Engineering Failure Analysis 14 (2007) 209–217

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Failure analysis of stress corrosion cracking in aircraft bolts

Hong-Chul Lee a, Jae-man Choi a, Bokwon Lee a, Tae-Gu Kim b,*

a Engine Division, ATRI (Aero Technology Research Institute), RoKAF, P.O. Box 304-160, Gumsadong, Donggu,

Daegu 701-799, Republic of Koreab Department of Occupational Safety and Health, College of Biomedical Science and Engineering, Inje University, Gimhae, Gyeongnam

621-749, Republic of Korea

Received 5 September 2005; accepted 24 October 2005Available online 3 March 2006

Abstract

This research was conducted on the failure analysis of the failed clamp bolt from a helicopter engine in the RoKAF.Through the fractography, metallography, and stress analysis of the failed part, it was found that the clamp bolt was frac-tured by stress corrosion cracking due to the interaction of tensile residual stress and corrosive environment. The stresscorrosion crack is a phenomenon that occurs in susceptible alloys and is caused by the conjoint action of a surface tensilestress and the presence of a specific corrosive environment. Therefore, it is recommended that the material of the clampbolt should be changed to prevent similar failures.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Clamp bolt; Stress corrosion cracking; Residual stress

1. Introduction

This paper analyzes the causes of the failure of stainless steel bolts that have been installed in certain kindsof the helicopters operated by the RoKAF. When the engine of one of these helicopters was started using theauxiliary power unit (APU) for a training mission, an abnormal noise was detected and the mission wasaborted. The engine was inspected to identify the cause of the abnormal noise and it was discovered that aclamp bolt connecting the bleed air valve, had failed. Furthermore, a bolt with a similar crack was also foundin the same location on another aircraft during a inspection. The failed clamp bolt was collected and the fail-ure mechanism was confirmed through chemical composition analysis and observation of the fracture surfaceusing a scanning electron microscope. The direct cause of cracking was revealed by analyzing the metallurgicalstructure, crack path and measuring the residual stress acting on the bolt.

1350-6307/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2005.10.021

* Corresponding author. Tel.: +82 55 320 3539; fax: +82 55 325 2471.E-mail address: tgkim@inje.ac.kr (T.-G. Kim).

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2. Analysis

2.1. Bleed air system assembly

Clamp bolts are used to connect the ducts and valves that transfer the bleed air generated from the APU tothe engine start motor as shown in Fig. 1. There are a total of 13 clamps installed in a helicopter engine. Thebleed air generated from the APU maintains an internal pressure of approximately 40–50 psig and a temper-ature ranging from 149 to 249 �C. Moreover, in order to fasten the clamp firmly, the clamp bolt is tightenedwith a torque wrench to between 35–45 in -lbs. During the engine start, the clamp was loosened by the boltfailure shown in Fig. 2, which led to leakage of the bleed air and the abnormal noise.

2.2. Chemical and mechanical properties of the clamp bolt

The result of the chemical components analysis (ICP, Table 1) and the hardness test (Table 2) shows thatthe material of the failed bolts was a stainless steel 431. STS 431, martensitic stainless steel gets a variety of itsmechanical properties through the heat treatment to form when martensite, is formed. This is similar to theway plain carbon steel gains many of its mechanical properties. As a result of these properties, it is generallyused in parts requiring high strength, such as airplane fittings, pump shafts and bolts [1].

Fig. 1. Photograph of engine bleed air system.

Fig. 2. Photograph showing the failed clamp bolt.

Table 1Chemical analysis of failed clamp bolt

Part Composition (wt%)

C Si Mn P S Ni Cr Mo Cu Fe

STS 431 0.20 max 1.00 max 1.00 max 0.040 max 0.030 max 1.25–2.50 15.00–17.00 Remain

Clamp bolt 0.17 0.59 0.73 0.020 0.001 1.48 15.65 0.02 0.17 Remain

Table 2Mechanical properties of failed clamp bolt

Material Tensile strength (ksi) Yield strength (ksi) Elongation (%) Hardness (HRC)

STS 431 150 130 18 42

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2.3. Macro examination

As shown in Figs. 3 and 4, a macro examination of the failed clamp bolt clearly indicated a brittle fracturein which there was no plastic deformation. Corrosion products were observed on the fracture surface inFig. 4A, which were considered to be formed as the corrosion progressed from the surface to the inside ofthe bolt. This corrosion covered up to 70% of the fracture surface. The crack initiated in the area of bendingand reaming which was designed to allow the bolt to be fitted easily. Three additional micro cracks weredetected adjacent to the fracture surface of the bolt as shown in Fig. 5. The residual stress may have actedon the bent area of the clamp bolt near the machining. When the bolt and nut were fastened normally, thebent area can be a stress raiser due to its unbalanced shape.

2.4. Fractographic evaluation

The fracture surface of the failed clamp bolt was observed using a scanning electron microscope and theresult indicated several different fracture modes as the crack propagated from origin to the final rupturedregion (Fig. 6):

� Crack origin area: cleavage or quasi-cleavage.� Transitional area: intergranular.� Final ruptured area: dimple rupture.

Fig. 3. Photograph showing the fracture surface of bolt.

Fig. 4. Macro-examination of failed bolt.

Fig. 5. SEM photograph showing three micro cracks resident in bolt surface.

Fig. 6. SEM micrograph showing the various crack mechanisms.

