Failure Analysis of a Cracked CH-47 Swashplate 

Rotating Ring, S/N BCW-1379 

by Scott Grendahl and Russell Yocum 

ARL-TR-3826 June 2006 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3826 June 2006 

Failure Analysis of a Cracked CH-47 Swashplate 

Rotating Ring, S/N BCW-1379 

Scott Grendahl 

Weapons and Materials Research Directorate, ARL 

Russell Yocum

U.S. Army Aberdeen Test Center 

Approved for public release; distribution is unlimited.
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June 2006 

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1 January 2005–1 March 2005 

5a. CONTRACT NUMBER 

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4. TITLE AND SUBTITLE 

Failure Analysis of a Cracked CH-47 Swashplate Rotating Ring, S/N BCW-1379 

5c. PROGRAM ELEMENT NUMBER 

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M042-489Y31 

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6. AUTHOR(S) 

Scott Grendahl and Russell Yocum*

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U.S. Army Research Laboratory 

ATTN: AMSRD-ARL-WM-MC 

Aberdeen Proving Ground, MD 21005-5069 

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ARL-TR-3826 

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U.S. Army Aviation and Missile Command 

Redstone Arsenal, AL 35898-5000 11. SPONSOR/MONITOR'S REPORT 

 NUMBER(S) 

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Approved for public release; distribution is unlimited. 

13. SUPPLEMENTARY NOTES 

*

Inspector, Physical Test Unit, U.S. Army Aberdeen Test Center, Aberdeen Proving Ground, MD 21005 

14. ABSTRACT 

The U.S. Aviation and Missile Command sent the U.S. Army Research Laboratory’s Weapons and Materials Research 

Directorate three swashplate rotating rings from the CH-47 helicopter for investigation. The components were examined due to 

the presence of cracks found during routine overhaul. These cracks were on and adjacent to the bearing retention shoulder. One 

of these rotating rings, S/N BCW-1379, was included for a detailed failure analysis to determine the cause of the cracking. This 

component was QDR exhibit number W45N7V-04-0307. The components were required to be forged from aluminum alloy 

2014-T6. The focus of this investigation, BCW-1379, was characterized by nondestructive inspection, chemical analysis, 

optical microscopy, scanning electron microscopy, and mechanical testing. The holes of the rotating ring were numbered 0–36 

around the perimeter, consistent with the convention of the MELRs from Boeing helicopters. 

15. SUBJECT TERMS

failure analysis, 9310 steel, fatigue, helicopter gear 

16. SECURITY CLASSIFICATION OF: 

19a. NAME OF RESPONSIBLE PERSON

Scott Grendahl 

a. REPORT 

UNCLASSIFIED 

b. ABSTRACT 

UNCLASSIFIED

c. THIS PAGE 

UNCLASSIFIED

17. LIMITATION 

OF ABSTRACT 

UL 

18. NUMBER 

OF PAGES 

28 

19b. TELEPHONE NUMBER (Include area code) 

410-306-0819 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18
iii

Contents 

List of Figures iv

List of Tables iv

1. Introduction 1

2. Fluorescent Penetrant Inspection 2

3. Visual Inspection and Light Optical Microscopy 5

4. Scanning Electron Microscopy 12

5. Metallography 14

6. Chemical Analysis 14

7. Hardness and Conductivity Testing 16

8. Discussion 17

9. Recommendations 18

10. Conclusions 18 

Distribution List 20 
iv

List of Figures 

Figure 1. As-received component BCW-1379 swashplate rotating ring. .......................................1 

Figure 2. White light indication of the circumferential crack at hole 5..........................................2 

Figure 3. Black light indication of the circumferential crack at hole 5. .........................................3 

Figure 4. Black light indication of the crack at the thumbnail fretted area at hole 30....................3 

Figure 5. White light macrograph of the corrosive attack near hole 30. ........................................4 

Figure 6. Black light macrograph of the corrosive attack near hole 30..........................................4 

Figure 7. Typical fretted area on the bearing retention shoulder (hole 9). .....................................6 

