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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
������������ ���������� %
Grip Plate (Aluminium)
Grip Pad (Steel)
Toughened epoxy adhesive
BLADE CONSTRUCTION
D-spar
Bushing
ADHESIVELY BONDED METAL LAMINATE STRUCTURE
������������ ���������� &
� ��� � ��$�� �' � �' ( �������� ���� ��� � ��$�� �' � �' ( �������� ���� ��� � ��$�� �' � �' ( �������� ���� ��� � ��$�� �' � �' ( �������� ���
FRACTURE SURFACE
CHORDWISE C/S
Fatigue crack limits
������������ ���������� )
GRIP PAD DISBONDING
FATIGUE CRACK
SAW CUT
TIPL.E.
UNDER-SURFACE OF CRACKED
BLADE
GRIP PAD REMOVEDAND BLADE CUT UP FOR FRACTURE SURFACE ANALYSIS
������������ ���������� *
� $ �� � � �� �� � +� $ �� � � �� �� � +� $ �� � � �� �� � +� $ �� � � �� �� � +
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
������������ ���������� ��
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)
������������ ���������� ��
� 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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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
)
������������ ���������� �%
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
������������ ���������� �&
� ��� � �� ��� ���-���� ����� � ����� ��" �$� ��� � �� ��� ���-���� ����� � ����� ��" �$� ��� � �� ��� ���-���� ����� � ����� ��" �$� ��� � �� ��� ���-���� ����� � ����� ��" �$
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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� ��� ' � �� ��� $ � � ����� ���, �� � �� ���� ��� ' � �� ��� $ � � ����� ���, �� � �� ���� ��� ' � �� ��� $ � � ����� ���, �� � �� ���� ��� ' � �� ��� $ � � ����� ���, �� � �� ���
# �.��/�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
������������ ���������� ��
MB
P
Resultant Horizontal Shear
Resultant Horizontal Shear
Steel Grip PadAdhesiveBond Line
OutboardUp
MAIN ROTOR BLADE ATTACHMENT FITTING
Aluminium Grip Plate
ADHESIVEPEEL
- LOAD TRANSFER
������������ ���������� ��
• HIGH PEEL STRESS IN THE ADHESIVE
• DEMANDING LOADS SPECTRUM?
FEA: ADHESIVE STRAIN IN Z-AXIS
FATIGUE?
Analysis by S K Campbell (DTA)
������������ ���������� ��
��������####�� � , �' � �� ���� � � ����� � , �' � �� ���� � � ����� � , �' � �� ���� � � ����� � , �' � �� ���� � � ���
SAFETY BY INSPECTION
• STRAIN NDI FOR DISBOND MONITORING
• CONTINUOUS ANALYSIS OF RESULTS
• REDUCED BLADE LIFE CONFIRMED
• INSPECTION METHOD IMPOSES AN UNAVOIDABLE MAINTENANCE PENALTY
������������ ���������� �
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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