Title: Quantifying Corrosion in Fuselage Lap Joints
Objective:
(1) To show examples of the type of damage that can be present in
corroded 2024-T3 lap joints and (2) to illustrate the process for determining the
equivalent initial flaw sizes for corroded fuselage lap joints.
General
Description:
This problem focuses on the methods to determine the equivalent
initial flaw sizes that can be used to carry out a damage tolerance assessment of corroded
fuselage lap joints. To differentiate the use
of the EIFS concept in a corrosion fatigue situation, the phrase Equivalent
Corrosion Damage (ECD) is used. A study
was carried out on dog-bone coupons fabricated from naturally and artificially corroded
lap joints. These coupons were subjected to a
cyclic load and the fracture surfaces were examined with the aid of a scanning electron
microscope to determine the location and size of the nucleation site. Also, using x-ray techniques to map the thickness
loss in these coupons as well as in corroded lap joints that were disassembled and cleaned
of corrosion products, the coefficient of variation, which is a measure of the scatter,
was determined.
Topics Covered: Corrosion,
equivalent corrosion damage
Type of Structure:
Fuselage lap joints
Relevant
Sections of Handbook: Section 8
Author:
Nicholas C. Bellinger
Company Name: National
Research Council Canada
Institute for Aerospace Research
Montreal Road, Ottawa, Ontario Canada K1A 0R6
613-993-2410
www.nrc.ca/iar
Contact Point:
Jerzy P. Komorowski
Phone: 613-993-3999
e-mail: jerzy.komorowski@nrc.ca
Overview of Problem Description
This problem focuses on one of the critical areas in transport
aircraft, lap joints, which can be of various designs usually consisting of two or more
layers of material with two or three rows of rivets, Figure
NRC-1.1. These joints are known to be
susceptible to crevice corrosion, which is a form of attack that occurs when a corrosive
liquid gains access to crevices in, or between components.
Studies have shown that the corrosion products which form in lap joints fabricated
from clad 2024-T3 aluminum material have a molecular volume of approximately 6.5 times
greater than the aluminum alloy from which it originated (Bellinger et al., 1994). This increase in volume causes the skins between
the rivets to bulge, an effect referred to as pillowing, Figure
NRC-1.2, and is often used by inspectors to identify areas in lap joints affected by
corrosion.
Figure
NRC-1.1. Typical Fuselage Lap
Joint
Figure
NRC-1.2. Effect Of Rivet
Spacing on Pillowing in Lap Joints
Damage Characterization of Faying Surface
Damage characterization studies
that were carried out on naturally corroded lap joints have shown the presence of
corrosion pits, exfoliation, intergranular corrosion and environmentally assisted
cracking, which occurred in the presence of a sustained stress caused by corrosion
pillowing, Figure NRC-1.3 (Bellinger et al., 1999). It should be noted that very few large
pits were found in the lap joints that were studied although some areas had an average
thickness loss of 20% or higher. Those large
pits that were found occurred in an area that contained very little corrosion damage other
than the pit. The presence of intergranular
corrosion and exfoliation may explain why there were very few large pits given the high
thickness loss. As the intergranular
corrosion or exfoliation progressed causing pits to link-up to other edges, the material
between the pits could be removed by way of flaking or dissolution decreasing the existing
pit depths. This, in turn would cause the
remaining damage to appear less severe although the average thickness loss would be
higher.
Figure
NRC-1.3. Examples of damage
present in corroded lap joints; (a), (b) pitting,
(c) exfoliation along
faying surface, (d) intergranular corrosion and
(e), (f)
environmentally assisted cracking that occurred near the rivet holes
Determination of ECD Values
ECD values can be determined either by examining failed corroded
lap joints or by conducting tests on coupons fabricated from corroded lap joints taken
from in-service or retired aircraft. Ideally,
the tests should be carried out using stress levels similar to that experienced by lap
joints in service. Prefabricated lap joints
artificially corroded in a salt fog chamber may also be used if the resulting ECD values
are similar to those determined from naturally corroded joints (that is the damage
produced by the artificial process is similar to those produced by the natural process). This example addresses this issue (Bellinger et
al., TBP; Bellinger et al., 1999a).
