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Failure investigation of antenna wire of helicopter
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 4408–4415
www.materialstoday.com/proceedings
ICMPC-2019
Failure investigation of antenna wire of helicopter Rajesh Sharmaa* and Ashok Kumar Singha a
Defence metallurgical research laboatory, kanchanbagh, hyderabad-500058, India
Abstract Present report describes the failure investigation of high frequency (HF) antenna cable of the helicopter. The incidents of HF antenna cable being broken near finger insulator were reported by helicopter operating unit. The present work includes thorough failure investigation to find out the reason for this failure. The in-depth analysis includes visual observation, stereo microscopy, chemical analysis by energy dispersive spectroscopy, fractography, microstructural characterisation and hardness measurement. The experimental evidences suggest the main cause of the HF antenna cable failure is overload. © 2019 Published by Elsevier Ltd. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: antenna cable; energy dispersive spectroscopy; fractograph; microstructure; hardness.
1. Introduction The incidents of failure of high frequency (HF) antenna cable have occurred near finger insulator. The HF antenna cable was broken near finger insulator on starboard side of the helicopter. This cable has completed 1008:04 hrs of life. After extensive visual checks on HF antenna cable and airframe, following points have emerged: (i) rear end of HF antenna cable is connected to stabilizer and front end to front stands above tail boom on both the port and starboard sides of the helicopter. Both ends of the HF antenna cable are insulated from helicopter body with the help of finger insulators and connected to airframe support via antenna cable. In addition, both the sides of HF cables are interconnected to each other near the front end with the help of tensionless insulated cable.
* Corresponding author. Tel.: 09441427205; fax: 91-40-24340266. E-mail address: [email protected]
2214-7853© 2019 Published by Elsevier Ltd. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
R. Sharma et al./ Materials Today: Proceedings 18 (2019) 4408–4415
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The tensionless insulated cable forms the whole length of HF cable into one single antenna cable. HF cables on both sides are kept tightened at a tension varying from 6.7 to 11.3 Kgf in accordance with temperature. (ii) Visual inspection revealed that the cable sub-part near finger insulator on starboard side had broken which resulted in releasing up the tension in starboard side HF cable. The cable was still connected to port side HF cable through tensionless insulated cable at front end and to the stabilizer at rear end. (iii) The failure of cable was inside the rubber cap protective cover which is not visible during normal routine inspection. Generally, antennas for helicopters are mounted close to the body of helicopter [1]. The antenna consists of a wire stretched between two spacers. This is used to keep antenna away from the helicopter body. The required tension sometimes exceeds the load capacity of wire and results in overload failure. This in turn is related to applied load, operating outside temperature and wire material. A popular band of frequencies for operation of several communications is the HF band of frequencies and thus these antenna are termed as HF antenna. The primary investigation has revealed that the cable strands had broken. The thorough failure analysis was carried out to find out the reason for failure. The as-received photographs are displayed in Fig. 1(a-c). The HF broken and normal cables are displayed in these figures.
Fig. 1: The as-received photographs: (a) and (b) broken HF and (c) normal and broken cables.
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2. Experimental Visual observation of HF antenna cable was carried out and few photographs of the failed component were taken to capture the initial information. Also, close views of HF antenna cable were photographed. Subsequently, fracture surfaces were recorded using stereo microscopy. The fracture surfaces were separated by sectioning. Small pieces of HF antenna cable were also sectioned for microstructural evaluation. The cable containing fracture surface was ultrasonically cleaned by acetone. Fracture surfaces were examined under scanning electron microscope (SEM). Microstructures of the sample were examined under optical microscope. The chemical compositions of HF antenna cable have been determined using energy dispersive spectroscopy (EDS) facility attached with SEM. 3. Results 3.1. Visual observation The photographs of as-received broken HF cable are shown in Fig. 2. This displays that the cable has failed near the rubber cap. The free broken end of cable is observed here (Fig.2a). The construction of cable is defined as np where n and p are numbers of strands and number of wires in each strand, respectively [2]. Incidentally, the n and p are same in present case and equal to 7. The diameter of the cable wire in each strand is 200 m. The as-received single strand wire (separated) is also seen here. The broken cable near rubber cap is displayed in Fig. 2b. The free end of broken cable consisting of sheared fracture surface on individual strands as encircled is seen in Fig. 3a. Almost all cable strands have slant fracture feature. The slant feature near partially broken region as marked by an arrow is exhibited in Fig. 3b. The other broken individual strands have similar features.
