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Experimental investigation of fatigue destruction of CFRP using the electrical resistance change method
Measurement 87 (2016) 236–245
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Experimental investigation of fatigue destruction of CFRP using the electrical resistance change method Jacek Gadomski, Paweł Pyrzanowski ⇑ Institute of Aeronautics and Applied Mechanics, Warsaw University of Technology, Nowowiejska 24, 00-665 Warsaw, Poland
a r t i c l e
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Article history: Received 30 July 2015 Received in revised form 25 February 2016 Accepted 15 March 2016 Available online 19 March 2016 Keywords: Carbon fibers Polymer-matrix composites (PMCs) Electrical properties Failure
a b s t r a c t CFRP (Carbon Fiber Reinforced Polymer) composites are one of the most popular composites used as the structural material in high-loaded structures. Important disadvantage of this material is the difficulty in predicting the time of fatigue destruction. These changes decrease the stiffness of the structure, but in practice it is usually not possible to measure it. Therefore, it is necessary to find another method to determine the moment when destruction of the composite begins. In this paper, one of possible methods – measuring of the electrical resistance change was investigated. Bending beam-shape specimens were tested under static and periodically varied load. Bending moment, deflection in the center of the probe and 8 voltages were measured. Performed experiment shows that measuring of the electrical resistance change provides better information about destruction of CFRP structure than a measure of the bending moment. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced composites (FRP) are used for years as a construction for the responsible structure in the aerospace, automotive, shipbuilding etc. Structures made of FRP, like other construction materials, are subject to irreversible negative physical changes appearing in the entire volume during the period of use. Damages, such as fiber breaks, cracks of matrix and delaminations are scattered and difficult to observe by visual methods, and can potentially threaten the composite structure. The long, cyclicallyvarying levels of stress in the structure plays the biggest role among the factors contributing to the formation of defects in composites. Non-destructive diagnostic methods (NDE) are applied in order to monitor the condition of FRP. These methods make it possible to localize damage in FRP, identify their type and receive information about their density. Methods ⇑ Corresponding author. Tel.: +48 22 2347512. E-mail address: [email protected] (P. Pyrzanowski). http://dx.doi.org/10.1016/j.measurement.2016.03.036 0263-2241/Ó 2016 Elsevier Ltd. All rights reserved.
based on the acoustic techniques, X-ray, thermal, magnetic and optic methods are used to detect defects in the composite structure. They are characterized by a more or less limited area monitored at a time. The methods differ in the type of transducers, which are in contact with the investigated object or collect a signal or image from a distance [12]. In many of these methods, diagnostics is not possible to perform during operation, and the area monitored at a time is often very small compared to the dimensions of the structure of FRP. The method consisting in measuring the change of resistance in CFRP can be a system that would provide information on failures of CFRP on a regular basis. Literature does not provide information about the use of the method in practice. Many advantages of the method were found: the possibility of continuous monitoring of CFRP, the ability to simultaneously detect defects throughout the volume of the tested element, and the possibility of deformation investigation of the composite structure in real time. In the 80th of the last century, research on this method was initiated in several research centers. It was
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based on the use of the properties of conduction of electrical current through the fibers of the CFRP [2,3,6,7,13,14]. The process of destruction of structures made of CFRP involves the damage to the reinforcement and matrix. This process increases the resistance of the laminate pieces and, consequently, the entire composite part. Such a resistance change can be measured by external measuring devices connected to the composite by means of element being permanently in contact with the CFRP. Available publications present results of CFRP research in the scope of the resistivity, both for laminates and for an isolated fiber [8]. Information on this subject is also provided by carbon fiber manufacturers, e.g. Toray, Toho Tenax etc. Resistivity measurements of unidirectional CFRP were also performed. Resistivity was studied in different directions with respect to the direction of the fibers depending on the fiber volume fraction Vf [19]. Non-zero conductance in an unidirectional CFRP in the lateral direction is the result of the actual construction of the laminate. Fibers are arranged in the FRP at random way, which creates the possibility of direct contacts between adjacent fibers [19]. This phenomenon is the cause for the appearance of a network of electrical connections in the entire volume of CFRP. When the fiber volume fraction increases, the fiber density distribution becomes more uniform in the cross section perpendicular to the direction of fiber arrangement. The best method for determining the resistance of CFRP is a method consisting in measuring the potential. At least four contacts arranged on the test element are used for the measurement of voltage. A stabilized current source is connected to two external contacts, and between other pairs of contacts the voltage is measured. The advantage of this method is high accuracy due to the fact that the quality of the connection between CFRP and the test meter does not significantly affect the value of the measured voltage [4,10]. The possible positions of the current and voltage contacts on the laminate sections are proposed by Wang and Chung [20]. The first proposal with contacts on one side of the laminate would be the easiest to apply in real CFRP structure (large elements for aircrafts, ships etc). However, one continues to search for contacts of new shape and size in order to improve the quality of the method of measuring the change in resistance [21,22]. However, the contacts must be on both sides if it is necessary to examine the CFRP in the through-thickness direction for delamination investigation [23]. Materials for the contacts are silver, copper, and substances containing particles of silver such as glue, paint or epoxy binder [22]. The new integrated copper plating method was proposed in order to create durable connection by soldering between lead wire and CFRP [17]. The most often used method to connect the conductor to reinforcement of the laminate is based on bonding with an electro – conductive glue. In order to expose the fiber reinforcement, it is proposed to grind the top layer of matrix [1,16]. Therefore, it is necessary to conduct experimental research in order to extend knowledge on the methods of measuring the change of resistance in CFRP, under the influence of static and periodically variable loads.
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Performed experiments and analysis can be in the future used for analytical and numerical modeling, especially for most complicated cases [24].