Figure 8. Typical raised fretted areas associated with 16.19-in surface damage (hole 30). ...........6 

Figure 9. The damage causing the raised fretted areas associated with 16.19-in surface...............7 

Figure 10. Photograph 1 of the circumferential crack at hole 5......................................................7 

Figure 11. Photograph 2 of the circumferential crack at hole 5......................................................8 

Figure 12. Cross section of the crack at hole 5...............................................................................8 

Figure 13. Fractograph of the opened crack at hole 5 (arrows indicate origins). ...........................9 

Figure 14. Fractograph of the opened crack at hole 5 with propagation lines..............................10 

Figure 15. Crack at a raised fretting site located at hole 30..........................................................11 

Figure 16. Corrosion pits and raised fretted areas at hole 30........................................................11 

Figure 17. SEM fractograph of the crack at hole 5.......................................................................12 

Figure 18. SEM micrograph depicting cyclic crack propagation at hole 5. .................................13 

Figure 19. EDS spectrum from the black product on the fracture surface at hole 5.....................13 

Figure 20. Microstructure near the wear step. ..............................................................................15 

Figure 21. Typical microstructure of aluminum 2014-T6 base material......................................15 

Figure 22. Macro-etch of a radial section depicting forging flow lines........................................16 

List of Tables 

Table 1. Chemical composition of swashplate rotating rings by weight-percent.........................16 
1

1. Introduction 

The U.S. Aviation and Missile Command (AMCOM) sent the U.S. Army Research Laboratory’s 

(ARL’s) Weapons and Materials Research Directorate (WMRD) three swashplate rotating rings 

(S/Ns BCW-828, BCW-1379, and W1718240-1) from the CH-47 helicopter for investigation. 

The components were examined due to the presence of cracks found during routine overhaul. 

These cracks were on and adjacent to the bearing retention shoulder. One of these rotating rings, 

S/N BCW-1379, was included for a detailed failure analysis to determine the cause of the 

cracking. This component was QDR exhibit number W45N7V-04-0307. The components were 

required to be forged from aluminum alloy 2014-T6. The focus of this investigation, BCW1379, was characterized by nondestructive inspection, chemical analysis, optical microscopy, 

scanning electron microscopy (SEM), and mechanical testing. The holes of the rotating ring 

were numbered 0–36 around the perimeter, consistent with the convention of the MELRs from 

Boeing helicopters. Figure 1 depicts the as-received BCW-1379 rotating ring from Corpus 

Christi Army Depot, TX. Additional investigators from Boeing, Mr. Kirk Grassett, and from 

AMCOM, Mr. Gary Wechsler, took part in the investigation at ARL-WMRD. 

The objective of this work was to determine the root cause of the cracking associated with the 

bearing retention shoulder on the CH-47 swashplate rotating ring BCW-1379. 

Figure 1. As-received component BCW-1379 swashplate rotating ring.

1 

5 

10 

15 

20 

25 

30 

35 
2

2. Fluorescent Penetrant Inspection 

The bearing retention shoulder of BCW-1379 was inspected utilizing Magnaflux ZL-37, 

fluorescent, post emulsifiable hydrophilic with a 4.0 level sensitivity penetrant in an attempt to 

detect the indications that had been previously noted by other investigators and any other 

possible indications. Two crack indications were observed. A circumferential crack was 

observed near hole 5 measuring approximately 0.35 inches in length. The crack was located 

approximately 0.015 in from the 16.19-in inner diameter (ID) bore. This indication can be 

observed under white light in figure 2 and in black light in figure 3. Additionally, an indication 

was observed near hole 30. This indication appeared to be related to a raised fretted area. The 

crack is depicted under black light in figure 4. This crack measured approximately 0.035 in on 

the shoulder face and 0.005 in onto the 16.19-in-diameter surface. Two areas of corrosive attack 

were also revealed near hole 30. However, there were no cracks associated with these attacked 

areas. One of the two areas with corrosive attack is shown in figure 5 under white light and in 

figure 6 under black light. 

