characterization of defects and damage in rivet holes slide

0

CHARACTERIZATION OF DEFECTS AND

DAMAGE IN RIVET HOLES IN A CROWN

LAP JOINT OF A COMMERCIAL AIRCRAFT

AT DESIGN SERVICE GOAL

Ramesh Ramakrishnan

Douglas Jury

Enabling Technologies, Technical Operations

Delta Air Lines, Inc.

9th Joint FAA/DoD/NASA Aging Aircraft Conference

March 9, 2006

1

Acknowledgements

This project was conducted under FAA R&D Contract No. DTFA03-02-C-00044,

“DESTRUCTIVE EVALUATION AND EXTENDED FATIGUE TESTING OF A RETIRED

PASSENGER AIRCRAFT (B727)”.

2

Damage Characterization

Scope:

• Crown lap joint along stringer 4 R removedfrom a retired narrow body aircraft near it’sdesign service goal was destructivelycharacterized to determine the state ofdamage in the joint. This joint has knownsusceptibility to multi-site damage (MSD)

Purpose:

• This study fills a hole in otherwise abundantliterature on MSD by providing state-of-the-damage data from actual service article.

3

Damage Characterization – Target Area

• This presentation shows the results ofdestructive characterization of the crown lapjoint along stringer 4R betweenfuselage/frame stations FS 480 – 600 & FS720B – 720D, covering eight lap joint bays.

4

Damage Characterization – Joint Details

• Three row single shear lap joint, with lower skin atthe lower row of fasteners known to be susceptibleto cracking.

5

Damage Characterization - Methodology

• The lap joints within a frame – bay were cut out

and initial documentation of rivets and skin done.

Outer skin, outboard surface Lower skin, inner surface

Tilted view

of lower half of rivet.

Cracks observed

6


• ~1” coupons of the joint containing the lower row

rivets were cut out.

• Examination of the inner surface of the lower skin in

these coupons, under the stereomicroscope usually

provided some indications of crack locations.

• Vee cuts were made into the coupons away from the

crack locations and the fastener liberated.

• The coupons were soaked in a solvent to soften the

faying surface sealant after which the upper and

lower skins were separated.

• The cleaned faying surfaces were then examined for

cracks, with more focus on the faying surface of the

lower skin.

7


• Lower skin faying surfaces examined and any crack

present were documented and measured.

8


• Scanning Electron Microscope examination of the lower skinshowed presence of hole and faying surface defects near or atcrack locations.

Hole 1Hole 1

Hole 2Hole 2

9


• After crack lengths on the faying surface were measured, thecracks were opened by notching the lower skin to within 0.050”of the crack tip, cooling the coupon in LN2, and breaking theligament with pliers and hand pressure.

Crack tip

Notch tip

Lower skin inner surfaceLower skin inner surface

Lower skin faying surfaceLower skin faying surface

HoleHole

surfacesurface

Example crack surface view under the Example crack surface view under the

stereomicroscope; dashed line shows crack extents.stereomicroscope; dashed line shows crack extents.

Opened up cracks were measuredOpened up cracks were measured

as shown below.as shown below.

10

Damage Characterization – Crack Arrays

Crack measurements of opened and unopened andCrack measurements of opened and unopened and

documentation of crack locations around the holes were useddocumentation of crack locations around the holes were used

to construct schematics of crack arrays found across eachto construct schematics of crack arrays found across each

bay. bay.

Crack array in lower row holes in bay FS 540 - 560 Crack array in lower row holes in bay FS 540 - 560

FS 540FS 540

FS 560FS 560

11


FS 580FS 580

FS 600FS 600

Crack array in lower row holes in bay FS 580 - 600 Crack array in lower row holes in bay FS 580 - 600

12


Crack array in lower row holes in bay FS 720B Crack array in lower row holes in bay FS 720B –– 720C 720C

FS 720CFS 720C

FS 720BFS 720B

13


Summary chartsSummary charts

showing percent crackedshowing percent cracked

holes and max. crackholes and max. crack

lengthlength

in each of the bays.in each of the bays.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

480-

500

500-

520

520-

540

540-

560

560-

580

580-

600

600-

620

620-

640

640-

660

660-

680

680-

700

700-

720

720-

720A

720A

-720

B72

0B-7

20C

720C

-720

D72

0D-7

20E

Frame Bays

Pe

rce

nt

of

Ho

les

Cra

ck

ed

Max Crack Length Vs. Bay Position

0

0.05

0.1

0.15

0.2

0.25

0.3

480-

500

520-

540

560-

580

600-

620

640-

660

680-

700

720-

720A

720B

-720

C72

0D-7

20E

Ma

xim

um

Cra

ck

Le

ng

th (

in)

