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Fatigue failure of a hydraulic filter head
Engineering Failure Analysis 9 (2002) 435–450 www.elsevier.com/locate/engfailanal
Fatigue failure of a hydraulic filter head Myounggu Park* Engine Division, ATRI(Aero-Tech Research Institute), ROKAF, PO Box 304-160, Kumsa dong, Dong gu, Deagu, 701-799, South Korea Received 15 May 2001; accepted 30 June 2001
Abstract An in-service failure of a hydraulic filter installed on a combat aircraft has been investigated. Cracking had occurred at the filter head and no sign of plastic deformation was found. Chemical examination and micro-hardness measurement revealed that the filter head material was Al 6061-T6. But the base material of the randomly chosen filters from the field was Al 2024-T4. So the wrong material selection was confirmed. Metallography showed microstructural anomalies which contributed to the early fracture of hydraulic filter head. A surface examination of the fracture region revealed striations, and it was confirmed that the fracture mode was fatigue. At the initiation site there were corrosion pits providing initiation sites for fatigue cracks. The corrosion pits and microstructural anomalies might be generated by applying the manufacturing processes designed for Al 2024 to the wrong material (Al 6061). It was verified by fatigue analysis that with pre-existing defects and/or flaws, the fracture should occur far earlier than the expected cyclic life. The right selection of the material can be the most important remedial action in this case. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aircraft failures; Material selection; Fatigue; Corrosion fatigue; Microstructures
1. Background During a flight, a combat aircraft aborted the mission and made an emergency landing due to illumination of the right hand hydraulic caution light and the sudden drop of hydraulic pressure to zero. On subsequent in-field investigation, it was reported that there was hydraulic leakage from a cracked region of the hydraulic filter head which was installed on the utility system in the aft fuselage section. Fig. 1 shows the installation of the hydraulic filter schematically. The hydraulic filter had been used for the filtration of hydraulic fluid in the utility system of the aircraft for about 4597 hours reportedly. The utility system together with the control system supplied hydraulic power (3000 200 psi) to the flight controls including ailerons, rudder and horizontal tail, etc. by pistontype engine driven pumps. Table 1 shows the leading particulars of the hydraulic filter [1].
* Tel.: +82-053-980-3931, 3932; fax: +82-053-819-1271. E-mail address: [email protected] (M. Park). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(01)00029-2
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Fig. 2 shows the as-received hydraulic filter. The filter head was separated completely due to cracking (see arrows). Fig. 3 shows the fracture region. Plastic deformation such as irregularity of diameter was not found in the fracture region.
2. Chemical analysis The chemical composition of the hydraulic filter head was found by ICP (inductively coupled plasma spectrometer: Model JY 38 PLUS) analysis. The analysis revealed that the filter head material is an Al alloy 6061 [2]. The results are given in Table 2 along with the specified chemical composition. The actual
Fig. 1. The installation of the hydraulic filter.
Table 1 Leading particulars of hydraulic filter Service fluid
Hydraulic fluid (military specification MIL-H-83282)
Capacity
3.5 gpm at 32.2 to 43.3 C with maximum differential pressure of 30.0 psig; 2.0 gpm (after 5 min stabilized flow) at a stabilized temperature of 28.9 C with a maximum differential pressure of 95psig
Pressure ratings
Operating: 3000 psig Proof: 4500 psig Burst: 7500 psig
Filtration medium
Chemically treated paper
Filtration rating
10 mm per military specification MIL-F-5504
Relief valve operation
Cracking pressure: 15015 psig Rated flow pressure: 200 psig max 150% rated flow pressure : 240 psig max Reseat pressure: 100psig min
Temperature range
59.3 C to +135 C
Port connections
9/16/18 UNF/3B thread per AND10050-6
Weight
0.6 lb maximum
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Fig. 2. The as-received hydraulic filter. The scale is in cm.
Fig. 3. Normal view of fractured hydraulic filter head.
