View
832
Download
23
Category
Preview:
Citation preview
BYJ30-SDT-P00-00x Page 1-101/01/00/SAM
Structural Damage Tolerance — Part 1 —
Concepts and Overview
Matthew MillerStructural Damage Technology
(425) 266-5091
Course Number: 6CV40051
BYJ30-SDT-P00-00x Page 1-201/01/00/SAM
Damage Tolerance Overview
BYJ30-SDT-P00-00x Page 1-301/01/00/SAM
Damage Tolerance Overview
Concepts and Overview Outline
IntroductionKey elements of damage toleranceKey elements of residual strengthKey elements of crack growthKey elements of damage detection and maintenance planningDocumentation
BYJ30-SDT-P00-00x Page 1-401/01/00/SAM
Damage Tolerance Overview
Crack in Pressure Bulkhead
BYJ30-SDT-P00-00x Page 1-501/01/00/SAM
Damage Tolerance Overview
IntroductionResponsibilities of Structures Engineers
Static strengthVerify that the static strength of undamaged structure meets load
requirementsDurability (fatigue)
Verify that structural detail fatigue quality matches the required quality to meet the Design Service Objective
Damage toleranceVerify that an economically feasible inspection program can be
implemented to detect fatigue, corrosion, or accidental damage before the residual strength of the structure falls below the required fail-safe load level
BYJ30-SDT-P00-00x Page 1-601/01/00/SAM
Damage Tolerance Overview
IntroductionDefinitions of Damage Tolerance
Structure is damage tolerant if damage can be detected and repaired before residual strength falls below the regulatory load level
Damage tolerance means that the structure has been evaluated to ensure that should serious fatigue, corrosion or accidental damage occur within the operating life of the airplane, the remaining structure can withstand reasonable loads without the failure of excessive structural deformation until the damage is detected
Damage tolerance is the attribute of the structure than permits it to retain its required residual strength for a period of use after the structure has sustained specific levels of fatigue, corrosion, accidental or discrete source damage
BYJ30-SDT-P00-00x Page 1-701/01/00/SAM
Damage Tolerance Overview
SafetyDamage tolerance• Residual strength• Crack growth• Damage detection
EconomicsDurability• Fatigue• Corrosion prevention• Maintenance
Introduction Basic Concepts
BYJ30-SDT-P00-00x Page 1-801/01/00/SAM
Damage Tolerance Overview
Introduction Durability and Damage Tolerance Analysis Philosophy
XCritical Crack
Length(Allowable Damage)
Detectable Crack Length
Damage Tolerance
Durability(includes fatigue, corrosion and manufacturing quality)
Flights, N
N = Inspection Frequency
Cra
ck L
engt
h, L
Crack Initiation
Crack Propagation
N N N
MDSO X FRF< 5% of Fleet
Damage Detection
Period
N N
Crack growth analysis
Residual strength analysis
Inspection program
BYJ30-SDT-P00-00x Page 1-901/01/00/SAM
Damage Tolerance Overview
Introduction Damage Tolerance Technology Standards
D6-24958 “Damage Tolerance Methods and Allowables” (Books 3 & 3A)
Commercial Airplanes standards are based on 30 years of transport airplane development and service experience
Comprehensive methods and allowables are based on traditional aerospace industry fracture mechanics techniques
Audited and approved by the FAA as acceptable means to comply with FAR 25.571 requirements
BYJ30-SDT-P00-00x Page 1-1001/01/00/SAM
Damage Tolerance Overview
Introduction Damage Tolerance Methods and Allowables (Book 3)
PurposeAid in the development of a maintenance program to inspect for potential
fatigue damage in a fleet of airplanes
Approach optionsToo simple
Loss of credibility Loss of accuracy
Too complex Excess resources and flow time required Excess probability of human error Loss of visibility and control by management
Just right Sufficiently accurate Easy to use
BYJ30-SDT-P00-00x Page 1-1101/01/00/SAM
Damage Tolerance Overview
Safety requires diligent performance by all participants
Airlines
IntroductionAirplane Safety
Design, analysis and
manufacturing
