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Microstructural Characterization of Failed Mounting Studs of Aero-engine Bevel Gear Box
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 2380–2387
www.materialstoday.com/proceedings
ICMPC-2019
Microstructural Characterization of Failed Mounting Studs of Aero-engine Bevel Gear Box Premkumar Manda* and A K Singh Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad – 500 058, India.
Abstract This paper reports the microstructural characterization of failed mounting studs of aero-engine bevel gear box. The chemical compositions obtained by EPMA indicate that the studs are manufactured from stainless carbon steel. The optical microstructures of studs show very fine microstructure and display the presence of very fine chromium carbide precipitates. The overall hardness of the stud-2 is marginally higher than that of the stud-1. It has been found that the cadmium coating was applied on the studs. This has been confirmed by EDS spectra and line scan profile of EPMA. The fracture surfaces of the broken mounting studs of bevel gear box of aero-engine portray the beach marks and overload fracture / final rupture. The presence of beach marks indicates that the studs are failed by fatigue. The material is free from inhomogeniety, discontinuity or any other defects. The fracture surface is also free from any corrosion products. There is no apparent cause of the failure like material defect of excess load etc. observed. The studs have failed due to loose-fitting of the studs leading to excessive vibration resulting in fatigue failure. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords:Mounting Stud; Microstructure; Hardness; Fatigue
1. Introduction The broken mounting studs (2 nos) of bevel gear box that belongs to aero-engine were received in damaged condition for defect investigation. Initially, it has been observed that the aero-engine was stripped into two subparts and two mounting studs of bevel gear box were found broken from the thread end. The application of screw thread to suds provide the ability to join two or more pieces of material together securely, easily and more importantly it is not permanent joining [1-2]. In general gears are toothed members that transmit power / motion between two shafts
* Corresponding author. Tel.: +91-9949663492; fax: +91-40-24340681. E-mail address: [email protected], 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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by meshing without any slip. The smaller and larger ones of any pair of gears are called as pinion and gear (irrespective of which is driving other), respectively. Among the different types of gear, the bevel gears have direct line contact and these are regularly manufactured to have localized contact in order to permit more tolerance in mounting. Present work thus describes the detailed microstructural characterization of failed mounting studs of aeroengine bevel gear box 2. Experimental Procedure Visual examination was carried out on broken mounting studs followed by photography in as-received condition. The chemical compositions of the broken mounting studs were determined using Scanning Electron Microscopy (SEM) and Electron Probe Micro Analyzer (EPMA). The microstructural characterization was carried out using optical microscope (OM), SEM and EPMA. Line scan profiles of constituent elements of the studs were carried out using EPMA. Fracture surfaces of the broken mounting studs were examined in secondary electron (SE) mode in SEM. EDS spectra were recorded from the fractured area in different regions. The hardness values were measured using Vickers Hardness tester (AFFRI) on both the studs on longitudinal and transverse sections using 10 Kg load. 3. Visual Examination The photographs of the two broken mounting studs (designated as stud-1 and stud-2) in as-received condition are shown in Fig. 1. It seems that the studs are broken from the one of the threaded end and the other side it contains the nuts. The yellowish colour of studs indicates that some coating was applied on surface of the studs.
Fig. 1: The photographs of the broken mounting studs in as-received condition. The close views of both the ends of the studs are displayed in Fig. 2 (a and b). The surfaces of the studs exhibit the yellowish and red colour and removal of coating / thin layer at a few locations. Stud-1 has more threads near fracture than the stud-2 (Fig. 2a). The close view of the Fig. 1 shown in black rectangular box is displayed in Fig. 2b. The surfaces of both the studs exhibit the rough surface.
Fig. 2: Close view of rectangular box regions of the Fig. 1; (a) white and (b) black.
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4. Results 4.1. Stereo-microscopy The stereo-microscope images of the fracture surfaces of the broken studs are shown in Fig. 3 (a and b). The fracture surfaces clearly exhibit the presence of beach marks on both the mounting studs. The enlarged view of the stereo-microscope image of the fracture surface of the broken stud-2 is shown in Fig. 3c. The stud-2 exhibits the presence of ratchet mark (indicated by black arrow).
