Maintenance July
19-21, 2006
Composites @ Airbus
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Main principles in Damage tolerance methodologyR
EG
UL
AT
ION
RE
GU
LA
TIO
N
ANALYSISANALYSIS--
FATIGUEFATIGUE& DAMAGE& DAMAGE
TOLERANCETOLERANCEEVALUATIONSEVALUATIONS
ININ--SERVICESERVICEEXPERIENCEEXPERIENCE
TEST RESULTS TEST RESULTS
BUILDING BLOCK APPROACHBUILDING BLOCK APPROACH
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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1- AIRBUS Damage tolerance philosophy
DT Philosophy to answer to requirement and means of
compliance
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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1.1- Damage detectability
Damage detectability
Damage metric
BVID definition
Large VID definition
Supporting tests and analysis
Relaxation behaviour
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1.1- Damage detectability
4For Airbus composite parts (CWB, Keel Beam, aileron, spoiler,
HTP, VTP, LGD, etc)relevant impacts for DT analysis are from
maintenance i.e. tool drop, removable panel drop, and in a smaller
extent from operation by runway debris (LGD), 4Shape of damage can
be simulated by low impactor diameter (diameter generally used for
composite test and DT substantiation is from 6 to 25mm), and
4Resulting damages have similar diameter, mainly the dent depth
(and crack length for edges), and depend on the impact energy
For transverse impact, the damage metric used for detectability
is the
dent depth
For edge impact, the damage metric used for detectability is the
dent
depth and/or cracks length
Has to be revisited for composite fuselage application for
consistency
with impact sources (ground handling)Damage metric
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1.1- Damage detectability
The minimum impact damage surely detectable by scheduled
inspection
4Dent depth criterion as a damage metric is widely used for
composites. (It is acceptable to use additional criteria (not just
dent depth) when establishing the limit of detectability, if this
is justified by appropriate testing)4It corresponds to a
probability of detection of 90% with an interval of confidence of
95%.4It provides a reasonable level of robustness for the structure
design
the aim is to sustain UL with BVID
Two values for the BVID criterion are established dependent on
the visual inspection type : DET and GVI
BVID definition
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1.1- Damage detectability
is technology and structure dependant
4Damage size associated to walk-around is considered on a
case-by-case basis
4 Typically penetration
Example for a sandwich structure
Large VID definition
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1.1- Damage detectability
DET InspectionDetection of damages on different composite panels
(size: from 100*100mm to 0.8m, painted or not, glossy or mat,white,
grey, blue or green paint, primer) Duration of inspection : not
limitedDistance of inspection : 50 cmLighting condition : available
lighting+grazing light (if required)Several impactor diameter : 6mm
and 16mmA total of 902 inspections
GVI InspectionInspection on large panel (8m*1.2 m)Two
configurations : horizontal or vertical panelsDistance of
inspection : 1mDuration of inspection : 30sec/panelArtificial
lighting representative of Natural daylightSeveral impacts on
painted panel: from 0.3mm deep to perforationSeveral impactor
diameter : from 6 to 120mmA total of 240 inspections
FOR BVID TRANSVERSE IMPACT
Supporting tests and analysis and in-service survey
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1.1- Damage detectability
( )
-
-
-
---
==>j
jd
md
ymd
j dyeddeddPs
s
psp
log
22
log
det
2
2
2
21
)(log.2
1)(
33.2)95/50()95/99(
)95/50(
:
aLogaLogaLogm
depthdentd
-=
=
sBVID
Results of inspection were statistically processed using a
search for maximum plausibility type approach.
The analytical POD function used is the Log Normal cumulative
distribution
Supporting tests and analysis and in-service survey
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1.1- Damage detectability
85% of collected impact damages (dent) (around 1000 damage
records) detected through GVI inspection (A, C check, daily,
weekly, etc) are below Airbus established detectability
threshold
Airbus BVID(GVI) is consistent with Airline survey findings
Survey in European airlineCumulative curve of dent depth
0,00%
20,00%
40,00%
60,00%
80,00%
100,00%
120,00%
0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 4,00 4,50 5,00 5,50 6,00
6,50
Dent depth (mm)
Po
urc
enta
ge
of
dam
ages
wit
h d
ent
less
than
d
Example for GVI inspection
Supporting tests and analysis and in-service survey
Airbus BVID (GVI)
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1.1- Damage detectability
The relaxation is the phenomenon that leads to damages that
become less detectable over time: a damage being detectable at time
of impact, can become undetectable after an interval of inspection
due to mechanical, thermal cycling, wet and ambient ageing and
temperature.
