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Airframe Design Philosophies,Airframe Design Philosophies,Guidelines for Damage ToleranceGuidelines for Damage Tolerance

(or intolerable damage?)(or intolerable damage?)

Withstand WellWithstand Well--Defined Load CasesDefined Load Cases

Proof of adequate strength (analysis, tests)Proof of adequate strength (analysis, tests)

Accidental failure extremely improbableAccidental failure extremely improbable

I am constructing my machine to sustain about 5 times my weightand I am testing every piece. I think there is no possible chance of itsbreaking while in the air, Wilbur Wright (1900)===YetDuring test-phase the propeller shaft failed due to fatigue

Intolerable damage: http://www.findarticles.com/p/articles/mi_m0UBT/is_1_18/ai_111868142

References:De Jonge, Raymer, Schijue, Salvetti et al.

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StrengthStrength

StaticStatic

Resistance against veryResistance against very

high loads (Design Statichigh loads (Design StaticLoads) that haveLoads) that have lowlowprobability of occurrenceprobability of occurrence

FatigueFatigue

Resistance againstResistance against

repeated loadsrepeated loads

Loads much lower thanLoads much lower thanthe Design Static Loadsthe Design Static Loads

may fail structuremay fail structure ififrepeated sufficiently oftenrepeated sufficiently often

Major Parts (IF NOT ALL) of modern airframes are designed againstMajor Parts (IF NOT ALL) of modern airframes are designed againstfatigue stressfatigue stress::

FATIGUE LOAD SPECTRUM:FATIGUE LOAD SPECTRUM: ALL SERVICE LOADS HAVE TO BE DETERMINEDALL SERVICE LOADS HAVE TO BE DETERMINED

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AIRCRAFT LOADSAIRCRAFT LOADS

Aerodynamic loads (Lift, Drag, Propulsion,Aerodynamic loads (Lift, Drag, Propulsion,Aeroelastic effects)Aeroelastic effects)

Gravity loadsGravity loads

Ground Reaction Loads (wheel struts)Ground Reaction Loads (wheel struts)

PressurizationPressurization

Apparent LoadsApparent Loads:: amFaTT

!

0TTT

!7a

FF

LOAD FACTORLOAD FACTORmg

Fn aer

TT 7!

InternalInternal

(structural)(structural)LoadsLoads::Bending,Bending,

Torsion, ShearTorsion, Shear

External LoadsExternal Loads(maneuvers,(maneuvers,gusts, taxigusts, taxi--loads)loads)

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Ground LoadsGround Loads

KingAir ~20000 hours ~7000 flights ~10k miles ofKingAir ~20000 hours ~7000 flights ~10k miles ofridingriding

nnzz Number ofNumber ofcycles per 1000cycles per 1000

landingslandings

110.050.05 300,000 300,000

110.350.35 2,000 2,000

110.450.45 90 90

110.050.05 0.150.15

110.750.75 0.0050.005

Bending Moment

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IN-FLIGHTLOADS

MANEUVERS GUSTS PRESSURIZATION

StationaryNon-stationary orAbrupt (checkedand unchecked)

Symmetric Asymmetric

Symmetric

Non-symmetric

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Loads in steady horizontal flightLoads in steady horizontal flight

LOADS AND STRUCTURAL STRESSES CHANGE WITH TIME

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Loads in steady horizontal flightLoads in steady horizontal flight

Bending Moment>0

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NonNon--stationary maneuverstationary maneuver

checkedmaneuver

Tail load

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Asymmetric maneuver:Asymmetric maneuver:yaws and rollsyaws and rolls

Un/Checked maneuvers can be due to efforts toUn/Checked maneuvers can be due to efforts toovercome turbulence and up/downdraftsovercome turbulence and up/downdrafts

Tail-load

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Steady Maneuver LoadsSteady Maneuver Loads

22

2

!