14


Number of Cracks at Hole in FS 480-600, 720B-720D

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8

Number of cracks at hole

Nu

mb

er

of

Ho

les

Summary chart showing frequency ofSummary chart showing frequency of

occurrence of zero, one, two or more cracks.occurrence of zero, one, two or more cracks.Histogram of the relative frequencyHistogram of the relative frequency

of occurrence of cracks at locationsof occurrence of cracks at locations

around the hole circumferencearound the hole circumference

15


Summary Conclusions:

• Each of the eight lap joint bays exhibitedclassic multi-site damage;

• They therefore provided eight different “real”distributions of possible multi-site damagescenarios that could be used in MSDsimulations;

• None of the bays however exhibited theidealized form of MSD of one or two cracksper hole; instead in every bay there were atleast two or more holes which each hadthree or more cracks present in them;

16


Summary Conclusions (continued):

• Some of the holes had up to five to eightcracks present in a half star-burst pattern.

• Crack sizes (lengths) across bays did notalways correlate to the expected stressdistribution across a bay.

• In none of the bays had adjacent crackslinked up and or reached a critical size; thelargest size of any crack observed in all eightbays was about 0.259 in.

• All the cracks were found within the lower240 deg. sector around the holecircumference.

17

Damage Characterization – Crack Growth

A brief description of the methodology used to

backtrack the crack growth (length) as a

function of aircraft flight cycles is provided here

using an example crack shown below.

SEM micrograph ofSEM micrograph of

the crack surface is shownthe crack surface is shown

with the crack origin at thewith the crack origin at the

hole corner identified.hole corner identified.

Striation measurements wereStriation measurements were

made along the longitudinalmade along the longitudinal

count path from the origin tocount path from the origin to

the crack front (dashed line).the crack front (dashed line).

Circled area shows a deep Circled area shows a deep

groove in the hole surface.groove in the hole surface.

18


SEM micrographs of striations observed along the longitudinal striation count path.SEM micrographs of striations observed along the longitudinal striation count path.

Striations at a distance of 0.107Striations at a distance of 0.107””

from the origin.from the origin.

Striations at a distance of 0.011Striations at a distance of 0.011”” from from

the origin.the origin.

Striation spacing was measured at two locations on each of the micrographsStriation spacing was measured at two locations on each of the micrographs

and the average striation spacing at the micrograph location obtained.and the average striation spacing at the micrograph location obtained.

19


1.00E-06

1.00E-05

1.00E-04

0.001 0.01 0.1 1

Distance from the Origin (in)

Str

iati

on

Sp

ac

ing

(in

/cy

cle

)

0

0.02

0.04

0.06

0.08

0.1

0.12

0 5000 10000 15000 20000 25000

Cycles

Cra

ck len

gth

(in

)

0

0.05

0.1

0.15

0 10000 20000 30000 40000 50000 60000 70000

Total Aircraft Cycles

Cra

ck L

en

gth

(in

)

Plot of striation spacing as a function of Plot of striation spacing as a function of

crack length.crack length.

Plot of crack length as a function of cyclesPlot of crack length as a function of cycles

obtained by piecewise integration (andobtained by piecewise integration (and

summation) of the striation spacing plot.summation) of the striation spacing plot.

Final plot of crack lengthFinal plot of crack length

as a function of aircraftas a function of aircraft

cycles obtained by shiftingcycles obtained by shifting

the above curve such thatthe above curve such that

the final crack lengththe final crack length

corresponds to the flightcorresponds to the flight

cycles at the aircraftcycles at the aircraft’’ss

retirement (59,497 cycles).retirement (59,497 cycles).

20


Crack Backtracking Conclusion:

• The backtracked crack lengths as a function

of aircraft flight cycles was obtained for the

two largest cracks at each of the holes in bay

540 – 560.

• This data is being presented by D. Steadman

in a companion paper on “Simulation of

Multiple Site Damage Growth”, in this

conference.

21

Joint Installation Characterization

• Fasteners in joint: Al 2017-T4 5/32 inch 100O shear

head rivets

• Driven tail dimensions measured during disassembly

in order to determine the condition of the rivet fit.