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The morphology of the fracture surface with respect to the crack propagation is shown in Figs. 7–9. EDSanalysis detected corrosive materials such as chlorine and sulfur in the fracture surface in the same areas wherethe corrosion products had been observed macroscopically as shown in Fig. 10. These results confirm that theclamp bolt failed catastrophically after a corrosion assisted intergranular crack propagated to a critical crack

Fig. 7. SEM micrograph showing the cleavage or qui-cleavage fracture in Fig. 6.

Fig. 8. SEM micrograph showing the intergranular fracture.

Fig. 9. SEM micrograph showing the dimple rupture.

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Fig. 10. EDS spectra of the corroded area.

Fig. 11. Optical micrograph showing the intergranular crack propagation.

Fig. 12. Optical micrograph showing the branch crack (L/H) along the grain boundary.

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length. Similar cracks were observed in the bolts of other aircrafts, during a special inspection, and showed thesimilar intergranular crack pattern as shown in Fig. 8, and the discoloring was observed inside the fracturesurface.

2.5. Metallographic examination

To identify the primary cause of the cracks, the fracture surface was sectioned, polished and etched(Vilella) and the crack growth path was examined. The crack propagated along the grain boundary asshown in Fig. 11, which shows the side view of the fracture surface in Fig. 12A shows the branch cracks,which are connected with the fracture surface. The cracks that propagated from the fracture surface to theinside of the material were also observed, as shown in Fig. 12B and C. Therefore, it was confirmed thatthe intergranular crack propagated towards the subsurface of the bolt fracture along the grainboundary.

3. Discussion

3.1. Stress analysis

The residual stress of the cracked area of the clamp bolt resulted from machining in the form of acurve. This was done to provide for easier assembly. When the nut is fastened with a torque of 35–45in -lbs, the maximum tension of 123 N and the bending moment (M) corresponding to it are inducednear the cracked area [2]. Therefore, the stress acting on the cracked area of the clamp bolt is higherthan that of the opposite surface. The stress level acting on the cracked area was analyzed using CATIAV5, the finite element analysis program, because it is too difficult to calculate the stress by applying thetheoretical equations. Fig. 13 shows the boundary condition where the uniaxial load of 123 N is appliedto the area of the nut fastening, while the opposite side is fixed at the axial direction but is freed fromthe rolling direction. There was relatively high stress acting on the lower part of the bolt where the bend-ing moment was applied and a maximum tensile stress of 51 MPa acted on the same position as thecracked area. Therefore, the stress in the cracked area was increased by two due to the stress concentra-tion resulting from configuration, yet it was only 6% of its material yield strength (see Figs. 14and 15).

3.2. Material and environmental conditions

The material of the clamp bolt, stainless steel 431, has considerable resistance against corrosion in theatmosphere. However, when it is exposed to relatively high temperatures (149–249 �C) as in the case of theclamp bolt, elements such as chlorine and sulfur can easily penetrate the material [3]. Moreover, it has beenreported that high temperature prevents the activity of corrosion inhibitors and accelerates the corrosion rateconspicuously [4]. For example, the J79 engine V-band clamp bolts, which are of the same material as thefailed clamp bolt, have failed continuously due to stress corrosion crack. A technical order has been publishedto say that the STS 431 V-band clamp bolt should be urgently replaced with the STS A286 clamp bolt that hasstrong corrosion resistance properties within a short time [5].

Fig. 13. Boundary conditions for FE analysis of bolt.

Fig. 14. Stress distributions of the bolt (left: upper surface, right: lower surface).

Fig. 15. FE analysis result showing the maximum stress.

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The stress corrosion crack as shown in Fig. 16 is a phenomenon that occurs in susceptible alloys and iscaused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment.It is difficult to detect the stress corrosion crack before reaching the critical crack length and it can be initiatedby only residual stress [6]. Therefore, it is recommended that the material of the clamp bolt should be changedinto a more corrosion resistance material rather than inspection methods and procedures should be improvedto prevent similar failures.

Fig. 16. Schematic diagram showing the factors concerning the SCC.

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4. Conclusions

After performing fractography, metallographic examination, and stress analysis on the cracked clamp bolt,installed in certain kinds of the helicopters operated by the Air Force, the following conclusions can besummarized:

(1) The fracture surface shows the morphologies of intergranular fracture but has no evidence of fatigue.Corrosion products had spread up to 70% of the fracture surface area.

(2) Corrosive materials such as chlorine and sulfur were detected in the fracture surface through the EDXanalysis and metallographic examination confirmed that the intergranular crack propagated towards thesubsurface of the bolt fracture along the grain boundary.

(3) The tensile stress induced by the torque during fastening and the residual stress formed by machiningacted on the fractured area.

(4) The clamp bolt failed due to the stress corrosion crack caused by the conjoint action of surface tensilestress and the presence of a specific corrosive environment. Therefore, it is recommended that the mate-rial of the clamp bolt should be changed into more corrosion resistant material to prevent similarfailures.

References

[1] Smith WF. Structure and properties of engineering alloys. McGraw-Hill; 1993. p. 303–312.[2] Juvinall RC, Marshek KM. Fundamentals of machine component design. Wiley; 1991. p. 365.[3] Jones RH. Stress-corrosion cracking; materials performance and evaluation. ASM; 1999. p. 121.[4] ASM metals handbook, vol. 13. ASM International; 1992. p. 489.[5] TCTO 1F-4-1616. Inspection of V-band coupling; 1997.[6] Davis JR. Corrosion: understanding the basics. ASM; 2003. p. 256.