Table 2 Chemical composition of hydraulic filter head material Element
Specified percentage
Actual percentage
Silicon Iron Copper Manganese Magnesium Chromium Zinc Titanium Others Others Aluminum
0.40–0.80 0.70 Maximum 0.15–0.40 0.15 Maximum 0.8–1.2 0.04–0.35 0.25 Maximum 0.15 Maximum Each 0.05 Maximum Total 0.15 Maximum Balance
0.45 0.48 0.47 0.09 0.87 0.17 0.20 – – Balance
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chemical composition shows copper content on the high side. However, the slight difference in copper content is unlikely to influence the performance of the hydraulic filter to a significant extent. From the outer appearance and brass-like surface color of the hydraulic filter, it can be guessed that it was anodized as a surface treatment.
3. Mechanical properties Several micro-vickers hardness measurements were made on a polished and unetched surface of the sample (Fig. 4). The average value of the test results was 122.86. Micro-vickers hardness 122.86 is equivalent to HB 119. The HB value is within the range of the Al 6061-T6 [3], so it can be assumed that it was in the T6 condition. T6 means that it was solution-treated and artificially aged from O (annealed) condition.
4. Metallography Longitudinal and transverse sections were prepared from the cracked region of the filter head for optical microscopy. The specimens were polished and etched with Kellar’s etchant (HF:HCl:HNO3=2:3:5 ml+water 190 ml) to reveal the microstructures. To compare them with the fractured filter head, samples were taken from standard tensile specimens made from Al 6061-T6 and also polished and etched. Fig. 5 shows the microstructure of longitudinal sections. Compared with tensile specimens, the cracked hydraulic filter head has bigger and more elongated constituent particles. Also the constituent particles are not equally distributed. Fig. 6 shows the microstructure of cross sections. From Fig. 6(a) elongated grain (preferred orientation) and bigger constituent particles than those of the tensile specimen were found. Because of preferred orientation, non-homogeneous distribution and the bigger size of constituent particles, the fatigue strength might be lowered. It is well known that when there is preferred orientation, the ST-oriented specimen has the lowest fatigue crack initiation resistance among the three directions—LT, SL, ST. Consequently, the microstructure of the two samples—fractured and standard—was different, which indicated that a microstructural anomaly was one of the likely contributors to the early fracture of the hydraulic filter head.
Fig. 4. The micro-vickers hardness measurement sites.
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5. Surface examination The fractured region was carefully observed both visually and with the aid of the scanning electron microscope SEM. Fig. 7 shows the overall view of the fracture surface. Visually there are two distinctive regions on the fracture surface. The flat region (A and B) and the 45 slant region. Except for the flat region, the remaining part is 45 slant region. Typical SEM photographs of the fracture surface of these two regions are shown in Figs. 8–10. At the flat region, typical fatigue striation markings are observed. On the other hand, the 45 slant region shows dimples which indicates that overload fast fracture had occurred. From the surface examination, the cracking procedures/paths can be determined. The crack initiated from the inner surface propagating along the surface of the wall and moved forward though the wall at the same time. Fig. 11 shows the cracking procedures/paths schematically. From experience and the configuration
Fig. 5. The micrographs of longitudinal sections.
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of the small fatigue fracture region including the origin 2 (O2), it is suggested that the crack inclined to propagate along the surface of the wall rather than through the wall in the case of the tube-like part. This feature can be related to the fatigue striation pattern consisting of many parallel lines from the crack starting region unlike the typical fatigue striation pattern of a solid body converging to an origin. Observation of the initiation region of the fatigue crack revealed a corrosion pit (Figs. 12 and 13). From the fractography it is clear that crack initiation can be related to the corrosion pits. Also the crack followed a linear groove (tool mark) made by the cutting tool (Fig. 14) which is at the thinnest region of the cracked filter head. The corrosion pits and the tool mark underneath might act as stress raisers, and it is considered that as the crack propagates along the surface, corrosion pits were connected to accelerate the rate of crack growth (Fig. 15).
Fig. 6. The micrograph of a transverse section.
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Fig. 7. Overall view of fracture surface of hydraulic filter head.