Material A
0,00
0,10
0,20
0,30
0,40
0,50
0,60
0,70
0,80
0,90
After impact After 20 mn After 48H After WA Beforefatigue
After110cycles
0,6Fr
After fatigue
Event
Den
t dep
th e
volu
tion
(mm
)
18J impact+WA70/95%HR1500h and fatigue (r=10c/c) at 20
18J impact+WA70/95%HR1500h and fatigue (r=10c/c) at -40
18J impact+WA70/95%HR1500h and fatigue (r=-1 t/c)at 20
20J impact+WA70/95%HR1500h and fatigue (r=10c/c) at 20
20J impact+WA70/95%HR1500h and fatigue (r=10c/c) at -4020J
impact+WA70/95%HR1500h and fatigue (r=-1 t/c)at 20
23J impact+WA70/95%HR1500h and fatigue (r=10c/c) at 20
23J impact+WA70/95%HR1500h and fatigue (r=10c/c) at -4023J
impact+WA70/95%HR1500h and fatigue (r=-1 t/c)at 20
Influent parameters were studied, the wet ageing until
saturation covers all environmental and mechanical effects during
the aircraft life.For tests, impact inflicted to the structure
takes into account the relaxation of the dent under environmental
conditions.
Hot/wet ageing
Relaxation behaviour
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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1.2- Impact threat
Impact threat
Impact threat definition
Typical impact threat
Supporting data and analysis
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1.2- Impact threat
The impact threat is the mathematical description of impact
severities associated to their probability of occurrence. It is
supported by extensive survey of in-service incidents.
External partTypical impact threat:
35J 10-5 /FH (static cut-off)90J 10-9 /FH (damage tolerance
cut-off)
HTP root/Rear fuselage skin140J 10-5 /FH (static cut-off)
Doorway zones132,5J 10-5 /FH (static cut-off)238,5J 10-9 /FH
(damage tolerance cut-off)
Note : for some structures where a low impact threat can be
anticipated (eg x >2,7), then the energy associated to a
realistic event could be low.
1510)(jEx
jj EEp--
=
fhJEPj /10)30(5-=
fhJEPj /10)90(9-=
with x=3, giving
Impact threat definition
Typical impact threatRef: Effect of low velocity impact damage
on primary aircraft structures the certification issue; Aug 1999,
J. Rouchon
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1.2- Impact threat
4A survey on wing impact damage, covering the whole Airbus
types, totalling 18,740,000 flight hours and 9,800,000 flight
cycles4A similar survey extended the data to the fuselage, covering
A320 family, totalling 1,140,000 flight hours 4A similar survey
covering the whole aircraft covering A320 family, totalling 500,000
flight hours4And another source of data was a survey, totalling
10,330,000 flight hours
Extensive survey available from which the current impact threat
is derived.
Impact threat parameters have a solid foundation, new in-service
data, additional applications (A380 for example) and associated
in-service history should lead to future updates with a more
complete understanding of damage threats.
Supporting data and analysis
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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1.3- Large Damage
Large Damage Capability, LDC: not realistic damage
Design precautions to protect against the unknown.
Design precautions
4 Fail Safe demonstration on main joint areas: hinged
structures, high load introduction (disconnection of one load
path)
4 In addition, for each typical technology / design, arbitrary
typical damages are assumed for LDC assessment, such as: Stringer
disbond analysis for co-bonded technology Missing fasteners at load
introduction area Large hole in typical area
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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1.4- Hail
Hailstorms data is based on meteorological survey defining:
4 Size of hailstones : Standard hailstorm, (Dia 10mm) for a P of
50% of hailstorms Rare hailstorm, (Dia 25mm) for a P of 5% of
hailstorms Extremely rare hailstorm, (Dia 50mm) for a P of 0.1% of
hailstorms.
4 Concentration per unit area: number of hailstones impacting a
surface based on the size of the storm.
4 Velocities for the energy of hails impact on ground and flight
conditions.
Structure Damage tolerance approach, 2 points are
considered:
4 Unloaded Structure, hail on ground for commercial aspect
Showers of Dia 10 and 50 mm ( 33m/s; 32 Joules)
4 Loaded structure, hail in flight considered in damage
tolerance analysis (Energy, loading, risk analysis) Tests determine
the structure behaviour
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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1.5- Manufacturing defects
Allowable manufacturing defects accounted for in the static
demonstration
Size and type4 Inherent to manufacturing process4 Established
through quality assurance plan4 Quantified for each sizing
criteria
Manufacturing defects included in the building block
demonstration from coupon to full scale test
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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1.6- No-growth / fatigue
Means of compliance AMC25-6034 6.2.1 Structural details,
elements, and subcomponents of critical structural
areas should be tested under repeated loads to define the
sensitivity of the structure to damage growth. This testing can
form the basis for validating a no-growth approach to the damage
tolerance requirements.[]
4 6.2.3 The evaluation should demonstrate that the residual
strength of the structure is equal to or greater than the strength
required for the specified design loads For the no-growth concept,
residual strength testing should be performed after repeated load
cycling.
