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Testing and Prediction of Structural Failures Caused by Fretting
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 3 (2016) 1103 – 1107
DAS 2015
Testing and prediction of structural failures caused by fretting Milosav Ognjanovica*, Marko Milosa, Nenad Kolarevica a
University of Belgrade, Faculty of mechanical engineering, 11120 Belgrade, Serbia
Abstract Fretting is an insidious failure difficult to predict when it will occur along the service life of a mechanical structure and to take it into consideration in the design stage. The article contains presentation of some of these failures, the analysis of causes which produce fretting fatigue and fretting wear. Testing methodology for fretting fatigue is developed but not in a full sense for fretting wear testing. The main objective of the work is the development of the testing device for fretting wear failure probability identification and analysis of the possibility to include this probability in reliability for design of the mechanical structure. © 2015 Elsevier Ltd. All rights reserved. © 2016 Elsevier Ltd. All rights reserved. Selection andPeer-review Peer-review under responsibility the Committee Members 32nd DANUBIA ADRIA SYMPOSIUM on Selection and under responsibility of theofCommittee Members of 32ndof DANUBIA ADRIA SYMPOSIUM on AdvancedininExperimental Experimental Mechanics (DAS 2015). Advanced Mechanics (DAS 2015) Keywords: Fretting; Testing device; Reliability for design; Engineering design
1. Introduction Fretting is a phenomenon which produces surface failures and significant material wear without visible causes [1]. Relative micro-motions in the surface contacts are the result of elastic deformations of machine parts exposed to dynamic loads, vibrations, misalignments of rotation components, etc. In general, two kinds of failure can be the result of the fretting process: fretting fatigue and fretting wear. Figure 1 displays two examples. The shaft from the automotive gearbox (Fig.1a) is damaged by both kinds of failure. Contact surface of the spline joint is deeply worn and then the cracks have taken out a piece from the shaft. The claws of the claw coupling (Fig.1b) are outworn by fretting caused by misalignment and rotation. The fretting wear produces significant loss of material from the contacting surfaces. The fretting fatigue is acceleration of crack nucleation and reduction of fatigue strength. According to [2] a high level of pressure and lower displacement amplitude produce fretting fatigue and clean fretting wear is the result of higher motion amplitude with the same or lesser contact pressure (Fig.1c). A very small displacement amplitude or total absence of micro-motion eliminates any failure of the contact surfaces. * Corresponding author: Milosav Ognjanović Tel.: +381-62-295-890; fax: +381-11-3370-364. E-mail address: [email protected]
2214-7853 © 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of 32nd DANUBIA ADRIA SYMPOSIUM on Advanced in Experimental Mechanics (DAS 2015) doi:10.1016/j.matpr.2016.03.056
1104
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
a)
b)
c) Fig. 1. Fretting failures: (a) shaft failed by fretting fatigue and fretting wear; (b) glow coupling with glows failed by fretting wear, c) areas of fretting fatigue and fretting wear.
2. Testing methodology The theory of the fretting process is analysed using analytic or numerical methodology [3, 4] but for practical application experimental data have the main role. For this purpose, the basic principles of the fretting failure testing are standardized [2]. The surface contacts in these testing principles can be the point contact, the line contact and the surface contact. In order to provide normal force balance in the testing device, suggested contacts are symmetrical in relation to the testing sample. Contacts can be in one section of the testing sample (single clamp) or in two sections in the form of the bridge-type and grip-type of the device. One of these tests’ principles with the bridge surface contact is used for presentation in Fig. 2. The principle provides possibility to vary the normal force (contact pressure) and oscillatory micro-motion. The testing device for fretting fatigue, presented in Fig. 2a, is a combination of fatigue test sample and bridge components which produce surface pressure in the sample. Elastic deformations caused by force fluctuation produce fretting in the clamp contacts. The pressure and amplitude of relative motion (Fig.1c) are in such ratio that the beginning of fretting wear initiates surface cracks which propagate inside of the testing sample volume [5]. For the testing of fretting wear it is necessary to provide oscillatory motion with a corresponding amplitude and also a corresponding pressure in the contact. One of the possible testing devices for this purpose, with one test sample (contact), is developed and presented in [6]. Figure 2b shows another possibility. The testing sample and the clamp device, in relation to Fig.2a, change the role. The central bar is exposed to oscillatory motion and four contacts of the clamp present four test samples. Due to the difference in the surface hardness, the wear is transferred to the samples only. Test results present the worn layer thickness in the test sample. Force
a)
Displacement
Fig. 2. Principles of the fretting failures testing: a) fretting fatigue testing, b) fretting wear testing.