Artificial
Process
Since ECD values are material dependent, any lap joint
configuration can be used in the artificial process.
However, it should be emphasized that only lap joints should be used and not just
single sheets of material since crevice corrosion results in a more severe attack than
just pitting corrosion. The fabricated
joints were placed in a salt fog chamber and subjected to a modified copper assisted salt
spray (CASS) process. The joints were
periodically removed and inspected using the enhanced optical inspection technique, D
SightTM and multi-frequency eddy current techniques to estimate the average
thickness loss (Komorowski et al., 1991; Forsyth et al., 2000). Once the desired thickness loss was achieved, the
joints were removed from the chamber, disassembled and the corrosion products removed
using a chemical/ultrasonic process. Studies
have shown that the damage produced by the artificial process was similar to that produced
by the natural one (Eastaugh et al., 1999).
Test
Coupon Design and Fabrication
One of the main problems in generating ECD values is the design of
the test coupon since the majority of the lap joints consist of 2024-T3 clad aluminum
material, 1.27 mm (0.05 inch) thick. The ASTM
E466-96 standard was used to design a coupon since it did not place any restriction on the
specimen dimensions. The coupons were
machined such that the loading axis was perpendicular to the material rolling direction as
shown in Figure NRC-1.4.
Figure
NRC-1.4. Schematic Showing Test Coupon Configuration And
Photograph Showing Orientation In Lap Joint Used For This Problem.
Using thickness maps developed from radiographs of the coupons, the
thickness in the gauge length of each coupon was determined. Based on these results, ten coupons from the
naturally corroded skins and twenty from the artificially corroded ones were machined such
that the average thickness loss in the gauge length was approximately 2%. Coupons were also machined from both the naturally
and artificially corroded skins so that the average thickness loss within the gauge length
was 5%. Figure
NRC-1.5 shows the x-ray thickness maps for the coupons fabricated from the naturally
corroded lap joints.
Figure
NRC-1.5. X-Ray Thickness Maps Of Naturally Corroded Coupons
Experimental Procedures
All tests were carried out under load control in a servo-hydraulic
load frame with hydraulic grips. Given the
effect that humidity has on the fatigue life of aluminum alloys, the relative humidity was
recorded throughout the duration of the test. Each
coupon was subjected to cyclic loading until failure with a load ratio of 0.02 and a test
frequency of 10 Hz. The maximum load was
adjusted for each coupon to maintain a maximum stress level of 207 MPa (30 ksi) in the
gauge length area. This stress was the approximate value that the lap joints experience in
the vicinity of the critical rivet hole where multi-site damage occurs. To prevent premature failure at the corners, the
edges of each coupon in the gauge length were broken using a machinist sanding
stone.
Experimental Results
The average number of cycles to failure and the standard deviation
for each set of results is shown in Table NRC-1.1. The combined average number of cycles to failure
for the 5% corroded test coupons is shown in this table along with the average value for
the artificially and naturally corroded 5% results. The
total number of coupons tested for each condition is also given.
To determine the ECD values for the different test condition, the
majority of the fracture surfaces for the 2% artificial and natural coupons were examined
with the aid of a scanning electron microscope (SEM) while values for the 5% coupons were
determined by back-calculations. All the
nucleation sites found using the SEM were semi-elliptical in shape and the majority of the
sites were located along the corroded surface (faying surface of the joint). It should be pointed out that the coupons did not
always fail at the thinnest area of the gauge length or at the maximum pit depth as shown
in Figure NRC-1.5 by the black lines, which indicated where
the coupons failed. For those coupons that
did not have a single nucleation site, different scenarios were used to obtain the ECD
value. The average semi-elliptical ECD values
that were measured with the aid of a scanning electron microscope for the 2% artificial
and natural coupons and the calculated average semi-circular ECD value for the 5% combined
coupons are shown in Table NRC-1.2.
