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Failure Analysis of Cooler Fan Drive Gear System of Helicopter
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 5254–5261
www.materialstoday.com/proceedings
ICMPC 2017
Failure Analysis of Cooler Fan Drive Gear System of Helicopter Premkumar Manda* , Satyapal Singh and A. K. Singh Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad – 500 058, India.
Abstract This paper presents the failure analysis of cooler fan drive gear system of helicopter. The work involves failure analysis of two components of main gear box assembly i.e. intermediate gear and drive fan pinion. The materials of the gears are made up of case carburizing steel. The microstructure of both the components is martensite. The microstructure of un-failed teeth is finer than that of the failed teeth. The crack is observed near the root of the failed teeth. It seems that cracks are formed only at the roots of the teeth and propagated inside the material. It has been observed that the fatigue cracks are formed at the roots of many teeth during service. These cracks have been sharply revealed by etching. The large crack length introduces simultaneous failure of several teeth as observed in intermediate gear while the short crack of drive pinion results in successive failure of the same. Both the teeth of intermediate gear and drive fan pinion fail by bending fatigue due to over / shock loads although the associated mechanisms are quite different. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization. Keywords: Failure Analysis; Gear; Fatigue; Hardness; Microstructure.
Introduction The components such as ball bearings, gears, bushings, gun barrels, cam shafts and axles are surface hardened by chemical heat treatment to obtain different properties in core and surface. A large number of components requires hard and wear resistant on outer surface while the inner core should be ductile and tougher. Such a combination of properties ensures that the component has sufficient wear resistance to give long service life and at the same time has sufficient toughness to withstand shock loads [12]. Case carburizing is the most widely used process for surface hardening of steels. The main attributes of the case carburized steels are high superficial hardness and an increased mechanical, fatigue and wear strength. The microstructure of case carburized steel is quite complex particularly close to surface. It consists of high carbon tempered martensite, retained austenite and carbides [3-7].
*Corresponding author. Tel.: +91-9949663492; fax: +91-40-24340681. E-mail address: [email protected], [email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization.
Premkumar Manda et al./ Materials Today: Proceedings 5 (2018) 5254–5261
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The combination of properties as mentioned above is recommended for applications wherein high stress and cyclic loading are involved. The case carburized steels are often used to fabricate gears in different mechanical components. Present work thus describes the failure analysis of cooler fan drive gear system of a helicopter. The failed gear components of the helicopter were received from Hindustan Aeronautics Limited, Bengaluru to find out the root cause of failure. The helicopter was flying at local operations. The repetitive chip warning has been noticed during flight at LH and RH in the cockpit instrument and immediately flight was aborted. The gearbox was disassembled to component level. One of the gears of cooler fan drive system of main gearbox has been found in damaged condition. The function of the cooler fan is to suck the air by suction / by forcing the air into the cooler circuit of MGB assembly. It is a closed loop circuit wherein the heat generated in the transmission system due to various frictional contacts, is carried away by air via transmission heat exchanger unit. The materials of the gears are made up of case carburizing steel. The chemical composition is given in Table 1. Table 1: Chemical composition (wt. %) of the as-received failed components.
2. Experimental details Visual examination was carried out on MGB cooler fan intermediate gear and drive pinion followed by photography in as-received condition. The chemical composition of the failed parts were determined using X-Ray fluorescence (XRF) for the elements such as Fe, Ni, Cr, Mn, Si, Cu and Mo. The damaged parts of cooler fan drive gear system were examined under optical microscope (OM) for microstructural analysis and scanning electron microscope (SEM) for fracture surface studies. Fractography was carried out on the damaged specimens. Bulk and micro hardness values were measured on the MGB cooler fan intermediate gear and drive pinion. 3.Results and discussion Visual examination: The photographs of damaged parts in as-received condition are shown in Figures 1 and 2. During the visual examination, it has been observed that complete shaving of gear teeth of cooler fan intermediate gear (Figure 3) and shaving of four teeth followed with partial damage on the other teeth of drive pinion (Figure 4).
