FATIGUE FAILURE IN A MILITARY HELICOPTER MAIN ROTOR BLADE

FATIGUE FAILURE IN A MILITARY HELICOPTER MAIN ROTOR BLADE

DESCARGAR PDF

FATIGUE FAILURE IN A MILITARY HELICOPTER MAIN ROTOR BLADE
N.J. Lourenço, M.L.A. Graça, L.A.L.Franco, O.M.M.Silva
IAE - Divisão de Materiais
Pça. Mal. Eduardo Gomes, 50, V. das Acácias, S. J. dos Campos – SP,
CEP 12228-904
[email protected]
Abstract
The helicopters have a lot of moving parts that are potentially source
of material failure. Among these moving parts rotor blades plays a
special role acting as wings. They are composed from different
materials and components working under vibratory cycles and fatigue
loads providing ideal conditions for fatigue failure. This work
describes a potentially dangerous incident without human losses that
occurred during an experience flight. The crew felt an abnormal
vibration and proceeded an emergency landing. After landing a visual
inspection was performed and was noticed a crack covering more than 70
% of the rotor blade surface from trailing edge to leading edge. The
blade failure was due to fatigue process promoted by corrosion pits
which acted as stress concentration site.
Keywords: Rotor blade; Corrosion; Fatigue failure.
1. Background
The Brazilian military helicopter fleet is growing due to
strategically importance of this kind of aircraft in maintain border
control as well save and rescue activities. However helicopters are a
special kind of aircraft without space for mistakes on operation and
maintenance. The helicopters have a lot of moving parts that are
potentially source of material failure. Among these moving parts rotor
blades plays a special role acting as wings. They are composed from
different materials and components working under vibratory cycles and
fatigue loads and as textually said by Symonds and Pitt [1]“military
helicopters provide ideal conditions for the nucleation and
propagation of failure damage”. This case study describes the failure
analysis of a fractured main rotor blade from a military rotary-wing
aircraft, known as UH-1 Huey. This helicopter type was initially used
during Vietnam War [2] and until now there is an expressive number
flying all over the world. In Brazil this helicopter is used by
Brazilian Air Force and is called kindly as “Big Frog”.
This work describes a potentially dangerous incident without human
losses that occurred during an experience flight. The crew felt an
abnormal vibration and proceeded an emergency landing. After landing a
visual inspection was performed and was noticed a crack covering more
than 70 % of the rotor blade surface from trailing edge to leading
edge.
2. Main Rotor Blade Examination
Initially a visual examination was carried out. The as-received blade
is shown in Fig1.

Fig. 1. The as-received rotor blade. The black arrow shows the crack
site.
There was a crack with 30 cm long that had propagated from trailing
edge to leading edge as shown in Fig.2.

Fig. 2. Section of the fractured blade.
A section of the failed blade is shown schematically in Fig. 3. The
crack initiated at the filler aluminium alloy at trailing edge and
progressed to the skin metal in direction to the leading edge.
Basically the origin of the fatigue crack initiation is identified at
“I “and the final position is identified at “II”. After this point the
crack was due to overload until position “III” and further progressed
by the metallic skin to the leading edge until reaches the steel
reinforcing bar and finally stops. The arrows in Figure 3 show the
path of the crack from initiation site to the end.

Fig. 3. Squematic representation of the fractured blade. I –
Initiation crack site, I-II crack propagation due to fatigue, II-III
crack propagation due to overload, from III to the end of the crack,
propagation due to overload .
The crack was opened and a detail of the fracture surface with aspects
of fatigue failure is shown in Fig.4.

Fig. 4. Crack surface after opening.
In order to identify the crack mechanism a fractographic examination
was made using a Leo 435VPi - Oxford scanning electron microscopy
(SEM). Fig.5 shows a corrosion pit at the initiation crack site.

Corrosion pit

Fracture initiation site
Fig. 5. Initiation crack site with corrosion pit.
An EDS analysis was carried out and this analysis showed significant
amount of oxygen (O) and chlorine (Cl), Fig. 6. These elements are
common as corrosion product. It was also found beach marks typical
from fatigue failure, Fig. 7.

Fig.6. EDS spectrum showing considerable amounts of Cl (clorine) and O
(oxygen).

Fig 7. SEM micrograph of the fracture zone showing beach marks.
At higher magnification fatigue striations was seen Fig. 8. In
addition a metallographic examination near the crack initiation site
showed secondary cracks Fig 9.

Fig. 8. Higher SEM magnification showing fatigue striations.

Fig.9. Secondary cracks near the fracture surface.
3. Discussion
In this study it was noticed that several cracks were found at the
trailing edge, more precisely at the metallic soul inside the
propeller. It´s quite difficult for nondestructive testing such
dye-penetrating , eddy-current or X-ray examination, detect a crack in
this area in order to proceed maintenance. This aircraft was in
service for a long time, more than 20 years working in a military
environment where frequently the operation is harder than civil
aviation. An interesting report from Australian Transport Safety
Bureau [3] points out that define how age is one aircraft is a very
difficult task bearing in mind that the age is a relation that’s
include chronological age, a complete take-off and landing procedure,
known as flight cycles as well the number of the flight hours
accumulated. In this case study was recommended to pay attention on
maintenance in order to observe the limits of operation of the failed
blade.
4. Conclusions
1. It was found pit and intergranular corrosion near the fracture
surface.
2. The cause of accident was a blade failure due to fatigue process
promoted by corrosion pits which acted as stress concentration site.
References
1. SYMONDS N. and Pitt C. Military helicopters: Have the seeds of
future
accidents already been sown? Engineering Failure Analysis Volume 13,
Issue 3, (2006) Pages 493-515. Available in:
http://www.ewp.rpi.edu/hartford/~ernesto/S2008/SMRE/Papers/Symonds.pdf
access in 10/09/2012
2. McGOWEN S. S., Helicopters: an illustrated history of their impact.
ABC-CLIO. California. 2005
3. Australian Transport Safety Bureau. How Old is Too Old? The impact
of ageing aircraft on aviation safety. Aviation research and analysis
report - B20050205.2007. Available in:
http://www.atsb.gov.au/publications/2007/b20050205.aspx access in
10/09/2012