b)
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
1105
3. The new test device development For the purpose of statistical prediction of fretting wear in mechanical structures and for design calculation reliability, it is necessary to obtain a lot of test results in a relatively short time. The idea for the new test device is based on the increase of the number of testing contacts to be simultaneously tested. Also, the frequency of oscillatory motion has to be increased as much as possible. This will increase the temperature in the testing surface contacts, and cooling is necessary. The provision of equal pressure in the contacts, the high level of oscillatory motion control, cooling etc. make this mechatronic structure really complex (Fig.3). This installation is developed starting from the principle presented in Fig.2b. Electromagnetic motion actuator produces oscillatory motion of the slider which is in contact with test samples 1 (Fig.3). Those six samples are distributed around the slider and provide 12 fretting wear contacts. Every sample is exposed to the force from hydraulic pistons 2 under controlled hydraulic (oil) pressure. In order to transfer fretting wear to the testing samples only and to protect the slider from wear, the slider has to have high surface hardness. The increase of oscillatory motion frequency and amplitude and also the pressure in the contacts significantly increase the temperature in contacts and change conditions in the contacts in relation to service conditions. In order to provide and keep service conditions, cooling and lubrication in the contacts carry a separate flow of oil. This two oil flows are separated and pressure space and cooling-lubrication spaces are sealed by corresponding seals. Fixation of test samples and prevention of sample motion together with the slider provide lateral screws. Hydraulic oil system contains oil aggregate which provides cold and clean oil in both systems for force provision, cooling and lubrication. The subsystem for oil pressure control provides oil pressure set up in advance. The surplus of oil returns to the oil aggregate to be cooled and cleaned. During the experiment, this oil circulates and inside of the test device there is the same oil. In the cooling and lubrication flow the oil circulates through the space of the test contacts. The action of the pump for oil suction from the device provides permanent oil circulation and cooling. Measurement of oscillatory motion is performed by the contactless sensor. The controller for electromagnetic motion actuator control compares the measured amplitude and frequency and maintains the desired values of motion amplitude and frequency. The increase of frequency testing process can be significantly accelerated. However, the frequency has to be harmonized with the amplitude, necessary force, actuator power, temperature, etc. The process of fretting wear testing is already significantly accelerated by simultaneous testing of twelve fretting contacts. Simultaneous testing provides the set of results in one attempt and in a relatively short time provides enough data for statistical processing. The test starts with the selected contact pressure in the fretting contacts and for the selected number of oscillations N. Measurement of fretting worn layer thickness 'h in every twelve contacts follows after device disassembly. In Fig.4a, for a certain pressure and certain number N, it is presented the number of failures z in relation to worn layer thickness 'h and amplitude xa. Using the set of those presentations, in Fig 4b (right side), it is established the range of fretting wear failure probability distribution bounded by the lines with the failure probability PF=0.1 and 0.9. The range corresponds to a certain worn layer thickness 'h and to a certain pressure in contact.