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The analyzed chemical compositions of the intermediate gear and drive pinion are given in Table 2. The chemical compositions of both the fan intermediate gear and drive pinion are nearly same. The optical microstructures of the un-failed teeth, failed teeth and drive fan pinion are shown in Figure 5. These microstructures are taken away from the teeth portions which clearly indicate the presence of tempered martensite. The microstructure of un-failed teeth is fine in comparison to that of the failed teeth. Bulk Vickers hardness values of unfailed (459 VHN) and failed (455 VHN) intermediate gear and drive pinion (434 VHN) are shown in Figure 5d. It is to be noted that the hardness value of failed teeth is lower than the un-failed one. This can be attributed to overheating of the component during operation. The hardness value of drive pinion is lower than both the un-failed and failed teeth (Figure 5d). Interestingly, microstructure of drive pinion is coarser than both the failed and un-failed teeth. Table 2: Analyzed chemical compositions (wt.%) of the MGB cooler fan intermediate gear and drive pinion.
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The optical microstructure of the root of the failed gear teeth is shown in Figure 6. It is important to mention here that the contrast of the microstructure is different near the root and rim. This indicates that microchemical compositions of both the regions near root and rim are slightly different although Energy Dispersive Spectrometer (EDS) and Wavelength Dispersive Spectrometer (WDS) techniques attached with SEM and Electron Probe Micro Analyser (EPMA) could not reveal the same particularly the carbon content. It is to be noted that this microstructural difference is also present in teeth. Both the surface and core of the teeth have different contrast. Therefore, Vickers micro hardness profiles of two teeth (marked as A (A1 and A2) and B (B1 and B2) in inset) are measured across the un-failed gear teeth (Figure 7). The hardness decreases from surface to core and then increases towards the surface. It has been reported that the effective case depth is defined as the distance below the surface where the hardness is equal to 550 VHN. This can be ascribed to the presence of case carburized layer which has been given to enhance hard and wear resistance of low carbon steels. As a result, the microstructural contrasts of the surface and core of the teeth as well as near root and rim are different. The crack is observed near the root of the failed teeth (Figure 6). It seems that cracks are formed only at the roots of the teeth and propagated inside the material. It has been observed that the fatigue cracks are formed at the roots of many teeth after the service. These cracks have been sharply revealed by etching. The micro hardness near the crack is considerably low i.e. ∼ 650 VHN (0.3Kg load). It is known that the hardness of the martensite depends mainly on the carbon content. The low hardness near the crack appears to be due to low carbon content. The presence of cracks near the root fillet of teeth can therefore be attributed to the insufficient hardening which is not extended to the root fillet of tooth. The optical microstructure of the drive fan pinion consisting of crack near the root is shown in Figure 8. The microstructure appears to be tempered martensite and it is coarser than the intermediate gear near the case carburized layer. The size of the cracks observed in drive fan pinion near the root of the teeth is relatively finer than that of intermediate gear. The micro hardness profile of the teeth of drive pinion also reflects the presence of case carburized layer (Figure 9). As a result, micro hardness values at the core are lower in comparison to the surface of the teeth. The hardness values at surface and core of drive pinion is lower than the intermediate gear at similar locations. The fracture surface of the failed intermediate gear is shown in Figure 10. The fracture surface of the intermediate gear shows the presence of rubbed regions, elliptical beach marks at the centre of the fracture surface as well as jagged regions near the periphery. The appearance of jagged regions near periphery indicates that this is the last portion of the tooth to break away. High magnification of elliptical beach mark displays flat regions with very fine and shallow dimples, micro cracks and uneven shaped regions consisting of curved / distorted flat surfaces. This indicates that the fatigue cracks have initiated at different locations. Based on the microstructural evidences, it appears that the cracks initiate at the fillet and moves towards the root of the individual tooth. This is due to the fact that hardening is not extended to the root of the tooth. It is to be noted that high stress concentration exists at fillets. The crack slowly propagates over a large part of the life. These cracks also simultaneously initiate and propagate in the roots of other teeth. The crack then propagates fast and suddenly results in fracture.
