Summary
In one type of helicopter flying at high altitudes in mountainous regions, a few failures had been reported at the mixing unit at-tachment lug joint. The mixing unit is held through fasteners to four lugs, two on top and two at the bottom, welded to two hori-zontal members of a tubular frame. The tubular frame is charged with nitrogen gas. The tube had cracked below the lug, resulting in gas leakage. It was a fatigue fracture due to excessive flexural load.
Background
In one type of helicopter, a few failures were experienced at the lug joint of the mixing unit attachment. The mixing unit assembly is a critical part of the flight control system. It is attached through fasteners to four lugs, two on top and two at the bottom, which are welded to two horizontal members of a tubular frame. The tubular frame is charged with nitrogen at a pressure of 1.8 bars.
Any leakage of gas from the tubular frame due to cracks is revealed by a telltale “VIDEOPT” indicator.
Out of ten incidents reported, there were eight cases of such gas leakage, and in the other two, the lug had fractured above the weldment. It was reported that in all these cases, the helicopters were flying at high altitudes in the mountainous regions. In one such incident, there was gas leakage at the welded joint of the bottom lug, on the port side.
Pertinent Specifications
The tubular frame and the lugs were made of a chromium-mo-lybdenum-vanadium steel equivalent to French specification 15CDV6.
Visual Examination of General Physical Features
The tubular structure holding the two bottom lugs cut from the body structure of the helicopter is sketched in Fig. CH19.1. The tube was cut near the region of leakage to expose the interior. The inner surface was cleaned. A crack was observed on the tube wall, directly below the lug. The location of this crack is shown sche-matically in Fig. CH19.2.
The lug had been welded with a tack weld on one side and a continuous weld on the other side, penetrating the entire tube wall thickness. The crack had occurred in the heat-affected zone (HAZ) in between these two welds. This crack was opened. The features of this fracture surface were indicative of fatigue crack. The crack had originated on the inner surface of the tube and propagated through the wall thickness toward the lug. This indicates that the
Fig. CH19.1 Tubular structure holding the two bottom lugs Failure Analysis of Engineering Structures: Methodology and Case Histories
V. Ramachandran, A.C. Raghuram, R.V. Krishnan, S.K. Bhaumik, p110-112 DOI:10.1361/faes2005p110
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tube had been subjected to upward flexure about the welded re-gion.
Testing Procedure and Results
Scanning Electron Microscopy
The thickness side of this cut piece was polished and examined in a SEM. A number of other cracks parallel to the crack that was opened were observed. These are sketched in Fig. CH19.2 and shown in Fig. CH19.3 and CH19.4. These also confirm the flexure of the tube in this region.
The leakage of gas from the tube was due to the crack at the port side lug attachment. In order to determine whether a similar situation had prevailed on the starboard side weldment, a cut was made in that region also and the tube wall examined. A crack was observed, emanating from the weld toe (Fig. CH19.5, CH19.6).
This again confirms flexure of the tube.
Discussion
Presence of cracks below the lug clearly indicates the tube had been subjected to a two-point load at the two bottom attachment lugs, resulting in the flexure of the tube and subsequent cracking due to fatigue.
tube
Full weld
Lug
tack weld
Cracks
Fig. CH19.2 Sketch showing the location of the cracks on the tube
20 m
Fig. CH19.3 SEM photograph showing cracks in the tube wall
10 m
Fig. CH19.4 SEM photograph showing cracks in the tube wall
1 mm
Fig. CH19.5 SEM photograph showing a crack emanating from the weld toe at the starboard lug
112 / Failure Analysis of Engineering Structures: Case Histories
Conclusion
The tube cracked below the lug by fatigue due to excessive flexural load. The crack initiated on the inner surface of the tube below the lug and propagated through the tube thickness, causing leakage of gas and pressure drop.
Recommendation
It is necessary to redistribute the loads on the tube to avoid failures. However, before carrying out any modification, it is de-sirable to know the nature and magnitude of loads at this area under flying conditions. This is best done by instrumenting a helicopter and actually measuring the strains and loads experienced in the region.
Once the data are obtained, suitable modifications to redistribute the loads can be made. The modified designs should be rig tested under simulated loading conditions to select the most optimal de-sign modification.
20 m
Fig. CH19.6 SEM photograph showing a crack emanating from the weld toe at the starboard lug