Fig. 3. The new installation for probabilistic fretting wear testing
1106
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
4. Prediction of fretting wear failures Prediction of fretting wear presents a very complex problem. This failure in the contact surfaces of mechanical structure can be eliminated if in the contact there is no relative oscillatory motion or/and no contact pressure. Figure 4a shows one from the collection of such diagrams obtained by testing using the device in Fig.3. The procedure for these data processing, which provide failure probability distribution PF=0.1...0.9 (Fig.4b) for a certain thickness of worn layer, is not presented. Since the amplitude of oscillatory motion is also random, on the left side of Fig.4b the spectrum of the amplitude is also shown. Elementary reliability for design R is the result of relation between these two probabilities: probability of displacement amplitude pi, and failure probability for this amplitude PFi § xai ·
E
¨ ¸ n6i ; R=1-FP (1) PFi 1 e © K ¹ ; pi Fp ¦ pi PFi ; n6 i 1 In these relations i denotes the order of xa displacement amplitude value and n6i is the number of oscillations with this amplitude in the total number of oscillations n6Amplitude cycles to failure is N. The failure probability PFi for each value of amplitude xai and cycles’ number n6i is presented by Waybills’ distribution function with the parameters Kand E . A simpler approach implies unification of displacement amplitude values when i=1, pi=1 and unreliability Fp=PF for the total number n6The third simplest approach is reliability testing of a certain mechanical joint. k
a)
b) Fig. 4. Fretting wear probability and reliability for design prediction
5. Conclusion In order to predict the fretting wear in mechanical structures and identify reliability for the structures design, a lot of test results are necessary. For this purpose, the testing device for simultaneous testing of 12 samples is developed. This is a mechatronic system which provides the load, oscillatory motion and temperature control. Testing and measurement of motion cycles’ number and surface layer thickness of worn out material can provide data for statistical analysis. The three approaches for design prediction reliability in relation to the fretting wear are presented. Acknowledgements This work is a contribution to the Ministry of Education, Science and Technological development of Serbia project TR-035006 and TR-035044. References [1] [2] [3] [4]
J. Murugesan, Y. Mutoh, Fatigue strength prediction of dovetail joint and bolted joint by using the generalized tangential stress range– compressive stress range diagram, Tribology International, vol. 76 (2014) 116–121. R.W. Neu: Progress in standardization of fretting fatigue terminology and testing, Tribology International, 44(2011)1371–1377. H. Attia, A generalized fretting wear theory, Tribology International 42 (2009) 1380–1388. I. Chung, S. Lee, J. Kwon, Fretting wear volume calculation on cylindrical surface: Inaccuracy due to misalignment of measured object with respect to 3D profiler, Wear 297 (2013) 1074–1080
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
[5] [6]
1107
G.Q. Wu, Z. Li, W. Sha, H.H. Li, L.J. Huang: Effect of fretting on fatigue perform. of Ti-1023 titanium alloy, Wear 326-327 (2015) 20– 27. R.Ramesh, R. Gnanamoorthy, Development of a fretting wear test rig and preliminary studies for understanding the fretting wear properties of steels, Materials and Design, vol 27 (2006) 141–146.
ScienceDirect Materials Today: Proceedings 3 (2016) 1103 – 1107
DAS 2015
Testing and prediction of structural failures caused by fretting Milosav Ognjanovica*, Marko Milosa, Nenad Kolarevica a
University of Belgrade, Faculty of mechanical engineering, 11120 Belgrade, Serbia
Abstract Fretting is an insidious failure difficult to predict when it will occur along the service life of a mechanical structure and to take it into consideration in the design stage. The article contains presentation of some of these failures, the analysis of causes which produce fretting fatigue and fretting wear. Testing methodology for fretting fatigue is developed but not in a full sense for fretting wear testing. The main objective of the work is the development of the testing device for fretting wear failure probability identification and analysis of the possibility to include this probability in reliability for design of the mechanical structure. © 2015 Elsevier Ltd. All rights reserved. © 2016 Elsevier Ltd. All rights reserved. Selection andPeer-review Peer-review under responsibility the Committee Members 32nd DANUBIA ADRIA SYMPOSIUM on Selection and under responsibility of theofCommittee Members of 32ndof DANUBIA ADRIA SYMPOSIUM on AdvancedininExperimental Experimental Mechanics (DAS 2015). Advanced Mechanics (DAS 2015) Keywords: Fretting; Testing device; Reliability for design; Engineering design
1. Introduction Fretting is a phenomenon which produces surface failures and significant material wear without visible causes [1]. Relative micro-motions in the surface contacts are the result of elastic deformations of machine parts exposed to dynamic loads, vibrations, misalignments of rotation components, etc. In general, two kinds of failure can be the result of the fretting process: fretting fatigue and fretting wear. Figure 1 displays two examples. The shaft from the automotive gearbox (Fig.1a) is damaged by both kinds of failure. Contact surface of the spline joint is deeply worn and then the cracks have taken out a piece from the shaft. The claws of the claw coupling (Fig.1b) are outworn by fretting caused by misalignment and rotation. The fretting wear produces significant loss of material from the contacting surfaces. The fretting fatigue is acceleration of crack nucleation and reduction of fatigue strength. According to [2] a high level of pressure and lower displacement amplitude produce fretting fatigue and clean fretting wear is the result of higher motion amplitude with the same or lesser contact pressure (Fig.1c). A very small displacement amplitude or total absence of micro-motion eliminates any failure of the contact surfaces. * Corresponding author: Milosav Ognjanović Tel.: +381-62-295-890; fax: +381-11-3370-364. E-mail address: [email protected]
2214-7853 © 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of 32nd DANUBIA ADRIA SYMPOSIUM on Advanced in Experimental Mechanics (DAS 2015) doi:10.1016/j.matpr.2016.03.056
1104
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
a)
b)
c) Fig. 1. Fretting failures: (a) shaft failed by fretting fatigue and fretting wear; (b) glow coupling with glows failed by fretting wear, c) areas of fretting fatigue and fretting wear.
