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Fracture criterion for tested CFRP specimens under tension
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Procedia Structural Integrity 13 00 (2018) 1285–1290 Structural Integrity Procedia (2016) 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Fracture criterion for tested CFRP specimens under tension Fracture criterion for tested CFRP specimens under tension
XV Portuguese Conference on 10-12 February Arcos, Portugal a Fracture, PCF 2016, b* c* 2016, Paço de d*
A.A. Lukyanchuka*, A.G. Kalininb*, A.V. Pankovc*, Yu.A.Svirskiyd* A.A. Lukyanchuk *, A.G. Kalinin , A.V. Pankov , Yu.A.Svirskiy
*Central aerohydraudynamic institute n.a. Prof. N.E. Zhukovsky (TsAGI),Russia, Thermo-mechanical modeling of a [email protected]; high [email protected] turbine blade of an *Central aerohydraudynamic institute n.a. Prof. N.E. Zhukovsky (TsAGI),Russia, [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected] airplane gas turbine engine a
b
c
d
a
b
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Abstract Abstract P. Brandãoa, V. Infanteb, A.M. Deusc* The development of the theory-experimental methodology to evaluate the fatigue life is one of the essential stages to substantiate the a Department Mechanical Engineering, Instituto Técnico, Universidade Pais, 1,a1049-001 Lisboa,itthe The development of of thethe theory-experimental methodology evaluate the fatigue lifedeisLisboa, one ofAv. theRovisco essential stages to substantiate strength and to verify certification of aircraft fromSuperior the to POV of fatigue and service lives. To develop such methodology, is Portugal strength and to verify the certification of aircraft from the POV of fatigue and service lives. To developstress suchcycle a methodology, it is necessary to obtain the S-N curves for different stress ratios. However, the fatigue test by zero-to-tension of an open-hole b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, necessary manufactured to obtain the S-N curves for stress ratios. However, the fatigue test by(broken zero-to-tension stressshows cycle of an open-hole specimen of carbon fiberdifferent reinforced polymer (CFRP) until final fracture into 2 parts) a big scatter of Portugal c specimen manufactured of fiberEngineering, reinforced polymer (CFRP) until final (broken into parts) shows a1049-001 big scatter of fatigue lives and sometimes it is impossible due toInstituto fibers long lives despite offracture fractured For example, in 1, our research, the CeFEMA, Department of carbon Mechanical Superior Técnico, Universidade dematrix. Lisboa, Av.2Rovisco Pais, Lisboa, Portugal fatigue lives and sometimes it is impossible due to fibers long lives despite of fractured matrix. For example, in our research, the root-mean-square deviation of the logarithm of fatigue life SlgN was in the range 0.7 ÷ 0.9. In order to obtain reliable resource the range 0.7 ÷ 0.9. order to obtain reliable resource root-mean-square deviation of at thethis logarithm fatigue life SlgNa was characteristics of the structure, kind of of dispersion values, lot ofintime and material costsInare required. In order to solve this characteristics of the at to this kindaofnew dispersion a lot of time material costs are required. In order to solve this problem, the task wasstructure, formulated create fracture values, criterion, which will and allow reducing dispersion of fatigue life. Our analysis Abstract problem, the the taskfracture was formulated to create new fracture criterion, which allow dispersion analysis showed that process starts froma interlayer delamination at holewill edge andreducing propagates towardsof thefatigue outer life. side Our of specimen showed thatThe thedelamination fracture process starts atfrom interlayer delamination edge and propagates outer specimen under test. initiation the hole edge is proposed toatbehole taken as failure initiation towards moment;the then the side data of obtained are During operation, aircraft engine components increasingly demanding operating conditions, under The delamination initiation at To the hole edge is proposed be subjected taken as failure initiation moment; then the data obtained are to be test. usedtheir to estimate the modern S-N curve. fix the appearance oftoare delamination, atospecial sensor was developed. A method was especially high pressure turbine (HPT) Such conditions cause theseaparts to undergo different types ofAtime-dependent to be usedtotothe estimate the S-N curve. To fixblades. the appearance special sensor method was developed detect the delamination initiation while monitoring of thedelamination, transverse displacement near thewas freedeveloped. edge of hole. This method degradation, one of is creep. A modelwhile using the finite element method (FEM) wasnear developed, in order to be This able to predict developed detect thewhich delamination the transverse displacement edge of hole. that is acceptable to estimate the the free fatigue life; at this, twomethod modes provided a to drastic decrease of fatigue initiation life scatter up tomonitoring SlgN = 0.2÷0.3 the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation 0.2÷0.3 that ispropagation acceptable to estimate the fatigue life; atcharacterized this, two modes provided a drastic decrease ofare fatigue lifeinto scatter up to SlgN =initiation of fracture process in CFRP taken consideration: and of delamination, which are by company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model of fracture process in CFRP are taken into consideration: initiation and propagation of delamination, which are characterized by different neededfracture for themechanisms. FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were different fracture mechanisms. obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D © rectangular 2018 The Authors. Published by Elsevier B.V. block shape, inby order to better © 2018 The Authors. Published Elsevier B.V. establish the model, and then with the real 3D mesh obtained from the blade scrap. The © overall 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a Peer-review under responsibility of the ECF22 organizers. Peer-review under responsibility of the ECF22 model can be useful in the goal of predicting organizers. turbine blade life, given a set of FDR data. Keywords: Сarbon fiber reinforced polymer; open-hole specimen; delamination; fracture criterion; fatigue life; fatigue scattering parameter. Keywords: fiber reinforced polymer; open-hole specimen; delamination; fracture criterion; fatigue life; fatigue scattering parameter. © 2016 Сarbon The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016.
