FATIGUE FRACTURE OF A MAIN

LANDING GEAR SWINGING LEVER

IN A CIVIL AIRCRAFT

Pramod Kawade

2012AMD122613

Submitted to-

Prof. R.K.Pandey

Submitted By

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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.

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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.

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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.

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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 1.8 0.08 0.03 0.02 0.02 0.02 0.02

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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.

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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

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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

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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

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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,

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Fracture Analysis

•Fractography-

SEM micrograph showing one defect as crack origin

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Fracture Analysis

•Fractography-

SEM micrographs related to the non-uniform fatigue beach

marks spacing at different distances from the crack origin

1.5 mm 4 mm

8 mm 12 mm

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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

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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.

Optical micrograph of the Swinging lever microstructure.

Silicon content Aluminum content

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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.

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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.

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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.

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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.

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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.

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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).

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Thank You…..

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