Maintenance and
inspection
Regulatory agencies
• FAA• JAA• CAA• CAAC• AR
BYJ30-SDT-P00-00x Page 1-1201/01/00/SAM
Damage Tolerance Overview
Structural Loads
Limit loadHighest load encountered by a fleet during its operational life
Ultimate loadsLimit load exceeded by a factor of safety, usually 50%
Operating loadsLoads normally encountered in day to day operations
Regulatory fail-safe loadsUsually limit load or equivalentDetailed in FAR 25.571 and JAR 25.571
BYJ30-SDT-P00-00x Page 1-1301/01/00/SAM
Damage Tolerance Overview
Stru
ctur
al s
treng
th
Normal operating loads
Period of service
Structure is damage tolerant if damage can be detected and repaired before residual strength falls below the regulatory load level
Key Elements of Damage ToleranceDamage Tolerance Definition
Regulatory load requirement per FAR 25.571
Damage detection and restoration
Ultimate load capability required after damage detection and repair
NDI detection
period
Normal deterioration due to undetected damage
Detectable by NDI
Dam
age
size
Visual detection period
Visually detectable
Maximum allowable undetected damage
BYJ30-SDT-P00-00x Page 1-1401/01/00/SAM
Damage Tolerance Overview
•Cyclic stress spectrum•Material•Geometry•Multiple Site Damage•Environment•Load distribution
Key Elements of Damage ToleranceDamage Tolerance Parameters
Crack growth resistance
Inspection program
Allowable damage
• Access / visibility• Inspection intervals• Inspection methods• Damage detection period• Multiple cracking in the
fleet• Damage detection
requirement
•Material•Geometry•Fracture toughness•Multiple Site Damage•Load distribution
BYJ30-SDT-P00-00x Page 1-1501/01/00/SAM
Damage Tolerance Overview
+ + =
Key Elements of Damage ToleranceSummary
12
3
1. Residual strength analysis
2. Crack growth analysis
3. Inspection program
Safety of flight
Lcritical
Cra
ck le
ngth
Lcritical
Inspectable crack length vs. flights to
critical curve
Skin crack
Chord crack
Flights to critical
3
2
1Chord crack
TOTAL DTRBased on assumed inspection program
REQUIRED DTRBased on successful service experience
Compare Damage Tolerance Rating
Are additional inspections required?
BYJ30-SDT-P00-00x Page 1-1601/01/00/SAM
Damage Tolerance Overview
Key Elements of Damage ToleranceDefinitions
Primary structureThat structure which carries flight, ground or pressure loads
Secondary structureThat structure which carries only air or inertial loads generated on or
within the secondary structureStructural Significant Item (SSI)
Any structural detail, element or assembly judged to be significant because its failure reduces airplane residual strength or results in the loss of function
Principal Structural Element (PSE)Structure that contributes significantly to the carrying of flight,
ground and pressurization loads, and whose failure could result in catastrophic failure of the airplane (Ref. AC 25.571 - 1b)
BYJ30-SDT-P00-00x Page 1-1701/01/00/SAM
Damage Tolerance Overview
Key Elements of Damage Tolerance Structural Classification
All p rim ary structure notincluded in two
categories above
Structural CategoryTechnology
ControlMethod
Technique ofEnsuring Safety
S econdaryS tructure
OtherStructure
D esign for sa fesepara tion or loss o ffunction
C ontinuedsafe fligh t
D am ageO bvious orM a lfunction
Evident
Adequate residua lstrength w ith extensivedam age-obviousduring wa lkaround orind icated bym a lfunction
R es idualstrength
R es idualstrength
Crack growthInspectionprogram
Inspection programm atched to structu ra lcharacte ristics
D am ageDetection
by P lannedInspection
S afe-L ifeC onservative fa tiguelife Fatigue
StructuralClassification
Examples
W ing spoile r segm ent(sa fe separation or sa fe
loss o f function)
Typ ica l w ing sk in /stringersurface (fue l leak)
Landing gear s tructure
Dam
age
Tole
rant
Des
ign
Safe
-Life
Des
ign
Stru
ctur
ally
Sig
nific
ant I
tem
s (P
rimar
ySt
ruct
ure)
BYJ30-SDT-P00-00x Page 1-1801/01/00/SAM
Damage Tolerance Overview
Obvious Damage!
