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Domination of self-heating effect during fatigue of polymeric composites
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Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
Procedia Structural Integrity 00 5 (2017) Structural Integrity Procedia (2016)93–98 000–000
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
2nd International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, Madeira, Portugal
of self-heating effect fatigue ofPaço polymeric XVDomination Portuguese Conference on Fracture, PCF 2016, during 10-12 February 2016, de Arcos, Portugal composites Thermo-mechanical modeling of a high pressure turbine blade of an a, Andrzej * engine airplane gasKatunin turbine a Institute
Institute of Fundamentals of Machinery Design, Silesian University of Technology, Konarskiego 18A, Gliwice 44-100, Poland
P. Brandãoa, V. Infanteb, A.M. Deusc*
a
Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal ThecCeFEMA, followingDepartment study focuses on investigation of influence of a self-heating temperature fatiguePais, process of polymeric of Mechanical Engineering, Instituto Superior Técnico, Universidade de value Lisboa,on Av.aRovisco 1, 1049-001 Lisboa, composites and, in particular, on a criticality of this effect. A Portugal main goal of the study is to find a temperature value at which its
Abstract b
growth becomes non-stationary (and thus dominates the fatigue processes) and investigate an influence of selected self-heating temperature values on fatigue life of a composite structure. The investigation is based on experimental studies, during which Abstract were subjected to cyclic loading with simultaneous measurement of loading force, deflection velocity, surface specimens temperature and acoustic emission. Such measurements allow for accurate evaluation of differences between particular loading During their operation, modern aircraft engine subjectedinitiation, to increasingly demanding conditions, cases as well as determination of characteristic points components (moments) ofare degradation and finally analysis ofoperating all of the measured especiallywithin the high pressureofturbine blades. Such cause these partsfound to undergo different typeseffect of time-dependent parameters a number cycles (HPT) to failure. Based on conditions the obtained results it was that the self-heating is possibly degradation, one of which is creep. A model using even the finite element low method (FEM) was developed, in order be significantly able to predict dangerous to cyclically loaded composite structures at relatively self-heating temperature values and to may the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation shorten their structural lifetime. company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model © 2017 The Authors. Published by Elsevier B.V. the FEM analysis, a HPT © needed 2017 Thefor Authors. Published by Elsevier B.V.blade scrap was scanned, and its chemical composition and material properties were Peer-review under responsibility of the Scientific Committee of ICSI 2017. Peer-review responsibility the Scientific of FEM ICSI 2017 obtained.under The data that wasofgathered was Committee fed into the model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The Keywords: fatigue of polymeric composites; self-heating effect; accelerated degradation overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
1.©Introduction 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016.
A self-heating effect, occurring in polymeric composites during vibrations or cyclic loading due to a viscoelastic Keywords: High Pressure Blade; Creep; Finiteis Element 3D Model; Simulation. which, under certain conditions, may of polymers usedTurbine for such composites, a veryMethod; dangerous phenomenon, nature
* Corresponding author. Tel.: +48-32-237-2741; fax:+48-32-237-1360. E-mail address: [email protected] 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review underauthor. responsibility the Scientific Committee of ICSI 2017. * Corresponding Tel.: +351of 218419991. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017 10.1016/j.prostr.2017.07.073
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dominate the fatigue process and significantly intensify degradation processes of a loaded polymeric structure. Due to dissipative processes of mechanical energy delivered to the structure, an intensive heating occurs there, which is a main part of the dissipated mechanical energy. The heat is additionally stored in the structure since most polymers used in manufacturing of industrial composites are characterized by a very low thermal conductivity. The resulting temperature is strongly related to stress, thus the higher the stress the higher value of a self-heating temperature it is. According to this, the self-heating effect may develop following two scenarios: the first scenario assumes a growth of a self-heating temperature until reaching a specific value, and then, its stabilization or a very slow linear growth (caused by mechanical degradation); while the second scenario assumes domination of the self-heating effect in the fatigue process, which results in sudden degradation of the structure until reaching a limit strength and failure. Both of the mentioned cases are presented in Fig. 1.
Fig. 1. Experimental data representing possible scenarios of the self-heating effect development.