Tests performed for compliance4 No initiation of damages checked
defining good design practices4 Critical Non detectable
damage/defects under repeated loads
during one DSG4 Critical detectable damage under repeated loads
during at least one
interval of inspection4 A residual test after cycling to
validate required design loads
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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2- Test Pyramid
Verify analysis methods
Verify FEM predicted stress/strain distribution
Verify predicted failure modes
BUILDING BLOCK APPROACH
COUPONS
DETAILS
ELEMENT
SUBCOMPONENT
COMPONENT
FULL SCALE
Allowable validation against coupon and smaller specimen
At detail level, B values are determined if test results are
used in the analysis. (1 or more typical feature per specimen)
Statistical treatment: large and small populations B value
In general 1 typical feature per specimen (hole,lay up, impact
damage)
Determine environmental effects (moisture, thermal)
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Purpose4 Assess laminate design value (CAI, TAI, ShAI &
failure criterion
including environmental conditions)4 hundred of specimens4
Statistical treatment to obtain design values based on
MIL-HDBK-17
2- Test Pyramid for Damage tolerance
Coupons & details tests
ShAI specimen after failure
CAI or TAI specimens after impact
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2- Test Pyramid for Damage tolerance
Purpose4 Verify strength of critical design details (hole edge
impact, top stringer
impact, ply drop off with impact, etc)4 Obtain design values for
these critical designs (Statistical treatment
based on small sample law)4 Tenths of specimens
Element tests
Top stringer impacted after compression failure Compression
specimen with impact in the
hole radius
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2- Test Pyramid for Damage tolerance
Purpose4 Verify design concept4 Validate method
(analytical, complex loading, etc)
4 Validate fatigue behaviour
4 Few specimens
Sub-Component tests
Stiffened panel with stringer edge impact loading with
combined compression/pressure
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2- Test Pyramid for Damage tolerance
Component & Full-scale tests Purpose
4 Validate the stress GFEM analysis4 Prove the behaviour of the
structure4 Show compliance with Regulations. For instance
Limit load strength without detrimental deformations Ultimate
load strength (with BVID damages and allowable manufacturing
defects in
critical location) Fatigue and damage tolerance requirements (no
generation of new damages and no
growth of damages) with BVID, manufacturing defect, VID and
large damage in critical location
4 Validate in-service repair solutions
Example of full scale test
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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3- Analysis
The damage tolerance method4 Dent depth versus impact energy4
Damage size versus impact energy4 Residual strength versus damage
size4 Failure criterion
Relies on coupons&detail tests of the test pyramidAnd is
enhanced at higher level of the test pyramid
Parameters accounted for4 Material differences4 Laminate
thickness4 Lay-up and stacking sequence4 Hot/wet4 Support condition
for impact4 Net section for residual4 Scatter (B-value)4 etc
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3- Analysis
Dent depth prediction example
( )conditionsboundarythMatEfd .,,,=+ Relationship between Dent
depth after relaxation and dent depth just after impact
Qualification test results QI(4mm) AR/RT
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 10 20 30 40 50 60 70
Energy (J)
Den
t de
pth
afte
r im
pact
(m
m)
prediction material 1
Test points Material 1
prediction material 2
Test points Material 2
Material 2: thickness effect
0
0,5
1
1,5
2
2,5
0 10 20 30 40 50 60 70
Impact energy (J)
dent
dep
th a
fter
impa
ct (
mm
)
test points 4mmprediction 4mmtest point 4,5mmprediction
4,5mmtest points 5mmprediction 5mm
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3- Analysis
Delaminated area prediction example
( )uplayconditionsboundarythMatEfSd -= ,.,,,Qualification test
results QI(4mm) AR/RT
0
200
400
600
800
1000
1200
1400
1600
0 10 20 30 40 50 60 70
Energy (J)
Del
amin
ated
are
a (m
m)
prediction material 1
Test points Material 1
prediction material 2Test points Material 2
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3- Analysis
Compression after impact prediction example
( )uplayngconditionithMatSdfEpsCAI -= ,,,, Test results
AR/RT
2000
3000
4000
5000
6000
7000
8000
0 500 1000 1500 2000 2500 3000
Delaminated area (mm)
Lo
ss o
f st
rain
in c
om
pre
ssio
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Material 1 prediction QI 4mm thick
Material 1 Test points QI 4mm thick
Material 2 prediction oriented lay-up 8mm thick
Material 2 Test points oriented lay-up 8mm thick
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CONTENT
1. AIRBUS Damage tolerance philosophy1. Damage Detectability2.
Impact threat3. Large Damage4. Hail5. Manufacturing defects6.
No-growth / Fatigue
2. Test Pyramid3. Analysis4. Key messages
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4- Key messages
Airbus Damage tolerance methodology relies on
Mature design practices Extensive tests to support analysis
Robust impact survey based on in-service experience
Airlines cooperation, by rigorous inspections reporting ,
enables Airbus to design more durable and damage tolerant Composite
Structures
Impact threat understanding Detectability assessment
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