Accessory Gear Box in an Aircraft Engine - Failure Analysis of Engineering Structures Methodolo

Summary

There have been a few cases of premature failure of quill shafts in the accessory gear boxes of aircraft engines. The failure was found to be due to gradual wear resulting from inadequate lubri-cation.

Background

It was reported that there have been a number of premature failures of quill shafts that transmit the drive from the engine ac-cessory gear box to the oil unit. The failed quill shaft was not available for investigation. However, its mating splines in the splined bore were available.

Visual Examination of General Physical Features

The splines in the splined bore on the drive side were found covered with dry red powder. The splines were cleaned. They had an odd shape compared with the splines on other drives. The splines were cut open for detailed examination. They showed ex-cessive wear. There was some blueing on the splines indicating heating effects.

Testing Procedure and Results

The reddish powder collected from the worn out splines was found to contain all the elements from the gear material. This pow-der is thus a result of wear of the splines. The reddish color is due to formation of iron oxide.

The proﬁles of the drive side spline and a good spline were examined in a contourograph and the proﬁle traces are shown in Fig. CH35.1.

Scanning Electron Microscopy

The cross section of these splines was observed in a SEM. Fig-ure CH35.2 shows the spline proﬁles in the worn spline and in a

good spline. It was estimated that about 30% of the effective spline volume had become worn.

Metallography and Hardness

The cross sections of the splines were metallographically pol-ished and observed in the microscope. The splines were found to be case hardened to a depth of 0.25 mm. The hardness of the case was 63 HRC and that of the core was 38 HRC. These values appeared to be within the normal spline speciﬁcations.

From these observations, it may be concluded that the failure of the spline in the spline bore resulted from wear due to inade-quate lubrication.

Conclusion

The failure of the spline was due to gradual wear resulting from inadequate lubrication.

Recommendation

Adequate lubrication should be ensured at all times by periodic inspection.

Fig. CH35.1 Traces of the proﬁles of the splines taken in a contourograph Failure Analysis of Engineering Structures: Methodology and Case Histories

V. Ramachandran, A.C. Raghuram, R.V. Krishnan, S.K. Bhaumik, p148-149 DOI:10.1361/faes2005p148

www.asminternational.org

200 ␮m

(a) (b) 200 ␮m

Fig. CH35.2 SEM photographs showing cross sections of (a) good and (b) worn splines

CASE 36

Failure of a Compressor

Blade in an Aircraft Engine

Summary

A compressor blade in an aircraft engine failed in service. The failure was due to fatigue fracture originating from nonmetallic inclusions present at a critical location near the blade root.

Background

A compressor blade of an aircraft engine failed in service. One-half of the blade containing the root portion was available for investigation.

Visual Examination of General Physical Features

The blade had fractured at the root, with the fracture surface showing a conchoidal pattern indicating the delayed nature of frac-ture. The fracture had originated from the convex surface of the blade, almost from the middle portion, and progressed to about

80% of the cross section before ﬁnal fracture, with a shear lip.

Figure CH36.1 shows the macrograph near the origin. Well-delin-eated beach marks are present on the fracture surface (Fig.

CH36.2).

Testing Procedure and Results

Scanning Electron Fractography and EDAX

Examination of the cleaned fracture surface in the SEM revealed coarsely spaced striations near the origin with cracks running par-allel to the striations (Fig. CH36.3). Away from the origin but still within the region of beach marks, the striation appeared much ﬁner and without cracks (Fig. 36.4). The rapid fracture region showed the presence of dimples.

Detailed examination of the fracture surface near the origin re-vealed the presence of inclusions (Fig. CH36.5). The inclusions

1 mm

Fig. CH36.1 Fracture surface showing the fracture origin

300 ␮m

Fig. CH36.2 Well-deﬁned beach marks on the fracture surface Failure Analysis of Engineering Structures: Methodology and Case Histories

V. Ramachandran, A.C. Raghuram, R.V. Krishnan, S.K. Bhaumik, p150-151 DOI:10.1361/faes2005p150

www.asminternational.org

by in situ EDAX were found to be rich in silicon. They are possibly silicates. Similar inclusions were seen on the fracture surface away from the origin also.

Metallography

A section from the root was cut and prepared for metallographic examination. It was found to contain a few large silicon-rich in-clusions.

Discussion

It is obvious that the primary cause of fracture of the blade was fatigue, originating from inclusions present in a critical area. The change in the nature of striations could best be explained as due to a change from low-cycle fatigue condition in the initial stages to a high-cycle fatigue condition after the crack had grown to a critical length.

Conclusion

The failure of the compressor blade was due to fatigue fracture originating from inclusions present at critical location near the blade root on the convex side.

Recommendation

Blades manufactured from this batch of raw material should be reviewed.

5 ␮m

Fig. CH36.3 SEM fractograph near the origin showing fatigue striations along with cracking

2 ␮m

Fig. CH36.4 SEM fractograph farther away from the origin showing very-ﬁne fatigue striations

6 ␮m Inclusion

Fig. CH36.5 SEM fractograph of region close to the origin showing inclu-sions that were analyzed to be rich in silicon

CASE 37

Failure of an Aileron

Control Cable in an Aircraft

Summary

The aileron control cable of an aircraft had failed. It was found damaged. Investigation revealed that the cable had been damaged by a shearing tool.

Background

The cable controlling the aileron movement in an aircraft had failed. The two mating portions of the cable were made available for the investigation to ﬁnd the type of failure.

50 ␮m

Fig. CH37.1 SEM photographs of broken ends of individual wires showing shear fracture Failure Analysis of Engineering Structures: Methodology and Case Histories

V. Ramachandran, A.C. Raghuram, R.V. Krishnan, S.K. Bhaumik, p152-153 DOI:10.1361/faes2005p152

www.asminternational.org

Visual Examination of General Physical Features

The control cable was made of galvanized plain carbon steel wires of diameter 0.23 to 0.25 mm. It contained seven strands of 19 wires in each strand.

Testing Procedure and Results

Scanning Electron Fractography

The fractured ends of the strands of the cable were examined in a SEM. The majority of the wires showed sheared ends. Some of the sheared ends are shown in Fig. CH37.1. A few wires were found to have fractured after necking under a tensile load (Fig.

CH37.2).

Discussion

The localized failure of the cable and the evidence that the ma-jority of the wires showed sheared ends indicated that the cable had been damaged by a shearing tool. The general wear and tear and overload were not the cause of failure.

Conclusion

The aileron cable failed because of damages caused by a shear-ing tool.

Fig. CH37.2 SEM photograph of broken end of a wire showing tensile fracture preceded by necking (typical cup and cone fracture)

CASE 38

In document Failure Analysis of Engineering Structures Methodology and Case Histories (Page 148-154)