When the self-heating effect dominates the fatigue process, damage initiation and propagation occurs at much lower value of temperature than the temperature reached during failure, which intensifies this process. Moreover, in case of the non-stationary self-heating, one can clearly observe three characteristic phases of a temperature growth (confirmed in numerous studies – see e.g. Ferreira et al. (1999), Toubal et al. (2006), Naderi and Khonsari (2012), Katunin (2012a)): the first one is heating following the exponential characteristic (according to the thermodynamic laws), the second phase is connected with the temperature stabilization or monotonic linear growth resulting from progressive damage accumulation, and the third phase is connected with an initiation and development of a macrocrack in a location of the highest stress concentration, which, in consequence, leads to a rapid self-heating temperature growth and a failure of the structure. In case of the stationary self-heating, while a relatively low number of cycles (or a short loading time period) is considered, only the first and the second phase of the temperature growth are observed, i.e. after reaching a certain value the self-heating temperature distribution stabilizes. Thus, it can be interpreted that the self-heating effect influences on fatigue, however does not dominate it. Numerous studies on influence of the self-heating effect on fatigue have been performed to-date. The mentioned duality of evolution of a self-heating temperature, namely, the stationary and non-stationary self-heating, has been observed in many studies (see e.g. Liu et al. (2004), Moisa et al. (2005), Karama (2011), Katunin (2012a)). From practical reasons it is essential to investigate a criticality of the self-heating effect occurring during fatigue of polymeric composites, i.e. a point (or temperature value) at which the self-heating dominates the fatigue process, which leads to the sudden degradation and failure of the loaded structure. Recently, several attempts in evaluation of the self-heating effect criticality have been made by Naderi et al. (2012), Kahirdeh and Khonsari (2014). In their studies they analyzed a temperature history curve together with an intensity of acoustic emission events in order to determine the criticality of self-heating. Previous studies of the author of the present paper focused on evaluation of the criticality of the selfheating effect covered approximation of self-heating temperature history curves (Katunin (2012b)) in order to
Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 93–98 Andrzej Katunin et al. / Structural Integrity Procedia 00 (2017) 000–000
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determine a difference between the measured temperature and the approximation model (this difference indicated a beginning of macrocrack development, and thus a critical value of a self-heating temperature) and Raman spectroscopy analysis (Katunin et al. (2012)) that allowed for evaluation of residual cross-linking processes during heat generation in polymeric composite structures. Recent studies of the author’s research group (Katunin et al. (2017)) have been focused on a multiphysical approach to evaluation of the criticality of self-heating. In this study, tested specimens were cyclically loaded until reaching a specific self-heating temperature value on their surfaces in a range of 40÷100°C with a step of 5°C, and further, parameters obtained directly from fatigue tests (acceleration of vibrations, loading force, surface temperature, acoustic emission) as well as parameters obtained from microscopic observations and tensile tests (residual elastic modulus, ultimate tensile strength, maximal force at failure) were used for evaluation of a critical self-heating temperature value. The obtained results showed that the self-heating effect initiates cracking of a composite matrix at 65-70°C, which is a much lower temperature value in comparison to the glass-transition temperature of the same composite (Katunin and Gnatowski (2012)), which is in a range of 124-157°C, depending on loading parameters. The above-described studies, however, were performed in a non-stationary self-heating regime, i.e. the self-heating dominated the fatigue process. Therefore, the presented results seem to be underdrawn and do not provide a full picture of the criticality of the self-heating phenomenon. Following this, it is essential to perform a series of experimental studies in a regime of the stationary self-heating, i.e. stabilized at certain temperature values. Performing such studies allows for determination of both influence of particular self-heating temperature values on intensity of the structural degradation as well as determination of the critical self-heating temperature by comparison of a number of loading cycles to failure between particular cases. Such an analysis allows for full qualitative and quantitative description of influence of the self-heating effect on fatigue of polymeric composites and developing a relation between self-heating temperature values and the structural lifetime of polymeric composites. 2. Specimens and testing procedure The specimens used for fatigue tests were manufactured from a 14-layered unidirectional glass/epoxy composite and supplied by Izo-Erg S.A. (Gliwice, Poland). A description of manufacturing process as well as basic mechanical and dynamic properties of these specimens can be found in Katunin and Gnatowski (2012). The composite sheet of a thickness of 2.5 mm was cut to specific dimensions of specimens: width of 10 mm and length of 100 mm. An effective length, i.e. the length between specimen holders which participated in loading, of each specimen equaled 40 mm. The specimens were loaded with a constant frequency of 30 Hz. The tests were performed on a laboratory test rig, which is presented on a scheme (Fig.2a) and a photography (Fig.2b).
Fig. 2. Experimental test rig for performing fatigue tests: (a) scheme, (b) photography.