Fig. 8. SEM photograph of the flat region near the inside showing the fracture occurred by fatigue (500).
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Fig. 9. SEM photograph of the flat region near the outside (end of fracture surface) showing fracture occurred by fatigue (1500).
Fig. 10. SEM photograph of 45 slant region showing fracture occurred by overload (300).
Fig. 11. Cracking procedure/path.
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Fig. 12. SEM photograph showing corrosion pit at the origin of the fatigue crack (700).
Fig. 13. Enlarged view of corrosion pits (1200).
Fig. 14. Stereoscopic micrograph showing tool marks.
Fig. 15. Surface crack propogation connection of corrosion pits.
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6. Stress calculation Fig. 16 shows the cross-section of the hydraulic filter schematically. The rated pressure of the hydraulic filter is 3000 psi ( 200 psi). Since the filter head is a pressure vessel, the longitudinal stress ( 1) is calculated as follows [4] where r=radius of the head and t=thickness of the wall: 1 ¼
pr 3000 psi 25:13 mm ¼ ¼ 14442 psi614:4 ksi 2t 2 2:61 mm
ð1Þ
It is not easy to obtain an accurate figure for the number of loading cycles experienced by the hydraulic pressure. The following estimate is based on a reasonable guess. The hydraulic filter has been used for about 4597 h. Here 1 h equals approximately one flight. So the filter head cracked after 4597 flights— 4.6103 cycles. During the flight, the hydraulic filter was at the pressure of 3000 psi continuously and dropped to 0 psi after landing. Only the longitudinal stress was acted on the cracked region of the filter head (=14.4 ksi). Therefore the load cycle is assumed as following (Fig. 17). The fatigue cycle is therefore 7.2 ksi about a mean tensile stress of 7.2 ksi.
Fig. 16. Cross-section of hydraulic filter.
Fig. 17. Load spectrum of hydraulic filter head at the cracking region.
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The service temperature range of the hydraulic fluid lies between 65 F and 160 F (53 71 C). Also the cracked region is not directly affected by hydraulic flow. Because it was not used in a high temperature environment it can be assumed that there is no holding effect on the cracked region.
7. Fatigue analysis The polished surface of the cracked region of the hydraulic filter head was observed (Fig. 18). The thickness (t1, t2) and the radius of the corner (r1, r2) were measured by image-pro plusR and the result was shown in Table 3. At the cracked region, the stress concentration factor (kt) is 2.2 (Fig. 19) [5]. From the S– N curve (Fig. 20) [6], failure will be expected after about 108 cycles. In aircraft structures fatigue critical locations FCLs are decided and non-destructive inspection NDI is done periodically. But for the hydraulic filter head, there is no mention of NDI and critical crack length in the technical manual. In order to evaluate the fatigue life of the hydraulic filter head in the case where an initial crack exists, a computer simulation was done by crack 98 software. It is assumed that fatigue failure was caused by initial cracks/flaws to a critical length without detection. The initial flaw sizes are determined by USAF (United States Air Force) specifications for damage tolerance requirements (MIL-A83444) [7]. The parameters of the crack growth analysis are shown in Table 4. The results of the crack growth analysis are shown in Figs. 21 and 22. The number of cycles at which crack depth exceeds the thickness of the filter head is 3104 cycles (Fig. 21). The number of cycles at which
Fig. 18. The cross-section of the cracked region.
Table 3 The dimension of the cracked region Thickness
t1 t2 t3
2.61 mm 5.22 mm 3.06 mm
Radius
r1 r2
0.24 mm 1.38 mm
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Fig. 19. Stress concentration factor.
Fig. 20. Axial notched fatigue life of 6061-T6 aluminium (load ratio R=o).
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instability of crack growth occurred is 4.4104 cycles (Fig. 22). So if initial cracks and/or flaws pre-existed for other reasons, the fatigue life of the filter head will drop dramatically.