Fig. 2: (a) The photograph of as-received HF antenna cable and (b) the close view photograph near partially broken end.
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Fig. 3: The photograph of cable: (a) at the broken end showing slant fracture as encircled and (b) few broken strands containing slant fractures as marked by arrow.
3.2. Stereo microscopy The stereo micrographs of broken HF cable are presented in Figs. 4 and 5. The sheared surfaces containing multiple steps are shown in Fig. 4 (a - c) within red circle. The pressed cable is displayed in Fig. 4c as encircled. The magnified micrographs of sheared cable are exhibited in Fig. 5. The sheared surfaces of cable strands are marked as 1 (Fig. 5a) and 2, 3 (Fig. 5b). On the other hand, the pressed cable strands are reflected as 4 and 5 (Fig. 5b). The other cable strands (not shown here) have similar features i.e. sheared and pressed.
Fig. 4: The stereo micrographs of HF antenna cable taken at different locations showing slant feature and multiple pressed regions (encircled by red colour).
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Figure 5: The stereo micrographs of failed cable surfaces marked by arrows.
3.3. Scanning electron microscopy of fracture surface The SEM fractographs of broken cable are displayed in Figs. 6 - 8. The pressed and rubbed regions (heavy dents) on cable surface are shown in Fig. 6a. These heavy dents might have formed during tightening. The failed cable containing slant fracture surface on multiple cable strands are exhibited in Fig. 6b indicating that the failure mode is overload. The SEM fractographs containing shear dimples formed during failure are displayed in Fig. 7. The throughout fracture surface contains shear dimples. The shear dimples are typical feature of overload failure. The cable is wrapped with a thicker wire with 500 m diameter. Both the wires (200 and 500 m) are displayed in Fig. 8 (a and b). The wire with 200 µm diameter consists of heavy dents which have resulted in localised reduction in cross sectional area (Fig. 8a). The load bearing capacity of cable decreases due to reduction in cross sectional area. This in turn led to failure of cable due to overload.
Fig. 6: The SEM fractographs showing (a) pressed and rubbed cable surface and (b) slant fracture surface on multiple cable strands.
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Fig. 7: The SEM fractographs showing shear dimples at different regions.
Fig. 8: The SEM fractographs: (a) localised reduction in cross sectional area and (b) other strand of cable having larger diameter.
3.4. Microstructural Characterisation: The optical microstructures of HF cable in un-etched and etched conditions are displayed in Figs. 9 and 10, respectively. The optical microstructures in un-etched condition along longitudinal and transverse directions are shown in Fig. 9 (a and b). The small dent having approximately equal depth and width of 50 µm is observed on cable strand surface along longitudinal direction (Fig. 9a). Localised dents on strand surface have weakened the cable and reduced the overall strength. Very fine globular oxide inclusions are seen along longitudinal direction (Fig. 9a). The optical microstructures in etched condition along longitudinal and transverse directions are shown in Fig. 10 (a and b). The microstructure is very fine consisting of pearlite along with fine carbide particles. Fine elongated grains are also observed along longitudinal direction. The microstructures in un-etched and etched conditions appear to be normal.
Fig. 9: The optical microstructures of HF cable along: (a) longitudinal and (b) transverse directions in un-etched condition.
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Fig. 10: The optical microstructures of HF cable along: (a) longitudinal and (b) transverse directions. Etchant: Nital
3.5. Chemical analysis and hardness measurement The chemical analysis of the cable was carried out qualitatively by EDS attached with SEM. Area analysis was accomplished at two different locations (Fig. 11(a and b)). The EDS spectra reveal that the cables are made from Fe - base alloy consisting of Cr and Si as alloying elements (Fig. 11(c and d)). The other alloying elements are either in small quantity or low atomic number and therefore may not be visible in EDS spectra due to its inherent spectral resolution. The macro hardness measurement of individual wire could not be performed due to small cross section. Thus, the micro hardness was measured by applying 500 gm load in Vickers scale. The average micro hardness value of HF cable is 537 ± 7 Hv.