2. Experimental procedure 2.1. Specimen The specimen that was made of CFRP as a bending beam, designed for load of constant bending moment, is shown in Fig. 1a. On both sides of the beam, electrical contacts were attached. The tested CFRP layers were separated from each other by the core made of GFRP (Glass Fiber Reinforced Polymer), what in shown in Fig. 1b. Similar type of specimen was used already by Todoroki at al. [18]. Novelty in this type of specimen consists in using the cross-ply laminate of [0/90] in the parts containing CFRP layers. In these layers, unidirectional tapes with a weight of 300 g/m2 (Havel Composites) arranged along and across the specimen were used. Examination the FRP as a cross-ply or quasi-isotropic laminate shows that the first damage appears in the form of matrix cracks in the transverse layers, or at the interfaces between the layers of fibers perpendicular to each other [11]. The GFRP core was made of glass fabric STR 028-150110 with a weight of 150 g/m2 in a technological process separate from that of the cross-ply laminate [±708], and it had the fiber volume fraction of approximately 0.4. The glass fabric was set in such a direction as to minimize the core stiffness. The core, whose thickness is 70% of the thickness of the entire specimen meets the task of an electrical insulator between examined layers. The specimen constructed in this way allows the testing of the CFRP in which the same number of fibers is extended and compressed at the same time. It was assumed that the stiffness of the core is low relative to the stiffness of the CFRP layers. The Young’s modulus of the core was found by measuring the stiffness using the three-point bending method. The outer GFRP thin layer ensures durable adhesion of contacts and carbon fibers. Interglass 02037 fabric with weight of 49 g/m2 was used for this layer. The specimen was formed in a special mold using hand-operated process. The controlled vacuum process made it possible to obtain the specimens of identical size and proper fiber volume fraction in each series. The matrix of the specimen contains Epidian 53 resin and Z1 hardener. Electrical contacts were made of the pure silver tape of 0.07 mm thickness and width 1.3 mm. They were attached in the configuration similar to that presented in previous work [21]. These tapes were added to the specimens in the manufacturing process, as it was described in [5]. Eight of contacts (two pairs for both side of the specimen – extended and compressed) were applied in the area not subjected to bending, that is at the free ends of the specimen, as shown in Fig. 1a. This allows us to maintain the same quality of electrical connection between the carbon fibers (reinforcement) and the metal (contacts) for the whole duration of the test. Owing to such a montage of electrodes, the information about the resistance is
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Fig. 1. Structure of the specimen: (a) general view; (b) internal structure.
gathered on the whole length of specimen, and the electric current can be properly supplied. The remaining eight contacts (4 for tensile side and 4 for compressed side) were applied in the bending zone. Enumeration of the contacts is shown in Fig. 2. The idea of embedded contacts, buildin into the specimen during production has been also used for examination of carbon fiber reinforced cement based composite [15]. Carbon fiber bundles were locally deformed by the contacts and CFRP fiber volume fraction increased in this place. A notch was made at the place around which the destruction process was initiated. Similar distortions occur in the real composite structures. For example, they exhibit local changes in the thickness of the laminate (higher local pressure during the lamination process), or the edges of additional layers of the laminate, etc. After preparing the specimen, one removed the small piece of external GFRP at the center of contact to expose the metal. Electric wire was soldered to this place, what enabled the measurement of the electrical parameters. The resistance between selected pairs of contacts was measured before and after soldering. The average value of changes in resistance for pairs of contacts 2–8 and 12–18 for contacts before and after soldering was 0.053 X, and the population standard deviation was 0.0089 X. It meant that the soldering process didn’t lead to destruction of the specimen’s structure or the contact, because the initial average resistance for 8 tested specimens was about 1 X. 24 specimens were made in six series. Eight specimens from first two series were used for static tests, another sixteen for fatigue testing. The fiber mass fraction and the densities of carbon fiber and matrix were used for calculation of the fiber volume fraction. The thermogravimetric analyzer was used to find the fiber mass fraction. The fiber
volume fraction Vf ± standard deviation for each series is described below.
series series series series series series
1: 2: 3: 4: 5: 5:
Vf = 60.5 ± 1.22% Vf = 70.3 ± 1.06% Vf = 68.0 ± 0.26% Vf = 62.3 ± 0.48% Vf = 64.9 ± 0.27% Vf = 58.2 ± 0.39%
2.2. Experimental set-up The loading system of experimental stand enables implementation of the static and the periodically-variable pure bending of the specimen for a stress ratio R between 0 and 1. The novel system allows for obtaining a large curvature of the specimen. Schematic diagram of the specimen bending is shown in Fig. 3a, and the photograph of the specimen during test in Fig. 3b. Levers driven by the cord were used to apply the bending. This cord passes through a small hole in the fixing and in the specimen. Thanks to the fixing made of GFRP, the specimen was insulated from outer elements and the load frame. One end of the cord was attached to the load system and the other to the force gauge. The system that measure deflection was mounted in the geometric center of the specimen and it used inductive position sensor Burster 87244-000. The system allowed for obtaining the deflection of the specimen in the range of 0–40 mm. The advantage of this arrangement is that the pair of forces that produce the bending moment are always perpendicular to the specimen surface regardless of the radius of curvature. In order to eliminate the loads other than the constant bending moment, loading system (levers) was on
Fig. 2. Sequence and enumeration of contacts on the specimen.
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Fig. 3. Load frame: (a) schematic diagram of bending; (b) photograph.
rollers in order to allow free movement in the horizontal direction. The bending moment of the specimen was determined as a function of the force in the cord, the deflection and geometrical sizes of the loading device. The recording system allows for saving values of the load in the cord and the deflection in center of the specimen and the eight voltages, as shown in Fig. 4. The constant current of 20 ± 0.001 mA supplied to the pairs of contacts 1–9 and 11–19 was controlled by an independent highly-stable constant current source. For voltage measurement in fatigue tests, one used 16-bit NI-6211 multifunction card (maximal error about 0.015%). Measured voltages were recalculated to resistances using the Ohm’s law. For static measurements, we used Agilent 34401A 6½ digits 0.0015% basic DC Volt accuracy multimeter. 2.3. Experimental procedure The static and fatigue tests were performed using the pure bending of specimens. The purposes of the static tests were: (i) analyze structural changes in the CFRP during bending test, (ii) determine the average bending moment at which the specimen underwent destruction, and (iii)
analyze the strain of the CFRP based on the results obtained from measurements of changes in the resistance change and as a function of the bending moment and deflection of the specimen. The obtained results were analyzed as a function of the deflection, which made it possible to compare specimens. It was assumed that the specimen was substantially damaged when the bending moment was reduced rapidly and the curvature increased locally at the fracture site. The purposes of the fatigue testing of the stress ratio equal to R = 0 were: (i) the analysis of structural changes of the CFRP based on the results obtained from measurements of changes in the resistance and as a function of the bending moment and deflection of the specimen during the periodically-variable load and (ii) the durability test of contacts on CFRP (mechanical). The specimens were bent so as to obtain the controlled deflection at a fixed value during the test. In one cycle, 10 measurements were performed during increasing load and 10 measurements during its reduction. It was assumed that the maximum bending moment at the beginning of the fatigue test should be about 80% of the average of bending moment at which the destruction of the
Fig. 4. Schematic diagram of voltage recording system.