A tested specimen 5 was clamped in a specimen holder 4 and excited by the TIRA® TV-51120 electrodynamic shaker 1 through a stinger 3 with a specimen holder 6 connected to a force sensor PCB Piezotronics® 208C03 7 at the
Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 93–98 Andrzej Katunin et al. / Structural Integrity Procedia 00 (2017) 000–000
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end. The specimen holder 4 was made of bakelite in order to provide a thermal insulation of the heat generated during the tests. For ensuring repeatable conditions, each specimen was clamped with a constant torque of 20 Nm. The excitation signal was measured by the accelerometer PCB Piezotronics® T352C34 2. A velocity of vibration of the specimen was measured on its surface near the clamp 4 using the single point laser Doppler vibrometer (LDV) Polytec ® PDV-100 9. In order to detect damage initiation in the structure due to the self-heating effect, besides measuring the surface temperature and excitation parameters, acoustic emission (AE) was observed. The AE signal was measured by means of the system Vallen® AMSY-5. In particular, an AE sensor 12, an AE signal preamplifier, a dual channel AE signal processor board 13 and a dedicated software for AE signal acquisition and processing were used. The force sensor and accelerometer were connected through a conditioning module to the multi-channel data acquisition card NI® DAQ Card 6062E, which was connected to a PC 14 and controlled by an application developed in LabView®. Force and vibration signals were acquired with a sample rate of 2 kHz. The application allows controlling of the excitation signal parameters through the analogue output of multi-channel signal acquisition module 11 and controls the shaker amplifier 10 TIRA® BAA 500. The temperature measurements were carried out using the InfraTec ® VarioCAM® hr infrared camera (IRC) 8. A frame rate of the IRC was set to 2 frames per second. The fatigue tests were performed as follows. The specimens were loaded in such a way that a maximal self-heating temperature on their surfaces reached a certain value in a range of 30÷55°C with a step of 5°C. The upper limit of 55°C was selected based on the results of previous tests, where the self-heating temperature growth became nonstationary in the second phase of its development. As a criterion of examination of non-stationarity of temperature growth the following assumption was made: if the temperature growth in the second phase increases with a rate of less than 1°C per 3000 cycles (100 s), then the self-heating temperature growth is assumed to be stationary. After reaching the certain temperature the specimens were subjected to fatigue cyclic loading until failure. For each maximal selfheating temperature value 5 specimens were tested for obtaining statistically valid results. During such a study all of the parameters available to be acquired using the above-described measurement equipment were collected. The collected data allow for evaluation of influence of the self-heating effect in the stationary regime on fatigue lifetime of polymeric composites. 3. Results and discussion The results of performed test indicated that for the considered dimensions of specimens and loading parameters the self-heating temperature was stable up to 50°C (according the assumed criterion of a temperature growth), which can be observed in Fig. 3.
Fig. 3. Selected self-heating temperature history curves for various temperatures of stabilization.
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From direct observations one can conclude that the increased temperature resulting from the self-heating effect significantly influences on a residual life of the polymer-based composite structure. Comparing the results for the stabilization temperature close to 30°C one can see that the increase of 3°C (cf. red lines in Fig. 3) may shorten a residual life of the structure two times, or more. Assuming the case of temperature stabilization on a level of 30°C and comparing the obtained temperature history curves with the results obtained for higher values of temperature stabilization one can observe that a difference of residual life in these cases is even of one order, which has a crucial meaning during the design and operation stage of composite structures subjected to cyclic loading or forced vibrations. The results of the self-heating temperature growth during fatigue tests indicate a dependence between a residual life of a composite structure and an observed self-heating temperature, which allows for modelling and prediction of a residual life based on observed stabilized self-heating temperature as well as evaluation of the criticality of the selfheating effect in case of appearance of the stationary self-heating during fatigue loading of polymer-based composite structures. One can also observe that the lower the stationary self-heating temperature during fatigue the lower the temperature at failure of a structure, which is clearly observable in Fig. 3. These observations prove that the criticality of the selfheating effect described in the previous studies (Katunin (2012a), Katunin (2012b), Katunin et al. (2017)) depends on loading conditions, which is directly connected with a generated heat. However, this dependence is noticeable well for the stationary regime of self-heating, while in case of non-stationary self-heating and domination of the self-heating effect the influence of loading conditions on the residual life is much less, and in many cases can be neglected due to intensive degradation of a composite structure in such conditions. The maximal values of a self-heating temperature for non-stationary self-heating at failure usually place in a range of 110÷130°C (see Katunin (2012a), Katunin (2012b), Katunin et al. (2017)), which is similar with the results for a temperature of stabilization of 55°C obtained in the presented study. This additionally proves that the loading parameters in case of non-stationary self-heating have little influence on a resulting critical self-heating temperature. In case of stationary self-heating, i.e. when the self-heating effect does not dominate the fatigue processes, the selfheating effect accelerates the degradation processes, but the main factor, which influences on a final failure of a structure is mechanical degradation with accompanying crack formation. The previous studies (Katunin et al. (2017)) showed that the critical self-heating temperature during non-stationary self-heating is in a range of 65÷70°C, which is connected with cracking of a polymeric matrix of a composite, while formation of a macrocrack in case of stationary self-heating appears at much lower temperature values (see Fig. 4).