8. Discussion The base material of three randomly chosen filter heads was chemically analysed using chips from the surface. The results showed that the material was Al 2024. It is different from the result of the cracked hydraulic filter head—Al 6061. Table 5 shows the typical mechanical properties of Al 2024-T4 and Al 6061-T6 [8]. Although not as strong as 2024, 6061 alloy has better formability and weldability, and corrosion resistance than 2024. Due to its unique features, it is not easy to directly compare these two materials only by the strength of materials. Though the material was not 2024, 6061 also has enough reason for usage as is shown already by the stress and fatigue analysis in this paper. Most of all, 6061 has excellent
Fig. 21. The result of crack growth–crack depth vs flying hours.
Table 4 The parameters of crack growth analysis Geometric properties
Surface crack in hollow cylinder Loading type=tension Thickness=2.61 mm Head diameter=50.26 mm Initial crack length=0.100 inch Initial crack depth=0.050 inch
Load interaction
None
Spectrum
Constant amplitude analysis Maximum stress=14.400 ksi Minimum stress=0.000 ksi Cycles per flight hour=1.00
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SCC (stress corrosion cracking) resistance. So the change of material itself is not considered to be a reason for the early fracture of the hydraulic filter head. However, if all the filter head manufacturing processes are designed for Al 2024, the change of base material without changing the manufacturing process can significantly affect the properties of the filter head. As for the corrosion pits on the surface of the fractured hydraulic filter head (Fig. 23), these are distinguished from the pores in the anodic coating. Generally the surface of engineering aluminum alloys is anodized. Fig. 24 shows the porous film (anodic coating) growth on the Al substrate in the anodizing process [9]. The diameter of the pores is about 200–300 A˚. So pits (Fig. 25) with diameter 20–30 mm on the surface of the filter head are not pores but corrosion pits. It is believed that due to these corrosion pits the fatigue crack initiation life is shortened significantly. Also pit formation is closely related to the electrochemical reaction between the constituent particles and the matrix in electrolytes [10]. Therefore the surface treatment process might be involved in pit formation. But in reality, it is almost impossible for an end-user to discover which processes and/or parameters were wrong.
Fig. 22. The result of crack growth–crack length vs flying hours.
Table 5 The mechanical properties of Al 2024-T4 and Al 6061-T6
Al 2024-T4 Al 6061-T6
Tensile strength, psi
Tensile yield strength, psi
Elongation, %
Hardness, HB
Fatigue strength, psi
68 45
47 40
20 12
120 95
18 14
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Fig. 23. Corrosion pits on surface: —(a) cracked filter, (b) normal filter (25).
Fig. 24. Porous film growth by anodizing process.
Fig. 25. Corrosion pit near the fracture surface (500).
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9. Conclusions 9.1. Most probable cause The root cause of the hydraulic filter head failure can be attributed to wrong material selection. If all the manufacturing processes designed for Al 2024 were applied to the wrong material (Al 6061), the following consequences might be induced. . Improper surface treatment might generate the corrosion pits which acted as stress raisers, nucleated fatigue cracks and accelerated the crack growth rates. . Microstructural anomalies might increase the brittleness of the hydraulic filter head
9.2. Remedial action Right material choice is the most important and fundamental remedial action in this case.
Acknowledgements The author would like to acknowledge the assistance and help of Dr. Young-Ha Hwang, Hyoung-Dea Ju, and Kyoung-Sook Son of ATRI, Dr. Yoon-Sung Choi and all the staff members in Engine Engineering Center of the Samsung Techwin, and Philip Le Page of Rolls-Royce International Ltd at various stages in this investigation.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
T.O. 9H3-3-58-3, ‘Pressure fluid filter’, 1 July, 1981. AMS Handbook Vol. 2, ASM International, 1990, p.102. Heat Treater’s Guide Practices and Procedures for Nonferrous Alloys, ASM International, 1996, p.143. Gere JM, Timoshenko SP. Mechanics of Materials. 4th ed. PWS publishing company, 1997 p. 556-557. Hertzberg RW. Deformation and fracture of Mechanics of Engineering Materials. Fourth edition. John Wiley & Sons, Inc., 1996. Structural Alloys Handbook, CINDAS/Purdue University, 1997, p. 78 (Wrought Aluminum). T-37 FMS Durability and damage tolerance update final data report—T-37C Republic of Korea Air Force, Southwest Research Institute, 1996, p.3–2. Smith WF. Structure and properties of engineering alloys, 2nd ed. Mcgraw-Hill, Inc., 1993, p. 207. Aluminum Surface vol 36, Korea Aluminum Surface Treatment Association, 1995, pp. 12–18. Pao P, Feng C, Gill S. 1999. Corrosion-fatigue crack initiation in 7000-series Al Alloys, Proc. of The Second NASA/FAA/DoD Conference on Aging Aircraft, NASA, Part 2, p.831–840.