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specimens occurs during the static tests. The test was carried out until the destruction of the specimen, and for some time after the destruction (determined individually for each specimen). It was assumed that, if the bending moment of the specimen decreases by 1 Nm, one can consider that the specimen failed.
3. Results 3.1. Results of static test The measured signals were saved every 1 ms, which correspond to approx. 7 10 5 mm of deflection. The time of one cycle was about 4 min, which means that for one specimen about a quarter of million records were registered. Some selected results are presented in Fig. 5a for one specimen from the first series (fiber volume fraction Vf = 60%) and in Fig. 5b for the second one (Vf = 70%). In the Fig. 5 noises are mainly due to high levels of electrical interference. Courses of the resistance had not been filtered. The amplitude of the noise frequency of 50 Hz is about 6 times higher than the amplitude of components with different frequency. Larger noise amplitude in Fig. 5b than in the Fig. 5a, is due to the different resistance scale. In the Fig. 6 large blue areas do not show noises, but the range of variation of the resistance. The upper boundary shows the maximal resistance during the cycles (for the maximal value of the bending moment) and the lower boundary shows the minimal resistance during the cycles (for the minimal value of the bending moment). Great number of cycles in the graph (5000) makes it impossible to show the change of the resistance during a single cycle. One can observe that the nature of the destruction of the laminate is different in both series of specimens. In the specimens of the first series appeared the small jumps of the bending moment before the final fracture of the beam. Abrupt changes in the resistance between contacts 14 and 15 during these jumps were seen. The first break
of bending moment appeared for deflection of about 24 mm, what corresponded to about 40% increase in resistance between contacts 14 and 15. The final damage, and a decrease in bending moment of 4.5% appeared for a deflection of about 27 mm. At the same moment, very high increase in resistance (about 350%) between contacts 14– 15 was observed. The change in resistance for outer contacts in the compressed side (12–18) was not very big – about 4% – but also visible. It was remarkable that for the tension side (contacts 2–8) no change in resistance was observed. The noticed change in the resistance at the beginning of composite rupture is not big, but in practice it can be measured easier than the change in the stiffness of the structure. For a laminate with a very high fiber volume fraction, that is for the second series of specimens, fracture appeared suddenly and without any sign in the form of a change of bending moment and resistance. Damage appeared immediately, in the weakest place within one of the contacts. Delamination between longitudinal and transverse carbon layer was always accompanied by the damage. Its range was different for each specimen. The separation between the transverse CFRP and GFRP core has never been observed. The process that took place between the longitudinal CFRP and the outer GFRP had a similar course. There were no signs of destruction in extension layers, neither observed visually, nor detected as a change in resistance. Average levels of strain and stress in longitudinal CFRP layers were determined for specimens at the time of their destruction. For the first series of specimens, the strain was 0.85% and the normal stress on the assumption of homogenous material was 1030 MPa. For the second series of specimens, it was 0.55% and 670 MPa, respectively. The change in the resistance between external contacts (2–8 for tensile and 12–18 for compressed layer) was quasi-linear, and it could be also used for the strain measurement. This can be explained by the increase in resistance of the a conductor during its elongation and the
Fig. 5. Bending moment and resistance between selected contacts as a function of deflection for chosen specimen from: (a) first series; (b) second series.
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Fig. 6. Envelope of the maximum bending moment and resistance vs. number of cycles between: (a) external contacts; (b–d) contacts of the compression layers of the same specimen.
decrease during contraction, a phenomenon that is used in typical resistive strain gauges. The electrical resistance increases by about 0.15% for the strain growth of about 0.05%, and is approximately 2 times greater than that observed by Todoroki et al. [18]. This difference may be due to different specimen geometry and structure, especially to the presence of more sensitive transverse fiber layer. 3.2. Results of fatigue test The results of fatigue tests for a selected specimen, for the fourth series, are shown in Fig. 6. Distributions of the resistance between the external contacts 2–8 and 12–18 are shown together with the distribution of the maximum bending moment in each cycle (Fig. 6a). Distributions of the resistance between adjacent contacts of the compression layers (measuring sections 13–14, 14–15 and 15–16) are shown in Fig. 6 b.
Fig. 7 shows images of cracks on the compressed side of the specimen from the Fig. 6 for about: 10; 3000; 3500 and 4500 cycle. It is visible, that the number of cracks and their length increase simultaneously with the number of cycles. White lines parallel to the specimen length represent transverse cracks of the delaminated layer, which reached the surface. The white color results from a thin layer of glass fabric, which covers the outside sides of the sample (see Fig. 1). After the test, the specimens were tested using the active thermographic method. The performed analysis confirmed that during destruction the layers were delaminated, as shown in Fig. 8. White color near the left contact in Fig. 8b means that there is a space between carbon layers caused by delamination. White lines, visible in Fig. 8a, were observed simultaneously during the test. Their length grew with increasing number of cycles, which indicated a progressive fatigue damage process.
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Fig. 7. Images of cracks on the compressed side of the specimen from Fig. 6 for about: (a) 10 cycle; (b) 3000 cycle; (c) 3500 cycle; (d) 4500 cycle.
25 mm
(a)
(b) Fig. 8. Compressed layer of one of destroyed specimen from fourth series: (a) view; (b) thermographic examination.
For the specimen presented above, the distribution can be divided into several stages in terms of ranges of the recorded parameters:
not change. Rapid change in parameters shows that there appears the first, very small damage of composite near the contact 14. The change in resistance is more visible than change in maximal bending moment.
1. Cycles from 0 to about 2200. 3. Cycles from about 3250 to about 3900. Every parameters (maximal bending moment, resistances) are almost constant. Specimen shows no sign of destruction. 2. Cycles from about 2200 to about 3250. A slight decrease in maximal bending moment, which appears as the slope of its curve. The maximal value of resistance of external contacts 12–18 slightly increases by about 1.0%, the minimal value does not vary. Similar phenomenon is observed for contacts 13–14 and 14–15 (change of about 4.2%). The remaining resistance does
Abrupt decrease in maximal bending moment (to about 20%) indicates serious damage of the composite, although the specimen still has quite a good load rating. At the same time, great change in maximal value and range of variation of resistance is observed. For external contacts it is about 15% for maximal value and about 1000% for the range of variation. The same phenomenon is observed for internal contacts, and the corresponding changes are 63% and 500% for contacts 13–14 and 96% and 1400% for contact 14–15, respectively. In this period, delamination between layers grows up.