Retention 

shoulder 

surface 

Figure 2. White light indication of the circumferential crack at hole 5. 
3

Retention 

shoulder 

surface 

Figure 3. Black light indication of the circumferential crack at hole 5. 

Retention 

shoulder 

surface 

Figure 4. Black light indication of the crack at the thumbnail fretted area at hole 30.
4

16.19” 

diameter 

surface 

Retention 

shoulder 

surface 

Figure 5. White light macrograph of the corrosive attack near hole 30. 

16.19” 

diameter 

surface 

Retention 

shoulder 

surface 

Figure 6. Black light macrograph of the corrosive attack near hole 30.
5

3. Visual Inspection and Light Optical Microscopy 

The bearing retention shoulders of all three rings were inspected visually and with light optical 

microscopy. For ring BCW-1379, cracks and crack measurements were verified at hole 5 and 

hole 30. The bearing shoulder wear pattern appeared discontinuous, with the most severe wear 

occurring between pitch link locations. At all other locations, the shot peening impressions 

appeared uniform and continuous. The discontinuous wear step was caused from contact with 

the swashplate bearing and the contact loads between the two components. The other two rings 

had a much more severe, continuous wear step around the bearing retention shoulder. 

Differences in the severity of the wear step were most likely due to the loads applied to the 

components, slight differences in dimension, and time in service coupled with the service 

conditions. Although the wear step on BCW-1379 appeared less severe than the wear step 

observed on the other two rings, examples of similar damage mechanisms were apparent. 

Significantly fretted areas were observed at holes 3–4, 6, 9, 15–17, 26, 27, 32, and 35. An 

example of a significantly fretted area is depicted in figure 7. Raised areas where damage on the 

16.19 in diameter caused material to be pushed up on the bearing retention shoulder could be 

observed at holes 9, 11, 13, 16, 20, 23, 30, and 34. These raised areas only had considerable 

fretting at holes 20, 30, and 34; however, the raised areas at hole 30 can be observed in figures 8 

and 16. The damage to the 16.19-in-diameter surface is highlighted for clarity in figure 9. 

Compare figure 9 with figure 8. The damage at the left in both photos is the most obvious. 

The crack at hole 5 was atypical of the previous cracking observed on other rotating rings. There 

was no apparent fretting wear in the area. There did exist, however, a circumferential score line 

that ran along the path where the crack initiated. This score line extended approximately 3 in 

along the circumference in this area and is clearly visible in figures 2, 9, and 10. A similar score 

line was apparent at other locations around the circumference of the bearing retention shoulder 

surface. It appeared to be an impression left by the edge of the outer race of the mating bearing 

and the contact loading between the components. Although the bearing that mated with this 

rotating ring was not available for inspection, the team examined a similar bearing from another 

rotating ring. This “representative” bearing exhibited raised machining marks on the edges of 

the inner and outer races of the bearing. The crack at hole 5 was circumferential, approximately 

0.35 in long, and only 0.015 in from the 16.19-in-diameter surface. It initiated and propagated 

circumferentially (see figures 10 and 11). The circumferential cracks observed on other rotating 

rings initiated near the outer edge of the wear step (approximately 0.90 in away from the 

16.19-in-diameter surface) and quickly turned radially in favor of the service stresses on the 

component. To determine if the crack was the edge of a machining burr rolled over by 

subsequent shot peening, a section was made through one end of the crack and examined 

metallographically. Figure 12 shows the resulting cross section clearly indicating that the crack 
6

Retention 

shoulder 

surface 

Figure 7. Typical fretted area on the bearing retention shoulder (hole 9). 

16.19” 

diameter 

surface 

Retention 

shoulder 

surface 

Figure 8. Typical raised fretted areas associated with 16.19-in surface damage (hole 30). 
7

16.19” 

diameter 

surface 

Retention 

shoulder 

surface 

Figure 9. The damage causing the raised fretted areas associated with 16.19-in surface. 