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The presence of elliptical beach marks at the centre of the fracture and jagged regions near periphery supports the same. In this case, the whole tooth or part of the tooth breaks away. As a result, several teeth can fail together. The teeth failure of the intermediate gear is therefore by bending fatigue. This occurs from excessive load on teeth. This results in root stresses higher than the endurance limit of the material. When the gears are loaded and subjected to enough repeated cycles during operation, the teeth fail due to excessive loads.
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The fracture surface of the failed tooth of drive pinion is shown in Figure 11. It resembles the presence of three features namely oval type beach (C1), jagged regions (C2) and clean break lines. The high magnification fracture surface displays largely the appearance of dimples and small regions of flat facets (Figure 11b). The regions consisting of dimples also reveal the presence of coalescence of dimples (Figure 11c). A sudden shock loads generally display break lines. This indicates that the drive pinion teeth are failed by bending fatigue followed by shock loads. It appears that the crack initiates from the fillets and it moves towards the root. The travel distance of the crack in drive pinion is relatively less than the intermediate gear. The beach mark in failed teeth therefore appears in drive pinion on one side of the fracture (marked C1) whereas it is observed at the centre in intermediate gear. It is clear from the Figures 4 and 11 that failure of teeth of drive pinion occurs by initiation of cracks from fillets and these cracks moves a short distance toward the root during bending fatigue. The teeth then break due to shock loads. The presence of jagged regions supports the same. It appears that one tooth fails from shock and then load is transferred to successive teeth. This has resulted in the stripping of several teeth as observed in Figure 4. It appears that the length of the cracks initiated at fillets plays a very important role. If the length is large enough to reach the centre of the root, then the oval beach mark appears at the centre of the fracture surface as observed in case of intermediate gear. This allows the initiation and movement of cracks simultaneously in several teeth and they can fail together. On the other hand, if the crack length is too short as in case of drive pinion, the oval beach mark appears on one side of the fracture surface (marked C1). This caused the failure of individual tooth and load is being transferred to successive tooth. The stripping of several teeth takes place one by one.
The problem of teeth failure in both the intermediate gear and drive pinion can be overcome by (i) designing the gear tooth elements which can bear the transmitted load within endurance limit, (ii) using high strength material for the fabrication of gear, (iii) appropriate case carburizing of the teeth with sufficient hardness and (iv) proper heat treatment to minimize residual stresses. 4. Conclusions • Both the teeth of intermediate gear and drive fan pinion fail by bending fatigue due to over / shock loads although the associated mechanisms are quite different. • The length of the crack in carburized layer in turn defines the nature of failure. The large crack length introduces simultaneous failure of several teeth as observed in intermediate gear while the short crack of drive pinion results in successive failure of the same. • The cracks are initiated from fillet and propagate towards the centre of root due to insufficient hardening which has not extended properly to the roots of teeth.
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Acknowledgements Authors acknowledge Defence Research and Development Organisation for financial support and Dr Samir V Kamat, Director, DMRL for his kind encouragement. Authors thank Photography, SFAG and EMG groups of DMRL for their kind help. References [1] S.H. Avner: Introduction to Physical Metallurgy, 2nd edition, Tata McGraw-Hill edition, New Delhi, 1997. [2] T.V. Rajan, C.P. Sharma and A. Sharma: Heat Treatment: Principles and Techniques, 2nd edition. PHI learning private limited, New Delhi, 2011. [3] L.J. Ebert, Metallurgical Transactions A, 9 (1978) 1537. [4] R.G. Luther, T.R.G. Williams, Metallurgia and Metal Forming, 41 (1974) 72. [5] K. Genel, M. Demirkol, International Journal of Fatigue, 21 (1999) 207. [6] G. Krauss, Metallurgical Transactions A, 9 (1978) 1527. [7] M. Preciado, P.M. Bravo, J.M. Alegre, Journal of Materials Processing Technology 176 (2006) 41.