BYJ30-SDT-P00-00x Page 1-1901/01/00/SAM
Damage Tolerance Overview
Ground Accident
BYJ30-SDT-P00-00x Page 1-2001/01/00/SAM
Damage Tolerance Overview
Key Elements of Damage Tolerance FAR/JAR 25.571
Certification requirements pre & post Amendment 45
Analysis Old FAR 25.571(Pre-1978)
New FAR 25.571(Post-1978)
Residualstrength
Single element orobvious partial failure
Multiple active cracks
Crack growth No analysis required Extensive analysis required
Inspectionprogram
Based on service history FAA air carrier approval
Related to structural damagecharacteristics and pastservice history
Initial FAA engineering and aircarrier approval
BYJ30-SDT-P00-00x Page 1-2101/01/00/SAM
Damage Tolerance Overview
• Redundant load path.• Capable of
withstanding regulatory loads with a single element failed
(Pre - 1978) (Post -1978)
FAIL SAFE DAMAGE TOLERANT SAFE LIFE
Key Elements of Damage Tolerance FAR/JAR 25.571 Certification Requirements
• Capable of withstanding regulatory loads with partial or failed element with the presence of cracks in adjacent and attaching structure
• Able to detect and repair damage prior to structural strength loss below regulatory load capability (Amendment 45)
• Allowable crack size is very small (undetectable)
• Adequate inspections are impractical
• Parts are removed from service when “safe life” has been reached
BYJ30-SDT-P00-00x Page 1-2201/01/00/SAM
Damage Tolerance Overview
Principal Damage SourcesPrincipal Structural Elements
Fatigue damage
Environmental damage
Accidental and discrete damage
Damage tolerance
Environmental damage evaluations
Approved inspection and maintenance program
Damage detection evaluations
Types of inspections
Typical accidental
Discrete source
Corrosion prevention plan
Corrosion inspection
Safe life
Accidental damage evaluations
Types of inspections
Types of inspections
1
1
Fatigue cracking is assumed to have occurred in the most difficult inspection/access location independent of calculated fatigue life or full scale test results
BYJ30-SDT-P00-00x Page 1-2301/01/00/SAM
Damage Tolerance Overview
DETECTABLE
Key Elements of Damage Tolerance Inspection and Maintenance Philosophy
Environmental deterioration and accidental damage
Supplemental fatigue inspections
Flee
t dam
age
rate
Detectable size fatigue damage
Detectable fatigue
damage
Design Service ObjectiveYears of service
Thresholds are based on:• Fatigue approach for 727,737,747.
Typically 50% to 75% of DSO• Initial flaw approach for 737NG, 757,
767, 777 and any new program
Corrosion prevention and control program inspections
Scheduled maintenance check intervals
Fleet actions for WFD
Mandatory SB modifications
Repair assessments / inspections
BYJ30-SDT-P00-00x Page 1-2401/01/00/SAM
Damage Tolerance Overview
+ + =
Key Elements of Damage Tolerance Residual Strength
12
3
1. Residual strength analysis
2. Crack growth analysis
3. Inspection program
Safety of flight
Strength? Flights? Inspections?
BYJ30-SDT-P00-00x Page 1-2501/01/00/SAM
Damage Tolerance Overview
Static behavior
Transition behavior
Regulatory Requirement
LEFM behavior
Maximum allowable damage
Key Elements of Residual Strength Residual Strength Parameters
L
GeometryGeometry Correction
Factor
Y B
Thickness
MaterialFracture Toughness
KAKA
Y
Crack Length
Res
idua
l Stre
ngth
BYJ30-SDT-P00-00x Page 1-2601/01/00/SAM
Damage Tolerance Overview
Stress,
L
K is the defining parameter for crack growth and residual strength predictions
Key Elements of Residual StrengthStress Intensity Factor
tk
CYnLK 3tk
Stress,
3
Stress concentration factor Stress intensity factor
BYJ30-SDT-P00-00x Page 1-2701/01/00/SAM
Damage Tolerance Overview
Key Elements of Residual Strength Fracture Toughness
Fracture toughness is a function of:ThicknessOrientationAlloy, temper & formTemperature