1. Background 1. Keywords: Background High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. To develop theory-experimental methodology for estimating the fatigue life of CFRP specimens, it is necessary To develop theory-experimental methodology estimating the fatigue life of CFRP it is necessary to obtain experimentally S-N curves. For metal for specimens, fracture of a specimen intospecimens, two parts is used as the to obtainofexperimentally S-N fracture of astress specimen two parts is used as the criterion destruction, due to curves. the fact For that metal in the specimens, case of zero-to-tension cycle, into the period of development of criterion due to thewith factthe thatperiod in thebefore case of stress cycle, thethe period of testing development of the crackof is destruction, small in comparison itszero-to-tension occurrence. However, during fatigue of openthe crack is small in comparison with the period before its occurrence. However, during the fatigue testing of open2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. 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 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.262
A.A. Lukyanchuk et al. / Procedia Structural Integrity 13 (2018) 1285–1290 A.A. Lukyanchuk and others / Structural Integrity Procedia 00 (2018) 000–000
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hole CFRP specimens with zero-to-tension stress cycle the delamination effect was obtained. Delamination was observed visually on the edges of the specimen at 90°-th layers. Thus, the application of the "classical" fracture criterion into two parts led to a large scatter of fatigue life. The root-mean-square deviation of the logarithm of fatigue life SlgN, characterizing the scatter, ranged from 0.7 to 0.9. Such data are unacceptable for obtaining reliable methods of estimating fatigue life of aviation constructions. Figure 1 shows the delamination of an open-hole specimen under zero-to-tension cycle. Since the S-N curves must be obtained for different stress rations, it was necessary to develop another failure criterion to carry out the fatigue tests of open-hole specimens in the case of a zero-to-tension loading, which makes it possible to reduce the scattering of fatigue. In particular, it was necessary to solve a number of tasks for registration: the moment of the appearance of the delamination; the place where the delamination begins; the propagation velocity of delamination; methods for fixing the onset and growth of the delamination; what moment is considered the destruction of the specimen. To solve them, a series of works was performed.
Figure 1 - Delamination on the edges of an open-hole specimen under tensile fatigue test
2. First stage: investigation of the moment of the beginning of delamination using a wire sensor At the first stage of the research the wire sensor was chosen as one of the simplest solutions. Triggering (rupture) of the wire sensor (shown in Fig. 2(A)) occurred after the appearance of a delamination on the side surface of the specimen and its further increase to an actual opening of about 0.1 mm, followed by a stop of the testing machine.
(A)
(B)
Figure 2 – A)Broken wire sensor, specimen under tensile load; B) Delamination zones after fatigue runtime by zero-to-tension cycle
Investigation of delaminated, but not destroyed by the "classical" criterion, specimens by ultrasonic testing showed: the delamination appears not uniformly in the area of the specimen, but in sections of different length and shape. Figure 2(B) shows the delamination zones. Obviously, usage of a wire sensor does not give a vision of the moment and place of the beginning of the delamination, and therefore more precise and detailed control methods are required. A picture similar to Figure 2(B) can be found in some sources [1-3]. For example, in Figure 3, the X-ray diffraction patterns of the specimen at the beginning of the test (A), during the test (B) and after the application of 106 load cycles (C) with a cycle maximum of 80% of the breaking load in static tests.