Fatigue failure of a hydraulic filter head Myounggu Park* Engine Division, ATRI(Aero-Tech Research Institute), ROKAF, PO Box 304-160, Kumsa dong, Dong gu, Deagu, 701-799, South Korea Received 15 May 2001; accepted 30 June 2001
Abstract An in-service failure of a hydraulic filter installed on a combat aircraft has been investigated. Cracking had occurred at the filter head and no sign of plastic deformation was found. Chemical examination and micro-hardness measurement revealed that the filter head material was Al 6061-T6. But the base material of the randomly chosen filters from the field was Al 2024-T4. So the wrong material selection was confirmed. Metallography showed microstructural anomalies which contributed to the early fracture of hydraulic filter head. A surface examination of the fracture region revealed striations, and it was confirmed that the fracture mode was fatigue. At the initiation site there were corrosion pits providing initiation sites for fatigue cracks. The corrosion pits and microstructural anomalies might be generated by applying the manufacturing processes designed for Al 2024 to the wrong material (Al 6061). It was verified by fatigue analysis that with pre-existing defects and/or flaws, the fracture should occur far earlier than the expected cyclic life. The right selection of the material can be the most important remedial action in this case. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Aircraft failures; Material selection; Fatigue; Corrosion fatigue; Microstructures
1. Background During a flight, a combat aircraft aborted the mission and made an emergency landing due to illumination of the right hand hydraulic caution light and the sudden drop of hydraulic pressure to zero. On subsequent in-field investigation, it was reported that there was hydraulic leakage from a cracked region of the hydraulic filter head which was installed on the utility system in the aft fuselage section. Fig. 1 shows the installation of the hydraulic filter schematically. The hydraulic filter had been used for the filtration of hydraulic fluid in the utility system of the aircraft for about 4597 hours reportedly. The utility system together with the control system supplied hydraulic power (3000 200 psi) to the flight controls including ailerons, rudder and horizontal tail, etc. by pistontype engine driven pumps. Table 1 shows the leading particulars of the hydraulic filter [1].
* Tel.: +82-053-980-3931, 3932; fax: +82-053-819-1271. E-mail address: [email protected] (M. Park). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(01)00029-2
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M. Park / Engineering Failure Analysis 9 (2002) 435–450
Fig. 2 shows the as-received hydraulic filter. The filter head was separated completely due to cracking (see arrows). Fig. 3 shows the fracture region. Plastic deformation such as irregularity of diameter was not found in the fracture region.
2. Chemical analysis The chemical composition of the hydraulic filter head was found by ICP (inductively coupled plasma spectrometer: Model JY 38 PLUS) analysis. The analysis revealed that the filter head material is an Al alloy 6061 [2]. The results are given in Table 2 along with the specified chemical composition. The actual
Fig. 1. The installation of the hydraulic filter.
Table 1 Leading particulars of hydraulic filter Service fluid
Hydraulic fluid (military specification MIL-H-83282)
Capacity
3.5 gpm at 32.2 to 43.3 C with maximum differential pressure of 30.0 psig; 2.0 gpm (after 5 min stabilized flow) at a stabilized temperature of 28.9 C with a maximum differential pressure of 95psig
Pressure ratings
Operating: 3000 psig Proof: 4500 psig Burst: 7500 psig
Filtration medium
Chemically treated paper
Filtration rating
10 mm per military specification MIL-F-5504
Relief valve operation
Cracking pressure: 15015 psig Rated flow pressure: 200 psig max 150% rated flow pressure : 240 psig max Reseat pressure: 100psig min
Temperature range
59.3 C to +135 C
Port connections
9/16/18 UNF/3B thread per AND10050-6
Weight
0.6 lb maximum
437
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Fig. 2. The as-received hydraulic filter. The scale is in cm.