0
10
20
30
40
50
60
70
80
0 0.2 0.4 0.6 0.8 1
Thickness (in)
KA (k
si
in)
Plane stress
Plane strain
Mixed mode (plane stress and plane strain)
KIC
7150-T77511
Extrusion
L-T orientation
BYJ30-SDT-P00-00x Page 1-2801/01/00/SAM
Damage Tolerance Overview
Key Elements of Residual Strength Failure Criterion - Plane Stress Fracture
2L
ionsetoK:A
2L
cAoK:C
c
i
Lo Lc
Crack Length, L
A
BC
L
KA is the apparent fracture toughness associated with initial crack length at maximum load
KC is the fracture toughness associated with actual crack length at maximum load
Konset, KC and KA are not constants like KIC Most practical is KA - constant to first
approximation for given t
KA
Konset
KC
A
BC
2L
cCcK:B
Engineering approach to fracture assumes failure occurs when K = KA
BYJ30-SDT-P00-00x Page 1-2901/01/00/SAM
Damage Tolerance Overview
Key Elements of Residual Strength Failure Criterion - Plane Strain Fracture
c
c
Lo LcCrack Length, L
Little or no plasticity involved when brittle failure occursFlat fracture surface
KIC
2L
cICcK
Failure occurs when K = KIC
L
BYJ30-SDT-P00-00x Page 1-3001/01/00/SAM
Damage Tolerance Overview
Key Elements of Residual Strength Failure Criterion
Applies to all cracks in Titanium and all through thickness cracks in Aluminum and Steel
For a single corner crack (e.g., a lug) 0.05 inches or less, or two corner cracks (e.g., a fastener hole) 0.035 inches or less in Steel (< 240 ksi), use static strength analysis. For corner cracks of 0.01 Inches or less in Steel (> 240 ksi), use static strength analysis
For corner cracks in Aluminum 0.02 in. < L < thickness, use a straight line fit between the static strength at L= 0.02 inches and residual strength for L=thickness
Use typical yield stress; when data is not available use 2.228*”B basis” - 1.228*”A basis” or 1.10*”B basis” yield stress
AN/AG is the Net Area / Gross Area ratio for the damaged structure
(I/c)N / (I/c)G is the Net / Gross section modulus ratio for the damaged structure
In both the above cases, the Net properties = Gross - (holes & cracked out) properties
The “” is a shape factor for transition failures, = 0.63 for all materials
1
2
3
4
BYJ30-SDT-P00-00x Page 1-3101/01/00/SAM
Damage Tolerance Overview
F FAA
FW L
WRS TYN
GTY
F
KL Y 1
, Y 1.99 0.41LW
...RSA
Key Elements of Residual Strength Failure Criterion - Transition Mode
Net section yield Linear Elastic Fracture Mechanics
B =0.5 inW =10 inMat’l 2024-T351 (L-T)KA = 125 ksi inFTY = 55 ksiW
B
L
Crack
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10
Crack length (in)
Res
idua
l str
engt
h (k
si)
Net section yieldLEFM
Transition equation
Full static strength
L=t
Edge Cracked Panel Example
BYJ30-SDT-P00-00x Page 1-3201/01/00/SAM
Damage Tolerance Overview
Cracking Patterns Recommended crack configurations Based on experience and engineering judgement
BYJ30-SDT-P00-00x Page 1-3301/01/00/SAM
Damage Tolerance Overview
Y Factor Geometry correction factor in the stress intensity factor calculation Uses the superposition principle to include one or more geometry effects (J factors)
BYJ30-SDT-P00-00x Page 1-3401/01/00/SAM
Damage Tolerance Overview
J and C FactorsJ factors account for differences between structure being analyzed and infinite plate
C factors account for increase in load due to cracking of attached structure
BYJ30-SDT-P00-00x Page 1-3501/01/00/SAM
Damage Tolerance Overview
Y and C Factors
L L L
2LK YK 2
L CYK 2L
0.1Y
0.1C
0.1JY twidtheffec
0.1C
0.1Y
0.1C
BYJ30-SDT-P00-00x Page 1-3601/01/00/SAM
Damage Tolerance Overview
+ + =
IntroductionKey Elements of Damage Tolerance
12
3
1. Residual strength analysis
2. Crack growth analysis
3. Inspection program
Safety of flight
Strength? Flights? Inspections?
