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S C I E N C E C(~DIRECT° Engineering Failure Analysis 11 (2004) 305 312

ENGINEERING FAILURE ANALYSIS

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Failure of aircraft propeller assembly

Hong-Chul Lee a, Young-Ha Hwang a, Tae-Gu Kim b'* ~Engine Division, A TRI(Aero-Tech Research Institute), ROKAF, PO Box 304-160, Kumsa dong, Dong gu, Deagu 701-799, South Korea

bDeparmwnt of Safety Engineering, INJE University, Gimhae, Gyeongnam, 621-749, South Korea

Received 8 August 2003; accepted 12 August 2003

Abstract

This paper analyses the causes of the incident of a Cessna trainer whose propeller was separated due to the cracking of the propeller blade hub during the take off roll. Beach marks and fatigue striations, typical of fatigue cracks, were observed on the fracture surface and corrosive oxides were detected in the center of beach marks that are considered to be the crack origin. The stress acting on the fracture surface under a corrosive environment forms corrosive oxides, such as mud cracks. By analyzing the fractography and metallography of the failed parts, it is found that the propeller blade hub nucleated stress corrosion cracking (SCC) as a result of residual stress and corrosive environment and the SCC was the cause of the fatigue crack. Moreover, a fatigue crack reaches its critical length by repeated cyclic stress, which occurs during the rotation of the propeller blade and then, the rest of the fracture occurred instantaneously. © 2003 Elsevier Ltd. All rights reserved.

Keywords: Stress corrosion cracking; Fatigue; Aircraft failures; Corrosion; Fatigue markings

1. Introduction

This study describes the analysis and investigation of the causes of an accident of a Cessna trainer whose propeller separated during the take off roll. Dur ing take off at full power, the trainer forcibly stopped within 900 feet due to the separation o f the propeller assembly at 400 feet. The separated propeller was severely deformed because o f the impact on the runway. The failed propeller hub and the engine crankshaf t to which the propeller assembly was connected were observed.

For the failure analysis o f the cracked parts, R O K A F first inspected chemical components of the pro- peller blade hub materials and examined the mechanism of the cracking by observing the fracture surface with stereoscope and SEM. Also the direct cause o f the cracking was investigated by analyzing the micro- structure and crack path in the fracture surface.

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

1350-6307/$ - see front matter © 2003 Elsevier Ltd. All rights reserved. doi: 10.1016/j.engfailanal.2003.08.002

306 H.-C. Lee et al./ Engineering Failure Analysis 11 (2004) 305~12

2. Propeller assembly

The Cessna trainer has a single engine and propeller attached at the front of the airframe. The propeller assembly, consisting of two blades and a hub, was attached to the engine crankshaft. Also, the blade pitch is regulated automatically by a counter weight balanced hydraulic power and spring system applying force according to the engine power for a constant propeller rotation speed. As shown in Fig. 1, the two blades broken away were severely deformed and impact damage in the direction perpendicular to the blade span was observed in the curled blade tip. The fracture surface of the failed crankshaft shows a slant fracture; a shear fracture surface forms at an angle of 45 °, which occurred by abnormal torsion force [1]. As shown in Fig. 2, by visual examination of the failed parts, including the propeller hub, blade, and crankshaft, it was found that one blade separated as a result of a blade hub crack, and then the crankshaft fractured catastrophically because of the abnormal torsion force due to the imbalance of the propeller assembly.

Fig. 1. Failed propeller hub and blade.

Fig. 2. Visual examination; twisted blade (left), shear fractured crankshaft (right).

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3. Cause of the accident

307

3.1. Material properties o f propeller blade hub

The result of the chemical components analysis (ICP, Table 1) and the hardness test (Table 2) shows that the cracked propeller hub was a die forging aluminum alloy (A1 2014-T6). A1 2014-T6 is widely used in structural materials for aircraft components owing to the high strength. Because it has poor resistance to SCC, its fracture toughness against SCC is very low along the ST grain direction [2]. To make up for this, an anodizing coating is applied to the surface of the propeller hub to prevent corrosion.

3.2. Fractographic analysis

After observing the crack surface with the naked eye and with a stereoscopic microscope, there were beach marks broadly spread over the fracture surface. The crack nucleated in third thread area inside the hub where the blade is installed and then the crack propagated outward as shown in Figs. 3 and 4. The center of the beach marks considered to be the origin appears dark, which indicates corrosion. All the fracture surface beach marks show fine striations typical of a fatigue crack as shown in Fig. 5. The fatigue crack propagated to a length of 85 mm. On the other hand, the fracture surface elsewhere (Fig. 6) shows dimples from an overload rupture. Also the mud cracks observed in the initial crack area, as shown in Fig. 7, are formed by the action of the tensile stress under the corrosive environment, which is generally found in a stress corrosion crack and corrosion fatigue [3]. Results of surface component analysis by using EDS showed S, CI, O, which are not contained in the original material (Fig. 8). Such components are often seen in corrosion on components of aluminum alloy and are thought to provide the cause of a crack or to accelerate the crack growth rate [4]. The inside of the propeller hub is filled with engine oil to operate the blade pitch. Compressive residual stress exists in the area around the failed thread where the blade is installed with a torque of 55-60 ft-lb as shown in Fig. 9. Moreover, whenever the propeller is rotated, additional cyclic stress is imposed on the thread. So the surface of thread contact area in which the wear occurred due to the residual stress combining with the cyclic stress was damaged and then the anodizing coating layer was removed as shown in Fig. 10. After considering all the results from the fracture surface analysis of the crack surface, the fatigue crack was nucleated in the third thread area inside the propeller hub where the stress is concentrated by the propeller rotation and blade installation with the rest being cracks that fractured instantaneously.