Fig. 3: (a) Stereo-microscope images of the fracture surface of the broken studs: (a) stud-1; (b) stud-2; (c) enlarged view of the stereo-microscope image of the fracture surface of the broken stud-2. 4.2. EDS Analysis and Chemical Composition The chemical compositions of both the studs determined by EPMA are given in Table 1. The EDS patterns taken from the stud-1 and stud-2 are shown in Fig. 4(a and b). Both the stud-1 and stud-2 exhibit the presence of Fe, Ni and Cr as major alloying elements along with small amount of Si. This indicates that both the studs are manufactured from the same material. The Cr content along with other elements (Si, Ti, Mn and Ni) indicates that the material is stainless carbon steel (GOST 5949-61) [3]. Table 1: Chemical compositions of the studs (wt. %). Si
Ti
Mn
Cr
Ni
Fe
Stud-1
0.62
0.01
0.16
16.30
2.6
Bal.
Stud-2
1.31
0.01
0.14
17.29
2.4
Bal.
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Fig. 4: The EDS patterns taken from the fracture surface of the broken studs: (a) stud-1; (b) stud-2. 4.3. Fractography The low magnification SE SEM image of the fracture surface of the broken bevel gear box mounting stud-1 is shown in Fig. 5a. The features observed in various locations on the fracture surface are marked with numbered circles (1, 2 and 3) are displayed in Fig. 5 (b, c and d). The SE SEM image taken from the area marked with circle 1 portrays the rubbed surfaces and small featureless region (Fig. 5b). There appears a small dent at the initiation site.
Fig. 5: (a) Low magnification SE SEM image taken from the fracture surface of the broken stud-1: (a) low and high magnification images: area marked with (b) circle 1; (c) circle 2; (d) circle 3 of Fig. a. The high magnification SE SEM image taken from the area marked with circle 2 is shown in Fig. 5c. It displays the presence of very fine dimples along with a few coarser dimples. The high magnification SE SEM image taken from the area marked with circle 3 is displayed in Fig. 5d. It exhibits the features which are typical of the fatigue [4].
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The low magnification SE SEM image of the fracture surface of the broken bevel gear box mounting stud-2 is shown in Fig. 6a. As mentioned above, it also displays the presence of beach marks and overload fracture similar to that of the stud-1. The area encircled with the black line is displayed in Fig. 6b. It exhibits the presence of hills and valleys features consisting of very fine ductile dimples. The high magnification image taken from the same region is displayed in Fig. 7a. The dimples contain the particles / inclusions within it.
Fig. 6: SE SEM image taken from the fracture surface of the broken stud-2: (a) low magnification; (b) High magnification image from black encircled area of Fig. a. The high magnification image containing inclusions / particles is shown in Fig. 7a. The EDS pattern taken from the inclusion is depicted in Fig. 7b. It exhibits the presence of Al and O as major alloying elements along with small fraction of S, Cr, Mn and Fe. The particle is Al rich (Alumina).
Fig. 7: (a) High magnification SE SEM image taken from the overload fracture showing inclusion / particle within dimple; (b) EDS pattern taken from the area marked in red colour of the Fig. a. 4.4. Microstructural Analysis The optical microstructures are taken from both the studs away from the damaged region along longitudinal direction in un-etched condition to observe the non metallic inclusions. A representative un-etched microstructure of these inclusions in stud-1 is shown in Fig. 8 (a and b). The inclusions are globular in shape. The detailed analyses of these are carried out by EPMA line scan. The high magnification BSE EPMA image with single inclusion is displayed in Fig. 8b. The corresponding line scans (marked by double arrows in Fig. 8b) obtained from the inclusion is shown in Fig. 8c. These are enriched with O and Al indicating the presence of aluminium oxide inclusions.
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Fig. 8: (a) The optical image taken from the stud-1 along longitudinal section showing inclusions (black in colour); (b) BSE EPMA image showing single inclusion (in black colour); (c) EPMA line scan taken across the inclusion (marked by double arrow) in Fig. a. The optical microstructures obtained from the longitudinal and transverse sections of the broken mounting studs are shown in Figs. 9 and 10. The optical microstructures of the studs are very fine. The low magnification optical microstructures of the stud-1 both in longitudinal and transverse sections are shown in Figs. 9a and 10a. The low magnification images exhibits very fine microstructure. The high magnification images of the same are shown in Figs. 9b and 10b. The microstructure displays the presence very fine precipitates. The optical microstructures taken from stud-2 along longitudinal and transverse sections are similar to those of the Stud-1 (not shown). The microstructure is finer in both the studs along transverse sections than that of the longitudinal.