2. Testing methodology The theory of the fretting process is analysed using analytic or numerical methodology [3, 4] but for practical application experimental data have the main role. For this purpose, the basic principles of the fretting failure testing are standardized [2]. The surface contacts in these testing principles can be the point contact, the line contact and the surface contact. In order to provide normal force balance in the testing device, suggested contacts are symmetrical in relation to the testing sample. Contacts can be in one section of the testing sample (single clamp) or in two sections in the form of the bridge-type and grip-type of the device. One of these tests’ principles with the bridge surface contact is used for presentation in Fig. 2. The principle provides possibility to vary the normal force (contact pressure) and oscillatory micro-motion. The testing device for fretting fatigue, presented in Fig. 2a, is a combination of fatigue test sample and bridge components which produce surface pressure in the sample. Elastic deformations caused by force fluctuation produce fretting in the clamp contacts. The pressure and amplitude of relative motion (Fig.1c) are in such ratio that the beginning of fretting wear initiates surface cracks which propagate inside of the testing sample volume [5]. For the testing of fretting wear it is necessary to provide oscillatory motion with a corresponding amplitude and also a corresponding pressure in the contact. One of the possible testing devices for this purpose, with one test sample (contact), is developed and presented in [6]. Figure 2b shows another possibility. The testing sample and the clamp device, in relation to Fig.2a, change the role. The central bar is exposed to oscillatory motion and four contacts of the clamp present four test samples. Due to the difference in the surface hardness, the wear is transferred to the samples only. Test results present the worn layer thickness in the test sample. Force
a)
Displacement
Fig. 2. Principles of the fretting failures testing: a) fretting fatigue testing, b) fretting wear testing.
b)
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
1105
3. The new test device development For the purpose of statistical prediction of fretting wear in mechanical structures and for design calculation reliability, it is necessary to obtain a lot of test results in a relatively short time. The idea for the new test device is based on the increase of the number of testing contacts to be simultaneously tested. Also, the frequency of oscillatory motion has to be increased as much as possible. This will increase the temperature in the testing surface contacts, and cooling is necessary. The provision of equal pressure in the contacts, the high level of oscillatory motion control, cooling etc. make this mechatronic structure really complex (Fig.3). This installation is developed starting from the principle presented in Fig.2b. Electromagnetic motion actuator produces oscillatory motion of the slider which is in contact with test samples 1 (Fig.3). Those six samples are distributed around the slider and provide 12 fretting wear contacts. Every sample is exposed to the force from hydraulic pistons 2 under controlled hydraulic (oil) pressure. In order to transfer fretting wear to the testing samples only and to protect the slider from wear, the slider has to have high surface hardness. The increase of oscillatory motion frequency and amplitude and also the pressure in the contacts significantly increase the temperature in contacts and change conditions in the contacts in relation to service conditions. In order to provide and keep service conditions, cooling and lubrication in the contacts carry a separate flow of oil. This two oil flows are separated and pressure space and cooling-lubrication spaces are sealed by corresponding seals. Fixation of test samples and prevention of sample motion together with the slider provide lateral screws. Hydraulic oil system contains oil aggregate which provides cold and clean oil in both systems for force provision, cooling and lubrication. The subsystem for oil pressure control provides oil pressure set up in advance. The surplus of oil returns to the oil aggregate to be cooled and cleaned. During the experiment, this oil circulates and inside of the test device there is the same oil. In the cooling and lubrication flow the oil circulates through the space of the test contacts. The action of the pump for oil suction from the device provides permanent oil circulation and cooling. Measurement of oscillatory motion is performed by the contactless sensor. The controller for electromagnetic motion actuator control compares the measured amplitude and frequency and maintains the desired values of motion amplitude and frequency. The increase of frequency testing process can be significantly accelerated. However, the frequency has to be harmonized with the amplitude, necessary force, actuator power, temperature, etc. The process of fretting wear testing is already significantly accelerated by simultaneous testing of twelve fretting contacts. Simultaneous testing provides the set of results in one attempt and in a relatively short time provides enough data for statistical processing. The test starts with the selected contact pressure in the fretting contacts and for the selected number of oscillations N. Measurement of fretting worn layer thickness 'h in every twelve contacts follows after device disassembly. In Fig.4a, for a certain pressure and certain number N, it is presented the number of failures z in relation to worn layer thickness 'h and amplitude xa. Using the set of those presentations, in Fig 4b (right side), it is established the range of fretting wear failure probability distribution bounded by the lines with the failure probability PF=0.1 and 0.9. The range corresponds to a certain worn layer thickness 'h and to a certain pressure in contact.