Fig. 11: The SE SEM microstructures containing rectangular selected area for overall analysis (a), (b)and their corresponding EDS spectra in (c), (d).
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4. Discussion The HF cable has failed near the rubber cap. As a result, one end of the joint cable has come out (called as broken free end of the cable). Visually, several dent marks on cable surface with shear fracture end have been observed. The dent marks on the surface of the cable strands might have come due to overload during excess tightening. The stereo microscopy confirm the visual observations. The dents on the cable surface and slant fracture features support the same. The fractographs reveal the presence of pressed and rubbed surface on cable strands as well as dents. The localised reduction in cross sectional area causes decrease in load bearing ability and finally resulted in failure by overload. The appearance of twisted features on cable surface is due to excess tightening. The slant feature observed on fractographs is present in almost all strands of the cable. This also indicates that the cable has failed by shear process due to overload. The presence of shear dimples on fracture surface of cable also point towards overload mode of failure. The microstructure of cable is fine pearlite with equiaxed and elongated grains in transverse and longitudinal directions, respectively. The chemical analysis indicates that it is made from Fe base alloy containing Cr and Si as alloying elements. The micro hardness of cable is 537 Hv [3]. The microstructure in un-etched condition reveals the presence of dent having dimension of 50 µm. The dent reduces localised diameter of the cable strands and weaken its load carrying capacity significantly. 5. Concluding remark The cable has failed due to overload. The appearances of dents, slant feature and shear dimples point towards the same. Acknowledgements Authors are grateful to The Director DMRL for his kind support and encouragement. Reference [1] L. E. Brown, G. Luck and T. K. Gibbs, US Patent No. 5745081A, 1998. [2] G. A. Costello, Theory of wire rope. Chapter 1: Mechanical engineering series, 2nd edition. New York (1990). [3] N. Liu, Microstructure and mechanical properties of cold drawn steel wire (thesis), Edith Cowan University, Australia, 2012.
ScienceDirect Materials Today: Proceedings 18 (2019) 4408–4415
www.materialstoday.com/proceedings
ICMPC-2019
Failure investigation of antenna wire of helicopter Rajesh Sharmaa* and Ashok Kumar Singha a
Defence metallurgical research laboatory, kanchanbagh, hyderabad-500058, India
Abstract Present report describes the failure investigation of high frequency (HF) antenna cable of the helicopter. The incidents of HF antenna cable being broken near finger insulator were reported by helicopter operating unit. The present work includes thorough failure investigation to find out the reason for this failure. The in-depth analysis includes visual observation, stereo microscopy, chemical analysis by energy dispersive spectroscopy, fractography, microstructural characterisation and hardness measurement. The experimental evidences suggest the main cause of the HF antenna cable failure is overload. © 2019 Published by Elsevier Ltd. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: antenna cable; energy dispersive spectroscopy; fractograph; microstructure; hardness.
1. Introduction The incidents of failure of high frequency (HF) antenna cable have occurred near finger insulator. The HF antenna cable was broken near finger insulator on starboard side of the helicopter. This cable has completed 1008:04 hrs of life. After extensive visual checks on HF antenna cable and airframe, following points have emerged: (i) rear end of HF antenna cable is connected to stabilizer and front end to front stands above tail boom on both the port and starboard sides of the helicopter. Both ends of the HF antenna cable are insulated from helicopter body with the help of finger insulators and connected to airframe support via antenna cable. In addition, both the sides of HF cables are interconnected to each other near the front end with the help of tensionless insulated cable.