• Rivet fit condition affects the clamping force at the

joint – may affect latent conditions leading to

cracking.0.050 in

Maximum driven tail height Minimum driven tail height

Driven tail diameter

Deformed rivet shank, revealing

non-uniform expansion

22


• Rivet fit classified into one of five categories of fit based on tail

dimensions

• Majority of the examined rivets were either under driven (22% +

38%) or were within specification, but tending towards under

driven (30%). Minority of the rivets were within the middle of the

specification or were overdriven.

22%

38%

30%

8% 2%

Significantly under driven

Marginally under driven

Within specification - lowelimit

Within specification

Within specification -upper limit

23


• Similarly, large majority of cracked holes had

under driven rivets

• About 90% of cracked holes at sites with

under driven rivets, or rivets close to being

under driven

0

5

10

15

20

25

30

35

40

Significantly

under driven

Marginally

under driven

Within

specification -

lower limit

Within

specification

Within

specification -

upper limit

Nu

mb

er

of

cra

ck

ed

ho

les

24

Defect Characterization: Motivation

• Damage characterization findings suggest crack

initiation may be driven by multiple parameters.

– Crack length distribution not similarly

distributed as expected stress level

– Cracks observed both in presence of and in

absence of macro defects

• Effects of observed defects warranted closer

analysis to determine influence of defect severity

on cracking

25

Defect Characterization

• Typical defects observed, severity was ranked:

– Hole surface helical grooves

– Fretting/galling on the faying surface

– Faying surface defects (gouges, scratches)

– Hole/faying surface edge deformation

• Since cracking only observed in lower 240O around

hole, ranking also limited to this region.

• The following results are presented from defect

characterization from lower skin of the lower fastener

row from the eight fully examined bays.

26

Defect Characterization: Ranking assignments

• Degree of defect severity assigned numerical

value based on typical severity observed

over a large number of holes.

Fretting/Galling

No appreciable 1

Mild galling 2

Heavy galling 3

Mild fretting, with or without galling 4

Heavy fretting 5

Edge Deformat ion

Clean, no deformed edge 1

Edge deformat ion like a volcano, no overflow 2

Edge deformat ion with little metal overflow 3

Edge deformat ion with significant overflow 4

Faying Surface Defects

Clean 1

Light scratches 2

Heavy scratches 3

Light gouges 4

Heavy gouges 5

Hole Quality

Clean 1

Circumferential gouge 2

27

Fretting/Galling Defect Characterization

Faying surface free of galling or

fretting

Faying surface with

widespread fretting around the

hole

0

20

40

60

80

100

Cracked Uncracked

Co

un

t

Heavy/widespread fretting

Mild fretting, with or without g

Heavy galling

Mild galling

No significant

Cracked holesCracked holes

typically exhibitedtypically exhibited

slightly moreslightly more

severesevere

Fretting/gallingFretting/galling

than non-crackedthan non-cracked

holesholes

28

Hole/Faying Surface Edge Deformation Defect Characterization

Edge bulged around the hole like a

volcano with no material overflow

Significant edge deformation

0

20

40

60

80

100

Cracked Uncracked

Co

un

t

Edge deformation w ith

signif icant metal

overflow

Edge deformation w ith a

little metal overflow

Edge deformation like a

volcano

Clean - no deformed

edge

Unexpected result: Non-Unexpected result: Non-

cracked holes had slightlycracked holes had slightly

greater percentage ofgreater percentage of

significant edgesignificant edge

deformation than crackeddeformation than cracked

holes holes –– suspected due to suspected due to

small sample of non-small sample of non-

cracked holescracked holes

29

Faying Surface Defect Characterization

Faying surface heavily gouged

0

20

40

60

80

100

Cracked Uncracked

Heavy gouges

Light gouges

Heavy scratches

Light scratches

Clean

Similar unexpectedSimilar unexpected

result: percentage of non-result: percentage of non-

cracked holes withcracked holes with

significant faying surfacesignificant faying surface

defects slightly greaterdefects slightly greater

than cracked holes.than cracked holes.

30

Hole Surface Defect Characterization

Hole surface typically either had pronounced helical groove or did notHole surface typically either had pronounced helical groove or did not

0

20

40

60

80

Cracked Uncracked

Co

un

t

Signif icant helical

groove

Clean or w ith minor

circumferential

f law s

Large percentage ofLarge percentage of

cracked holes hadcracked holes had

helical groove (~80%),helical groove (~80%),

while about 50% ofwhile about 50% of

non-cracked holes hadnon-cracked holes had

similar groove.similar groove.

