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Fatigue fracture of a main landing gear swinging lever in a civil aircraft
1. 1
FATIGUE FRACTURE OF A MAIN
LANDING GEAR SWINGING LEVER
IN A CIVIL AIRCRAFT
Submitted By
Submitted toProf. R.K.Pandey
Pramod Kawade
2012AMD122613
2. 2
Introduction
• This is the case study performed by F Bagnoli, F. Dolce, M. Colavita,
M. Bernabei to investigate the failure of a Left Hand Main Landing
Gear (LH-MLG) swinging lever in an aircraft ATR-42.
• On April 3, 1996, at 1338 Eastern Standard Time, an American eagle,
ATR-42, N421MQ, operated by Simmons Airlines, sustained minor
damage during takeoff when the left main landing gear collapsed.
• The lever has completed about 26,139 CSN before failure.
3. 3
Introduction
• The left-hand main landing gear (LH-MLG) of a civil aircraft collapsed
during the takeoff due to the fracture of its swinging lever.
4. 4
Contents
1. Fracture Analysis.
a. Chemical Composition.
b. Visual Inspection.
c. Fractography.
d. Microstructure Examination.
e. Microanalysis.
2. FEA .
a. Stress Analysis.
b. Fatigue Analysis.
3. Results and Discussion.
5. 5
Fracture Analysis
• Chemical Composition
• Material of swinging lever is 7010 T74 Aluminum Alloy.
• Composition of the material is as given below,
Al(%)
Zn(%) Mg(%) Cu(%) Zr(%)
Ti(%)
Fe(%) Mn(5) Si(%)
5.8
2.2
0.02
0.02
1.8
0.08
0.03
0.02
0.02
6. 6
Fracture Analysis
• Visual Inspection
• There was no evidence of macroscopic Plastic Deformation
• The fracture surface revealed the presence of three different
morphologies named A, B and C.
7. 7
Fracture Analysis
• Visual Inspection
• A- Details
• 20%of the total area.
• Smooth,
flat and bright
surface.
• Well-defined
concentric
thumb-nail-shaped
beach
marks due to fatigue failure.
• Radial
Marks
are
also
observed.
• Retracing radial and beach
mark we can find out the origin
of the fracture and its at the
surface.
Details of A
8. 8
Fracture Analysis
• Visual Inspection
• B- Details
• The sector B shows smooth zones which shows fatigue failure
alternated by coarse and dark ones characteristic of overload
failure and this was due to the unstable crack propagation
obtained when the crack depth had reached the thickness of the
swinging lever.
• C- Details
• Sector C exhibited a completely dark, coarse grainy morphology,
where the overload was the main fracture mechanism
9. 9
Fracture Analysis
• Fractography• FESEM examination of the A, B and C zones of the fracture surface
has been carried out to confirm the fatigue failure and identify the
related fatigue crack initiation
10. 10
Fracture Analysis
• Fractography• The presence of two points located on the external surface,
approximately 3 mm from each other is found from retracing of
the beach marks and fatigue evidences.
• Below given image shows one defect as crack origin,
12. 12
Fracture Analysis
• Fractography1.5 mm
4 mm
8 mm
12 mm
SEM micrographs related to the non-uniform fatigue beach
marks spacing at different distances from the crack origin
13. 13
Fracture Analysis
• Microstructure Analysis• Microstructure contained a forging fold with intermetallic phases
precipitates located at the grain boundaries. Also defects are observed
in region near to origin of the crack.
• The largest defect were measured to be about 600µm deep.
Optical micrograph of the
Swinging lever microstructure.
Defect in correspondence of the
crack origin region
14. 14
Fracture Analysis
• Microanalysis
• Microanalysis of the swinging lever composition was carried out in
correspondence to the deepest defect observed. It revealed an
abnormal presence of silicon in addition to the alloying elements found
in the base alloy.
Silicon content
Aluminum content
Optical micrograph of the Swinging lever microstructure.
15. 15
FEA Analysis
• Stress Analysis
• The FEA analysis confirmed the origin of the fracture is located in
correspondence of the most stressed area and the stress value is about
140 Mpa.
FEA model showing the most stressed
zone of the swinging lever.
16. 16
FEA Analysis
• Fatigue life analysis
• Then
a
crack
propagation
simulation by imposing a load
spectrum and using an AFGROW
analysis was carried out. Curves
obtained by measuring the following
beach marks spacing for two sets of
beach mark spacing and by
simulating the crack propagation
using the design load spectrum as
well as the load spectrum obtained
by imposing the value deriving from
the FEA (Smean = 140MPa) taking
into account an initial defect
approximately
600
µm
are
illustrated.
17. 17
FEA Analysis
• Fatigue life analysis
• From the AFGROW analysis corresponding to the load applied as per
FEA analysis the fatigue life of the lever is found to be 20,000 on the
other hand when the design load is used to calculate the fatigue life of
the lever the life of the lever obtain is larger.
18. 18
Results and Discussion
• Fractographic features revealed fatigue as the cause of failure of the
•
•
•
•
swinging lever.
The fatigue had originated on the external surface in correspondence
of the most stressed area of the forged, as confirmed by FEA.
The crack was also found to propagate to about 30% of the swinging
lever cross-section, indicating a low cycle fatigue.
No evidences of pitting corrosion as well as superficial damages were
found acting as a stress concentrator. Therefore, the fatigue crack
initiation seems to be due to exceeding of the swinging lever fatigue
life.
The initial defect was related to the presence of an abnormal silicon
content compared with the alloying elements, which had resulted in
stress concentrations at the external surface of the swinging lever
leading to the fatigue crack initiation.
19. 19
Results and Discussion
• FEA analysis also revealed that the origin of the crack was in a most
stressed region of the lever.
• Predicted life of the component was 33,316 and it failed at 26,139
CSN. The AFGROW analysis with load data obtained from FEA shows
the fatigue life of the component to be 20,000 cycles which is in well
agreement with the actual life. Therefore, the fatigue crack initiation
seems to be due to exceeding of the swinging lever fatigue life.
20. 20
References• [1] Reddy AV. Fatigue fracture of a main landing gear swinging lever in
a civil aircraft, CRC Press LLC; 2004. p. 208. F. Bagnoli *, F. Dolce, M.
Colavita, M. Bernabei. (Engineering Failure Analysis journal).