A.A. Lukyanchuk et al. / Procedia Structural Integrity 13 (2018) 1285–1290 A.A. Lukyanchuk and others / Structural Integrity Procedia 00 (2018) 000–000
(A)
(B)
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(C)
Figure 3 - Dynamics of damage development in an open-hole specimen according to X-ray data
In the works abovementioned, visual and X-ray inspection was used. X-ray monitoring makes it possible to determine the growth dynamics of interlayer delaminations and longitudinal cracks, and the visual method – the growth of the number and dimensions of transverse and longitudinal cracks on the specimen surface. It is obvious that even with successful establishment of a relationship between the rate of development of the above defects and the loading parameters, the use of a technique based on this dependence will have a very limited application, since the abovementioned nondestructive testing methods (NDT) is not usable for continuous monitoring of the state of the specimen. In addition, such kinds of monitoring the state of the specimen will increase the economic and time costs for conducting the experiment. This will call into question the possibility of carrying out numerous, multicyclic tests on fatigue life. In accordance with the data obtained by us, the starting point of the delamination process is the zone around the stress concentrator, in this case the hole. Due to the fact that in modern CFRP, the growth rates of the delaminations are high and their detection in the process of runtime by the methods of nondestructive testing is very difficult, according to the requirements of normative documentation [4], delaminations should not appear in aircraft structures and, if they arise, further use of the structure is prohibited. Thus, for the fatigue tests under tension, the criterion for the destruction of the specimen of the CFRP can be considered the moment of the beginning of the delamination in the zone of the concentrator. In the future it will be shown that this moment was recorded by monitoring the "opening" of the delamination. It should be noted that the inspiration for creating such a criterion was the methods based on measuring the degradation of the stiffness of the construction element. They are based on the experimentally discovered fact that a change in the stiffness of a construction element of the CFRP indicates the beginning of its destruction. Such methods are used, for example, to estimate the fatigue life of wind turbine blades [5]. The degradation of the stiffness of the construction element in this case is determined from the change in the slope in the "Load-displacement" relationship under regular loading. Thus, for carrying out fatigue tests and fix the whole picture of the stress-strain state change, a method of detecting the initiation of the delamination in the zone of hole is necessary, which must have the following key features: Continuous measurement; Indication of measured parameters in real time; Providing precise information about the onset of stratification and further growth rate. 3. The second stage: debugging of the growth sensor of the delamination of the specimen in the zone of the hole, the development of the criterion In the second stage of the investigation, a method was used to detect the onset of delamination in the hole zone using a delamination sensor specially developed in TsAGI. The work of the sensor is based on continuous fixing the change of the specimen thickness around the hole during fatigue loading. For the sensor, different thresholds were set: from 0.05mm to 0.5mm of thickening of the specimen in the zone of the hole. When the established threshold on thickening in the program of the testing machine associated with the delamination sensor is achieved, the test is suspended. The ultrasonic inspection of the
A.A. Lukyanchuk et al. / Procedia Structural Integrity 13 (2018) 1285–1290 A.A. Lukyanchuk and others / Structural Integrity Procedia 00 (2018) 000–000
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specimen was carried out and the changing of internal structure of the composite was fixed, after which the sensor was mounted back onto the specimen and the tests were continued. Data obtained by NDT method was used to develop a quantitative criterion value: tension . As a result of the implementation of the set of measures, a criterion for the destruction of the specimen was developed for fatigue tests under tension on the basement of a delamination sensor: tension 0.05 mm
(1)
The developed criterion provides a fatigue data with a dispersion SlgN not exceeding 0.3, which is acceptable for modern CFRPs and does not lead to an unreasonable increase in reliability factors for fatigue life.
Figure 4 - A specimen mounted on the test machine with a delamination sensor in the zone of the hole
To verify the fracture criterion, a delamination sensor was used in fatigue tests, which shown in Figure 4. An Instron extensometer with a 10 mm base was used with it. Figure 5 shows the typical dependence of the readings of the delamination sensor on the runtime (cycles). In the figure, the values min and max denote the values of the minimum and maximum values of the extensometer readings of each cycle.
Figure 5 - Graph of the dependence of the extensometer readings from the fatigue runtime
According to the experimental results, it was found that the failure criterion (1) (i.e. the displacement of the extensometer knives by 0.05 mm) corresponds to the largest number of specimens: 21 of 31. The remaining 10 specimens had different failures and errors during the development and debugging of the method. The results of ultrasonic testing, conducted after achievement of the developed criterion, showed the uniformity of the damages obtained: delamination in the open-hole zone of the same area in all the specimens studied (Figure 6). It should be noted that after reaching a thickening of 0.05 mm according to the readings of the delamination sensor, all the investigated specimens were subjected to further tests with a gradual increase of the thickening threshold to 0.075 and 0.1 mm. However, only nine specimens reached a thickening of 0.075 mm, while the area of emerging delaminations in the specimen became comparable with the area of the working space of the specimen. In remaining specimens, thickening stopped at a value of about 0.05 mm, while the delamination area continued to grow.