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Catastrophic decrease in the maximal bending moment and increase in resistance is observed. This is caused by rupture near the contact number 14. The specimen has only very small residual strength. The range of variation of the resistance, both for the internal and external contact, is more than 50 times greater than for a new specimen. The nature of the resistance distributions in the individual stages of the other specimens with different fiber volume fraction had a character more or less similar that described above. The differences consisted in different number of cycles at various stages. For the whole time of experiment, no significant changes in resistance of specimen was observed for the tension surface. Fig. 9 presents changes of the resistance and bending moment for beginning and late cycles for selected specimen. The deflection was controlled and fixed on the level of 13 mm. Other works present similar course of changes for single fiber [8], nanofiber [9] and classical multifiber composite [22]. It is visible, that for late cycle number (Fig. 9c and d), when damage of the specimen begins, the course of resistance changes are similar to the initial one, especially for tensioned side. For compressed side of the
specimen resistance changes are similar, but the upper part of line is more ‘‘flat” than for initial cycles. Also change of the electrical resistance is bigger that in the beginning of the experiment. For tension layer the change of bending moment and electrical resistance has the same phase. This is due to the fact that the conductive carbon fibers in this layer are stretched and the resistance is proportional to the length of the conductor, which increases with bending moment. In compressed layer the fibers are shortened and their resistance increase, what results in reverse phase of bending moment and electrical resistance. As expected, the resistance is subject to oscillations, whose period corresponds to one cycle of load. Within a single cycle of the specimen without damage, when the bending moment increases, the resistance increases in the expanded layers, and decreases in the compressed ones. The results show that the resistance change (amplitude) in the expanded layer before the appearance of damage is greater than that in the compression layer and this difference is 2.5% on average. One needs more investigations to find the cause for this phenomenon. For now, we can only hypothesize. Perhaps the lack of conformity in amplitude was caused by an increase in resistance in the
3501
3502
bending moment [Nm]
4. Cycles about 3900 and further.
0 3503
number of cycles
Fig. 9. Change of the resistance (continuous line) and bending moment (dotted line) for selected cycles for: (a) beginning cycles – compressed side; (b) beginning cycles – tensioned side; (c) late cycles – compressed side; (d) late cycles – tensioned side.
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transverse layer due to micro-cracking of the matrix during stretching. As mentioned above, many fibers were in contact with each other via the side surfaces. Probably the break of contact appeared during bending of specimens and the contact was restored after return to the original shape of the specimen. The average resistance for 16 specimens for the initial cycles equals 0.11 X, standard deviation in the population equals 0.01 X. The next stage of the analysis concerned the resistance changes when the damage appeared. The range of cycles, wherein the bending moment decreased catastrophically, was taken into account. The bundles of compressed longitudinal fiber have already been finally broken (stage No. 4 for the specimens described above). This allowed for the comparative analysis of changes in the composite structure in two cases: 1. The specimen is bent up i.e. the broken ends of the fiber in a destroyed cross-section are in contact with each other; 2. The specimen is unloaded (not flexed) i.e. the broken ends of the same fibers are not in contact. This second case will just show the scale of destruction. If more bundles are broken, the maximal resistance in the cycle becomes greater. Although the bundle’s cracks were appearing always close to any of internal contacts and apparently should have a similar nature, the destruction process was different for each specimen. Visual inspection of specimens and the level of maximal resistance in a cycle tells us about this phenomenon. The average values of maximal resistance in a cycle between adjacent internal contacts (25 mm range) in compressed layers for 10 specimens equals 0.56 X and standard deviation in the population equals 0.4 X. When the specimen is maximally bent, i.e. ends of broken fibers are in contact, the average resistance measured in the same area is 2.8 times smaller, and equals 0.2 X. The standard deviation in the population equals 0.14 X. 3.3. Quality of the contacts test The residual peel strength of the connections between the contact and the CFRP was tested. The test was performed on the specimen that was removed from the experimental stand after the fatigue test. The wire that was soldered to the contact was pulled by an electronic dynamometer and, at the same time, resistance was measured using the four-wire method. The wire that was soldered to the contact was broken off in each contact test, and the contacts remained intact. The average value of the force needed to brake off the wires equals 8.5 N. No changes in resistance were observed during this test. 4. Conclusions The experiments described in the paper showed that the method of measuring the change of resistance in CFRP allows one to gather information about the damage of the
laminate. The results indicate that small structural defects can change the resistance of the composite, despite the fact that they cannot be detected even using other traditional methods, like measuring of the stiffness of the laminate. This is particularly important when the composite element is assessed. Deformation of CFRP i.e. the change in the length of fibers affects the resistance change, which is easier to measure than the decrease in stiffness. It is meaningful that the method can be used to monitor the load level in structures made of CFRP. This procedure may be used in static and fatigue load. In both cases, the measurable change in resistance precedes visible changes in the stiffness. The obtained results confirm the findings of other researchers. The use of hybrid structures of specimens with a glass core allows for the study of both tension and compression of CFRP at the same time. Additionally, the structure of the specimen allows the large load on longitudinal layers of CFRP what means that the stress can reach high values. The special, novel experimental set-up enabled us to obtain pure bending of the specimen, regardless of the level of the curvature and deflection of the specimen. Durability and strength of connection between the proposed new type of contacts used in the test and the composite is very high, also in the case when the composite element reaches a large curvature. The use of deflection controlled load enables precise examination of changes in stiffness and analysis of resistance changes after the destruction of the carbon layer in the fatigue test. Future research should lead to a better understanding of the relationship between the changes in the structure and the changes in resistance, and should increase the sensitivity of the method. In particular, it is important to develop a method for determining an optimal location of contacts attachment points on the structure depending on its shape, structure and expected loads. Further work should also allow one to build appropriate numerical models, useful for determining the location, size and nature of the damage on the basis of the measured change in resistance.
Acknowledgements This work has been supported by the Scientific Research Communities of Poland, Project No. 4034/B/T02/2008/35.