Fig. 9: Optical microstructures of broken mounting stud-1 along longitudinal section: (a) low and (b) high magnifications. The SE SEM image taken from the longitudinal section of the broken mounting studs is shown in Fig. 11a. The studs exhibit the presence very fine precipitates (white contrast) in a dark matrix. These are nearly globular in shape and uniformly distributed throughout the matrix (Fig. 11a). The chemical composition of the precipitates is analyzed in detail by EPMA line scan. The representative line scans for single precipitate are shown in Fig. 11b. The precipitates are enriched with Cr and C based on the number of counts (intensity). This indicates the presence of chromium carbide precipitates within the matrix. As mentioned above, the yellowish colours observed on the surfaces of the studs indicate the presence of coating. This has been analysed by EPMA line scan. The presence of coating on the surface of the studs is analyzed by EPMA line scan. The results of line scans are shown within double arrow of Fig. 12a. This clearly reflects the presence of cadmium coating (blue colour line in Fig. 12b) on the surfaces of both the studs.
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Fig. 10: Optical microstructures of broken mounting stud-1 along transverse section: (a) low; (b) high magnifications.
Fig. 11: (a) SE SEM image taken from the longitudinal section of broken mounting stud-1; (b) representative EPMA line scans of the precipitate shown in Fig. a.
Fig. 12: (a) The representative BSE EPMA image for line scan; (b) the line scan of the coating along with matrix of the Fig. a.
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4.5. Hardness The Vickers Hardness Values (VHN) obtained from longitudinal and transverse sections are given in Table 2. The hardness values measured along transverse sections of both the studs are marginally higher than that of longitudinal. The fine microstructures obtained along transverse sections (Fig. 10) supports the same. The overall hardness of the stud-2 is marginally higher as compared to stud-1. The high hardness obtained in stud-2 can be attributed to the presence of different sizes, orientations and morphologies of the precipitates. Table 2: Vickers Hardness values of both the studs. Vickers Hardness (VHN) Longitudinal
Transverse
Stud-1
256 1
268 1
Stud-2
275 2
280 2
5. Discussion The chemical compositions obtained by EPMA from both the cross sections (longitudinal and transverse) of studs indicate that the studs are manufactured from stainless carbon steel. The optical microstructures of studs obtained from both the longitudinal and transverse sections show very fine microstructure. The microstructures of both the studs display the presence of very fine chromium carbide precipitates. The overall hardness of the stud-2 is marginally higher than that of the stud-1. The cadmium (Cd) coating has been applied on the studs. This has been confirmed by EDS and line scan profile of EPMA. The fracture surfaces of the broken mounting studs of aero-engine bevel gear box portray the beach marks and overload fracture / final rupture. The final overload fracture area is approximately 10-15% of the total cross section. It implies that the studs are lightly loaded as only 15% of the area is able to withstand the static load. The presence of beach marks indicates that the studs were failed by fatigue. In stud-1 a small featureless region is seen at the initiation of the fatigue crack. The overload fracture exhibits the presence of very fine ductile dimples in both the studs. The inclusion content of the material is very low and falls in the category D1 (thin series). The material is free from inhomogeniety, discontinuity or any other defects. The fracture surface is also free from any corrosion products. There is no apparent cause of the failure like material defect, excess load etc. Therefore, both the studs have failed due to loose-fitting of the studs leading to excessive vibration resulting in fatigue failure. 6. Conclusions The mounting studs of aero-engine bevel gear box of are failed by fatigue. The fatigue cracks have been initiated from the threaded portion. In absence of any material and surface defects, fatigue failure probably has been caused due to loose-fitting of the studs. Acknowledgements Authors acknowledge Defence Research and Development Organization for financial support and Dr. Vikas Kumar, Director, DMRL for his kind encouragement. Authors thank Photography, SFAG and EMG groups of DMRL for their kind help. References [1] K. Morling, Geometric and Engineering Drawing, 3rd edition, Elsevier, MA 01803, USA, 2010. [2] K.L. Narayana, P. Kannaiah and K. Venkata Reddy, Machine Drawing, 3rd edition, New Age International (P) Limited Publishers, New Delhi-110002, 2006. [3] www.splav-kharkov.com/ database of steel and alloy (Marochnik), 2003-2018. [4] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill Book Co., Singapore, 1988.