A.A. Lukyanchuk et al. / Procedia Structural Integrity 13 (2018) 1285–1290 A.A. Lukyanchuk and others / Structural Integrity Procedia 00 (2018) 000–000
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Figure 6 - : Specimens No.151, 157 after reaching the thickening of 0.05 mm
Figure 7 shows a photo of two specimens after an ultrasound inspection with the noted typical fatigue damage. Specimens are shown after reaching: a thickening of 0.05 mm and runtime of about 50 000 cycles; achieving a thickening of 0.1 mm and runtime of about 1 000 000 cycles.
(A).
(B)
Figure 7 - A) Specimens No.149 and 151 after reaching a thickening of 0.05mm; B) Specimens No.149 and 151 after reaching a thickening of 0.1mm
4. Results of fatigue tensile tests using the proposed failure criterion To check the results (sensor designed and criteria) additional fatigue tests of that kind of specimens were carried out. The main interest was the possibility of derivation S-N curve from the data obtained for further estimation of fatigue. Specimens were tested under regular loading with the stress ratio: R = 0. The tests were carried out at three loading levels. The loading levels were chosen to obtain the fatigue lives in the required range. The delamination occurred in the range of 103 ÷ 106 cycles. Complete destruction according to the "classical criterion" was not achieved by any single specimen. The values of the fatigue life were recorded only by the delamination sensor in the zone of the hole, criterion (1) was used for this. When criterial value was reached, the specimen was considered to be failed. The results of the inspection of specimens by ultrasonic methods corresponded to the data described in the section above. The results of fatigue tensile tests of open-hole specimens are shown in Figure 8. It also shows the S-N curve approximated. Fatigue scattering parameter SlgN is 0.21. The experimental data for the construction of the S-N curve were approximated by a power law in the form [6, 7]: lg N C mR lg , N с
mR
, C mR lg с
where C и mR – experimental constants; N и – fatigue and maximum stresses of regular loading.
(2)
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A.A. Lukyanchuk et al. / Procedia Structural Integrity 13 (2018) 1285–1290 A.A. Lukyanchuk and others / Structural Integrity Procedia 00 (2018) 000–000
Figure 8 - The S-N curve for fatigue tensile tests of open-hole specimens of CFRP
5. Conclusions The studies provided a method for experimental determining the fatigue life under tensile tests open-hole CFRP specimens. The method is based on the usage of a sensor, which fixes an increase in the thickness of the specimen in the zone of the hole (delamination). On the basis of the analysis of the obtained experimental data in the process of debugging the method, a failure criterion is proposed: an increase in the thickness of the specimen over 0.05 mm at the zone of concentrator. It is shown that the proposed failure criterion under cyclic loading allows to fix the appearance and propagation of delaminations in the zone of hole. The criterion demonstrates an acceptable repeatability of the physical picture of the initiation of macro destruction, while the fatigue scatter parameter SlgN did not exceed 0.31. Such a fatigue scatter parameter is acceptable for estimating the fatigue life and does not lead to an unreasonable increase in safety factors of fatigue, and hence the duration of certification tests for the aircraft. Acknowledgements The research was financed by Ministry of Education and Science of the Russian Federation, under agreement No.RFMEFI62518X0044 (FENIKS). The authors wish to greatly acknowledge the project partner of FENIKS, Irkut Corporation. References [1] Hahn, H.T., et. al. The Effect of Preloading on Fatigue Damage in Composite Structures: Part I. US Department of Transportation. Federal Aviation Administration. DOT/FAA/AR-95/79. 1996. 48 p. [2] Hahn, H.T., Cho J.B., Lim S.G. The Effect of Loading Parameters on Fatigue of Composite Laminates: Part II. US Department of Transportation. Federal Aviation Administration, DOT/FAA/AR-96/76. 1997. 92 p. [3] Hahn, H.T., Mitrovic M., Turkgenc O. The Effect of Loading Parameters on Fatigue of Composite Laminates: Part III. US Department of Transportation, Federal Aviation Administration. DOT/FAA/AR-99/22. 1999. 69 p. [4] Evaluation of damage tolerance and fatigue structural integration. AR CTs «Prochnost' - Strength» AR MAC, 2015, AC-AR25.571-1А (Russian). [5] Vassilopoulos, A.P., Keller T., Fatigue of Fiber-reinforced Composites. Springer-Verlag London Limited, 2011, p. 69, 102-104 [6] Wilson, T. Modeling Of In-plane and Interlaminar Fatigue Behavior Of Glass and Carbon Fiber Composite Materials. ‒ Montana State University, Bozeman, Montana, 2007. ‒ 202 p. [7] Konovalov V and Pankov A. Verification of CFRP Components Fatigue Evaluation Procedure under Irregular Cycling Loading. Proc Conference ICAS, Korea, 2016.