Fig. 4. Thermograms of the tested specimens after various number of cycles: with a self-heating temperature of stabilization of 30°C after a) 103455, b) 275355, c) 386835 cycles and a temperature of stabilization of 35°C after d) 16380, e) 147570, f) 187095 cycles.
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4. Conclusions The performed studies focused on determination of influence of the self-heating effect in the stationary regime on fatigue processes of polymeric composites allows for preliminary evaluation of structural degradation, based on which prediction of residual life of composite structures subjected to such a type of loading is possible. The obtained results show that the degradation initiation in case of stationary self-heating, in contrast to non-stationary self-heating, depends on loading parameters and the self-heating temperature of stabilization significantly influences on a residual life of a composite structure. Moreover, it was observed that the initiation of degradation during stationary self-heating takes place at much lower temperature values than the critical self-heating temperature range determined in the author’s previous studies for non-stationary self-heating. However, formation of a macrocrack in a polymeric matrix occurs after a large number of cycles, which points on domination of mechanical character of degradation. In order to investigate the degradation processes during stationary self-heating a deeper analysis of the collected data including AE and hysteresis evolution in a function of time should be performed. Further studies will be focused on analyzing of the mentioned data for evaluation of the criticality of the self-heating effect in the stationary regime as well as confrontation of the results of the present analysis with results of degradation evaluation during nonstationary self-heating using various destructive and non-destructive testing methods. Such an analysis will allow for global evaluation of safe ranges of self-heating temperature in designed and operated polymer-based composite structures and use of the self-heating effect in the determined ranges for non-destructive evaluation of such structures. Acknowledgements The results presented in this paper have been obtained within the framework of research grant No. 2015/17/D/ST8/01294 financed by the National Science Centre, Poland. References Ferreira, J.A.M., Costa, J.D.M., Reis, P.N.B., Richardson, O.W., 1999. Analysis of Fatigue and Damage in Glass-Fibre-Reinforced Polypropylene Composite Materials. Composites Science and Technology 59(10), 1461–1467. Toubal, L., Karama, M., Lorrain, B., 2006. Damage Evolution and Infrared Thermography in Woven Composite Laminates Under Fatigue Loading. International Journal of Fatigue 28(12), 1867–1872. Naderi, M., Khonsari, M.M., 2012. Thermodynamic Analysis of Fatigue Failure in a Composite Laminate. Mechanics of Materials 46, 113–122. Liu, Z.Y., Beniwal, S., Jenkins, C.H.M., Winter, R.M., 2004. The Coupled Thermal and Mechanical Influence on a Glassy Thermoplastic Polyamide: Nylon 6,6 Under Vibro-Creep. Mechanics of Time-Dependent Materials 8, 235–253. Moisa, S., Landsberg, G., Rittel, D., Halary, J.L., 2005. Hysteretic Thermal Behavior of Amorphous Semi-Aromatic Polyamides. Polymer 46(25), 11870–11875. Karama, M., 2011. Determination of the Fatigue Limit of Carbon/Epoxy Composite Using Thermographic Analysis. Structural Control and Health Monitoring 18, 781-789. Katunin, A., 2012a. Thermal Fatigue of Polymeric Composites Under Repeated Loading. Journal of Reinforced Plastics and Composites 31(15), 1037–1044. Kahirdeh, A., Khonsari, M.M., 2014. Criticality of Degradation in Composite Materials Subjected to Cyclic Loading. Composites: Part B 61, 375– 382. Naderi, M., Kahirdeh, A., Khonsari, M.M., 2012. Dissipated Thermal Energy and Damage Evolution of Glass/Epoxy Using Infrared Thermography and Acoustic Emission. Composites: Part B 43, 1613–1620. Katunin, A. 2012b. Critical Self-Heating Temperature During Fatigue of Polymeric Composites Under Cyclic Loading. Composites Theory and Practice 12(1), 72–76. Katunin, A., Krukiewicz, K., Turczyn, R., 2012. Evaluation of Residual Cross-Linking Caused by Self-Heating Effect in Epoxy-Based Fibrous Composites Under Cyclic Loading. Chemik 68(11), 957–966. Katunin, A., Wronkowicz, A., Bilewicz, M., Wachla, D., 2017. Criticality of Self-Heating in Degradation Processes of Polymeric Composites Subjected to Cyclic Loading: A Multiphysical Approach. Archives of Civil and Mechanical Engineering, in press, DOI: 10.1016/j.acme.2017.03.003. Katunin, A., Gnatowski, A., 2012. Influence of Heating Rate on Evolution of Dynamic Properties of Polymeric Laminates. Plastics, Rubber and Composites 41(6), 233–239.