ScienceDirect Materials Today: Proceedings 5 (2018) 5254–5261
www.materialstoday.com/proceedings
ICMPC 2017
Failure Analysis of Cooler Fan Drive Gear System of Helicopter Premkumar Manda* , Satyapal Singh and A. K. Singh Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad – 500 058, India.
Abstract This paper presents the failure analysis of cooler fan drive gear system of helicopter. The work involves failure analysis of two components of main gear box assembly i.e. intermediate gear and drive fan pinion. The materials of the gears are made up of case carburizing steel. The microstructure of both the components is martensite. The microstructure of un-failed teeth is finer than that of the failed teeth. The crack is observed near the root of the failed teeth. It seems that cracks are formed only at the roots of the teeth and propagated inside the material. It has been observed that the fatigue cracks are formed at the roots of many teeth during service. These cracks have been sharply revealed by etching. The large crack length introduces simultaneous failure of several teeth as observed in intermediate gear while the short crack of drive pinion results in successive failure of the same. Both the teeth of intermediate gear and drive fan pinion fail by bending fatigue due to over / shock loads although the associated mechanisms are quite different. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization. Keywords: Failure Analysis; Gear; Fatigue; Hardness; Microstructure.
Introduction The components such as ball bearings, gears, bushings, gun barrels, cam shafts and axles are surface hardened by chemical heat treatment to obtain different properties in core and surface. A large number of components requires hard and wear resistant on outer surface while the inner core should be ductile and tougher. Such a combination of properties ensures that the component has sufficient wear resistance to give long service life and at the same time has sufficient toughness to withstand shock loads [12]. Case carburizing is the most widely used process for surface hardening of steels. The main attributes of the case carburized steels are high superficial hardness and an increased mechanical, fatigue and wear strength. The microstructure of case carburized steel is quite complex particularly close to surface. It consists of high carbon tempered martensite, retained austenite and carbides [3-7].
*Corresponding author. Tel.: +91-9949663492; fax: +91-40-24340681. E-mail address: [email protected], [email protected]
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization.
Premkumar Manda et al./ Materials Today: Proceedings 5 (2018) 5254–5261
5255
The combination of properties as mentioned above is recommended for applications wherein high stress and cyclic loading are involved. The case carburized steels are often used to fabricate gears in different mechanical components. Present work thus describes the failure analysis of cooler fan drive gear system of a helicopter. The failed gear components of the helicopter were received from Hindustan Aeronautics Limited, Bengaluru to find out the root cause of failure. The helicopter was flying at local operations. The repetitive chip warning has been noticed during flight at LH and RH in the cockpit instrument and immediately flight was aborted. The gearbox was disassembled to component level. One of the gears of cooler fan drive system of main gearbox has been found in damaged condition. The function of the cooler fan is to suck the air by suction / by forcing the air into the cooler circuit of MGB assembly. It is a closed loop circuit wherein the heat generated in the transmission system due to various frictional contacts, is carried away by air via transmission heat exchanger unit. The materials of the gears are made up of case carburizing steel. The chemical composition is given in Table 1. Table 1: Chemical composition (wt. %) of the as-received failed components.
2. Experimental details Visual examination was carried out on MGB cooler fan intermediate gear and drive pinion followed by photography in as-received condition. The chemical composition of the failed parts were determined using X-Ray fluorescence (XRF) for the elements such as Fe, Ni, Cr, Mn, Si, Cu and Mo. The damaged parts of cooler fan drive gear system were examined under optical microscope (OM) for microstructural analysis and scanning electron microscope (SEM) for fracture surface studies. Fractography was carried out on the damaged specimens. Bulk and micro hardness values were measured on the MGB cooler fan intermediate gear and drive pinion. 3.Results and discussion Visual examination: The photographs of damaged parts in as-received condition are shown in Figures 1 and 2. During the visual examination, it has been observed that complete shaving of gear teeth of cooler fan intermediate gear (Figure 3) and shaving of four teeth followed with partial damage on the other teeth of drive pinion (Figure 4).