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Structural Integrity 00 (2018) 000–000 Available online at www.sciencedirect.com Available online atProcedia www.sciencedirect.com
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Structural Integrity Procedia 00 (2018) 000–000
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Procedia Structural Integrity 13 00 (2018) 1285–1290 Structural Integrity Procedia (2016) 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Fracture criterion for tested CFRP specimens under tension Fracture criterion for tested CFRP specimens under tension
XV Portuguese Conference on 10-12 February Arcos, Portugal a Fracture, PCF 2016, b* c* 2016, Paço de d*
A.A. Lukyanchuka*, A.G. Kalininb*, A.V. Pankovc*, Yu.A.Svirskiyd* A.A. Lukyanchuk *, A.G. Kalinin , A.V. Pankov , Yu.A.Svirskiy
*Central aerohydraudynamic institute n.a. Prof. N.E. Zhukovsky (TsAGI),Russia, Thermo-mechanical modeling of a [email protected]; high [email protected] turbine blade of an *Central aerohydraudynamic institute n.a. Prof. N.E. Zhukovsky (TsAGI),Russia, [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected] airplane gas turbine engine a
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c
d
a
b
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Abstract Abstract P. Brandãoa, V. Infanteb, A.M. Deusc* The development of the theory-experimental methodology to evaluate the fatigue life is one of the essential stages to substantiate the a Department Mechanical Engineering, Instituto Técnico, Universidade Pais, 1,a1049-001 Lisboa,itthe The development of of thethe theory-experimental methodology evaluate the fatigue lifedeisLisboa, one ofAv. theRovisco essential stages to substantiate strength and to verify certification of aircraft fromSuperior the to POV of fatigue and service lives. To develop such methodology, is Portugal strength and to verify the certification of aircraft from the POV of fatigue and service lives. To developstress suchcycle a methodology, it is necessary to obtain the S-N curves for different stress ratios. However, the fatigue test by zero-to-tension of an open-hole b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, necessary manufactured to obtain the S-N curves for stress ratios. However, the fatigue test by(broken zero-to-tension stressshows cycle of an open-hole specimen of carbon fiberdifferent reinforced polymer (CFRP) until final fracture into 2 parts) a big scatter of Portugal c specimen manufactured of fiberEngineering, reinforced polymer (CFRP) until final (broken into parts) shows a1049-001 big scatter of fatigue lives and sometimes it is impossible due toInstituto fibers long lives despite offracture fractured For example, in 1, our research, the CeFEMA, Department of carbon Mechanical Superior Técnico, Universidade dematrix. Lisboa, Av.2Rovisco Pais, Lisboa, Portugal fatigue lives and sometimes it is impossible due to fibers long lives despite of fractured matrix. For example, in our research, the root-mean-square deviation of the logarithm of fatigue life SlgN was in the range 0.7 ÷ 0.9. In order to obtain reliable resource the range 0.7 ÷ 0.9. order to obtain reliable resource root-mean-square deviation of at thethis logarithm fatigue life SlgNa was characteristics of the structure, kind of of dispersion values, lot ofintime and material costsInare required. In order to solve this characteristics of the at to this kindaofnew dispersion a lot of time material costs are required. In order to solve this problem, the task wasstructure, formulated create fracture values, criterion, which will and allow reducing dispersion of fatigue life. Our analysis Abstract problem, the the taskfracture was formulated to create new fracture criterion, which allow dispersion analysis showed that process starts froma interlayer delamination at holewill edge andreducing propagates towardsof thefatigue outer life. side Our of specimen showed thatThe thedelamination fracture process starts atfrom interlayer delamination edge and propagates outer specimen under test. initiation the hole edge is proposed toatbehole taken as failure initiation towards moment;the then the side data of obtained are During operation, aircraft engine components increasingly demanding operating conditions, under The delamination initiation at To the hole edge is proposed be subjected taken as failure initiation moment; then the data obtained are to be test. usedtheir to estimate the modern S-N curve. fix the appearance oftoare delamination, atospecial sensor was developed. A method was especially high pressure turbine (HPT) Such conditions cause theseaparts to undergo different types ofAtime-dependent to be usedtotothe estimate the S-N curve. To fixblades. the appearance special sensor method was developed detect the delamination initiation while monitoring of thedelamination, transverse displacement near thewas freedeveloped. edge of hole. This method degradation, one of is creep. A modelwhile using the finite element method (FEM) wasnear developed, in order to be This able to predict developed detect thewhich delamination the transverse displacement edge of hole. that is acceptable to estimate the the free fatigue life; at this, twomethod modes provided a to drastic decrease of fatigue initiation life scatter up tomonitoring SlgN = 0.2÷0.3 the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation 0.2÷0.3 that ispropagation acceptable to estimate the fatigue life; atcharacterized