Fig. 3. Normal view of fractured hydraulic filter head.
Table 2 Chemical composition of hydraulic filter head material Element
Specified percentage
Actual percentage
Silicon Iron Copper Manganese Magnesium Chromium Zinc Titanium Others Others Aluminum
0.40–0.80 0.70 Maximum 0.15–0.40 0.15 Maximum 0.8–1.2 0.04–0.35 0.25 Maximum 0.15 Maximum Each 0.05 Maximum Total 0.15 Maximum Balance
0.45 0.48 0.47 0.09 0.87 0.17 0.20 – – Balance
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chemical composition shows copper content on the high side. However, the slight difference in copper content is unlikely to influence the performance of the hydraulic filter to a significant extent. From the outer appearance and brass-like surface color of the hydraulic filter, it can be guessed that it was anodized as a surface treatment.
3. Mechanical properties Several micro-vickers hardness measurements were made on a polished and unetched surface of the sample (Fig. 4). The average value of the test results was 122.86. Micro-vickers hardness 122.86 is equivalent to HB 119. The HB value is within the range of the Al 6061-T6 [3], so it can be assumed that it was in the T6 condition. T6 means that it was solution-treated and artificially aged from O (annealed) condition.
4. Metallography Longitudinal and transverse sections were prepared from the cracked region of the filter head for optical microscopy. The specimens were polished and etched with Kellar’s etchant (HF:HCl:HNO3=2:3:5 ml+water 190 ml) to reveal the microstructures. To compare them with the fractured filter head, samples were taken from standard tensile specimens made from Al 6061-T6 and also polished and etched. Fig. 5 shows the microstructure of longitudinal sections. Compared with tensile specimens, the cracked hydraulic filter head has bigger and more elongated constituent particles. Also the constituent particles are not equally distributed. Fig. 6 shows the microstructure of cross sections. From Fig. 6(a) elongated grain (preferred orientation) and bigger constituent particles than those of the tensile specimen were found. Because of preferred orientation, non-homogeneous distribution and the bigger size of constituent particles, the fatigue strength might be lowered. It is well known that when there is preferred orientation, the ST-oriented specimen has the lowest fatigue crack initiation resistance among the three directions—LT, SL, ST. Consequently, the microstructure of the two samples—fractured and standard—was different, which indicated that a microstructural anomaly was one of the likely contributors to the early fracture of the hydraulic filter head.
Fig. 4. The micro-vickers hardness measurement sites.
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439
5. Surface examination The fractured region was carefully observed both visually and with the aid of the scanning electron microscope SEM. Fig. 7 shows the overall view of the fracture surface. Visually there are two distinctive regions on the fracture surface. The flat region (A and B) and the 45 slant region. Except for the flat region, the remaining part is 45 slant region. Typical SEM photographs of the fracture surface of these two regions are shown in Figs. 8–10. At the flat region, typical fatigue striation markings are observed. On the other hand, the 45 slant region shows dimples which indicates that overload fast fracture had occurred. From the surface examination, the cracking procedures/paths can be determined. The crack initiated from the inner surface propagating along the surface of the wall and moved forward though the wall at the same time. Fig. 11 shows the cracking procedures/paths schematically. From experience and the configuration
Fig. 5. The micrographs of longitudinal sections.