R

mVWL

22

1

!RgVnz

Jcos1!zn

nz is a measure of the maneuvers severity. At 45 bank nz=1.41

Structural load depends on:nzV

configuration (mass and mass distribution, flaps in/out etc)altitude

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Design Loading Cases (FAR 23/25)Design Loading Cases (FAR 23/25)

LIMIT LOADS (Airworthyness Requirements):LIMIT LOADS (Airworthyness Requirements):

No sign of permanent deformation under limit loadsNo sign of permanent deformation under limit loads

Occurrence: 10Occurrence: 10--55 times per flight hourtimes per flight hour(

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VV--n diagram (flight envelope)n diagram (flight envelope)

Adequate strength for all possible combinations of VAdequate strength for all possible combinations of V--n in then in the

maneuver envelope, for all possible weights and weightmaneuver envelope, for all possible weights and weightdistributions, at all altitudesdistributions, at all altitudes

Max WMax WTOTO nnmaxmax nnminmin

>50000 lbs>50000 lbs 2.52.5 --11

41004100--5000050000 2.1+24000/(W+10000)2.1+24000/(W+10000) --11

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VV--n diagram (flight envelope)n diagram (flight envelope)

altitude effectaltitude effect

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Expected aircraft usageExpected aircraft usage:: Definition of expected types of mission andDefinition of expected types of mission and

their occurrencetheir occurrence

Definition of a mission profile for eachDefinition of a mission profile for each

expected type of missionexpected type of mission

Design load spectrumDesign load spectrum

time

z cruise

ascent

descentcruise

air-ground

Step-climb cruise

time

z

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Design load spectrumDesign load spectrum

Maneuver loads are usually less problematicManeuver loads are usually less problematicthan gust loads with regard to fatigue, exceptthan gust loads with regard to fatigue, exceptfor military aircraftfor military aircraft

Transport Aircraft

Fighter Aircraft

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OurOur mission!!mission!!

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Gust (turbulence, most critical) LoadsGust (turbulence, most critical) Loads

NormalNormal--usage aircraft strength isusage aircraft strength is notnot sufficientsufficientto withstand thunderstorm turbulenceto withstand thunderstorm turbulence

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!

HUw deg

TX

2cos15.0

ggL

z KVwSW

Cn

(!(

/

5.0 EVE

cC

Sm

L

g

EVQ

/2!

wg

V

(E!wg/V

c

Vt!X

g

g

gK Q

Q

! 3.5

88.0

wg

Hc=25c

0.5Ude

AltitudeAltitude Ude [fps]Ude [fps]

@ V@ VBB @ V@ VCC @ V@ VDD

00--2000020000 6666 50 50 2525

20k20k--50k50k 3838 2525 12.512.5

Qg

Kg 0.88

90

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SW

UV

ndeEASL

z/

5.0 0 EV!( Pratts formula

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Gust (turbulence) LoadsGust (turbulence) LoadsPratts formula is often used in reverse mode

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Gust (turbulence) LoadsGust (turbulence) Loads

Analogous results are obtained with continuous turbulence spectrumBut AC dynamic response must be considered

21

21)(a

U

a

U

de

dede

eBeBUN

!

NON-storm turbulence

storm turbulence

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Lateral Gust LoadsLateral Gust Loads

The Vertical Tail is loaded similarly to the wingunder lateral gustsUse Pratts formula

A T-TAIL configuration is particularly vulnerableto lateral gusts

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FatigueFatigue

GAG cycle and moreGAG cycle and more

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Stress Concentration and FractureStress Concentration and FractureMechanicsMechanics

Notches cannot be avoided, but their effect is dramaticnom

peak

tW

W

!

Superposition of intensification factors: Kt=Kt1*Kt2

Kt=9

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Stress Concentration & Fracture MechanicsStress Concentration & Fracture Mechanics

A crack is a source for stress intensificationA crack is a source for stress intensification Through cracks vs part-through cracks Initiation at hole edge or elsewhere (dent, nicks etc) Different opening modes

Kt

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Cyclic slip Crack nucleation Growth of microcrack Growth of macrocrack

Final failure

occurs early,after a few%total life

Naked-eye visible(1cm? Can we miss it?)