* Corresponding author. Tel.: 09441427205; fax: 91-40-24340266. E-mail address: [email protected]
2214-7853© 2019 Published by Elsevier Ltd. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
R. Sharma et al./ Materials Today: Proceedings 18 (2019) 4408–4415
4409
The tensionless insulated cable forms the whole length of HF cable into one single antenna cable. HF cables on both sides are kept tightened at a tension varying from 6.7 to 11.3 Kgf in accordance with temperature. (ii) Visual inspection revealed that the cable sub-part near finger insulator on starboard side had broken which resulted in releasing up the tension in starboard side HF cable. The cable was still connected to port side HF cable through tensionless insulated cable at front end and to the stabilizer at rear end. (iii) The failure of cable was inside the rubber cap protective cover which is not visible during normal routine inspection. Generally, antennas for helicopters are mounted close to the body of helicopter [1]. The antenna consists of a wire stretched between two spacers. This is used to keep antenna away from the helicopter body. The required tension sometimes exceeds the load capacity of wire and results in overload failure. This in turn is related to applied load, operating outside temperature and wire material. A popular band of frequencies for operation of several communications is the HF band of frequencies and thus these antenna are termed as HF antenna. The primary investigation has revealed that the cable strands had broken. The thorough failure analysis was carried out to find out the reason for failure. The as-received photographs are displayed in Fig. 1(a-c). The HF broken and normal cables are displayed in these figures.
Fig. 1: The as-received photographs: (a) and (b) broken HF and (c) normal and broken cables.
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2. Experimental Visual observation of HF antenna cable was carried out and few photographs of the failed component were taken to capture the initial information. Also, close views of HF antenna cable were photographed. Subsequently, fracture surfaces were recorded using stereo microscopy. The fracture surfaces were separated by sectioning. Small pieces of HF antenna cable were also sectioned for microstructural evaluation. The cable containing fracture surface was ultrasonically cleaned by acetone. Fracture surfaces were examined under scanning electron microscope (SEM). Microstructures of the sample were examined under optical microscope. The chemical compositions of HF antenna cable have been determined using energy dispersive spectroscopy (EDS) facility attached with SEM. 3. Results 3.1. Visual observation The photographs of as-received broken HF cable are shown in Fig. 2. This displays that the cable has failed near the rubber cap. The free broken end of cable is observed here (Fig.2a). The construction of cable is defined as np where n and p are numbers of strands and number of wires in each strand, respectively [2]. Incidentally, the n and p are same in present case and equal to 7. The diameter of the cable wire in each strand is 200 m. The as-received single strand wire (separated) is also seen here. The broken cable near rubber cap is displayed in Fig. 2b. The free end of broken cable consisting of sheared fracture surface on individual strands as encircled is seen in Fig. 3a. Almost all cable strands have slant fracture feature. The slant feature near partially broken region as marked by an arrow is exhibited in Fig. 3b. The other broken individual strands have similar features.
Fig. 2: (a) The photograph of as-received HF antenna cable and (b) the close view photograph near partially broken end.
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Fig. 3: The photograph of cable: (a) at the broken end showing slant fracture as encircled and (b) few broken strands containing slant fractures as marked by arrow.
3.2. Stereo microscopy The stereo micrographs of broken HF cable are presented in Figs. 4 and 5. The sheared surfaces containing multiple steps are shown in Fig. 4 (a - c) within red circle. The pressed cable is displayed in Fig. 4c as encircled. The magnified micrographs of sheared cable are exhibited in Fig. 5. The sheared surfaces of cable strands are marked as 1 (Fig. 5a) and 2, 3 (Fig. 5b). On the other hand, the pressed cable strands are reflected as 4 and 5 (Fig. 5b). The other cable strands (not shown here) have similar features i.e. sheared and pressed.
Fig. 4: The stereo micrographs of HF antenna cable taken at different locations showing slant feature and multiple pressed regions (encircled by red colour).
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Figure 5: The stereo micrographs of failed cable surfaces marked by arrows.
3.3. Scanning electron microscopy of fracture surface The SEM fractographs of broken cable are displayed in Figs. 6 - 8. The pressed and rubbed regions (heavy dents) on cable surface are shown in Fig. 6a. These heavy dents might have formed during tightening. The failed cable containing slant fracture surface on multiple cable strands are exhibited in Fig. 6b indicating that the failure mode is overload. The SEM fractographs containing shear dimples formed during failure are displayed in Fig. 7. The throughout fracture surface contains shear dimples. The shear dimples are typical feature of overload failure. The cable is wrapped with a thicker wire with 500 m diameter. Both the wires (200 and 500 m) are displayed in Fig. 8 (a and b). The wire with 200 µm diameter consists of heavy dents which have resulted in localised reduction in cross sectional area (Fig. 8a). The load bearing capacity of cable decreases due to reduction in cross sectional area. This in turn led to failure of cable due to overload.