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The analyzed chemical compositions of the intermediate gear and drive pinion are given in Table 2. The chemical compositions of both the fan intermediate gear and drive pinion are nearly same. The optical microstructures of the un-failed teeth, failed teeth and drive fan pinion are shown in Figure 5. These microstructures are taken away from the teeth portions which clearly indicate the presence of tempered martensite. The microstructure of un-failed teeth is fine in comparison to that of the failed teeth. Bulk Vickers hardness values of unfailed (459 VHN) and failed (455 VHN) intermediate gear and drive pinion (434 VHN) are shown in Figure 5d. It is to be noted that the hardness value of failed teeth is lower than the un-failed one. This can be attributed to overheating of the component during operation. The hardness value of drive pinion is lower than both the un-failed and failed teeth (Figure 5d). Interestingly, microstructure of drive pinion is coarser than both the failed and un-failed teeth. Table 2: Analyzed chemical compositions (wt.%) of the MGB cooler fan intermediate gear and drive pinion.
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The optical microstructure of the root of the failed gear teeth is shown in Figure 6. It is important to mention here that the contrast of the microstructure is different near the root and rim. This indicates that microchemical compositions of both the regions near root and rim are slightly different although Energy Dispersive Spectrometer (EDS) and Wavelength Dispersive Spectrometer (WDS) techniques attached with SEM and Electron Probe Micro Analyser (EPMA) could not reveal the same particularly the carbon content. It is to be noted that this microstructural difference is also present in teeth. Both the surface and core of the teeth have different contrast. Therefore, Vickers micro hardness profiles of two teeth (marked as A (A1 and A2) and B (B1 and B2) in inset) are measured across the un-failed gear teeth (Figure 7). The hardness decreases from surface to core and then increases towards the surface. It has been reported that the effective case depth is defined as the distance below the surface where the hardness is equal to 550 VHN. This can be ascribed to the presence of case carburized layer which has been given to enhance hard and wear resistance of low carbon steels. As a result, the microstructural contrasts of the surface and core of the teeth as well as near root and rim are different. The crack is observed near the root of the failed teeth (Figure 6). It seems that cracks are formed only at the roots of the teeth and propagated inside the material. It has been observed that the fatigue cracks are formed at the roots of many teeth after the service. These cracks have been sharply revealed by etching. The micro hardness near the crack is considerably low i.e. ∼ 650 VHN (0.3Kg load). It is known that the hardness of the martensite depends mainly on the carbon content. The low hardness near the crack appears to be due to low carbon content. The presence of cracks near the root fillet of teeth can therefore be attributed to the insufficient hardening which is not extended to the root fillet of tooth. The optical microstructure of the drive fan pinion consisting of crack near the root is shown in Figure 8. The microstructure appears to be tempered martensite and it is coarser than the intermediate gear near the case carburized layer. The size of the cracks observed in drive fan pinion near the root of the teeth is relatively finer than that of intermediate gear. The micro hardness profile of the teeth of drive pinion also reflects the presence of case carburized layer (Figure 9). As a result, micro hardness values at the core are lower in comparison to the surface of the teeth. The hardness values at surface and core of drive pinion is lower than the intermediate gear at similar locations. The fracture surface of the failed intermediate gear is shown in Figure 10. The fracture surface of the intermediate gear shows the presence of rubbed regions, elliptical beach marks at the centre of the fracture surface as well as jagged regions near the periphery. The appearance of jagged regions near periphery indicates that this is the last portion of the tooth to break away. High magnification of elliptical beach mark displays flat regions with very fine and shallow dimples, micro cracks and uneven shaped regions consisting of curved / distorted flat surfaces. This indicates that the fatigue cracks have initiated at different locations. Based on the microstructural evidences, it appears that the cracks initiate at the fillet and moves towards the root of the individual tooth. This is due to the fact that hardening is not extended to the root of the tooth. It is to be noted that high stress concentration exists at fillets. The crack slowly propagates over a large part of the life. These cracks also simultaneously initiate and propagate in the roots of other teeth. The crack then propagates fast and suddenly results in fracture.
