UH-1H MAIN ROTOR BLADE FATIGUE Main Rotor Blade Fatigue.pdf · uh-1h main rotor blade (mrb) fatigue...

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UH-1H MAIN ROTOR BLADE FATIGUEby

Patrick Conor, Alan James, Michael Hollis

Defence Technology AgencyNew Zealand Defence Force

Auckland

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PRESENTATION OUTLINE

• BACKGROUND

• BLADE CRACKING

• GRIP PAD DISBONDING

• BLADE INSPECTION

• NDI RESULTS

• CAUSAL FACTORS

• IN-SERVICE MANAGEMENT

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UH-1H MAIN ROTOR BLADE (MRB) FATIGUE

BACKGROUND

• RNZAF AIRFRAME AVERAGE TSN APPROX 10,000 HRS

• US ARMY HOLD TYPE CERTIFICATE

• MAIN ROTOR BLADE SAFE LIFE 2,500 HRS (US ARMY SPECTRUM)

• RNZAF MAINTENANCE iaw US ARMY MANUALS

• RNZAF FREE OF SERIOUS MRB PROBLEMS SINCE 1965

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�� "�� "�� "�� " ####�" �� $ � � �� " �� $ � � �� " �� $ � � �� " �� $ � � ��

MEDIUM UTILITY HELICOPTER ROLE

Location of flight operations in 2000 - 2001:

• NEW ZEALAND•• EAST TIMOR

• BOUGAINVILLE

• ANTARCTICA

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DEFECT INDICATION – NZ3816, EAST TIMOR 2001

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Grip Plate (Aluminium)

Grip Pad (Steel)

Toughened epoxy adhesive

BLADE CONSTRUCTION

D-spar

Bushing

ADHESIVELY BONDED METAL LAMINATE STRUCTURE

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� �� $�� ' � �' ( �� $�� ' � �' ( �� $�� ' � �' ( �� $�� ' � �' ( ��

FRACTURE SURFACE

CHORDWISE C/S

Fatigue crack limits

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GRIP PAD DISBONDING

FATIGUE CRACK

SAW CUT

TIPL.E.

UNDER-SURFACE OF CRACKED

BLADE

GRIP PAD REMOVEDAND BLADE CUT UP FOR FRACTURE SURFACE ANALYSIS

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� $ �� +� $ �� +� $ �� +� $ �� +

RESULTS OF DESTRUCTIVE TESTS

TWO MRB CLASSES

Early-Model Blades

- MODERATE DISBONDS AT 2,500 HRS

Recent Blades

- SIGNIFICANT DISBONDS AT 1,000 HRS

- MANY BLADES IN SERVICE

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DAMAGE ASSESSMENT

� GRIP PLATE CRACK GROWTH RATE WAS RAPID

� CRACKS CONCEALED BY GRIP PAD

� CRACKING PRECEDED BY GRIP PAD DISBONDING

� SAFETY PREDICATED BY DISBONDING

SO,

� MANAGE SAFETY BY MONITORING DISBOND ONSET(SAFETY BY INSPECTION)

� PRESCRIBED INSPECTION – CAREFUL VISUAL CHECK

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�� , �� $ � ' � �� , �� $ � ' � �� , �� $ � ' � �� , �� $ � ' � ��

DISBONDING FOUND IN A BLADE REJECTED FOR OTHER REASONS AFTER 1100 HRS IN SERVICE -

- LARGE DEFECT WAS NOTDETECTED BY EITHER:

• BONDLINE VISUAL INSPECTION (IN A LAB ENVIRONMENT)

OR,

• COIN TAP-TESTING

Grip Plate (Aluminum)

Grip Pad (Steel)

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� EXISTING INSPECTION METHODS DID NOT PROVIDE ADEQUATE MEASUREMENT CAPABILITY

PERFORMANCE OF AVAILABLE INSPECTION METHODSFOR MEASURING GRIP PAD DISBONDING

INTERPRET- DIFFICULT TO ULTRASONIC TEST

- FAILEDCOIN TAP TEST

- UNRELIABLEDYE PENETRANT

- UNRELIABLEVISUAL

RESULTMETHOD

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NEW INSPECTION TECHNIQUE

- BONDLINE COMPLIANCE -

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� � �� ' � � $ ��' �� ' � � $ ��' �� ' � � $ ��' �� ' � � $ ��' �

STRAIN GAUGE MECHANICAL DISPLACEMENT

1 2 3 4 5 6 7 8 9 10

0

54.4

5

95.9

75

0

100

200

300

400

500

600

700

Mic

rost

rain

Strain Gauge NumberBending Moment

(metre.kg)