Fig. 3. The new installation for probabilistic fretting wear testing
1106
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
4. Prediction of fretting wear failures Prediction of fretting wear presents a very complex problem. This failure in the contact surfaces of mechanical structure can be eliminated if in the contact there is no relative oscillatory motion or/and no contact pressure. Figure 4a shows one from the collection of such diagrams obtained by testing using the device in Fig.3. The procedure for these data processing, which provide failure probability distribution PF=0.1...0.9 (Fig.4b) for a certain thickness of worn layer, is not presented. Since the amplitude of oscillatory motion is also random, on the left side of Fig.4b the spectrum of the amplitude is also shown. Elementary reliability for design R is the result of relation between these two probabilities: probability of displacement amplitude pi, and failure probability for this amplitude PFi § xai ·
E
¨ ¸ n6i ; R=1-FP (1) PFi 1 e © K ¹ ; pi Fp ¦ pi PFi ; n6 i 1 In these relations i denotes the order of xa displacement amplitude value and n6i is the number of oscillations with this amplitude in the total number of oscillations n6Amplitude cycles to failure is N. The failure probability PFi for each value of amplitude xai and cycles’ number n6i is presented by Waybills’ distribution function with the parameters Kand E . A simpler approach implies unification of displacement amplitude values when i=1, pi=1 and unreliability Fp=PF for the total number n6The third simplest approach is reliability testing of a certain mechanical joint. k
a)
b) Fig. 4. Fretting wear probability and reliability for design prediction
5. Conclusion In order to predict the fretting wear in mechanical structures and identify reliability for the structures design, a lot of test results are necessary. For this purpose, the testing device for simultaneous testing of 12 samples is developed. This is a mechatronic system which provides the load, oscillatory motion and temperature control. Testing and measurement of motion cycles’ number and surface layer thickness of worn out material can provide data for statistical analysis. The three approaches for design prediction reliability in relation to the fretting wear are presented. Acknowledgements This work is a contribution to the Ministry of Education, Science and Technological development of Serbia project TR-035006 and TR-035044. References [1] [2] [3] [4]
J. Murugesan, Y. Mutoh, Fatigue strength prediction of dovetail joint and bolted joint by using the generalized tangential stress range– compressive stress range diagram, Tribology International, vol. 76 (2014) 116–121. R.W. Neu: Progress in standardization of fretting fatigue terminology and testing, Tribology International, 44(2011)1371–1377. H. Attia, A generalized fretting wear theory, Tribology International 42 (2009) 1380–1388. I. Chung, S. Lee, J. Kwon, Fretting wear volume calculation on cylindrical surface: Inaccuracy due to misalignment of measured object with respect to 3D profiler, Wear 297 (2013) 1074–1080
Milosav Ognjanovic et al. / Materials Today: Proceedings 3 (2016) 1103 – 1107
[5] [6]
1107
G.Q. Wu, Z. Li, W. Sha, H.H. Li, L.J. Huang: Effect of fretting on fatigue perform. of Ti-1023 titanium alloy, Wear 326-327 (2015) 20– 27. R.Ramesh, R. Gnanamoorthy, Development of a fretting wear test rig and preliminary studies for understanding the fretting wear properties of steels, Materials and Design, vol 27 (2006) 141–146.