ScienceDirect Materials Today: Proceedings 18 (2019) 2380–2387
www.materialstoday.com/proceedings
ICMPC-2019
Microstructural Characterization of Failed Mounting Studs of Aero-engine Bevel Gear Box Premkumar Manda* and A K Singh Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad – 500 058, India.
Abstract This paper reports the microstructural characterization of failed mounting studs of aero-engine bevel gear box. The chemical compositions obtained by EPMA indicate that the studs are manufactured from stainless carbon steel. The optical microstructures of studs show very fine microstructure and display the presence of very fine chromium carbide precipitates. The overall hardness of the stud-2 is marginally higher than that of the stud-1. It has been found that the cadmium coating was applied on the studs. This has been confirmed by EDS spectra and line scan profile of EPMA. The fracture surfaces of the broken mounting studs of bevel gear box of aero-engine portray the beach marks and overload fracture / final rupture. The presence of beach marks indicates that the studs are failed by fatigue. The material is free from inhomogeniety, discontinuity or any other defects. The fracture surface is also free from any corrosion products. There is no apparent cause of the failure like material defect of excess load etc. observed. The studs have failed due to loose-fitting of the studs leading to excessive vibration resulting in fatigue failure. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords:Mounting Stud; Microstructure; Hardness; Fatigue
1. Introduction The broken mounting studs (2 nos) of bevel gear box that belongs to aero-engine were received in damaged condition for defect investigation. Initially, it has been observed that the aero-engine was stripped into two subparts and two mounting studs of bevel gear box were found broken from the thread end. The application of screw thread to suds provide the ability to join two or more pieces of material together securely, easily and more importantly it is not permanent joining [1-2]. In general gears are toothed members that transmit power / motion between two shafts
* Corresponding author. Tel.: +91-9949663492; fax: +91-40-24340681. E-mail address: [email protected], 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
P. Manda and A.K. Singh / Materials Today: Proceedings 18 (2019) 2380–2387
2381
by meshing without any slip. The smaller and larger ones of any pair of gears are called as pinion and gear (irrespective of which is driving other), respectively. Among the different types of gear, the bevel gears have direct line contact and these are regularly manufactured to have localized contact in order to permit more tolerance in mounting. Present work thus describes the detailed microstructural characterization of failed mounting studs of aeroengine bevel gear box 2. Experimental Procedure Visual examination was carried out on broken mounting studs followed by photography in as-received condition. The chemical compositions of the broken mounting studs were determined using Scanning Electron Microscopy (SEM) and Electron Probe Micro Analyzer (EPMA). The microstructural characterization was carried out using optical microscope (OM), SEM and EPMA. Line scan profiles of constituent elements of the studs were carried out using EPMA. Fracture surfaces of the broken mounting studs were examined in secondary electron (SE) mode in SEM. EDS spectra were recorded from the fractured area in different regions. The hardness values were measured using Vickers Hardness tester (AFFRI) on both the studs on longitudinal and transverse sections using 10 Kg load. 3. Visual Examination The photographs of the two broken mounting studs (designated as stud-1 and stud-2) in as-received condition are shown in Fig. 1. It seems that the studs are broken from the one of the threaded end and the other side it contains the nuts. The yellowish colour of studs indicates that some coating was applied on surface of the studs.
Fig. 1: The photographs of the broken mounting studs in as-received condition. The close views of both the ends of the studs are displayed in Fig. 2 (a and b). The surfaces of the studs exhibit the yellowish and red colour and removal of coating / thin layer at a few locations. Stud-1 has more threads near fracture than the stud-2 (Fig. 2a). The close view of the Fig. 1 shown in black rectangular box is displayed in Fig. 2b. The surfaces of both the studs exhibit the rough surface.
Fig. 2: Close view of rectangular box regions of the Fig. 1; (a) white and (b) black.
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4. Results 4.1. Stereo-microscopy The stereo-microscope images of the fracture surfaces of the broken studs are shown in Fig. 3 (a and b). The fracture surfaces clearly exhibit the presence of beach marks on both the mounting studs. The enlarged view of the stereo-microscope image of the fracture surface of the broken stud-2 is shown in Fig. 3c. The stud-2 exhibits the presence of ratchet mark (indicated by black arrow).