440
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of the small fatigue fracture region including the origin 2 (O2), it is suggested that the crack inclined to propagate along the surface of the wall rather than through the wall in the case of the tube-like part. This feature can be related to the fatigue striation pattern consisting of many parallel lines from the crack starting region unlike the typical fatigue striation pattern of a solid body converging to an origin. Observation of the initiation region of the fatigue crack revealed a corrosion pit (Figs. 12 and 13). From the fractography it is clear that crack initiation can be related to the corrosion pits. Also the crack followed a linear groove (tool mark) made by the cutting tool (Fig. 14) which is at the thinnest region of the cracked filter head. The corrosion pits and the tool mark underneath might act as stress raisers, and it is considered that as the crack propagates along the surface, corrosion pits were connected to accelerate the rate of crack growth (Fig. 15).
Fig. 6. The micrograph of a transverse section.
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Fig. 7. Overall view of fracture surface of hydraulic filter head.
Fig. 8. SEM photograph of the flat region near the inside showing the fracture occurred by fatigue (500).
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Fig. 9. SEM photograph of the flat region near the outside (end of fracture surface) showing fracture occurred by fatigue (1500).
Fig. 10. SEM photograph of 45 slant region showing fracture occurred by overload (300).
Fig. 11. Cracking procedure/path.
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Fig. 12. SEM photograph showing corrosion pit at the origin of the fatigue crack (700).
Fig. 13. Enlarged view of corrosion pits (1200).
Fig. 14. Stereoscopic micrograph showing tool marks.
Fig. 15. Surface crack propogation connection of corrosion pits.
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6. Stress calculation Fig. 16 shows the cross-section of the hydraulic filter schematically. The rated pressure of the hydraulic filter is 3000 psi ( 200 psi). Since the filter head is a pressure vessel, the longitudinal stress ( 1) is calculated as follows [4] where r=radius of the head and t=thickness of the wall: 1 ¼
pr 3000 psi 25:13 mm ¼ ¼ 14442 psi614:4 ksi 2t 2 2:61 mm
ð1Þ
It is not easy to obtain an accurate figure for the number of loading cycles experienced by the hydraulic pressure. The following estimate is based on a reasonable guess. The hydraulic filter has been used for about 4597 h. Here 1 h equals approximately one flight. So the filter head cracked after 4597 flights— 4.6103 cycles. During the flight, the hydraulic filter was at the pressure of 3000 psi continuously and dropped to 0 psi after landing. Only the longitudinal stress was acted on the cracked region of the filter head (=14.4 ksi). Therefore the load cycle is assumed as following (Fig. 17). The fatigue cycle is therefore 7.2 ksi about a mean tensile stress of 7.2 ksi.
Fig. 16. Cross-section of hydraulic filter.
Fig. 17. Load spectrum of hydraulic filter head at the cracking region.
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445
The service temperature range of the hydraulic fluid lies between 65 F and 160 F (53 71 C). Also the cracked region is not directly affected by hydraulic flow. Because it was not used in a high temperature environment it can be assumed that there is no holding effect on the cracked region.
7. Fatigue analysis The polished surface of the cracked region of the hydraulic filter head was observed (Fig. 18). The thickness (t1, t2) and the radius of the corner (r1, r2) were measured by image-pro plusR and the result was shown in Table 3. At the cracked region, the stress concentration factor (kt) is 2.2 (Fig. 19) [5]. From the S– N curve (Fig. 20) [6], failure will be expected after about 108 cycles. In aircraft structures fatigue critical locations FCLs are decided and non-destructive inspection NDI is done periodically. But for the hydraulic filter head, there is no mention of NDI and critical crack length in the technical manual. In order to evaluate the fatigue life of the hydraulic filter head in the case where an initial crack exists, a computer simulation was done by crack 98 software. It is assumed that fatigue failure was caused by initial cracks/flaws to a critical length without detection. The initial flaw sizes are determined by USAF (United States Air Force) specifications for damage tolerance requirements (MIL-A83444) [7]. The parameters of the crack growth analysis are shown in Table 4. The results of the crack growth analysis are shown in Figs. 21 and 22. The number of cycles at which crack depth exceeds the thickness of the filter head is 3104 cycles (Fig. 21). The number of cycles at which
Fig. 18. The cross-section of the cracked region.