Retention 

shoulder 

surface 

Figure 10. Photograph 1 of the circumferential crack at hole 5.
8

Retention 

shoulder 

surface 

Figure 11. Photograph 2 of the circumferential crack at hole 5. 

Retention 

shoulder 

surface 

16.19” 

diameter 

surface 

Figure 12. Cross section of the crack at hole 5.
9

had considerable depth and was not simply a rolled-over burr. It can also be seen in this figure 

that the crack roughly paralleled the 16.19-in-diameter surface and would have likely propagated 

considerably farther in both the axial and circumferential directions before contacting the 

exterior surface of the component. The crack was then opened for fractographic examination. 

Examination of the crack’s fracture surface revealed at least three origins, all initiated by fatigue 

on the bearing retention shoulder surface. The beach marks and radial lines suggested it was 

unlikely that more origins existed within the material consumed during metallographic 

preparation, prior to the crack being opened (observable in figures 13 and 14). The crack 

propagated by fatigue from these three origins to a total depth of 0.090 in and a total estimated 

length of 0.350 in. Figures 13 and 14 depict the fracture surface with red arrows and lines 

indicating the crack origins and propagation fronts, respectively. A black product was visible on 

approximately half of the fracture surface; this product was analyzed with Energy Dispersive 

Spectroscopy (EDS) (and discussed within the SEM section of this report). The depth of the 

wear step at hole 5 was approximately 0.001 in. No mechanical damage was noted on the 

16.19-in-diameter surface in the area of the crack and no other anomalies were observed in this 

area. 

16.19” 

diameter 

surface 

Figure 13. Fractograph of the opened crack at hole 5 (arrows indicate origins). 
10

16.19” 

diameter 

surface 

Figure 14. Fractograph of the opened crack at hole 5 with propagation lines. 

The crack at hole 30 was similar to some of the cracks observed on the other rotating rings. This 

crack formed at the edge of a raised fretted thumbnail induced by damage incident to the 

16.19-in-diameter surface from an unknown source. This crack can be observed in figures 4 

and 15. The crack was very small, measuring approximately 0.035 in long × 0.005 in deep. Due 

to its small size and apparent similarity with the cracks on the other rotating rings previously 

examined, no attempt was made to open this crack for fractographic examination. It was 

assumed that this crack initiated and propagated by fatigue as a result of localized high stresses 

due to the effect of the raised thumbnail on the load distribution between the rotating ring 

and the bearing. Pitting corrosion was observed at two locations in this area as well as of the 

16.19-in-diameter surface. The pitting corrosion can be observed in figure 16 along with the 

raised fretted thumbnails. There were no cracks associated with the pitting sites. It is not 

believed that they are associated with the damage incident to the 16.19-in-diameter surface. 
11

Retention 

shoulder 

surface 

Figure 15. Crack at a raised fretting site located at hole 30. 

16.19” 

diameter 

surface 

Retention 

shoulder 

surface 

Figure 16. Corrosion pits and raised fretted areas at hole 30.
12

4. Scanning Electron Microscopy 

SEM utilizing a JEOL-JSM 6460-LV was performed to investigate the fracture surfaces in 

greater detail. The crack at hole 5 is depicted in the electron fractograph in figure 17. At closer 

inspection, evidence of cyclic fatigue crack propagation was present (figure 18). The fracture 

morphology was transgranular throughout the cracked region. EDS was utilized to analyze a 

dark product on the fracture surface. The product appeared to have high peaks for oxygen and 

aluminum, indicative of oxidized aluminum and that this crack either took a long time to form or 

formed quite a while ago. No more specific information about the age of the crack could be 

determined. This spectrum can be observed in figure 19. Aside from the wear step and the score 

line previously described, there was no significant mechanical damage or corrosion present at the 

origins. 

16.19” 

diameter 

surface 

Figure 17. SEM fractograph of the crack at hole 5. 