Fig. 3: (a) Stereo-microscope images of the fracture surface of the broken studs: (a) stud-1; (b) stud-2; (c) enlarged view of the stereo-microscope image of the fracture surface of the broken stud-2. 4.2. EDS Analysis and Chemical Composition The chemical compositions of both the studs determined by EPMA are given in Table 1. The EDS patterns taken from the stud-1 and stud-2 are shown in Fig. 4(a and b). Both the stud-1 and stud-2 exhibit the presence of Fe, Ni and Cr as major alloying elements along with small amount of Si. This indicates that both the studs are manufactured from the same material. The Cr content along with other elements (Si, Ti, Mn and Ni) indicates that the material is stainless carbon steel (GOST 5949-61) [3]. Table 1: Chemical compositions of the studs (wt. %). Si
Ti
Mn
Cr
Ni
Fe
Stud-1
0.62
0.01
0.16
16.30
2.6
Bal.
Stud-2
1.31
0.01
0.14
17.29
2.4
Bal.
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Fig. 4: The EDS patterns taken from the fracture surface of the broken studs: (a) stud-1; (b) stud-2. 4.3. Fractography The low magnification SE SEM image of the fracture surface of the broken bevel gear box mounting stud-1 is shown in Fig. 5a. The features observed in various locations on the fracture surface are marked with numbered circles (1, 2 and 3) are displayed in Fig. 5 (b, c and d). The SE SEM image taken from the area marked with circle 1 portrays the rubbed surfaces and small featureless region (Fig. 5b). There appears a small dent at the initiation site.
Fig. 5: (a) Low magnification SE SEM image taken from the fracture surface of the broken stud-1: (a) low and high magnification images: area marked with (b) circle 1; (c) circle 2; (d) circle 3 of Fig. a. The high magnification SE SEM image taken from the area marked with circle 2 is shown in Fig. 5c. It displays the presence of very fine dimples along with a few coarser dimples. The high magnification SE SEM image taken from the area marked with circle 3 is displayed in Fig. 5d. It exhibits the features which are typical of the fatigue [4].
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The low magnification SE SEM image of the fracture surface of the broken bevel gear box mounting stud-2 is shown in Fig. 6a. As mentioned above, it also displays the presence of beach marks and overload fracture similar to that of the stud-1. The area encircled with the black line is displayed in Fig. 6b. It exhibits the presence of hills and valleys features consisting of very fine ductile dimples. The high magnification image taken from the same region is displayed in Fig. 7a. The dimples contain the particles / inclusions within it.
Fig. 6: SE SEM image taken from the fracture surface of the broken stud-2: (a) low magnification; (b) High magnification image from black encircled area of Fig. a. The high magnification image containing inclusions / particles is shown in Fig. 7a. The EDS pattern taken from the inclusion is depicted in Fig. 7b. It exhibits the presence of Al and O as major alloying elements along with small fraction of S, Cr, Mn and Fe. The particle is Al rich (Alumina).
Fig. 7: (a) High magnification SE SEM image taken from the overload fracture showing inclusion / particle within dimple; (b) EDS pattern taken from the area marked in red colour of the Fig. a. 4.4. Microstructural Analysis The optical microstructures are taken from both the studs away from the damaged region along longitudinal direction in un-etched condition to observe the non metallic inclusions. A representative un-etched microstructure of these inclusions in stud-1 is shown in Fig. 8 (a and b). The inclusions are globular in shape. The detailed analyses of these are carried out by EPMA line scan. The high magnification BSE EPMA image with single inclusion is displayed in Fig. 8b. The corresponding line scans (marked by double arrows in Fig. 8b) obtained from the inclusion is shown in Fig. 8c. These are enriched with O and Al indicating the presence of aluminium oxide inclusions.
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Fig. 8: (a) The optical image taken from the stud-1 along longitudinal section showing inclusions (black in colour); (b) BSE EPMA image showing single inclusion (in black colour); (c) EPMA line scan taken across the inclusion (marked by double arrow) in Fig. a. The optical microstructures obtained from the longitudinal and transverse sections of the broken mounting studs are shown in Figs. 9 and 10. The optical microstructures of the studs are very fine. The low magnification optical microstructures of the stud-1 both in longitudinal and transverse sections are shown in Figs. 9a and 10a. The low magnification images exhibits very fine microstructure. The high magnification images of the same are shown in Figs. 9b and 10b. The microstructure displays the presence very fine precipitates. The optical microstructures taken from stud-2 along longitudinal and transverse sections are similar to those of the Stud-1 (not shown). The microstructure is finer in both the studs along transverse sections than that of the longitudinal.