Table 3 The dimension of the cracked region Thickness
t1 t2 t3
2.61 mm 5.22 mm 3.06 mm
Radius
r1 r2
0.24 mm 1.38 mm
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Fig. 19. Stress concentration factor.
Fig. 20. Axial notched fatigue life of 6061-T6 aluminium (load ratio R=o).
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instability of crack growth occurred is 4.4104 cycles (Fig. 22). So if initial cracks and/or flaws pre-existed for other reasons, the fatigue life of the filter head will drop dramatically.
8. Discussion The base material of three randomly chosen filter heads was chemically analysed using chips from the surface. The results showed that the material was Al 2024. It is different from the result of the cracked hydraulic filter head—Al 6061. Table 5 shows the typical mechanical properties of Al 2024-T4 and Al 6061-T6 [8]. Although not as strong as 2024, 6061 alloy has better formability and weldability, and corrosion resistance than 2024. Due to its unique features, it is not easy to directly compare these two materials only by the strength of materials. Though the material was not 2024, 6061 also has enough reason for usage as is shown already by the stress and fatigue analysis in this paper. Most of all, 6061 has excellent
Fig. 21. The result of crack growth–crack depth vs flying hours.
Table 4 The parameters of crack growth analysis Geometric properties
Surface crack in hollow cylinder Loading type=tension Thickness=2.61 mm Head diameter=50.26 mm Initial crack length=0.100 inch Initial crack depth=0.050 inch
Load interaction
None
Spectrum
Constant amplitude analysis Maximum stress=14.400 ksi Minimum stress=0.000 ksi Cycles per flight hour=1.00
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SCC (stress corrosion cracking) resistance. So the change of material itself is not considered to be a reason for the early fracture of the hydraulic filter head. However, if all the filter head manufacturing processes are designed for Al 2024, the change of base material without changing the manufacturing process can significantly affect the properties of the filter head. As for the corrosion pits on the surface of the fractured hydraulic filter head (Fig. 23), these are distinguished from the pores in the anodic coating. Generally the surface of engineering aluminum alloys is anodized. Fig. 24 shows the porous film (anodic coating) growth on the Al substrate in the anodizing process [9]. The diameter of the pores is about 200–300 A˚. So pits (Fig. 25) with diameter 20–30 mm on the surface of the filter head are not pores but corrosion pits. It is believed that due to these corrosion pits the fatigue crack initiation life is shortened significantly. Also pit formation is closely related to the electrochemical reaction between the constituent particles and the matrix in electrolytes [10]. Therefore the surface treatment process might be involved in pit formation. But in reality, it is almost impossible for an end-user to discover which processes and/or parameters were wrong.
Fig. 22. The result of crack growth–crack length vs flying hours.
Table 5 The mechanical properties of Al 2024-T4 and Al 6061-T6
Al 2024-T4 Al 6061-T6
Tensile strength, psi
Tensile yield strength, psi
Elongation, %
Hardness, HB
Fatigue strength, psi
68 45
47 40
20 12
120 95
18 14
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Fig. 23. Corrosion pits on surface: —(a) cracked filter, (b) normal filter (25).
Fig. 24. Porous film growth by anodizing process.
Fig. 25. Corrosion pit near the fracture surface (500).
449
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9. Conclusions 9.1. Most probable cause The root cause of the hydraulic filter head failure can be attributed to wrong material selection. If all the manufacturing processes designed for Al 2024 were applied to the wrong material (Al 6061), the following consequences might be induced. . Improper surface treatment might generate the corrosion pits which acted as stress raisers, nucleated fatigue cracks and accelerated the crack growth rates. . Microstructural anomalies might increase the brittleness of the hydraulic filter head
9.2. Remedial action Right material choice is the most important and fundamental remedial action in this case.
Acknowledgements The author would like to acknowledge the assistance and help of Dr. Young-Ha Hwang, Hyoung-Dea Ju, and Kyoung-Sook Son of ATRI, Dr. Yoon-Sung Choi and all the staff members in Engine Engineering Center of the Samsung Techwin, and Philip Le Page of Rolls-Royce International Ltd at various stages in this investigation.
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