BYJ30-SDT-P00-00x Page 1-3701/01/00/SAM
Damage Tolerance Overview
Crack Growth ConceptsCrack Growth Parameters
X
Flights
Cra
ck le
ngth
Li (detectable)
Lf (critical)MATERIAL
GEOMETRYFLIGHT STRESS PROFILE
M
Y S
Stre
ss
BYJ30-SDT-P00-00x Page 1-3801/01/00/SAM
Damage Tolerance Overview
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1 10 100
Kmax (ksi in)da
/dN
(in/
cycl
e)
1
p
M
Crack Growth FundamentalsCrack Growth Equation
pmax4
MKZ10
dNdL
n1
Boeing Crack Growth Rate Equation (1979)
Based on P. C. Paris, 1961, and K. Walker, 1970, equations
M & p = Material crack growth rate parameters
Measures relative material resistance to crack growth
Reflects effect of environment
TEST
COUPON
0.1R00.1R1.1
0.0R0.1R1.010.1R0.0R1
Z
q
BYJ30-SDT-P00-00x Page 1-3901/01/00/SAM
Damage Tolerance Overview
Allowable MSection 5, Book 3
Alloy Temper FormGrain
Orientation
Finished
Gage
(inches)
Allowable
M
Airplane Environment
W C B F
2014
2024
2219
2224
2324
T652T652
T3T4T3
T351
T3511T851T851
T6
T3511
T39
Hand ForgingDie Forging
Bare Sheet
Clad SheetPlate
Extrusion
Bare Sheet
PlatePlate
Die Forging
Extrusion
Plate
T-ST-L
L-T, T-LL-T, T-LL-T, T-LL-T, T-L
L-T, T-L
L-T, T-L
T-LL-T
L-T
L-T
.75
.75
.04 - .25
.04 - .06
.04 - .13
.10 - .15
.15 - .35
.35 - .50
.10 - .35
.37 - 1.0
.37 - 1.0
-
.16 - .31
.10 - .15
.15 - .35
.35 - .50
21.721.7
24.825.626.026.924.522.626.324.320.2
18.3
26.8
26.925.925.0
24.924.9
27.928.829.230.227.625.429.627.723.1
20.6
30.1
30.329.228.1
20.620.6
23.724.524.825.623.321.525.023.419.4
17.4
25.5
25.624.723.8
22.222.2
25.125.926.327.324.822.926.721.417.9
18.6
27.2
27.326.325.3
.
...
.
...
.
...
.
...
.
.
(cold)(wet) (bilge) (fuel)
BYJ30-SDT-P00-00x Page 1-4001/01/00/SAM
Damage Tolerance Overview
Integration of Crack Growth Rate Equation
dLCYnL
n1
fZM10N
f
o
L
L
pp
max
4
pmax4
MKZ10
dNdL
n1
If loading is constant amplitude, the rate equation can be integrated and solved as follows for the number of cycles:
CYfK nL
maxmax
Note: Constant Z and fmax
X
N, number of cycles
Cra
ck le
ngth
Lo
Lf (critical)
fmaxS
tress
fmin
CdNMfZ10dLCY
n1 p
max4pn
L
BYJ30-SDT-P00-00x Page 1-4101/01/00/SAM
Damage Tolerance Overview
Integration of Crack Growth Rate Equation
Lower wing skin stress history for a “typical” flight
Stress(ksi)
Time
If loading is variable amplitude, and the crack growth rate does not depend on the sequence of varying load cycles, then the life integral can be discretized as shown:
ground
flight
C
o
N
0i
pimaxFp
4L
L
pn
L fZNM
10dLCYn1
p/1N
0i
pimax
C
fZS
dLCYn1
SM10N
L
L
pn
Lp
p4
F
o
NC = Number of cycles in a flightNF = Number of flights
Define stress rating, S, as:
BYJ30-SDT-P00-00x Page 1-4201/01/00/SAM
Damage Tolerance Overview
Load Sequence Effects on Crack Growth
Overloads produce crack growth retardation, which is caused by the presence of residual compressive stresses in the yielded zone ahead of the crack tip
Source: Broek, D. , Elementary Engineering Fracture Mechanics, 4th ed., Kluwer Academic Publishers, Dordrecht, 1986
N, number of cycles
Cra
ck le
ngth
BYJ30-SDT-P00-00x Page 1-4301/01/00/SAM
Damage Tolerance Overview
Integration of Crack Growth Rate Equation
Lower wing skin stress history for a “typical” flight
Stress(ksi)
Time
To account for the effects of overload and underload, every stress cycle is adjusted, similar to the Willenborg model
ground
flight NC = Number of cycles in a flightNF = Number of flightsfmax eff = Effective fmax for cycle iZeff = Uses effective fmax and fmin for cycle i
Define spectrum stress rating, Sspectrum, as:
dLCgYn1
SM10N
L
L
pn
Lpspectrum
p4
F
o
p/1Ns
0i
pieffmaxeffspectrum fZS
BYJ30-SDT-P00-00x Page 1-4401/01/00/SAM
Damage Tolerance Overview
Interfaced Durability and Damage Tolerance Analysis Software (IDTAS)
Complex flight operating stress profiles are analyzed using IDTAS