Kt, a large part of

fatigue life is spenthere

(t1 (t2 (t3

(t4

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Fatigue and Fracture MechanicsFatigue and Fracture Mechanics

Fatigue lifeFatigue life depends on:1. external factors

-cyclic loads-environment

2. layout of the structure-detail design

-material-surface quality

Stress amplitude & meanCycle frequency and shape

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Fatigue and Fracture MechanicsFatigue and Fracture Mechanics

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Fatigue and Fracture Mechanics.Fatigue and Fracture Mechanics.

Need to combineseveral load cycles intoan equivalent constantamplitude cycle: e.g.,Palmgren-Miner rule

!i i

i

N

nD

Woheler curve

ni

Ni

Wi

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Goodmans curve

Fracture Mechanics. Fatigue.Fracture Mechanics. Fatigue.

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Fracture Mechanics. Crack Propagation.Fracture Mechanics. Crack Propagation.

))(1( maxKKR

K

dn

da

C

(!

FE

da/dn

Stress Intensity Factor: K=S(Ta)0.5

accounts for geometry, crack length and load

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Examples of da/dn curves


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Prediction of Crack Growth


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Crack growth retardation due to isolated load spikes

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Design Philosophies for airframesDesign Philosophies for airframes

2. Accept that Fatigue Cracks and stress corrosion cracks DO OCCUR in service Try to achieve a long crack-free life Know where cracks will start (inspection), what K to use, how fast they

grow (inspection periods): (Tisps*(T{ac-a0} (s~0.5) Fatigue cracks will have limited impact on static strength: 2 bay max

length (2 stringers or body frames max length) than it has to stop! residual strength enough for LL (only)

1. Exclude possibility of occurrence of cracks

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Durability and Damage ToleranceDurability and Damage Tolerance

Lower the probability of crack nucleationLower the probability of crack nucleation FailFail--safesafe vs Safevs Safe--life design: maintain adequatelife design: maintain adequate

strength (100% LL,no catastrophic failure) untilstrength (100% LL,no catastrophic failure) untilthe next inspection will identify and repair thethe next inspection will identify and repair thedamagedamage Multiple load pathMultiple load path BackBack--up structuresup structures

CrackCrack--arrest solutionsarrest solutions Accidental events (scratches, corrosion, dentsAccidental events (scratches, corrosion, dents

etc.) are also consideredetc.) are also considered Inspection programInspection program promoted by thepromoted by the

manufacturer and established by the operatormanufacturer and established by the operator

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Damage ToleranceDamage Tolerance

Inspection interval is a function of:Inspection interval is a function of:

SSI, Significant Structure ItemSSI, Significant Structure Item

NDI, minimum detectable length aNDI, minimum detectable length a00 Fracture Mechanics results,Fracture Mechanics results,

Accidental eventsAccidental events

Load cycles and propertiesLoad cycles and properties

(!(c

a

a

dnfdn

daN

0

)(/

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Damage ToleranceDamage Tolerancedamage

damage

damage

flights

flights

flights

Safetyguaranteedby:

Certified life goal life

Durability +

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Damage ToleranceDamage Tolerance

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NDINDINon DestructiveNon Destructive

InspectionInspectionVISUAL detectable

undetectable

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NDINDINon Destructive InspectionNon Destructive Inspection

http://www.saic.com/nde/#procedures

http://www.reinhartassoc.com/services/inspection/aircraft/

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Durability and Damage ToleranceDurability and Damage Tolerance

Staticstrength stillpriority

(safe-life)

New

thinking

(damagetolerance)

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Guidelines for inspection programsGuidelines for inspection programsand reassessment of the structureand reassessment of the structure

FAA Advisory circular 91FAA Advisory circular 91--5656

CAA Airworthiness Notice 89CAA Airworthiness Notice 89

ATA document Airline/ManufacturerATA document Airline/ManufacturerMaintenance Program Planning Document,Maintenance Program Planning Document,MSGMSG--33

Supplemental Inspection ProgramSupplemental Inspection Programrequirements: FAR25.571 AC 91requirements: FAR25.571 AC 91--5656

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Examples of Intolerable DamageExamples of Intolerable Damage

Wing of transport aircraft: the 7075-t6 wing structure completely failedduring severe to extreme clear-air turbulence. The investigationrevealed:

the failure started from 2 fatigue cracks at either side of an access

door, crack lengths being 3.25 and 2.5 respectivelythe critical location was covered by X-ray inspections. 3 sets of X-ray pictures from previous inspections indicated that the aircrafthad flown with the cracks for more than a year. They had beenoverlooked.