The crack at hole 30 was also inspected with electron microscopy. A raised area (approximately 

0.002 in high) was fretted and a crack initiated along one side of this fretting site. The raised 

area was very similar to other raised thumbnails previously discussed. The crack observed at this 

location was very similar to the cracks investigated by Boeing on the other rotating rings (raised 

thumbnails with associated cracks were common on BCW-828). It appeared that mechanical 
13

Figure 18. SEM micrograph depicting cyclic crack propagation at hole 5. 

Figure 19. EDS spectrum from the black product on the fracture surface at hole 5.
14

damage on the 16.19-in-diameter surface caused these raised thumbnails. The mechanical 

damage was likely from a blunt hard instrument as the shot peening impressions were not 

completely marred by the damage. It was apparent that the raised areas were preventing normal 

load distribution along the bearing retention surface, as there was little disturbance of the 

surrounding shot peening impressions signifying reduced or no contact with the mating bearing. 

As previously mentioned, no attempt was made to open this crack due to its small size and 

similarity with other cracks associated with raised fretted sites examined on other rings. No 

other significant wear or corrosion (other than that previously described) was observed in the 

area. The SEM examination was limited to the cracked areas on the bearing retention shoulder. 

5. Metallography 

A radial cross section was prepared through the bearing retention shoulder. The section was 

ground and polished through 0.025-μm noncolloidal silica. Subsequently, the specimen was 

etched with Keller’s reagent. The microstructure of the rotating ring material was found to be 

consistent with 2014-T6 aluminum. The depth of the wear step was believed to be at its 

maximum in this cross section and measured approximately 0.001 in. The onset of strain 

hardening of the material below the wear can be observed as a light etching effect. The 

microstructure beneath the wear step can be observed in figure 20. The typical microstructure of 

the base material is depicted in figure 21. No other abnormalities were observed. A radial cross 

section was also evaluated for the grain flow induced by forging. The section was polished and 

macro-etched with NaOH. Forging flow lines were observed generally following the contours of 

the component, confirming that the component was forged as required by the engineering 

drawing. The grain flow pattern is shown in figure 22. 

6. Chemical Analysis 

The chemical composition of BCW-1379 was analyzed with direct current plasma emission 

spectroscopy (DC plasma). The sample was sectioned from an area adjacent to hole 5 where the 

circumferential crack was located. The results are listed in table 1 along with the chemical 

composition requirements for AMS-QQ-A-367 as specified by the engineering drawing. The 

chemical constituency of the component compared favorably with the requirements. 
15

Figure 20. Microstructure near the wear step. 

Figure 21. Typical microstructure of aluminum 2014-T6 base material. 
16

Figure 22. Macro-etch of a radial section depicting forging flow lines. 

Table 1. Chemical composition of swashplate rotating rings by weight-percent. 

Sample ID BCW-1379 

(%) 

AMS-QQ-A-367 

(%) 

Iron 0.43 0.70 max 

Manganese 0.70 0.40–1.20 

Silicon 0.77 0.50–1.20 

Copper 4.61 3.90–5.00 

Zinc 0.17 0.25 max 

Magnesium 0.48 0.20–0.80 

Chromium 0.015 0.10 max 

Titanium 0.028 0.15 max 

7. Hardness and Conductivity Testing 

Hardness and conductivity measurements were made near the cracked areas within the core of 

the rotating ring. The hardness of the component measured 82.3 Hardness Rockwell B-Scale 

(HRB) and the conductivity measured 38.5% International Annealed Copper Standard (IACS). 
17

These values compared favorably with a 2014-T6 aluminum forging as specified by Boeing 

Aircraft Company (BAC) 5946 (78.5–89.5 HRB and 35%–40% IACS). 

8. Discussion 

There appear to be four types/causes of the cracking associated with the rotating rings: 

1. Cracking associated with raised fretted areas from damage on the 16.19-in-diameter surface 

(numerous examples). 

2. Cracking associated with pure fretting (hole 21 on S/N W1718240-1). 

3. Cracking associated with extreme loads forming a network of cracks (hole 16 on S/N 

W1718240-1). 

4. Cracking associated with uneven load distribution caused by the sharp chamfer of the outer 

bearing race (e.g., the circumferential crack on S/N BCW-1379). 