Loads and flight segment spectra are input, and relative fatigue damage and crack growth are calculated
BYJ30-SDT-P00-00x Page 1-4501/01/00/SAM
Damage Tolerance Overview
IDTAS Damage Tolerance Output
SspectrumSspectrum/Slinear
IDTAS is the source of the stress rating, S, for use in the crack growth equation
BYJ30-SDT-P00-00x Page 1-4601/01/00/SAM
Damage Tolerance Overview
Damage Detection and Structural Maintenance Planning
ObjectiveMaintain acceptable level of structural airworthiness throughout
the operational life of the airplane
GuidelineAirline / manufacturer maintenance program planning document,
MSG-3 (Maintenance Steering Group) Air Transport Association of America, Oct. 1980 (Rev. 2005.1)FAA approved as a means of complying with FAR 25.571
BYJ30-SDT-P00-00x Page 1-4701/01/00/SAM
Damage Tolerance Overview
Damage Detection and Structural Maintenance Planning
MSG-3 Rating Systems
Rating system provides quantitative means of determining inspection requirements
Development of rating system is responsibility of manufacturerRating systems are based on past practice and manufacturer /
operator experience with similar structureRatings address three principal sources of damage
FatigueCorrosionAccidental damage
BYJ30-SDT-P00-00x Page 1-4801/01/00/SAM
Damage Tolerance Overview
Damage Detection and Structural Maintenance Planning
Structural Maintenance Planning
Initial inspection program development and documentationThe initial structural maintenance plan for a new model is directed
toward detecting corrosion, stress corrosion and accidental damage
A feasibility study for fatigue damage detection is made to determine if a practical program can be constructed
Supplemental inspection programAs the fleet matures, the risk of fatigue cracking increases and the
inspection program is reassessed at 10 to 15 yearsIf the initial inspection program is inadequate for finding fatigue
cracks in significant structure, additional or supplemental inspections will be required
BYJ30-SDT-P00-00x Page 1-4901/01/00/SAM
Damage Tolerance Overview
Damage Detection and Structural Maintenance Planning
Environmental Deterioration Rating (EDR) System
Exposure to Adverse EnvironmentsSusceptibility Index
Probable Possible Unlikely
Standard 0 1 2
Proven /improved 1 2 3Environmental
ProtectionSpecial
attention 2 3 4
Sensitivity to Damage SizeTimely Detection Index
High Medium Low
Poor 0 1 2
Adequate 1 2 3
Visibility of theSSI for inspectionduring scheduled
maintenancechecks Good 2 3 4
EDR = Susceptibility Index + Timely Detection Index
BYJ30-SDT-P00-00x Page 1-5001/01/00/SAM
Damage Tolerance Overview
Damage Detection and Structural Maintenance PlanningAccidental Damage Rating (ADR) System
Likelihood of Accidental DamageSusceptibility Index
Probable Possible Unlikely
Low 0 1 2
Medium 1 2 3
Estimated residual
strength after accidental damage High 2 3 4
Sensitivity to Damage GrowthTimely Detection Index
High Medium Low
Poor 0 1 2
Adequate 1 2 3
Visibility of theSSI for inspectionduring scheduled
maintenancechecks Good 2 3 4
ADR = Susceptibility Index + Timely Detection Index
BYJ30-SDT-P00-00x Page 1-5101/01/00/SAM
Damage Tolerance Overview
Structural Maintenance PlanningKey Considerations and Responsibilities
for EDR and ADR EvaluationsApplication Primary ResponsibilityRating
Category EDR ADR Operator BoeingKey Considerations
Visibility Visibility for inspection after access
Sensitivity todamage size
or growth
Relative sensitivity within zoneconsidered
External: multiple element damage Internal: single element damage
Environmentalprotection
Comparison with previous protectionsystems and recent service history
Corrosion experience in same zoneExposure toadverse
environment
Material susceptibility to stress corrosionand potential for preload
Likelihood ofaccidentaldamage
Operator experience in the same zone