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J. Gadomski, P. Pyrzanowski / Measurement 87 (2016) 236–245 [6] P.E. Irving, C. Thiagarajan, Fatigue damage characterization in carbon fibre composite materials using an electrical potential technique, Smart Mater. Struct. 7 (1998) 456–466. [7] K. Moriya, T.A. Endo, Study on flaw detection method for CFRP composite laminates (1st report) the measurement of crack extension in CFRP composites by electrical potential method, Aeronaut. Space Sci. Jpn. 36 (410) (1988) 139–146. [8] J.M. Park, S.I. Lee, K.L. DeVries, Nondestructive sensing evaluation of surface modified single-carbon fiber reinforced epoxy composites by electrical resistivity measurement, Compos. Part B 37 (2006) 612– 626. [9] J.M. Park, J.H. Jang, Z.J. Wang, D.J. Kwon, K.L. DeVries, Self-sensing of carbon fiber/carbon nanofiber–epoxy composites with two different nanofiber aspect ratios investigated by electrical resistance and wettability measurements, Compos. Part A 41 (2010) 1702–1711. [10] M. Olzak, P. Pyrzanowski, Numerical modelling of resistance changes in symmetric CFRP composite under the influence of structure damage, Compos. Sci. Technol. 88 (2013) 99–105. [11] Reifsnider R. L., Stinchcomb W. W., A critical-element model of the residual strength and life of fatigue-loaded composite coupons, Composite Materials: Fatigue and Fracture,-ASTM STP-907, ed. Hahn H. L., Philadelphia, 1986, 298–313. [12] M. Rodzewicz, A. Boczkowska, Investigations into the CFRP strength deterioration due to fatigue loads or impacts, review of Polish Aeronautical Fatigue Work, Trans. Inst Aviation 188 (2007) 25–31. [13] K. Schulte, C. Baron, Load and failure analysis of CFRP laminates by means of electrical resistivity measurements, Compos. Sci. Technol. 36 (1989) 63–76. [14] R.D. Sherman, L.M. Middleman, S.M. Jacobs, Electron transport process in conductor-filled polymers, Polym. Eng. Sci. 23 (1) (1986) 36–46.
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[15] E. Teomete, Measurement of crack length sensitivity and strain gage factor of carbon fiber reinforced cement matrix composites, Measurement 74 (2015) 21–30. [16] A. Todoroki, K. Omagari, Y. Shimamura, H. Kobayashi, Matrix crack detection of CFRP using electrical resistance change with integrated surface probes, Compos. Sci. Technol. 66 (2006) 1539–1545. [17] A. Todoroki, K. Suzuki, Y. Mizutani, R. Matsuzaki, Durability estimates of copper plated electrodes for self-sensing CFRP composites, J. Solid Mech. Mater. Eng. JSME 4 (6) (2010) 610–620. [18] A. Todoroki, K. Suzuki, Y. Mizutani, R. Matsuzaki, Electrical resistance change of CFRP under compression load, J. Solid Mech. Mater. Eng. JSME 4 (7) (2010) 864–874. [19] A. Todoroki, M. Tanaka, Y. Shimamura, Measurement of orthotropic electric conductance of CFRP laminates and analysis of the effect on delamination monitoring with an electric resistance change method, Compos. Sci. Technol. 62 (2002) 619–628. [20] D. Wang, D.D.L. Chung, Comparative evaluation of the electrical configurations for the two-dimensional electric potential method of damage monitoring in carbon fibre polymer–matrix composite, Smart Mater. Struct. 15 (2006) 1332–1344. [21] S. Wang, D.D.L. Chung, J.H. Chung, Impact damage of carbon fiber polymer–matrix composites, studied by electrical resistance measurement, Compos. Part A 36 (2005) 1707–1715. [22] S. Wang, D.D.L. Chung, Self-sensing of flexural strain and damage in carbon fiber polymer-matrix composite by electrical resistance measurement, Carbon 44 (2006) 2739–2751. [23] X. Wang, D.D.L. Chung, Sensing delamination in a carbon fiber polymer-matrix composite during fatigue by electrical resistance measurement, Polym. Compos. 18 (6) (1997) 692–700. [24] J. Wen, Z. Xia, F. Choy, Damage detection of carbon fiber reinforced polymer composites via electrical resistance measurement, Compos. Part B 42 (2011) 77–86.
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Measurement journal homepage: www.elsevier.com/locate/measurement
Experimental investigation of fatigue destruction of CFRP using the electrical resistance change method Jacek Gadomski, Paweł Pyrzanowski ⇑ Institute of Aeronautics and Applied Mechanics, Warsaw University of Technology, Nowowiejska 24, 00-665 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 30 July 2015 Received in revised form 25 February 2016 Accepted 15 March 2016 Available online 19 March 2016 Keywords: Carbon fibers Polymer-matrix composites (PMCs) Electrical properties Failure
a b s t r a c t CFRP (Carbon Fiber Reinforced Polymer) composites are one of the most popular composites used as the structural material in high-loaded structures. Important disadvantage of this material is the difficulty in predicting the time of fatigue destruction. These changes decrease the stiffness of the structure, but in practice it is usually not possible to measure it. Therefore, it is necessary to find another method to determine the moment when destruction of the composite begins. In this paper, one of possible methods – measuring of the electrical resistance change was investigated. Bending beam-shape specimens were tested under static and periodically varied load. Bending moment, deflection in the center of the probe and 8 voltages were measured. Performed experiment shows that measuring of the electrical resistance change provides better information about destruction of CFRP structure than a measure of the bending moment. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced composites (FRP) are used for years as a construction for the responsible structure in the aerospace, automotive, shipbuilding etc. Structures made of FRP, like other construction materials, are subject to irreversible negative physical changes appearing in the entire volume during the period of use. Damages, such as fiber breaks, cracks of matrix and delaminations are scattered and difficult to observe by visual methods, and can potentially threaten the composite structure. The long, cyclicallyvarying levels of stress in the structure plays the biggest role among the factors contributing to the formation of defects in composites. Non-destructive diagnostic methods (NDE) are applied in order to monitor the condition of FRP. These methods make it possible to localize damage in FRP, identify their type and receive information about their density. Methods ⇑ Corresponding author. Tel.: +48 22 2347512. E-mail address: [email protected] (P. Pyrzanowski). http://dx.doi.org/10.1016/j.measurement.2016.03.036 0263-2241/Ó 2016 Elsevier Ltd. All rights reserved.