The first seems easily rectified, and none have been observed to propagate radially. Nos. 2 and 3 

appear to cause primarily radial cracks, while no. 4 appears to cause circumferential cracks. 

The fractographic evidence, and the intuitively dominant hoop stresses on the component in 

service, suggest that circumferential cracks are slowly propagating and older than radial cracks 

(circumferential cracks are also more difficult to detect nondestructively). The number of 

circumferential cracks observed to date indicates they are a rare occurrence. The fracture 

surfaces of the horseshoe shaped cracks developing around raised areas caused from damage on 

the 16.19-in-diameter surface appear fresh and recent. These raised areas are most likely caused 

by damage incurred on the 16.19-in diameter during overhaul. The source of the damage to the 

16.19-in diameter should easily be determined and rectified. To date, the horseshoe shaped 

cracks emanating around the raised areas have not resulted in spalling at the 16.19-in diameter. 

Cracks of this type have not been observed to turn radially and propagate into the part. 

The rotating rings appear to have a wear pattern and crack concentration with 120° symmetry, 

with the areas between the pitch link locations possessing the greatest concentration of cracks 

and the most severe wear. This suggests that the assembly is deflecting considerably during 

service. This deflection is most likely not accounted for in the design and stress profile 

calculations of the rotating ring. In short, the assembly appears to be both overloaded and/or 

possess insufficient stiffness.
18

9. Recommendations 

Elimination of the procedure and/or tool causing the damage on the 16.19-in-diameter surface 

seems prudent. This is most likely occurring at overhaul and is the most prominent cause of 

localized high stresses on the bearing retention shoulder of the rotating rings. 

Although it is likely not the most common cause of local high stresses, it would seem that the 

surface finish and chamfer on the mating bearing should be scrutinized to determine if they 

detrimentally affect the rotating ring under contact and service loading. 

Finally, the high degree of fretting observed in areas free of the damage discussed in the previous 

paragraphs suggests that the service loading of rotating rings may be excessive. The alleviation 

of high service loads or the use of a material with a greater endurance limit would likely increase 

the service life of the rotating ring. The use of a material with a greater modulus of elasticity 

might also be beneficial, but the resulting higher alternating stresses would have to be less than 

the endurance limit of the alternate material. 

10. Conclusions 

The subject swashplate rotating ring exhibited two cracks. Both cracks were located within the 

wear pattern observed on the bearing retention shoulder surface. One crack was located near 

hole 5 and oriented in the circumferential direction. The other crack was observed along the 

edge of a raised thumbnail shaped area near hole 30. 

The circumferential crack was located approximately 0.015 in from the 16.19-in ID of the 

rotating ring and measured 0.35 in long × 0.090 in deep. The crack initiated by fatigue and high 

stress along a circumferential score line thought to be induced by the outer race of the mating 

swashplate bearing and subsequent contact (service and clamp-up) loads. The propagation in a 

nonradial direction, coupled with the oxidized fracture surface, suggested that this crack took 

longer to form than other cracks observed to date, on other rotating rings. 

The crack along the edge of the raised thumbnail area initiated by fretting fatigue and localized 

high stresses. The localized high stresses were due to the reduced contact area the 

approximately 0.002-in high thumbnail area created. The raised area appeared to be a direct 

result of impact damage on the 16.19-in ID of the rotating ring from an unknown source, 

possibly at overhaul. The impact damage on the ID pushed material into the rotating ring, 

subsequently causing a bulge on the mating surface shoulder between the ring and the bearing. 
19

Chemical, physical, and mechanical testing confirmed the rotating ring was forged from 

aluminum alloy 2014-T6 in accordance with drawing material requirements. The shot peening 

appeared uniform and consistent in all areas that were unaffected by wear. 

No microstructural abnormalities were observed. The microstructure was consistent with an 

aluminum 2014-T6 alloy. Forging lines were observed generally following the contours of the 

component, confirming that the component was forged as required by the engineering drawing. 
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