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Cracking in a Tail Rotor Blade in a Helicopter

By Shayan Siddique
MY-033

Abstract
Tail rotor of a helicopter fractured when in one flight. Failure analysis on it was finished in laboratory. Macroscopic observation of the tail rotor, macro and microscopic inspection on the fracture surface was carried out. Chemical components and metallurgical structure was also carried out
Experiment results showed that fracture mode of the tail rotor pivot is fatigue fracture. Fatigue area is over 80% of the total fracture surface, obvious fatigue band characteristics can be found at the fracture face. According to the results were analyzed from the macroscopic and microcosmic aspects, fracture reasons of the tail rotor were analyzed in details.
Introduction
As the rotor blades rotate, the fuselage will rotate the opposite direction if unopposed. Tail rotor system of helicopter as one of key components, not only provide anti-torque and achieve directional control, but also keep the helicopter’s yaw stability, its working condition directly affects the safety and reliability of the helicopter.
Tail rotor teeter pivot bolt as a helicopter tail rotor system of the core force component, need to bear a greater bending moment and experiencing a lot of alternating load. Tail rotor flapping hinge bolt fracture will directly lead to the helicopter yaw control failure, even cause serious catastrophic accidents.
March 2016, a general aviation of domestic was carrying out flight training in field. The helicopter was climbing out when severe vibration prompted the commander to carry out an immediate autorotation landing in a field. The helicopter incurred severe damage during the landing.
On inspection, The bolt attaching the tail rotor hub to the rotating fork assembly, about which the rotor is free to teeter, was found to have fractured.

Examination of the fracture face of the teeter pivot bolt showed that it had failed in a plane very close to the exposed face of the threaded insert, allowing the dust shield and washers on one side of the hub to be lost.

Analysis of Tail Rotor Bolt
Appearance Check:
The whole morphology of fractured tail rotor teeter pivot bolt is shown in Figure.

No obvious bending deformation was found in this section of the bolt. But, at ends of the surface visible obvious signs of corrosion, wear characteristic is not obvious. The fracture section is located on the left side of the bolt. The fracture face is located at the edge of the threaded end bearing, the corrosion feature of the fracture surface of bolt is serious.
Microscopic observation of fracture
Scanning Electron Microscope (SEM) examination of the bolt found that fatigue cracking had sourced at the below side surface of the fracture section, no obvious defect on the source location. Fatigue growth from the source area to the left and right sides of the fracture surface, fatigue strip fine, fatigue expansion full. Besides, according to the fracture feature, it can be seen that the cyclic stress of fatigue fracture is small and more fatigue expansion cycles.

Microscopic feature of fracture source section

Microscopic characteristics on the left fatigue expansion of bolt

a In initial period b In middle period c In late period
Microscopic characteristics on the right fatigue expansion of bolt

Discussion
The microscopic characteristics of fatigue expansion at the left and right sides, from the front, middle and late stages of the fatigue source of the bolt fracture, the fatigue characteristics of fracture are obvious, fatigue strip fine, fatigue expansion full. The fatigue had the characteristics of torsional shear stressing applied in two distinct directions. However, the torsional evidence was not orientated about an axis coincident with that of the bolt. Therefore, it can be concluded that the fracture property of the bolt is fatigue fracture. The fracture face of tail rotor teeter pivot bolt at the thread root, originated in the near surface position of the bolt fatigue fracture, Obvious fatigue band characteristics can be found at each location of the Fracture face, fatigue area is over 80% of the total fracture surface. Therefore, it can be concluded that the fracture property of the bolt was high cycle fatigue fracture.

Conclusion
The fracture face originated in the near surface position of the bolt, no obvious defect on the source location the fracture extends from the source area to the left and right sides, fatigue banding closely. The fracture of tail rotor teeter pivot bolt is caused by fatigue fracture, high cycle fatigue fracture due to high original stress and fatigue fracture development is more adequate. It shows that the fatigue crack has experienced a long time process from the initiation to

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