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The presence of elliptical beach marks at the centre of the fracture and jagged regions near periphery supports the same. In this case, the whole tooth or part of the tooth breaks away. As a result, several teeth can fail together. The teeth failure of the intermediate gear is therefore by bending fatigue. This occurs from excessive load on teeth. This results in root stresses higher than the endurance limit of the material. When the gears are loaded and subjected to enough repeated cycles during operation, the teeth fail due to excessive loads.
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Premkumar Manda et al./ Materials Today: Proceedings 5 (2018) 5254–5261
The fracture surface of the failed tooth of drive pinion is shown in Figure 11. It resembles the presence of three features namely oval type beach (C1), jagged regions (C2) and clean break lines. The high magnification fracture surface displays largely the appearance of dimples and small regions of flat facets (Figure 11b). The regions consisting of dimples also reveal the presence of coalescence of dimples (Figure 11c). A sudden shock loads generally display break lines. This indicates that the drive pinion teeth are failed by bending fatigue followed by shock loads. It appears that the crack initiates from the fillets and it moves towards the root. The travel distance of the crack in drive pinion is relatively less than the intermediate gear. The beach mark in failed teeth therefore appears in drive pinion on one side of the fracture (marked C1) whereas it is observed at the centre in intermediate gear. It is clear from the Figures 4 and 11 that failure of teeth of drive pinion occurs by initiation of cracks from fillets and these cracks moves a short distance toward the root during bending fatigue. The teeth then break due to shock loads. The presence of jagged regions supports the same. It appears that one tooth fails from shock and then load is transferred to successive teeth. This has resulted in the stripping of several teeth as observed in Figure 4. It appears that the length of the cracks initiated at fillets plays a very important role. If the length is large enough to reach the centre of the root, then the oval beach mark appears at the centre of the fracture surface as observed in case of intermediate gear. This allows the initiation and movement of cracks simultaneously in several teeth and they can fail together. On the other hand, if the crack length is too short as in case of drive pinion, the oval beach mark appears on one side of the fracture surface (marked C1). This caused the failure of individual tooth and load is being transferred to successive tooth. The stripping of several teeth takes place one by one.
The problem of teeth failure in both the intermediate gear and drive pinion can be overcome by (i) designing the gear tooth elements which can bear the transmitted load within endurance limit, (ii) using high strength material for the fabrication of gear, (iii) appropriate case carburizing of the teeth with sufficient hardness and (iv) proper heat treatment to minimize residual stresses. 4. Conclusions • Both the teeth of intermediate gear and drive fan pinion fail by bending fatigue due to over / shock loads although the associated mechanisms are quite different. • The length of the crack in carburized layer in turn defines the nature of failure. The large crack length introduces simultaneous failure of several teeth as observed in intermediate gear while the short crack of drive pinion results in successive failure of the same. • The cracks are initiated from fillet and propagate towards the centre of root due to insufficient hardening which has not extended properly to the roots of teeth.
Premkumar Manda et al./ Materials Today: Proceedings 5 (2018) 5254–5261
5261
Acknowledgements Authors acknowledge Defence Research and Development Organisation for financial support and Dr Samir V Kamat, Director, DMRL for his kind encouragement. Authors thank Photography, SFAG and EMG groups of DMRL for their kind help. References [1] S.H. Avner: Introduction to Physical Metallurgy, 2nd edition, Tata McGraw-Hill edition, New Delhi, 1997. [2] T.V. Rajan, C.P. Sharma and A. Sharma: Heat Treatment: Principles and Techniques, 2nd edition. PHI learning private limited, New Delhi, 2011. [3] L.J. Ebert, Metallurgical Transactions A, 9 (1978) 1537. [4] R.G. Luther, T.R.G. Williams, Metallurgia and Metal Forming, 41 (1974) 72. [5] K. Genel, M. Demirkol, International Journal of Fatigue, 21 (1999) 207. [6] G. Krauss, Metallurgical Transactions A, 9 (1978) 1527. [7] M. Preciado, P.M. Bravo, J.M. Alegre, Journal of Materials Processing Technology 176 (2006) 41.