Strength afteraccidentaldamage
Likely size of damage relative to criticaldamage size
BYJ30-SDT-P00-00x Page 1-5201/01/00/SAM
Damage Tolerance Overview
EXAMPLE
Structural Maintenance PlanningExample of EDR/ADR Application
Environmental deteriorationrating (EDR) or Accidental
Damage Rating (ADR)External Internal
1 2A 2A
2 5A 5A
3 3C C
4 2C
5 4C
6 or greater
2CAGE exploration: 4C interval
with 1/5 or 1/10 of fleet
Frequency of Structural Inspections ATA: Major Zone 300 (Section 48 & Empennage)
A-check frequency: 300 flight cyclesC-check frequency: 15 months or 3000 flight cycles, whichever comes first
BYJ30-SDT-P00-00x Page 1-5301/01/00/SAM
Damage Tolerance Overview
PDProbability of
detecting damage
Probability of inspecting an aircraft with
damage
Probability of inspecting detail
consideredProbability of
crack detection
Structural Maintenance PlanningDamage Detection Parameters
P1 P3P2
BYJ30-SDT-P00-00x Page 1-5401/01/00/SAM
Damage Tolerance Overview
Structural Maintenance PlanningDefinition of Inspection methods
InspectionMethod
Description for Boeing DTR System(See MSG-3)
TypicalApplications
Walkaround Observations from the ground to detect obviousdiscrepancies such as fuel leaks (see category 2 structure) Pre-flight
General visual(no MSG-3 credit)
Visual check of exposed area of wing lower surface, lowerfuselage, doors and door cutouts, and landing gear bays A-check
Surveillance(MSG-3 general
visual equivalent)
Visual examination of defined internal or external structuralarea from a distance considered necessary to carry out anadequate check. External includes structure visiblethrough quick-opening access panels or doors. Internalapplies to obscured structure requiring removal of filletsfairings, access panels or doors, etc. for visibility
C-check
Detailed(MSG-3 detailed
equivalent)
Close intensive visual inspections of highly definedstructural details or locations searching for evidence ofstructural irregularity
D-checkselected items
Special (seeBook 3 pg. 12-26)
Inspections of specific locations or hidden details usingspecified nondestructive inspection (NDI) procedures.
D-checkselected items
BYJ30-SDT-P00-00x Page 1-5501/01/00/SAM
Damage Tolerance Overview
Structural Maintenance PlanningDamage Detection Period
Crack Growth Interval, Flight Cycles
Cra
ck L
engt
h
Damage Detection Period, Flights to Critical
Cra
ck L
engt
h
CriticalCritical
Surveillance
Detailed
NDI
Surveillance
Detailed
NDI
Engineering Format Maintenance Planning Format
BYJ30-SDT-P00-00x Page 1-5601/01/00/SAM
Damage Tolerance Overview
Maintenance PlanningDetermining Probability of Detection Parameters for Inspections
Flights
N N NIn
spec
tabl
e C
rack
Len
gth
Need to relateInspection methodProbability of detectionCrack length
Safe damage detection period
0.0001
0.001
0.01
0.1
0.99
0.1 1 10
Inspectable Crack Length (in)
Pro
babi
lity
of D
etec
tion
Detailed
Surveillance
Audited by the FAA in 1980 Inspectable Crack Growth Curve
General Visual
n
1iDD i
P̂11P
m
1j
n
1iDD ij
P̂11P
For n inspections of the first crack
For n inspections on m cracked airplanes
BYJ30-SDT-P00-00x Page 1-5701/01/00/SAM
Damage Tolerance Overview
Relative Damage Detection Reliability for Visual Inspections
0.9
0.95
0.99
5 10
Inspectable Crack Length (in)P
roba
bilit
y of
Det
ectio
n
303 7
0.0001
0.001
0.01
0.1
1
0.1 1 10
Inspectable Crack Length (in)
Pro
babi
lity
of D
etec
tion
3050.5
BYJ30-SDT-P00-00x Page 1-5801/01/00/SAM
Damage Tolerance Overview
Inspectable Crack Length
Inspection access: Top
Inspection access: Bottom
A
CB
Cra
ck L
engt
h Critical
Actual Crack Growth Curve
FlightsNA NBNC
AC
B
X
Insp
ecta
ble
Cra
ck L
engt
h
Inspectable Crack Growth Curves
FlightsNA NBNC
BYJ30-SDT-P00-00x Page 1-5901/01/00/SAM
Damage Tolerance Overview
When do Supplemental Inspections Start?