ScienceDirect
Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
Procedia Structural Integrity 00 5 (2017) Structural Integrity Procedia (2016)93–98 000–000
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
2nd International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, Madeira, Portugal
of self-heating effect fatigue ofPaço polymeric XVDomination Portuguese Conference on Fracture, PCF 2016, during 10-12 February 2016, de Arcos, Portugal composites Thermo-mechanical modeling of a high pressure turbine blade of an a, Andrzej * engine airplane gasKatunin turbine a Institute
Institute of Fundamentals of Machinery Design, Silesian University of Technology, Konarskiego 18A, Gliwice 44-100, Poland
P. Brandãoa, V. Infanteb, A.M. Deusc*
a
Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal ThecCeFEMA, followingDepartment study focuses on investigation of influence of a self-heating temperature fatiguePais, process of polymeric of Mechanical Engineering, Instituto Superior Técnico, Universidade de value Lisboa,on Av.aRovisco 1, 1049-001 Lisboa, composites and, in particular, on a criticality of this effect. A Portugal main goal of the study is to find a temperature value at which its
Abstract b
growth becomes non-stationary (and thus dominates the fatigue processes) and investigate an influence of selected self-heating temperature values on fatigue life of a composite structure. The investigation is based on experimental studies, during which Abstract were subjected to cyclic loading with simultaneous measurement of loading force, deflection velocity, surface specimens temperature and acoustic emission. Such measurements allow for accurate evaluation of differences between particular loading During their operation, modern aircraft engine subjectedinitiation, to increasingly demanding conditions, cases as well as determination of characteristic points components (moments) ofare degradation and finally analysis ofoperating all of the measured especiallywithin the high pressureofturbine blades. Such cause these partsfound to undergo different typeseffect of time-dependent parameters a number cycles (HPT) to failure. Based on conditions the obtained results it was that the self-heating is possibly degradation, one of which is creep. A model using even the finite element low method (FEM) was developed, in order be significantly able to predict dangerous to cyclically loaded composite structures at relatively self-heating temperature values and to may the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation shorten their structural lifetime. company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model © 2017 The Authors. Published by Elsevier B.V. the FEM analysis, a HPT © needed 2017 Thefor Authors. Published by Elsevier B.V.blade scrap was scanned, and its chemical composition and material properties were Peer-review under responsibility of the Scientific Committee of ICSI 2017. Peer-review responsibility the Scientific of FEM ICSI 2017 obtained.under The data that wasofgathered was Committee fed into the model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The Keywords: fatigue of polymeric composites; self-heating effect; accelerated degradation overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
1.©Introduction 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016.
A self-heating effect, occurring in polymeric composites during vibrations or cyclic loading due to a viscoelastic Keywords: High Pressure Blade; Creep; Finiteis Element 3D Model; Simulation. which, under certain conditions, may of polymers usedTurbine for such composites, a veryMethod; dangerous phenomenon, nature
* Corresponding author. Tel.: +48-32-237-2741; fax:+48-32-237-1360. E-mail address: [email protected] 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review underauthor. responsibility the Scientific Committee of ICSI 2017. * Corresponding Tel.: +351of 218419991. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017 10.1016/j.prostr.2017.07.073
94 2
Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 93–98 Andrzej Katunin et al. / Structural Integrity Procedia 00 (2017) 000–000
dominate the fatigue process and significantly intensify degradation processes of a loaded polymeric structure. Due to dissipative processes of mechanical energy delivered to the structure, an intensive heating occurs there, which is a main part of the dissipated mechanical energy. The heat is additionally stored in the structure since most polymers used in manufacturing of industrial composites are characterized by a very low thermal conductivity. The resulting temperature is strongly related to stress, thus the higher the stress the higher value of a self-heating temperature it is. According to this, the self-heating effect may develop following two scenarios: the first scenario assumes a growth of a self-heating temperature until reaching a specific value, and then, its stabilization or a very slow linear growth (caused by mechanical degradation); while the second scenario assumes domination of the self-heating effect in the fatigue process, which results in sudden degradation of the structure until reaching a limit strength and failure. Both of the mentioned cases are presented in Fig. 1.
Fig. 1. Experimental data representing possible scenarios of the self-heating effect development.