based on the acoustic techniques, X-ray, thermal, magnetic and optic methods are used to detect defects in the composite structure. They are characterized by a more or less limited area monitored at a time. The methods differ in the type of transducers, which are in contact with the investigated object or collect a signal or image from a distance [12]. In many of these methods, diagnostics is not possible to perform during operation, and the area monitored at a time is often very small compared to the dimensions of the structure of FRP. The method consisting in measuring the change of resistance in CFRP can be a system that would provide information on failures of CFRP on a regular basis. Literature does not provide information about the use of the method in practice. Many advantages of the method were found: the possibility of continuous monitoring of CFRP, the ability to simultaneously detect defects throughout the volume of the tested element, and the possibility of deformation investigation of the composite structure in real time. In the 80th of the last century, research on this method was initiated in several research centers. It was
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based on the use of the properties of conduction of electrical current through the fibers of the CFRP [2,3,6,7,13,14]. The process of destruction of structures made of CFRP involves the damage to the reinforcement and matrix. This process increases the resistance of the laminate pieces and, consequently, the entire composite part. Such a resistance change can be measured by external measuring devices connected to the composite by means of element being permanently in contact with the CFRP. Available publications present results of CFRP research in the scope of the resistivity, both for laminates and for an isolated fiber [8]. Information on this subject is also provided by carbon fiber manufacturers, e.g. Toray, Toho Tenax etc. Resistivity measurements of unidirectional CFRP were also performed. Resistivity was studied in different directions with respect to the direction of the fibers depending on the fiber volume fraction Vf [19]. Non-zero conductance in an unidirectional CFRP in the lateral direction is the result of the actual construction of the laminate. Fibers are arranged in the FRP at random way, which creates the possibility of direct contacts between adjacent fibers [19]. This phenomenon is the cause for the appearance of a network of electrical connections in the entire volume of CFRP. When the fiber volume fraction increases, the fiber density distribution becomes more uniform in the cross section perpendicular to the direction of fiber arrangement. The best method for determining the resistance of CFRP is a method consisting in measuring the potential. At least four contacts arranged on the test element are used for the measurement of voltage. A stabilized current source is connected to two external contacts, and between other pairs of contacts the voltage is measured. The advantage of this method is high accuracy due to the fact that the quality of the connection between CFRP and the test meter does not significantly affect the value of the measured voltage [4,10]. The possible positions of the current and voltage contacts on the laminate sections are proposed by Wang and Chung [20]. The first proposal with contacts on one side of the laminate would be the easiest to apply in real CFRP structure (large elements for aircrafts, ships etc). However, one continues to search for contacts of new shape and size in order to improve the quality of the method of measuring the change in resistance [21,22]. However, the contacts must be on both sides if it is necessary to examine the CFRP in the through-thickness direction for delamination investigation [23]. Materials for the contacts are silver, copper, and substances containing particles of silver such as glue, paint or epoxy binder [22]. The new integrated copper plating method was proposed in order to create durable connection by soldering between lead wire and CFRP [17]. The most often used method to connect the conductor to reinforcement of the laminate is based on bonding with an electro – conductive glue. In order to expose the fiber reinforcement, it is proposed to grind the top layer of matrix [1,16]. Therefore, it is necessary to conduct experimental research in order to extend knowledge on the methods of measuring the change of resistance in CFRP, under the influence of static and periodically variable loads.
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Performed experiments and analysis can be in the future used for analytical and numerical modeling, especially for most complicated cases [24].
2. Experimental procedure 2.1. Specimen The specimen that was made of CFRP as a bending beam, designed for load of constant bending moment, is shown in Fig. 1a. On both sides of the beam, electrical contacts were attached. The tested CFRP layers were separated from each other by the core made of GFRP (Glass Fiber Reinforced Polymer), what in shown in Fig. 1b. Similar type of specimen was used already by Todoroki at al. [18]. Novelty in this type of specimen consists in using the cross-ply laminate of [0/90] in the parts containing CFRP layers. In these layers, unidirectional tapes with a weight of 300 g/m2 (Havel Composites) arranged along and across the specimen were used. Examination the FRP as a cross-ply or quasi-isotropic laminate shows that the first damage appears in the form of matrix cracks in the transverse layers, or at the interfaces between the layers of fibers perpendicular to each other [11]. The GFRP core was made of glass fabric STR 028-150110 with a weight of 150 g/m2 in a technological process separate from that of the cross-ply laminate [±708], and it had the fiber volume fraction of approximately 0.4. The glass fabric was set in such a direction as to minimize the core stiffness. The core, whose thickness is 70% of the thickness of the entire specimen meets the task of an electrical insulator between examined layers. The specimen constructed in this way allows the testing of the CFRP in which the same number of fibers is extended and compressed at the same time. It was assumed that the stiffness of the core is low relative to the stiffness of the CFRP layers. The Young’s modulus of the core was found by measuring the stiffness using the three-point bending method. The outer GFRP thin layer ensures durable adhesion of contacts and carbon fibers. Interglass 02037 fabric with weight of 49 g/m2 was used for this layer. The specimen was formed in a special mold using hand-operated process. The controlled vacuum process made it possible to obtain the specimens of identical size and proper fiber volume fraction in each series. The matrix of the specimen contains Epidian 53 resin and Z1 hardener. Electrical contacts were made of the pure silver tape of 0.07 mm thickness and width 1.3 mm. They were attached in the configuration similar to that presented in previous work [21]. These tapes were added to the specimens in the manufacturing process, as it was described in [5]. Eight of contacts (two pairs for both side of the specimen – extended and compressed) were applied in the area not subjected to bending, that is at the free ends of the specimen, as shown in Fig. 1a. This allows us to maintain the same quality of electrical connection between the carbon fibers (reinforcement) and the metal (contacts) for the whole duration of the test. Owing to such a montage of electrodes, the information about the resistance is
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Fig. 1. Structure of the specimen: (a) general view; (b) internal structure.
gathered on the whole length of specimen, and the electric current can be properly supplied. The remaining eight contacts (4 for tensile side and 4 for compressed side) were applied in the bending zone. Enumeration of the contacts is shown in Fig. 2. The idea of embedded contacts, buildin into the specimen during production has been also used for examination of carbon fiber reinforced cement based composite [15]. Carbon fiber bundles were locally deformed by the contacts and CFRP fiber volume fraction increased in this place. A notch was made at the place around which the destruction process was initiated. Similar distortions occur in the real composite structures. For example, they exhibit local changes in the thickness of the laminate (higher local pressure during the lamination process), or the edges of additional layers of the laminate, etc. After preparing the specimen, one removed the small piece of external GFRP at the center of contact to expose the metal. Electric wire was soldered to this place, what enabled the measurement of the electrical parameters. The resistance between selected pairs of contacts was measured before and after soldering. The average value of changes in resistance for pairs of contacts 2–8 and 12–18 for contacts before and after soldering was 0.053 X, and the population standard deviation was 0.0089 X. It meant that the soldering process didn’t lead to destruction of the specimen’s structure or the contact, because the initial average resistance for 8 tested specimens was about 1 X. 24 specimens were made in six series. Eight specimens from first two series were used for static tests, another sixteen for fatigue testing. The fiber mass fraction and the densities of carbon fiber and matrix were used for calculation of the fiber volume fraction. The thermogravimetric analyzer was used to find the fiber mass fraction. The fiber
volume fraction Vf ± standard deviation for each series is described below.