Current inspection threshold determinationBased on crack growth from an equivalent manufacturing flawFAA recommended (used for 757 and 767)
Initial flaw size regardless of material or manufacturing process of– 0.05” for open holes– 0.03” for filled holes– 0.005” for cold worked holes
Threshold life is crack growth life from initial flaw to critical divided by 2FAA mandated 100% inspection after reaching thresholdBoeing new method (used for 777,
737NG and new models and derivatives) Establish initial flaw sizes based on
fatigue quality of detail Crack growth life to detectable or
through thickness
DFR
Equivalent Manufacturing Quality Flaw Size
BYJ30-SDT-P00-00x Page 1-6001/01/00/SAM
Damage Tolerance Overview
Documentation for a New Airplane Program
Structural Inspection Planning Data Document (SIPD) Description of structure Completed EDR/ADR forms DTR forms from feasibility study
Maintenance Planning Data Document (MPD)Maintenance Review Board (MRB) Report
BYJ30-SDT-P00-00x Page 1-6101/01/00/SAM
Damage Tolerance Overview
Reported Discrepancies
Discrepancies to be reported All cracks and previously unreported occurrences of significant corrosion involving any
SSI or PSE shall be reported promptly to Boeing Boeing follow-up actions
All adverse SSI or PSE reports will be reviewed immediately to determine if they are findings directly applicable to the SSI being addressed.
If discrepancies are related to fatigue cracking of the SSI, the following actions are to be taken
– Report factual data available on finding, with appropriate priority, to operators and airworthiness authorities
– If required, develop a fleet inspection program to obtain additional data necessary to formulate a service bulletin
– Prepare and issue a service bulletin addressing recommended structural modifications and inspections
– Revise the Supplemental Inspection Program Document (SIPD) to eliminate this SSI or a portion of the SSI, and reference the new service bulletin in the safety of flight service bulletin list
BYJ30-SDT-P00-00x Page 1-6201/01/00/SAM
Damage Tolerance Overview
Supplemental Structural Inspection ProgramsMethods are based on current practice, accessibility, critical crack length and non-destructive inspection
procedures
707/720 SSIP 727/737/747 SSIP
DamageTolerance
Assessment
Assessment of all SSI's(with and without a historyof in-service fatiguecracking) assumingcracking may have occurred
Assessment of all SSI's (withno history of in-servicefatigue cracking) requiringinspection to ensure timelydetection of fatigue damage
InspectionProgram
Intervals determined bydividing life (from detectableto critical) by a scatter factor
Programs augment or modifyexisting maintenance plansand intervals are based onlife (from detectable tocritical), multiple inspectionsand multiple fleet cracking)
AirplaneEffectivity
All airplanes exceeding thethreshold for an SSI aresubject to the requirementsfor that SSIDetails with known fatigueproblems have post-terminating inspectionrequirements with uniquethresholds based on servicedata
Only candidate fleetairplanes subject to in theinspection programrequirementsDetails with known fatigueproblems covered by existingservice bulletins arereassessed with inspectionsadded as needed
BYJ30-SDT-P00-00x Page 1-6301/01/00/SAM
Damage Tolerance Overview
Need a quantitative measure of the probability of detecting a crack
Calculated DTR vs. Required DTR
DTR = Damage Tolerance Rating
Number of 50/50 chances of detecting damage
Maintenance PlanningDamage Detection Rating System
Structure Required DTR
Externally visible areas 4
Wings and nacelles Areas not externally visible 6
Primary flap structure 8
Empennage Primary structure 6
FuselageContribution of
cabin differential<50% 6
pressure to total fail-safe stress >= 50% 10
Recommended