When the self-heating effect dominates the fatigue process, damage initiation and propagation occurs at much lower value of temperature than the temperature reached during failure, which intensifies this process. Moreover, in case of the non-stationary self-heating, one can clearly observe three characteristic phases of a temperature growth (confirmed in numerous studies – see e.g. Ferreira et al. (1999), Toubal et al. (2006), Naderi and Khonsari (2012), Katunin (2012a)): the first one is heating following the exponential characteristic (according to the thermodynamic laws), the second phase is connected with the temperature stabilization or monotonic linear growth resulting from progressive damage accumulation, and the third phase is connected with an initiation and development of a macrocrack in a location of the highest stress concentration, which, in consequence, leads to a rapid self-heating temperature growth and a failure of the structure. In case of the stationary self-heating, while a relatively low number of cycles (or a short loading time period) is considered, only the first and the second phase of the temperature growth are observed, i.e. after reaching a certain value the self-heating temperature distribution stabilizes. Thus, it can be interpreted that the self-heating effect influences on fatigue, however does not dominate it. Numerous studies on influence of the self-heating effect on fatigue have been performed to-date. The mentioned duality of evolution of a self-heating temperature, namely, the stationary and non-stationary self-heating, has been observed in many studies (see e.g. Liu et al. (2004), Moisa et al. (2005), Karama (2011), Katunin (2012a)). From practical reasons it is essential to investigate a criticality of the self-heating effect occurring during fatigue of polymeric composites, i.e. a point (or temperature value) at which the self-heating dominates the fatigue process, which leads to the sudden degradation and failure of the loaded structure. Recently, several attempts in evaluation of the self-heating effect criticality have been made by Naderi et al. (2012), Kahirdeh and Khonsari (2014). In their studies they analyzed a temperature history curve together with an intensity of acoustic emission events in order to determine the criticality of self-heating. Previous studies of the author of the present paper focused on evaluation of the criticality of the selfheating effect covered approximation of self-heating temperature history curves (Katunin (2012b)) in order to
Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 93–98 Andrzej Katunin et al. / Structural Integrity Procedia 00 (2017) 000–000
95 3
determine a difference between the measured temperature and the approximation model (this difference indicated a beginning of macrocrack development, and thus a critical value of a self-heating temperature) and Raman spectroscopy analysis (Katunin et al. (2012)) that allowed for evaluation of residual cross-linking processes during heat generation in polymeric composite structures. Recent studies of the author’s research group (Katunin et al. (2017)) have been focused on a multiphysical approach to evaluation of the criticality of self-heating. In this study, tested specimens were cyclically loaded until reaching a specific self-heating temperature value on their surfaces in a range of 40÷100°C with a step of 5°C, and further, parameters obtained directly from fatigue tests (acceleration of vibrations, loading force, surface temperature, acoustic emission) as well as parameters obtained from microscopic observations and tensile tests (residual elastic modulus, ultimate tensile strength, maximal force at failure) were used for evaluation of a critical self-heating temperature value. The obtained results showed that the self-heating effect initiates cracking of a composite matrix at 65-70°C, which is a much lower temperature value in comparison to the glass-transition temperature of the same composite (Katunin and Gnatowski (2012)), which is in a range of 124-157°C, depending on loading parameters. The above-described studies, however, were performed in a non-stationary self-heating regime, i.e. the self-heating dominated the fatigue process. Therefore, the presented results seem to be underdrawn and do not provide a full picture of the criticality of the self-heating phenomenon. Following this, it is essential to perform a series of experimental studies in a regime of the stationary self-heating, i.e. stabilized at certain temperature values. Performing such studies allows for determination of both influence of particular self-heating temperature values on intensity of the structural degradation as well as determination of the critical self-heating temperature by comparison of a number of loading cycles to failure between particular cases. Such an analysis allows for full qualitative and quantitative description of influence of the self-heating effect on fatigue of polymeric composites and developing a relation between self-heating temperature values and the structural lifetime of polymeric composites. 2. Specimens and testing procedure The specimens used for fatigue tests were manufactured from a 14-layered unidirectional glass/epoxy composite and supplied by Izo-Erg S.A. (Gliwice, Poland). A description of manufacturing process as well as basic mechanical and dynamic properties of these specimens can be found in Katunin and Gnatowski (2012). The composite sheet of a thickness of 2.5 mm was cut to specific dimensions of specimens: width of 10 mm and length of 100 mm. An effective length, i.e. the length between specimen holders which participated in loading, of each specimen equaled 40 mm. The specimens were loaded with a constant frequency of 30 Hz. The tests were performed on a laboratory test rig, which is presented on a scheme (Fig.2a) and a photography (Fig.2b).
Fig. 2. Experimental test rig for performing fatigue tests: (a) scheme, (b) photography.