Table NRC-1.1. Experimental Results Showing Average Number of
Cycles To Failure
Percent Thickness Loss
[number of coupons] |
Average Number of
Cycles to Failure |
Standard
Deviation |
Artificial
2%
[20] |
237,679 |
25,734 |
Natural 2%
[8] |
231,086 |
30,729 |
Combine
5%
[13] |
212,248 |
21,998 |
Natural
[4] |
212,454 |
18,578 |
Artificial [9] |
212,156 |
24,241 |
Table NRC-1.2. Experimental and Calculated ECD Values
Coupon |
Experiment
ECD Values (mm) |
Equivalent
Radius (mm) |
2c |
a |
2%
Artificial |
0.1024 |
0.00610 |
0.05512 |
2%
Natural |
0.1215 |
0.05801 |
|
5%
Combine |
|
|
0.06736 |
The resulting ECD numbers can then be used to predict the residual
life of corroded lap joints, which is demonstrated in a separate example.
References
Bellinger, N.C., Krishnakumar, S. and Komorowski, J.P. (1994),
Modelling of Pillowing Due to Corrosion in Fuselage Lap Joints, CAS Journal,
Vol. 40, No. 3, September 1994, pp. 125-130.
Bellinger, N.C., Komorowski, J.P. and Benak, T.J. (TBP),
Residual Life Predictions of Corroded Fuselage Lap Joints, to be published in
the International Journal of Fatigue.
Bellinger, N.C., Cook, J.C. and Komorowski, J.P. (2000) The
Role of Surface Topography in Corroded Fuselage Lap Joints, to be published in the
proceedings of the 2000 USAF Aircraft Structural Integrity Program Conference, December 5
to 7, 2000, San Antonio, Texas.
Bellinger,
N.C., Benak, T.J., Chapman, C.E., Komorowski, J.P., Scarich, G., and Falugi, M. (1999a),
Equivalent Corrosion Damage Methodology for Corroded Fuselage Lap Joints,
Published in the proceeding of the 1999 USAF Aircraft Structural Integrity Program
Conference, San Antonio, Texas, 30 November 2 December 1999.
Bellinger, N.C., and Komorowski, J.P. (1999b),
Environmentally Assisted Cracks in 2024-T3 Fuselage Lap Joints, Published in
the proceeding of the Third Joint FAA/DoD/NASA Conference on Aging Aircraft, Albuquerque,
New Mexico, September 20-23, 1999.
Eastaugh, G.F., Merati, A.A. and Simpson, D.L. (NRC), Straznicky,
P.V., Scott, J.P., Wakeman, R.B. and Krizan, D.V. (Carleton University) (1999), An
Experimental Study of Corrosion/Fatigue Interaction in the Development of Multiple Site
Damage in Longitudinal Fuselage Skin Splices, RTO-MP-18 AC/323(AVT)TP/8 March 1999,
proceedings NATO-RTO Air Vehicle Technology Panel Workshop on Fatigue in the Presence of
Corrosion (Fatigue sous corrosion), Corfu, 7-8 October 1998.
Forsyth, D. S., Fahr, A., Chapman, C. E., Bellinger, N. C.,
Komorowski, J. P. (2000) NDI Capabilities for Airframe Corrosion Damage
Tolerance, in the proceedings of the 2000 USAF Aircraft Structural Integrity Program
Conference, December 5 to 7, 2000, San Antonio, Texas.
Komorowski
J.P., Simpson D.L., Gould R.W. (1991), Enhanced Visual Technique for Rapid
Inspection of Aircraft Structures, Materials Evaluation, December 1991, pp.
1486-1490.