31

Single Defect Parameter

• In order to compare the overall hole quality for alarge number of holes, a single parameter wasdefined as the product of the individual rankings ofdefect severity:

• Parameter = Hole Groove x Fretting/Galling x EdgeDef. x Faying Surf.

• Larger the defect parameter, more severe thedefects at the hole.

• Example: Faying surface with mild galling (2), heavygouges (5), edge deformation with little metaloverflow (3), and gouged hole surface (2)

– Defect parameter = 2 x 3 x 5 x 2 = 60

• Defect parameter normalized by maximum observedvalue, 160

32

Single Defect Parameter Study

• Single defect parameter determined for each

hole, compared to damage characterization

findings:

– Maximum crack length at a hole, crack

density (number cracks/hole), and total crack

length at hole

• Expected positive correlation between defect

parameter and extent of cracking

33

Influence of Defect Parameter on Cracking

0.03

0.05

0.07

0.09

0.11

0.13

0-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.5-1

Normalized Defect Parameter

Av

era

ge

ma

xim

um

cra

ck

len

gth

(in

)

1

2

3

4

0-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.5-1


Me

an

nu

mb

er

of

cra

ck

s

aro

un

d h

ole

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0-0.1 0.1-0.2 0.2-0.3 0.3-0.4 0.5-1


Me

an

to

tal

cra

ck

le

ng

th a

t h

ole

(in

)

Unexpected result:Unexpected result:

No positive or negativeNo positive or negative

correlation observed betweencorrelation observed between

single defect parameter andsingle defect parameter and

extent of crackingextent of cracking

Error bars are 90% conf. limit on meanError bars are 90% conf. limit on mean

34

Influence of rivet fit on cracking

• Similar investigation of rivet fit condition on

extent of cracking revealed similar results

• Expected result: crack length & density to

increase with more under driven rivets

• Inclusion of the rivet fit condition in the single

defect parameter was attempted. Rivet fit

assigned a numeric value (significantly under

driven = 1, over driven = 5). Defect

parameter divided by rivet fit condition value.

• No strong correlation between defect

para/rivet fit and cracking observed.

35

Influence of rivet fit on cracking

Maximum Crack Length

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Sig

nific

an

tly

un

de

r d

rive

n

Ma

rgin

ally

un

de

r d

rive

n

With

in

sp

ecific

atio

n

- lo

we

r lim

it

With

in

sp

ecific

atio

n

With

in

sp

ecific

atio

n

- u

pp

er

limit

Rivet installation condition

Me

an

Ma

xim

um

Cra

ck

(in

)

Average Crack Density by Rivet Installation

Condition

0

0.5

1

1.5

2

2.5

3

Significantly

under driven

Marginally

under driven

Within

specification

- lower limit

Within

specification

Within

specification

- upper limit


Av

era

ge

nu

mb

er

of

cra

ck

s p

er

ho

le

Average Total Crack Length by Rivet Installation

Condition

0

0.05

0.1

0.15

0.2

0.25

Sig

nifi

can

tly

under

driv

en

Marg

inally

under

driv

en

With

in

spe

cific

atio

n

- lo

we

r lim

it

With

in

spe

cific

atio

n

With

in

spe

cific

atio

n

- u

pp

er

limit


Av

era

ge

to

tal

cra

ck

le

ng

th a

t

ho

le (

in)

No strong correlationNo strong correlation

observed between rivetobserved between rivet

fit and extent offit and extent of

cracking.cracking.

36

Conclusions from rivet installation and defect characterization

• Preliminary investigation into influence of

defects and rivet installation indicates extent

of cracking is independent of the severity of

these attributes.

• Inclusion of damage & defect

characterization results from fasteners of

extended fatigue test fuselage panels will

provide additional data to be similarly

analyzed.

37

Conclusions

• All examined bays had classic MSD, with unique findings:

– Presence of half-starburst pattern of cracks

– Distribution of crack lengths not similarly distributed asexpected stress distribution across a bay

– Results provide eight different distributions of MSDscenarios based on findings from service article

• Preliminary study of severity of defects and rivetinstallation revealed cracking was independent of theseattributes.

Similar characterization and analysis of lap joints infuselage panels undergoing extended fatigue testing toprovide additional data.

characterization of defects and damage in rivet holes slide

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