series series series series series series
1: 2: 3: 4: 5: 5:
Vf = 60.5 ± 1.22% Vf = 70.3 ± 1.06% Vf = 68.0 ± 0.26% Vf = 62.3 ± 0.48% Vf = 64.9 ± 0.27% Vf = 58.2 ± 0.39%
2.2. Experimental set-up The loading system of experimental stand enables implementation of the static and the periodically-variable pure bending of the specimen for a stress ratio R between 0 and 1. The novel system allows for obtaining a large curvature of the specimen. Schematic diagram of the specimen bending is shown in Fig. 3a, and the photograph of the specimen during test in Fig. 3b. Levers driven by the cord were used to apply the bending. This cord passes through a small hole in the fixing and in the specimen. Thanks to the fixing made of GFRP, the specimen was insulated from outer elements and the load frame. One end of the cord was attached to the load system and the other to the force gauge. The system that measure deflection was mounted in the geometric center of the specimen and it used inductive position sensor Burster 87244-000. The system allowed for obtaining the deflection of the specimen in the range of 0–40 mm. The advantage of this arrangement is that the pair of forces that produce the bending moment are always perpendicular to the specimen surface regardless of the radius of curvature. In order to eliminate the loads other than the constant bending moment, loading system (levers) was on
Fig. 2. Sequence and enumeration of contacts on the specimen.
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Fig. 3. Load frame: (a) schematic diagram of bending; (b) photograph.
rollers in order to allow free movement in the horizontal direction. The bending moment of the specimen was determined as a function of the force in the cord, the deflection and geometrical sizes of the loading device. The recording system allows for saving values of the load in the cord and the deflection in center of the specimen and the eight voltages, as shown in Fig. 4. The constant current of 20 ± 0.001 mA supplied to the pairs of contacts 1–9 and 11–19 was controlled by an independent highly-stable constant current source. For voltage measurement in fatigue tests, one used 16-bit NI-6211 multifunction card (maximal error about 0.015%). Measured voltages were recalculated to resistances using the Ohm’s law. For static measurements, we used Agilent 34401A 6½ digits 0.0015% basic DC Volt accuracy multimeter. 2.3. Experimental procedure The static and fatigue tests were performed using the pure bending of specimens. The purposes of the static tests were: (i) analyze structural changes in the CFRP during bending test, (ii) determine the average bending moment at which the specimen underwent destruction, and (iii)
analyze the strain of the CFRP based on the results obtained from measurements of changes in the resistance change and as a function of the bending moment and deflection of the specimen. The obtained results were analyzed as a function of the deflection, which made it possible to compare specimens. It was assumed that the specimen was substantially damaged when the bending moment was reduced rapidly and the curvature increased locally at the fracture site. The purposes of the fatigue testing of the stress ratio equal to R = 0 were: (i) the analysis of structural changes of the CFRP based on the results obtained from measurements of changes in the resistance and as a function of the bending moment and deflection of the specimen during the periodically-variable load and (ii) the durability test of contacts on CFRP (mechanical). The specimens were bent so as to obtain the controlled deflection at a fixed value during the test. In one cycle, 10 measurements were performed during increasing load and 10 measurements during its reduction. It was assumed that the maximum bending moment at the beginning of the fatigue test should be about 80% of the average of bending moment at which the destruction of the
Fig. 4. Schematic diagram of voltage recording system.
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specimens occurs during the static tests. The test was carried out until the destruction of the specimen, and for some time after the destruction (determined individually for each specimen). It was assumed that, if the bending moment of the specimen decreases by 1 Nm, one can consider that the specimen failed.
3. Results 3.1. Results of static test The measured signals were saved every 1 ms, which correspond to approx. 7 10 5 mm of deflection. The time of one cycle was about 4 min, which means that for one specimen about a quarter of million records were registered. Some selected results are presented in Fig. 5a for one specimen from the first series (fiber volume fraction Vf = 60%) and in Fig. 5b for the second one (Vf = 70%). In the Fig. 5 noises are mainly due to high levels of electrical interference. Courses of the resistance had not been filtered. The amplitude of the noise frequency of 50 Hz is about 6 times higher than the amplitude of components with different frequency. Larger noise amplitude in Fig. 5b than in the Fig. 5a, is due to the different resistance scale. In the Fig. 6 large blue areas do not show noises, but the range of variation of the resistance. The upper boundary shows the maximal resistance during the cycles (for the maximal value of the bending moment) and the lower boundary shows the minimal resistance during the cycles (for the minimal value of the bending moment). Great number of cycles in the graph (5000) makes it impossible to show the change of the resistance during a single cycle. One can observe that the nature of the destruction of the laminate is different in both series of specimens. In the specimens of the first series appeared the small jumps of the bending moment before the final fracture of the beam. Abrupt changes in the resistance between contacts 14 and 15 during these jumps were seen. The first break
of bending moment appeared for deflection of about 24 mm, what corresponded to about 40% increase in resistance between contacts 14 and 15. The final damage, and a decrease in bending moment of 4.5% appeared for a deflection of about 27 mm. At the same moment, very high increase in resistance (about 350%) between contacts 14– 15 was observed. The change in resistance for outer contacts in the compressed side (12–18) was not very big – about 4% – but also visible. It was remarkable that for the tension side (contacts 2–8) no change in resistance was observed. The noticed change in the resistance at the beginning of composite rupture is not big, but in practice it can be measured easier than the change in the stiffness of the structure. For a laminate with a very high fiber volume fraction, that is for the second series of specimens, fracture appeared suddenly and without any sign in the form of a change of bending moment and resistance. Damage appeared immediately, in the weakest place within one of the contacts. Delamination between longitudinal and transverse carbon layer was always accompanied by the damage. Its range was different for each specimen. The separation between the transverse CFRP and GFRP core has never been observed. The process that took place between the longitudinal CFRP and the outer GFRP had a similar course. There were no signs of destruction in extension layers, neither observed visually, nor detected as a change in resistance. Average levels of strain and stress in longitudinal CFRP layers were determined for specimens at the time of their destruction. For the first series of specimens, the strain was 0.85% and the normal stress on the assumption of homogenous material was 1030 MPa. For the second series of specimens, it was 0.55% and 670 MPa, respectively. The change in the resistance between external contacts (2–8 for tensile and 12–18 for compressed layer) was quasi-linear, and it could be also used for the strain measurement. This can be explained by the increase in resistance of the a conductor during its elongation and the
Fig. 5. Bending moment and resistance between selected contacts as a function of deflection for chosen specimen from: (a) first series; (b) second series.