Fig. 6: The SEM fractographs showing (a) pressed and rubbed cable surface and (b) slant fracture surface on multiple cable strands.
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Fig. 7: The SEM fractographs showing shear dimples at different regions.
Fig. 8: The SEM fractographs: (a) localised reduction in cross sectional area and (b) other strand of cable having larger diameter.
3.4. Microstructural Characterisation: The optical microstructures of HF cable in un-etched and etched conditions are displayed in Figs. 9 and 10, respectively. The optical microstructures in un-etched condition along longitudinal and transverse directions are shown in Fig. 9 (a and b). The small dent having approximately equal depth and width of 50 µm is observed on cable strand surface along longitudinal direction (Fig. 9a). Localised dents on strand surface have weakened the cable and reduced the overall strength. Very fine globular oxide inclusions are seen along longitudinal direction (Fig. 9a). The optical microstructures in etched condition along longitudinal and transverse directions are shown in Fig. 10 (a and b). The microstructure is very fine consisting of pearlite along with fine carbide particles. Fine elongated grains are also observed along longitudinal direction. The microstructures in un-etched and etched conditions appear to be normal.
Fig. 9: The optical microstructures of HF cable along: (a) longitudinal and (b) transverse directions in un-etched condition.
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Fig. 10: The optical microstructures of HF cable along: (a) longitudinal and (b) transverse directions. Etchant: Nital
3.5. Chemical analysis and hardness measurement The chemical analysis of the cable was carried out qualitatively by EDS attached with SEM. Area analysis was accomplished at two different locations (Fig. 11(a and b)). The EDS spectra reveal that the cables are made from Fe - base alloy consisting of Cr and Si as alloying elements (Fig. 11(c and d)). The other alloying elements are either in small quantity or low atomic number and therefore may not be visible in EDS spectra due to its inherent spectral resolution. The macro hardness measurement of individual wire could not be performed due to small cross section. Thus, the micro hardness was measured by applying 500 gm load in Vickers scale. The average micro hardness value of HF cable is 537 ± 7 Hv.
Fig. 11: The SE SEM microstructures containing rectangular selected area for overall analysis (a), (b)and their corresponding EDS spectra in (c), (d).
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4. Discussion The HF cable has failed near the rubber cap. As a result, one end of the joint cable has come out (called as broken free end of the cable). Visually, several dent marks on cable surface with shear fracture end have been observed. The dent marks on the surface of the cable strands might have come due to overload during excess tightening. The stereo microscopy confirm the visual observations. The dents on the cable surface and slant fracture features support the same. The fractographs reveal the presence of pressed and rubbed surface on cable strands as well as dents. The localised reduction in cross sectional area causes decrease in load bearing ability and finally resulted in failure by overload. The appearance of twisted features on cable surface is due to excess tightening. The slant feature observed on fractographs is present in almost all strands of the cable. This also indicates that the cable has failed by shear process due to overload. The presence of shear dimples on fracture surface of cable also point towards overload mode of failure. The microstructure of cable is fine pearlite with equiaxed and elongated grains in transverse and longitudinal directions, respectively. The chemical analysis indicates that it is made from Fe base alloy containing Cr and Si as alloying elements. The micro hardness of cable is 537 Hv [3]. The microstructure in un-etched condition reveals the presence of dent having dimension of 50 µm. The dent reduces localised diameter of the cable strands and weaken its load carrying capacity significantly. 5. Concluding remark The cable has failed due to overload. The appearances of dents, slant feature and shear dimples point towards the same. Acknowledgements Authors are grateful to The Director DMRL for his kind support and encouragement. Reference [1] L. E. Brown, G. Luck and T. K. Gibbs, US Patent No. 5745081A, 1998. [2] G. A. Costello, Theory of wire rope. Chapter 1: Mechanical engineering series, 2nd edition. New York (1990). [3] N. Liu, Microstructure and mechanical properties of cold drawn steel wire (thesis), Edith Cowan University, Australia, 2012.