A tested specimen 5 was clamped in a specimen holder 4 and excited by the TIRA® TV-51120 electrodynamic shaker 1 through a stinger 3 with a specimen holder 6 connected to a force sensor PCB Piezotronics® 208C03 7 at the
Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 93–98 Andrzej Katunin et al. / Structural Integrity Procedia 00 (2017) 000–000
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end. The specimen holder 4 was made of bakelite in order to provide a thermal insulation of the heat generated during the tests. For ensuring repeatable conditions, each specimen was clamped with a constant torque of 20 Nm. The excitation signal was measured by the accelerometer PCB Piezotronics® T352C34 2. A velocity of vibration of the specimen was measured on its surface near the clamp 4 using the single point laser Doppler vibrometer (LDV) Polytec ® PDV-100 9. In order to detect damage initiation in the structure due to the self-heating effect, besides measuring the surface temperature and excitation parameters, acoustic emission (AE) was observed. The AE signal was measured by means of the system Vallen® AMSY-5. In particular, an AE sensor 12, an AE signal preamplifier, a dual channel AE signal processor board 13 and a dedicated software for AE signal acquisition and processing were used. The force sensor and accelerometer were connected through a conditioning module to the multi-channel data acquisition card NI® DAQ Card 6062E, which was connected to a PC 14 and controlled by an application developed in LabView®. Force and vibration signals were acquired with a sample rate of 2 kHz. The application allows controlling of the excitation signal parameters through the analogue output of multi-channel signal acquisition module 11 and controls the shaker amplifier 10 TIRA® BAA 500. The temperature measurements were carried out using the InfraTec ® VarioCAM® hr infrared camera (IRC) 8. A frame rate of the IRC was set to 2 frames per second. The fatigue tests were performed as follows. The specimens were loaded in such a way that a maximal self-heating temperature on their surfaces reached a certain value in a range of 30÷55°C with a step of 5°C. The upper limit of 55°C was selected based on the results of previous tests, where the self-heating temperature growth became nonstationary in the second phase of its development. As a criterion of examination of non-stationarity of temperature growth the following assumption was made: if the temperature growth in the second phase increases with a rate of less than 1°C per 3000 cycles (100 s), then the self-heating temperature growth is assumed to be stationary. After reaching the certain temperature the specimens were subjected to fatigue cyclic loading until failure. For each maximal selfheating temperature value 5 specimens were tested for obtaining statistically valid results. During such a study all of the parameters available to be acquired using the above-described measurement equipment were collected. The collected data allow for evaluation of influence of the self-heating effect in the stationary regime on fatigue lifetime of polymeric composites. 3. Results and discussion The results of performed test indicated that for the considered dimensions of specimens and loading parameters the self-heating temperature was stable up to 50°C (according the assumed criterion of a temperature growth), which can be observed in Fig. 3.
Fig. 3. Selected self-heating temperature history curves for various temperatures of stabilization.
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From direct observations one can conclude that the increased temperature resulting from the self-heating effect significantly influences on a residual life of the polymer-based composite structure. Comparing the results for the stabilization temperature close to 30°C one can see that the increase of 3°C (cf. red lines in Fig. 3) may shorten a residual life of the structure two times, or more. Assuming the case of temperature stabilization on a level of 30°C and comparing the obtained temperature history curves with the results obtained for higher values of temperature stabilization one can observe that a difference of residual life in these cases is even of one order, which has a crucial meaning during the design and operation stage of composite structures subjected to cyclic loading or forced vibrations. The results of the self-heating temperature growth during fatigue tests indicate a dependence between a residual life of a composite structure and an observed self-heating temperature, which allows for modelling and prediction of a residual life based on observed stabilized self-heating temperature as well as evaluation of the criticality of the selfheating effect in case of appearance of the stationary self-heating during fatigue loading of polymer-based composite structures. One can also observe that the lower the stationary self-heating temperature during fatigue the lower the temperature at failure of a structure, which is clearly observable in Fig. 3. These observations prove that the criticality of the selfheating effect described in the previous studies (Katunin (2012a), Katunin (2012b), Katunin et al. (2017)) depends on loading conditions, which is directly connected with a generated heat. However, this dependence is noticeable well for the stationary regime of self-heating, while in case of non-stationary self-heating and domination of the self-heating effect the influence of loading conditions on the residual life is much less, and in many cases can be neglected due to intensive degradation of a composite structure in such conditions. The maximal values of a self-heating temperature for non-stationary self-heating at failure usually place in a range of 110÷130°C (see Katunin (2012a), Katunin (2012b), Katunin et al. (2017)), which is similar with the results for a temperature of stabilization of 55°C obtained in the presented study. This additionally proves that the loading parameters in case of non-stationary self-heating have little influence on a resulting critical self-heating temperature. In case of stationary self-heating, i.e. when the self-heating effect does not dominate the fatigue processes, the selfheating effect accelerates the degradation processes, but the main factor, which influences on a final failure of a structure is mechanical degradation with accompanying crack formation. The previous studies (Katunin et al. (2017)) showed that the critical self-heating temperature during non-stationary self-heating is in a range of 65÷70°C, which is connected with cracking of a polymeric matrix of a composite, while formation of a macrocrack in case of stationary self-heating appears at much lower temperature values (see Fig. 4).