Fig. 9: Optical microstructures of broken mounting stud-1 along longitudinal section: (a) low and (b) high magnifications. The SE SEM image taken from the longitudinal section of the broken mounting studs is shown in Fig. 11a. The studs exhibit the presence very fine precipitates (white contrast) in a dark matrix. These are nearly globular in shape and uniformly distributed throughout the matrix (Fig. 11a). The chemical composition of the precipitates is analyzed in detail by EPMA line scan. The representative line scans for single precipitate are shown in Fig. 11b. The precipitates are enriched with Cr and C based on the number of counts (intensity). This indicates the presence of chromium carbide precipitates within the matrix. As mentioned above, the yellowish colours observed on the surfaces of the studs indicate the presence of coating. This has been analysed by EPMA line scan. The presence of coating on the surface of the studs is analyzed by EPMA line scan. The results of line scans are shown within double arrow of Fig. 12a. This clearly reflects the presence of cadmium coating (blue colour line in Fig. 12b) on the surfaces of both the studs.
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Fig. 10: Optical microstructures of broken mounting stud-1 along transverse section: (a) low; (b) high magnifications.
Fig. 11: (a) SE SEM image taken from the longitudinal section of broken mounting stud-1; (b) representative EPMA line scans of the precipitate shown in Fig. a.
Fig. 12: (a) The representative BSE EPMA image for line scan; (b) the line scan of the coating along with matrix of the Fig. a.
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4.5. Hardness The Vickers Hardness Values (VHN) obtained from longitudinal and transverse sections are given in Table 2. The hardness values measured along transverse sections of both the studs are marginally higher than that of longitudinal. The fine microstructures obtained along transverse sections (Fig. 10) supports the same. The overall hardness of the stud-2 is marginally higher as compared to stud-1. The high hardness obtained in stud-2 can be attributed to the presence of different sizes, orientations and morphologies of the precipitates. Table 2: Vickers Hardness values of both the studs. Vickers Hardness (VHN) Longitudinal
Transverse
Stud-1
256 1
268 1
Stud-2
275 2
280 2
5. Discussion The chemical compositions obtained by EPMA from both the cross sections (longitudinal and transverse) of studs indicate that the studs are manufactured from stainless carbon steel. The optical microstructures of studs obtained from both the longitudinal and transverse sections show very fine microstructure. The microstructures of both the studs display the presence of very fine chromium carbide precipitates. The overall hardness of the stud-2 is marginally higher than that of the stud-1. The cadmium (Cd) coating has been applied on the studs. This has been confirmed by EDS and line scan profile of EPMA. The fracture surfaces of the broken mounting studs of aero-engine bevel gear box portray the beach marks and overload fracture / final rupture. The final overload fracture area is approximately 10-15% of the total cross section. It implies that the studs are lightly loaded as only 15% of the area is able to withstand the static load. The presence of beach marks indicates that the studs were failed by fatigue. In stud-1 a small featureless region is seen at the initiation of the fatigue crack. The overload fracture exhibits the presence of very fine ductile dimples in both the studs. The inclusion content of the material is very low and falls in the category D1 (thin series). The material is free from inhomogeniety, discontinuity or any other defects. The fracture surface is also free from any corrosion products. There is no apparent cause of the failure like material defect, excess load etc. Therefore, both the studs have failed due to loose-fitting of the studs leading to excessive vibration resulting in fatigue failure. 6. Conclusions The mounting studs of aero-engine bevel gear box of are failed by fatigue. The fatigue cracks have been initiated from the threaded portion. In absence of any material and surface defects, fatigue failure probably has been caused due to loose-fitting of the studs. Acknowledgements Authors acknowledge Defence Research and Development Organization for financial support and Dr. Vikas Kumar, Director, DMRL for his kind encouragement. Authors thank Photography, SFAG and EMG groups of DMRL for their kind help. References [1] K. Morling, Geometric and Engineering Drawing, 3rd edition, Elsevier, MA 01803, USA, 2010. [2] K.L. Narayana, P. Kannaiah and K. Venkata Reddy, Machine Drawing, 3rd edition, New Age International (P) Limited Publishers, New Delhi-110002, 2006. [3] www.splav-kharkov.com/ database of steel and alloy (Marochnik), 2003-2018. [4] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill Book Co., Singapore, 1988.