characterization of defects and damage in rivet holes slide
TRANSCRIPT
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
Damage Characterization - Methodology
• ~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
Damage Characterization - Methodology
• Lower skin faying surfaces examined and any crack
present were documented and measured.
8
Damage Characterization - Methodology
• 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
Damage Characterization - Methodology
• 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
Damage Characterization – Crack Arrays
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
Damage Characterization – Crack Arrays
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
Damage Characterization – Crack Arrays
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
Damage Characterization – Crack Arrays
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
Damage Characterization – Crack Arrays
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
Damage Characterization – Crack Arrays
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
Damage Characterization – Crack Growth
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
Damage Characterization – Crack Growth
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
Damage Characterization – Crack Growth
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
Joint Installation Characterization
• 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
Joint Installation Characterization
• 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
Normalized Defect Parameter
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
Normalized Defect Parameter
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
Rivet installation condition
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
Rivet installation condition
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.