Table 1 Chemical analysis of failed propeller blade hub

Part Composition (wt. %) Designation

Si Cu Mn Mg Cr Zn Fe A1

Failed cylinder 0.71 4.22 0.67 0.27 0.02 0.14 0.49 Remain A1 2014

Table 2 Mechanical properties of failed blade hub

Material Tensile strength Yield strength Elongation Hardness (HB) Remarks

A1 2014-T6 70 ksi 60 ksi 13% 140 Die forging

308 H.-C. Lee et al./ Engineering Failure Analysis' 11 (2004) 305-312

Fig. 3. Photograph showing the propeller hub which failed by fatigue.

Fig. 4. Visual examination of arrow A showing the fatigue crack which initiated in the 3rd thread area.

3.3. Metallographic analysis

For the examination of the direct cause of the fatigue crack, the initial crack site which is thought to be the origin, was cut perpendicular and parallel to the fracture surface and abraded using a hard polishing pad. As shown in Fig. 11, the crack path and the degree of damage to the .surface by corrosion was examined by etching. The surface of the mud cracks has branching cracks along the crack growth. On the other hand, there was no evidence of corrosion on the fatigue fracture surface. It is said that the surface of the failed thread inside the hub was not immune to corrosion because the anodizing coating layer was removed by contact wear.

H.-C. Lee et al. / Engineering Failure Analysis 11 (2004) 305~12 309

Fig. 5. SEM micrographs: fatigue striations.

Fig. 6. SEM micrographs: dimpled ruptured surface.

4. Discussion

4.1, Cause of accident. corrosion

Fatigue striations, typical of a fatigue crack, were observed on the fracture surface. A corrosive oxide such as mud cracks was detected in the initial stage of the cracking and is considered to be the cause of cracking. The inside of the hub needs to be processed with an anodizing coating, but in this case, when the wear occurred on the surface of the thread, corrosion was easy due to the removal of the coating layer. High-strength aluminum alloy has good material properties but is susceptible to stress corrosion cracking and corrosion fatigue because the corrosion resistance is poor. After the anodizing coating inside the hub was removed by wear due to the thread contact, stress corrosion cracking occurred at the third thread where the stress is concentrated from the propeller rotation and the blade installation. Then the fatigue crack nucleated by the stress corrosion cracking propagated to 85 mm with the rest of the cracks being fractured instantaneously.

3 lO H.-C. Lee et aI./Engineering Failure Analysis 11 (2004) 305-312

Fig. 7. SEM micrograph showing the mud cracks.

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Fig. 8. SEM micrograph showing the EDS spectrum in the crack origin which revealed the mud cracks as corrosion products.

4.2. Stress corrosion cracking

SCC is a cracking phenomenon that occurs in susceptible alloys and is caused by the joint action of a surface tensile stress and the presence of a specific corrosive environment [5]. For SCC to occur on an aircraft structure, three conditions must be met simultaneously. A specific crack-promoting environment must be present, the metallurgy of the material must be susceptible to SCC, and the tensile stress must be above some threshold value. A common method to control SCC in aircraft structural materials is to select

H.-C. Lee et al./' Engineering Failure Analysis 11 (2004) 305 312

blade retention nut

Centrifugal force 55~.60 lb-ft + induced by blade rotation

R e p u l s i v e / ~ ~ ~ /

US I Propeller blade hub

-~Fracture i /

Fig. 9. Schematic drawing describing the stress state between blade hub and nut.

311

Fig. 10. Micrograph showing the wear damage in the contact thread.

a material with greater SCC resistance to a particular environment. Often, as the strength of an alloy increases, its susceptibility to SCC increases. Use of alloys with lower strength levels can thus be effective means of reducing the likelihood of SCC if the design/application permits.

In this case, it was not easy to find the stress corrosion crack in the failed surface or fracture surface in advance when the evidence of corrosion did not appear macroscopically. To overcome this problem, the aircraft manufacturer recommends evaluating the thread by nondestructive mspection at periodic intervals. However it is hard to detect the flaw in the thread.

Furthermore, to prevent possible stress corrosion cracking, selection of the material and its continued maintenance and management are highly critical.

312 H.-C. Lee e t al./ Engineering Failure Analysis 11 (2004) 305 312

Fig. 11. Optical micrographs showing the propagation of corrosion from the surface to the inside of material; perpendicular to the fracture surface (left), parallel to crack growth (right).

5. Conclusions

After performing the cause analysis and experimental study on the crack of the propeller blade hub, which led to the separation of the propeller assembly, conclusions can be summarized as follows:

(1) After the anodizing coating inside the hub was removed by wear f rom thread contact , stress cor- rosion cracking occurred in the third thread where the stress is concentrated by the propeller rotat ion and by the blade installation.

(2) A fatigue crack nucleated by the stress corrosion cracking propagated to 85 m m by the centrifugal force induced by the blade rotation.

(3) It is critical to unders tand the cause o f stress corrosion cracking, thus correct selection of the material and its continued maintenance and management is required.

References

[1] ASM metals handbook, vo1.11. ASM International; 1992. p. 20. [2] Lifka BW. Corrosion of aluminum and aluminum alloys, corrosion engineering handbook. Marcel Dekker, Inc.; 1996 p. 122. [3] ASM metals handbook, vo1.12. ASM International; 1992. p. 361. [4] Jones RH. Stress-corrosion cracking of aluminum alloys, stress-corrosion cracking; materials performance and evaluation. ASM

International; 1999 p. 243. [5] Davis JR. Corrosion; understanding the basics. ASM International; 2000 p. 164~173.