uh-1h main rotor blade fatigue

UH-1H MAIN ROTOR BLADE FATIGUE
by
Patrick Conor, Alan James, Michael Hollis
Defence Technology Agency
New Zealand Defence Force
Auckland
PRESENTATION OUTLINE
• BACKGROUND
• BLADE CRACKING
• GRIP PAD DISBONDING
• BLADE INSPECTION
• NDI RESULTS
• CAUSAL FACTORS
• IN-SERVICE MANAGEMENT
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
"# "
$
MEDIUM UTILITY HELICOPTER ROLE
Location of flight operations in 2000 - 2001:
• NEW ZEALAND
•
• EAST TIMOR
• BOUGAINVILLE
• ANTARCTICA
!
DEFECT INDICATION – NZ3816, EAST TIMOR 2001
BLADE CONSTRUCTION
ADHESIVELY BONDED METAL LAMINATE STRUCTURE
D-spar
Grip Plate
(Aluminium)
Toughened
epoxy adhesive
Grip Pad
(Steel)
Bushing
%
$
'
'(
FRACTURE SURFACE
Fatigue crack limits
CHORDWISE C/S
&
GRIP PAD DISBONDING
UNDER-SURFACE
OF CRACKED
BLADE
SAW CUT
GRIP PAD REMOVED
AND BLADE CUT UP
FOR FRACTURE
SURFACE ANALYSIS
FATIGUE CRACK
TIP
L.E.
)
$
+
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
,
DISBONDING FOUND IN A
BLADE REJECTED FOR
OTHER REASONS AFTER
1100 HRS IN SERVICE -
$ '
Grip Pad
(Steel)
- LARGE DEFECT WAS NOT
DETECTED BY EITHER:
• BONDLINE VISUAL
INSPECTION (IN A LAB
ENVIRONMENT)
OR,
• COIN TAP-TESTING
Grip Plate
(Aluminum)
PERFORMANCE OF AVAILABLE INSPECTION METHODS
FOR MEASURING GRIP PAD DISBONDING
METHOD
RESULT
VISUAL
- UNRELIABLE
DYE PENETRANT
- UNRELIABLE
COIN TAP TEST
- FAILED
ULTRASONIC TEST
- DIFFICULT TO
INTERPRET
EXISTING INSPECTION METHODS DID NOT PROVIDE ADEQUATE
MEASUREMENT CAPABILITY
NEW INSPECTION TECHNIQUE
- BONDLINE COMPLIANCE -
'
STRAIN GAUGE
$
'
MECHANICAL DISPLACEMENT
BONDLINE
LOAD
BONDLINE COMPLIANCE
700
500
400
RESULT WITH
MULTIPLE
STRAIN
GAUGES
300
200
Microstrain
600
100
1
0
54.45
Bending
Mom ent
(metre.kg)
95.975
0
2
3
4
5
6
7
8
9 10
Strain Gauge Number
!
$ '
BLADE GRIP PAD EDGE STRAIN vs TIME SINCE NEW
1600
Max. Strain (uS)
1400
1200
1000
800
600
400
200
0
0
500
1000
1500
TIME SINCE NEW (Flying Hours)
2000
2500
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
%
-
" $
1400
DISBOND
RUNAWAY
LIMIT
Max TE Strain (uS)
1200
1000
800
600
BLADE
QUARANTINE
LIMIT
400
200
0
0
20
40
60
80
100
120
140
Delaminated Area (cm 2)
&
CAUSAL FACTORS
)
A-FS11396
A-FS11421
A-FS11423
A-FS12014
A-11392
A-FS11365
A-FS11068
A-FS11416
A-FS11413
A-7619
A-6715
A-FS11343
A-6712
A-FS11347
A-FS12062
A-FS11415
A-11376
A-FS11271
A-FS11243
A-FS11449
A-FS11385
A-FS11419
A-11928
A-FS12026
A-FS12061
A-FS12584
A-9144
A-FS11349
A-FS12008
A-FS12768
A-FS12767
A-FS12007
A-FS12708
A-FS12874
A-FS12060
A-11389
A-FS12769
A-FS12051
A-FS12869
AMR01288
A-FS12797
A-FS12890
A-FS12921
A-5031
A-FS12866
A-FS12770
A-FS12828
A-FS12923
Blade TSN (hours)
# . / 0 11 /2 .
$
3
4 25 26 2 4
2500
2000
RNZAF DATA
1500
1000
800
1000
600
500
0
Blade
Strain ->
'
,
7.1
5
1600
1400
Ice Hrs
1200
Hrs NZ
400
Hot &
Wet
Hrs
Strain 1
200
0
Analysis by S/Ldr M Going
*
MAIN ROTOR BLADE ATTACHMENT FITTING
- LOAD TRANSFER
Resultant Horizontal Shear
Outboard
Up
MB
P
Aluminium Grip Plate
ADHESIVE
PEEL
Steel Grip Pad
Resultant Horizontal Shear
Adhesive
Bond Line
FATIGUE?
• HIGH PEEL STRESS IN
THE ADHESIVE
• DEMANDING LOADS
SPECTRUM?
FEA: ADHESIVE STRAIN IN Z-AXIS
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
SUMMARY
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
Acknowledgements
RNZAF
Wing Commander D C Howard
No. 3 Squadron Maintenance Flight
RNZAF Base Auckland Technical Support Unit (ATSU)
RNZAF Base Ohakea Technical Support Unit (OTSU)
DTA
Mr S K Campbell
Mr I P Gatehouse
Bell Helicopter Textron Incorporated
!