BONDLINE COMPLIANCE

RESULT WITH MULTIPLE

STRAIN GAUGES

BONDLINE

LOAD

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�� $ � ' � �� $ � ' � �� $ � ' � �� $ � ' � ��

BLADE GRIP PAD EDGE STRAIN vs TIME SINCE NEW

0

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500 2000 2500

TIME SINCE NEW (Flying Hours)

Max

. Str

ain

(uS

)

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OPTIONS TO KEEP FLYING

1. BUY MANY NEW BLADES AND REPLACE VERY EARLY

- VERY EXPENSIVE

- IMPOSSIBLE SUPPLY PROBLEM

2. ADOPT SAFETY BY INSPECTION

- DEFINE SAFE DISBOND SIZE

- DETECT AND MONITOR SUB-CRITICAL DEFECTS

- USING THE STRAIN NDI METHOD

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� �� -�� " �$� �� -�� " �$� �� -�� " �$� �� -�� " �$

0

200

400

600

800

1000

1200

1400

0 20 40 60 80 100 120 140

Delaminated Area (cm2)

Max

TE

Str

ain

(uS

)

DISBONDRUNAWAY

LIMIT

BLADEQUARANTINE

LIMIT

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CAUSAL FACTORS

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� �� ' � �� $ � � �� , �� ' � �� $ � � �� , �� ' � �� $ � � �� , �� ' � �� $ � � �� , ��

# �.��/�0 11/2�. �3��425 26�24 �7.1 5 �

0

500

1000

1500

2000

2500

A-F

S11

396

A-F

S11

421

A-F

S11

423

A-F

S12

014

A-1

1392

A-F

S11

365

A-F

S11

068

A-F

S11

416

A-F

S11

413

A-7

619

A-6

715

A-F

S11

343

A-6

712

A-F

S11

347

A-F

S12

062

A-F

S11

415

A-1

1376

A-F

S11

271

A-F

S11

243

A-F

S11

449

A-F

S11

385

A-F

S11

419

A-1

1928

A-F

S12

026

A-F

S12

061

A-F

S12

584

A-9

144

A-F

S11

349

A-F

S12

008

A-F

S12

768

A-F

S12

767

A-F

S12

007

A-F

S12

708

A-F

S12

874

A-F

S12

060

A-1

1389

A-F

S12

769

A-F

S12

051

A-F

S12

869

AM

R01

288

A-F

S12

797

A-F

S12

890

A-F

S12

921

A-5

031

A-F

S12

866

A-F

S12

770

A-F

S12

828

A-F

S12

923

Blade

Bla

de T

SN

(hou

rs)

0

200

400

600

800

1000

1200

1400

1600

Str

ain

->

Ice Hrs

Hrs NZ

Hot &WetHrs

Strain 1

RNZAF DATA

Analysis by S/Ldr M Going

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MB

P

Resultant Horizontal Shear

Resultant Horizontal Shear

Steel Grip PadAdhesiveBond Line

OutboardUp

MAIN ROTOR BLADE ATTACHMENT FITTING

Aluminium Grip Plate

ADHESIVEPEEL

- LOAD TRANSFER

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• HIGH PEEL STRESS IN THE ADHESIVE

• DEMANDING LOADS SPECTRUM?

FEA: ADHESIVE STRAIN IN Z-AXIS

FATIGUE?

Analysis by S K Campbell (DTA)

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SAFETY BY INSPECTION

• STRAIN NDI FOR DISBOND MONITORING

• CONTINUOUS ANALYSIS OF RESULTS

• REDUCED BLADE LIFE CONFIRMED

• INSPECTION METHOD IMPOSES AN UNAVOIDABLE MAINTENANCE PENALTY

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THE BLADE WAS AFFECTED BY AN EXCEPTIONALLY LARGE FATIGUE CRACK

- THE HELICOPTER WAS SAVED BY CONSERVATIVE DESIGN

- MULTIPLE LOAD PATHS

- REDUNDANT STRUCTURAL ELEMENTS

- FLIGHT SAFETY IS NOW MAINTAINED BY BONDLINE STRAIN INSPECTION

- THE INSPECTION IS BASED ON EXHAUSTIVE ANALYSIS OF IN-SERVICE DEFECTS

SUMMARY

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Acknowledgements

RNZAF Wing Commander D C HowardNo. 3 Squadron Maintenance FlightRNZAF Base Auckland Technical Support Unit (ATSU)RNZAF Base Ohakea Technical Support Unit (OTSU)

DTAMr S K CampbellMr I P Gatehouse

Bell Helicopter Textron Incorporated

UH-1H MAIN ROTOR BLADE FATIGUE Main Rotor Blade Fatigue.pdf · uh-1h main rotor blade (mrb) fatigue...

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