Fig. 4. Thermograms of the tested specimens after various number of cycles: with a self-heating temperature of stabilization of 30°C after a) 103455, b) 275355, c) 386835 cycles and a temperature of stabilization of 35°C after d) 16380, e) 147570, f) 187095 cycles.
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4. Conclusions The performed studies focused on determination of influence of the self-heating effect in the stationary regime on fatigue processes of polymeric composites allows for preliminary evaluation of structural degradation, based on which prediction of residual life of composite structures subjected to such a type of loading is possible. The obtained results show that the degradation initiation in case of stationary self-heating, in contrast to non-stationary self-heating, depends on loading parameters and the self-heating temperature of stabilization significantly influences on a residual life of a composite structure. Moreover, it was observed that the initiation of degradation during stationary self-heating takes place at much lower temperature values than the critical self-heating temperature range determined in the author’s previous studies for non-stationary self-heating. However, formation of a macrocrack in a polymeric matrix occurs after a large number of cycles, which points on domination of mechanical character of degradation. In order to investigate the degradation processes during stationary self-heating a deeper analysis of the collected data including AE and hysteresis evolution in a function of time should be performed. Further studies will be focused on analyzing of the mentioned data for evaluation of the criticality of the self-heating effect in the stationary regime as well as confrontation of the results of the present analysis with results of degradation evaluation during nonstationary self-heating using various destructive and non-destructive testing methods. Such an analysis will allow for global evaluation of safe ranges of self-heating temperature in designed and operated polymer-based composite structures and use of the self-heating effect in the determined ranges for non-destructive evaluation of such structures. Acknowledgements The results presented in this paper have been obtained within the framework of research grant No. 2015/17/D/ST8/01294 financed by the National Science Centre, Poland. References Ferreira, J.A.M., Costa, J.D.M., Reis, P.N.B., Richardson, O.W., 1999. Analysis of Fatigue and Damage in Glass-Fibre-Reinforced Polypropylene Composite Materials. Composites Science and Technology 59(10), 1461–1467. Toubal, L., Karama, M., Lorrain, B., 2006. Damage Evolution and Infrared Thermography in Woven Composite Laminates Under Fatigue Loading. International Journal of Fatigue 28(12), 1867–1872. Naderi, M., Khonsari, M.M., 2012. Thermodynamic Analysis of Fatigue Failure in a Composite Laminate. Mechanics of Materials 46, 113–122. Liu, Z.Y., Beniwal, S., Jenkins, C.H.M., Winter, R.M., 2004. The Coupled Thermal and Mechanical Influence on a Glassy Thermoplastic Polyamide: Nylon 6,6 Under Vibro-Creep. Mechanics of Time-Dependent Materials 8, 235–253. Moisa, S., Landsberg, G., Rittel, D., Halary, J.L., 2005. Hysteretic Thermal Behavior of Amorphous Semi-Aromatic Polyamides. Polymer 46(25), 11870–11875. Karama, M., 2011. Determination of the Fatigue Limit of Carbon/Epoxy Composite Using Thermographic Analysis. Structural Control and Health Monitoring 18, 781-789. Katunin, A., 2012a. Thermal Fatigue of Polymeric Composites Under Repeated Loading. Journal of Reinforced Plastics and Composites 31(15), 1037–1044. Kahirdeh, A., Khonsari, M.M., 2014. Criticality of Degradation in Composite Materials Subjected to Cyclic Loading. Composites: Part B 61, 375– 382. Naderi, M., Kahirdeh, A., Khonsari, M.M., 2012. Dissipated Thermal Energy and Damage Evolution of Glass/Epoxy Using Infrared Thermography and Acoustic Emission. Composites: Part B 43, 1613–1620. Katunin, A. 2012b. Critical Self-Heating Temperature During Fatigue of Polymeric Composites Under Cyclic Loading. Composites Theory and Practice 12(1), 72–76. Katunin, A., Krukiewicz, K., Turczyn, R., 2012. Evaluation of Residual Cross-Linking Caused by Self-Heating Effect in Epoxy-Based Fibrous Composites Under Cyclic Loading. Chemik 68(11), 957–966. Katunin, A., Wronkowicz, A., Bilewicz, M., Wachla, D., 2017. Criticality of Self-Heating in Degradation Processes of Polymeric Composites Subjected to Cyclic Loading: A Multiphysical Approach. Archives of Civil and Mechanical Engineering, in press, DOI: 10.1016/j.acme.2017.03.003. Katunin, A., Gnatowski, A., 2012. Influence of Heating Rate on Evolution of Dynamic Properties of Polymeric Laminates. Plastics, Rubber and Composites 41(6), 233–239.