this, two modes provided a drastic decrease ofare fatigue lifeinto scatter up to SlgN =initiation of fracture process in CFRP taken consideration: and of delamination, which are by company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model of fracture process in CFRP are taken into consideration: initiation and propagation of delamination, which are characterized by different neededfracture for themechanisms. FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were different fracture mechanisms. obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D © rectangular 2018 The Authors. Published by Elsevier B.V. block shape, inby order to better © 2018 The Authors. Published Elsevier B.V. establish the model, and then with the real 3D mesh obtained from the blade scrap. The © overall 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a Peer-review under responsibility of the ECF22 organizers. Peer-review under responsibility of the ECF22 model can be useful in the goal of predicting organizers. turbine blade life, given a set of FDR data. Keywords: Сarbon fiber reinforced polymer; open-hole specimen; delamination; fracture criterion; fatigue life; fatigue scattering parameter. Keywords: fiber reinforced polymer; open-hole specimen; delamination; fracture criterion; fatigue life; fatigue scattering parameter. © 2016 Сarbon The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016.
1. Background 1. Keywords: Background High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. To develop theory-experimental methodology for estimating the fatigue life of CFRP specimens, it is necessary To develop theory-experimental methodology estimating the fatigue life of CFRP it is necessary to obtain experimentally S-N curves. For metal for specimens, fracture of a specimen intospecimens, two parts is used as the to obtainofexperimentally S-N fracture of astress specimen two parts is used as the criterion destruction, due to curves. the fact For that metal in the specimens, case of zero-to-tension cycle, into the period of development of criterion due to thewith factthe thatperiod in thebefore case of stress cycle, thethe period of testing development of the crackof is destruction, small in comparison itszero-to-tension occurrence. However, during fatigue of openthe crack is small in comparison with the period before its occurrence. However, during the fatigue testing of open2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. 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 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.262
A.A. Lukyanchuk et al. / Procedia Structural Integrity 13 (2018) 1285–1290 A.A. Lukyanchuk and others / Structural Integrity Procedia 00 (2018) 000–000
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hole CFRP specimens with zero-to-tension stress cycle the delamination effect was obtained. Delamination was observed visually on the edges of the specimen at 90°-th layers. Thus, the application of the "classical" fracture criterion into two parts led to a large scatter of fatigue life. The root-mean-square deviation of the logarithm of fatigue life SlgN, characterizing the scatter, ranged from 0.7 to 0.9. Such data are unacceptable for obtaining reliable methods of estimating fatigue life of aviation constructions. Figure 1 shows the delamination of an open-hole specimen under zero-to-tension cycle. Since the S-N curves must be obtained for different stress rations, it was necessary to develop another failure criterion to carry out the fatigue tests of open-hole specimens in the case of a zero-to-tension loading, which makes it possible to reduce the scattering of fatigue. In particular, it was necessary to solve a number of tasks for registration: the moment of the appearance of the delamination; the place where the delamination begins; the propagation velocity of delamination; methods for fixing the onset and growth of the delamination; what moment is considered the destruction of the specimen. To solve them, a series of works was performed.
Figure 1 - Delamination on the edges of an open-hole specimen under tensile fatigue test
2. First stage: investigation of the moment of the beginning of delamination using a wire sensor At the first stage of the research the wire sensor was chosen as one of the simplest solutions. Triggering (rupture) of the wire sensor (shown in Fig. 2(A)) occurred after the appearance of a delamination on the side surface of the specimen and its further increase to an actual opening of about 0.1 mm, followed by a stop of the testing machine.
(A)
(B)
Figure 2 – A)Broken wire sensor, specimen under tensile load; B) Delamination zones after fatigue runtime by zero-to-tension cycle
Investigation of delaminated, but not destroyed by the "classical" criterion, specimens by ultrasonic testing showed: the delamination appears not uniformly in the area of the specimen, but in sections of different length and shape. Figure 2(B) shows the delamination zones. Obviously, usage of a wire sensor does not give a vision of the moment and place of the beginning of the delamination, and therefore more precise and detailed control methods are required. A picture similar to Figure 2(B) can be found in some sources [1-3]. For example, in Figure 3, the X-ray diffraction patterns of the specimen at the beginning of the test (A), during the test (B) and after the application of 106 load cycles (C) with a cycle maximum of 80% of the breaking load in static tests.