2004: a 28-inch long crack in a circumference fuselage splice on aB747 initiated from an accidental scratch was found during maintenanceto other components.

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Was this a critical SSI whence to expect a crack to depart?Was this a critical SSI whence to expect a crack to depart?

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ConclusionsConclusions In some places theIn some places the critical crack length is microscopiccritical crack length is microscopic and sophisticatedand sophisticated

inspection techniques are needed. Elsewhere cracks can be a meter long and visualinspection techniques are needed. Elsewhere cracks can be a meter long and visualinspections suffice.inspections suffice. KnowingKnowing where to look for cracks is always difficultwhere to look for cracks is always difficult. Clues are drawn from the. Clues are drawn from the

aircraft design and an airframe is tested to simulate several lifetimes and see whereaircraft design and an airframe is tested to simulate several lifetimes and see wherecracks start. Howevercracks start. However abnormal usage or minor damage can hasten fatigue andabnormal usage or minor damage can hasten fatigue andinvalidate both safe life and damage tolerance methodologiesinvalidate both safe life and damage tolerance methodologies..

For example,For example, justjusta scratcha scratch can causecan cause fatiguefatigue where it is not expectedwhere it is not expected and suchand such

cracks can grow more quickly than normal, which is what happened to the Chinacracks can grow more quickly than normal, which is what happened to the ChinaAirlines B747 and to the Qantas jet (see ASW, June 30).Airlines B747 and to the Qantas jet (see ASW, June 30). Wreckage from the China Airlines plane points to explosive decompression afterWreckage from the China Airlines plane points to explosive decompression after

fatigue failure of a repair. Thefatigue failure of a repair. The originaloriginal damage in 1980 was just a scrape on thedamage in 1980 was just a scrape on thefuselage, over which a large patch was riveted.fuselage, over which a large patch was riveted. For the next 22 years the fuselageFor the next 22 years the fuselageskin beneath the patch fatigued, cracked and corroded, but the very patch that wasskin beneath the patch fatigued, cracked and corroded, but the very patch that wasplaced to repair the damage hid the growth of greater damageplaced to repair the damage hid the growth of greater damage..

This was a wellThis was a well--understood hazard about which aviation experts had prevaricatedunderstood hazard about which aviation experts had prevaricatedever since 1988 when an Aloha Airlines B737 lost much of its fuselage. Wideever since 1988 when an Aloha Airlines B737 lost much of its fuselage. Wide--rangingranginginvestigations found that repairs were made without regard to their durability, butinvestigations found that repairs were made without regard to their durability, butreassessment of existing repairs got bogged down in legalistic arguments and notreassessment of existing repairs got bogged down in legalistic arguments and notuntil 2000 were checks enforced. Even then, the mandated checks affected relativelyuntil 2000 were checks enforced. Even then, the mandated checks affected relativelyfew aircraft and were phased in over a number of years.few aircraft and were phased in over a number of years. Fixing old repairs on theFixing old repairs on theChina Airlines plane was not due to occur until six months after it crashed; sixChina Airlines plane was not due to occur until six months after it crashed; sixmonths and 225 deaths too latemonths and 225 deaths too late..

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ConclusionsConclusionsThe aircraft operator is responsible forThe aircraft operator is responsible for

maintenance and inspection.maintenance and inspection.

The operator should be fully aware of theThe operator should be fully aware of thepossibility of fatigue cracks, also unexpected crackspossibility of fatigue cracks, also unexpected cracks

Inspection techniques are not easy especially toInspection techniques are not easy especially tolocate small crackslocate small cracks

Is the operator using the aircraft in agreementIs the operator using the aircraft in agreement

with the original design assumptions?with the original design assumptions?Load monitoring systems should be carried out byLoad monitoring systems should be carried out bythe operator (possibly in cooperation with thethe operator (possibly in cooperation with themanufacturer)manufacturer)

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