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(A)
(B)
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(C)
Figure 3 - Dynamics of damage development in an open-hole specimen according to X-ray data
In the works abovementioned, visual and X-ray inspection was used. X-ray monitoring makes it possible to determine the growth dynamics of interlayer delaminations and longitudinal cracks, and the visual method – the growth of the number and dimensions of transverse and longitudinal cracks on the specimen surface. It is obvious that even with successful establishment of a relationship between the rate of development of the above defects and the loading parameters, the use of a technique based on this dependence will have a very limited application, since the abovementioned nondestructive testing methods (NDT) is not usable for continuous monitoring of the state of the specimen. In addition, such kinds of monitoring the state of the specimen will increase the economic and time costs for conducting the experiment. This will call into question the possibility of carrying out numerous, multicyclic tests on fatigue life. In accordance with the data obtained by us, the starting point of the delamination process is the zone around the stress concentrator, in this case the hole. Due to the fact that in modern CFRP, the growth rates of the delaminations are high and their detection in the process of runtime by the methods of nondestructive testing is very difficult, according to the requirements of normative documentation [4], delaminations should not appear in aircraft structures and, if they arise, further use of the structure is prohibited. Thus, for the fatigue tests under tension, the criterion for the destruction of the specimen of the CFRP can be considered the moment of the beginning of the delamination in the zone of the concentrator. In the future it will be shown that this moment was recorded by monitoring the "opening" of the delamination. It should be noted that the inspiration for creating such a criterion was the methods based on measuring the degradation of the stiffness of the construction element. They are based on the experimentally discovered fact that a change in the stiffness of a construction element of the CFRP indicates the beginning of its destruction. Such methods are used, for example, to estimate the fatigue life of wind turbine blades [5]. The degradation of the stiffness of the construction element in this case is determined from the change in the slope in the "Load-displacement" relationship under regular loading. Thus, for carrying out fatigue tests and fix the whole picture of the stress-strain state change, a method of detecting the initiation of the delamination in the zone of hole is necessary, which must have the following key features: Continuous measurement; Indication of measured parameters in real time; Providing precise information about the onset of stratification and further growth rate. 3. The second stage: debugging of the growth sensor of the delamination of the specimen in the zone of the hole, the development of the criterion In the second stage of the investigation, a method was used to detect the onset of delamination in the hole zone using a delamination sensor specially developed in TsAGI. The work of the sensor is based on continuous fixing the change of the specimen thickness around the hole during fatigue loading. For the sensor, different thresholds were set: from 0.05mm to 0.5mm of thickening of the specimen in the zone of the hole. When the established threshold on thickening in the program of the testing machine associated with the delamination sensor is achieved, the test is suspended. The ultrasonic inspection of the
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specimen was carried out and the changing of internal structure of the composite was fixed, after which the sensor was mounted back onto the specimen and the tests were continued. Data obtained by NDT method was used to develop a quantitative criterion value: tension . As a result of the implementation of the set of measures, a criterion for the destruction of the specimen was developed for fatigue tests under tension on the basement of a delamination sensor: tension 0.05 mm
(1)
The developed criterion provides a fatigue data with a dispersion SlgN not exceeding 0.3, which is acceptable for modern CFRPs and does not lead to an unreasonable increase in reliability factors for fatigue life.
Figure 4 - A specimen mounted on the test machine with a delamination sensor in the zone of the hole
To verify the fracture criterion, a delamination sensor was used in fatigue tests, which shown in Figure 4. An Instron extensometer with a 10 mm base was used with it. Figure 5 shows the typical dependence of the readings of the delamination sensor on the runtime (cycles). In the figure, the values min and max denote the values of the minimum and maximum values of the extensometer readings of each cycle.
Figure 5 - Graph of the dependence of the extensometer readings from the fatigue runtime
According to the experimental results, it was found that the failure criterion (1) (i.e. the displacement of the extensometer knives by 0.05 mm) corresponds to the largest number of specimens: 21 of 31. The remaining 10 specimens had different failures and errors during the development and debugging of the method. The results of ultrasonic testing, conducted after achievement of the developed criterion, showed the uniformity of the damages obtained: delamination in the open-hole zone of the same area in all the specimens studied (Figure 6). It should be noted that after reaching a thickening of 0.05 mm according to the readings of the delamination sensor, all the investigated specimens were subjected to further tests with a gradual increase of the thickening threshold to 0.075 and 0.1 mm. However, only nine specimens reached a thickening of 0.075 mm, while the area of emerging delaminations in the specimen became comparable with the area of the working space of the specimen. In remaining specimens, thickening stopped at a value of about 0.05 mm, while the delamination area continued to grow.
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Figure 6 - : Specimens No.151, 157 after reaching the thickening of 0.05 mm
Figure 7 shows a photo of two specimens after an ultrasound inspection with the noted typical fatigue damage. Specimens are shown after reaching: a thickening of 0.05 mm and runtime of about 50 000 cycles; achieving a thickening of 0.1 mm and runtime of about 1 000 000 cycles.
(A).
(B)
Figure 7 - A) Specimens No.149 and 151 after reaching a thickening of 0.05mm; B) Specimens No.149 and 151 after reaching a thickening of 0.1mm
4. Results of fatigue tensile tests using the proposed failure criterion To check the results (sensor designed and criteria) additional fatigue tests of that kind of specimens were carried out. The main interest was the possibility of derivation S-N curve from the data obtained for further estimation of fatigue. Specimens were tested under regular loading with the stress ratio: R = 0. The tests were carried out at three loading levels. The loading levels were chosen to obtain the fatigue lives in the required range. The delamination occurred in the range of 103 ÷ 106 cycles. Complete destruction according to the "classical criterion" was not achieved by any single specimen. The values of the fatigue life were recorded only by the delamination sensor in the zone of the hole, criterion (1) was used for this. When criterial value was reached, the specimen was considered to be failed. The results of the inspection of specimens by ultrasonic methods corresponded to the data described in the section above. The results of fatigue tensile tests of open-hole specimens are shown in Figure 8. It also shows the S-N curve approximated. Fatigue scattering parameter SlgN is 0.21. The experimental data for the construction of the S-N curve were approximated by a power law in the form [6, 7]: lg N C mR lg , N с
mR
, C mR lg с
where C и mR – experimental constants; N и – fatigue and maximum stresses of regular loading.
(2)
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Figure 8 - The S-N curve for fatigue tensile tests of open-hole specimens of CFRP
5. Conclusions The studies provided a method for experimental determining the fatigue life under tensile tests open-hole CFRP specimens. The method is based on the usage of a sensor, which fixes an increase in the thickness of the specimen in the zone of the hole (delamination). On the basis of the analysis of the obtained experimental data in the process of debugging the method, a failure criterion is proposed: an increase in the thickness of the specimen over 0.05 mm at the zone of concentrator. It is shown that the proposed failure criterion under cyclic loading allows to fix the appearance and propagation of delaminations in the zone of hole. The criterion demonstrates an acceptable repeatability of the physical picture of the initiation of macro destruction, while the fatigue scatter parameter SlgN did not exceed 0.31. Such a fatigue scatter parameter is acceptable for estimating the fatigue life and does not lead to an unreasonable increase in safety factors of fatigue, and hence the duration of certification tests for the aircraft. Acknowledgements The research was financed by Ministry of Education and Science of the Russian Federation, under agreement No.RFMEFI62518X0044 (FENIKS). The authors wish to greatly acknowledge the project partner of FENIKS, Irkut Corporation. References [1] Hahn, H.T., et. al. The Effect of Preloading on Fatigue Damage in Composite Structures: Part I. US Department of Transportation. Federal Aviation Administration. DOT/FAA/AR-95/79. 1996. 48 p. [2] Hahn, H.T., Cho J.B., Lim S.G. The Effect of Loading Parameters on Fatigue of Composite Laminates: Part II. US Department of Transportation. Federal Aviation Administration, DOT/FAA/AR-96/76. 1997. 92 p. [3] Hahn, H.T., Mitrovic M., Turkgenc O. The Effect of Loading Parameters on Fatigue of Composite Laminates: Part III. US Department of Transportation, Federal Aviation Administration. DOT/FAA/AR-99/22. 1999. 69 p. [4] Evaluation of damage tolerance and fatigue structural integration. AR CTs «Prochnost' - Strength» AR MAC, 2015, AC-AR25.571-1А (Russian). [5] Vassilopoulos, A.P., Keller T., Fatigue of Fiber-reinforced Composites. Springer-Verlag London Limited, 2011, p. 69, 102-104 [6] Wilson, T. Modeling Of In-plane and Interlaminar Fatigue Behavior Of Glass and Carbon Fiber Composite Materials. ‒ Montana State University, Bozeman, Montana, 2007. ‒ 202 p. [7] Konovalov V and Pankov A. Verification of CFRP Components Fatigue Evaluation Procedure under Irregular Cycling Loading. Proc Conference ICAS, Korea, 2016.