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Fig. 6. Envelope of the maximum bending moment and resistance vs. number of cycles between: (a) external contacts; (b–d) contacts of the compression layers of the same specimen.
decrease during contraction, a phenomenon that is used in typical resistive strain gauges. The electrical resistance increases by about 0.15% for the strain growth of about 0.05%, and is approximately 2 times greater than that observed by Todoroki et al. [18]. This difference may be due to different specimen geometry and structure, especially to the presence of more sensitive transverse fiber layer. 3.2. Results of fatigue test The results of fatigue tests for a selected specimen, for the fourth series, are shown in Fig. 6. Distributions of the resistance between the external contacts 2–8 and 12–18 are shown together with the distribution of the maximum bending moment in each cycle (Fig. 6a). Distributions of the resistance between adjacent contacts of the compression layers (measuring sections 13–14, 14–15 and 15–16) are shown in Fig. 6 b.
Fig. 7 shows images of cracks on the compressed side of the specimen from the Fig. 6 for about: 10; 3000; 3500 and 4500 cycle. It is visible, that the number of cracks and their length increase simultaneously with the number of cycles. White lines parallel to the specimen length represent transverse cracks of the delaminated layer, which reached the surface. The white color results from a thin layer of glass fabric, which covers the outside sides of the sample (see Fig. 1). After the test, the specimens were tested using the active thermographic method. The performed analysis confirmed that during destruction the layers were delaminated, as shown in Fig. 8. White color near the left contact in Fig. 8b means that there is a space between carbon layers caused by delamination. White lines, visible in Fig. 8a, were observed simultaneously during the test. Their length grew with increasing number of cycles, which indicated a progressive fatigue damage process.
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Fig. 7. Images of cracks on the compressed side of the specimen from Fig. 6 for about: (a) 10 cycle; (b) 3000 cycle; (c) 3500 cycle; (d) 4500 cycle.
25 mm
(a)
(b) Fig. 8. Compressed layer of one of destroyed specimen from fourth series: (a) view; (b) thermographic examination.
For the specimen presented above, the distribution can be divided into several stages in terms of ranges of the recorded parameters:
not change. Rapid change in parameters shows that there appears the first, very small damage of composite near the contact 14. The change in resistance is more visible than change in maximal bending moment.
1. Cycles from 0 to about 2200. 3. Cycles from about 3250 to about 3900. Every parameters (maximal bending moment, resistances) are almost constant. Specimen shows no sign of destruction. 2. Cycles from about 2200 to about 3250. A slight decrease in maximal bending moment, which appears as the slope of its curve. The maximal value of resistance of external contacts 12–18 slightly increases by about 1.0%, the minimal value does not vary. Similar phenomenon is observed for contacts 13–14 and 14–15 (change of about 4.2%). The remaining resistance does
Abrupt decrease in maximal bending moment (to about 20%) indicates serious damage of the composite, although the specimen still has quite a good load rating. At the same time, great change in maximal value and range of variation of resistance is observed. For external contacts it is about 15% for maximal value and about 1000% for the range of variation. The same phenomenon is observed for internal contacts, and the corresponding changes are 63% and 500% for contacts 13–14 and 96% and 1400% for contact 14–15, respectively. In this period, delamination between layers grows up.
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Catastrophic decrease in the maximal bending moment and increase in resistance is observed. This is caused by rupture near the contact number 14. The specimen has only very small residual strength. The range of variation of the resistance, both for the internal and external contact, is more than 50 times greater than for a new specimen. The nature of the resistance distributions in the individual stages of the other specimens with different fiber volume fraction had a character more or less similar that described above. The differences consisted in different number of cycles at various stages. For the whole time of experiment, no significant changes in resistance of specimen was observed for the tension surface. Fig. 9 presents changes of the resistance and bending moment for beginning and late cycles for selected specimen. The deflection was controlled and fixed on the level of 13 mm. Other works present similar course of changes for single fiber [8], nanofiber [9] and classical multifiber composite [22]. It is visible, that for late cycle number (Fig. 9c and d), when damage of the specimen begins, the course of resistance changes are similar to the initial one, especially for tensioned side. For compressed side of the
specimen resistance changes are similar, but the upper part of line is more ‘‘flat” than for initial cycles. Also change of the electrical resistance is bigger that in the beginning of the experiment. For tension layer the change of bending moment and electrical resistance has the same phase. This is due to the fact that the conductive carbon fibers in this layer are stretched and the resistance is proportional to the length of the conductor, which increases with bending moment. In compressed layer the fibers are shortened and their resistance increase, what results in reverse phase of bending moment and electrical resistance. As expected, the resistance is subject to oscillations, whose period corresponds to one cycle of load. Within a single cycle of the specimen without damage, when the bending moment increases, the resistance increases in the expanded layers, and decreases in the compressed ones. The results show that the resistance change (amplitude) in the expanded layer before the appearance of damage is greater than that in the compression layer and this difference is 2.5% on average. One needs more investigations to find the cause for this phenomenon. For now, we can only hypothesize. Perhaps the lack of conformity in amplitude was caused by an increase in resistance in the
3501
3502
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4. Cycles about 3900 and further.
0 3503
number of cycles
Fig. 9. Change of the resistance (continuous line) and bending moment (dotted line) for selected cycles for: (a) beginning cycles – compressed side; (b) beginning cycles – tensioned side; (c) late cycles – compressed side; (d) late cycles – tensioned side.
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transverse layer due to micro-cracking of the matrix during stretching. As mentioned above, many fibers were in contact with each other via the side surfaces. Probably the break of contact appeared during bending of specimens and the contact was restored after return to the original shape of the specimen. The average resistance for 16 specimens for the initial cycles equals 0.11 X, standard deviation in the population equals 0.01 X. The next stage of the analysis concerned the resistance changes when the damage appeared. The range of cycles, wherein the bending moment decreased catastrophically, was taken into account. The bundles of compressed longitudinal fiber have already been finally broken (stage No. 4 for the specimens described above). This allowed for the comparative analysis of changes in the composite structure in two cases: 1. The specimen is bent up i.e. the broken ends of the fiber in a destroyed cross-section are in contact with each other; 2. The specimen is unloaded (not flexed) i.e. the broken ends of the same fibers are not in contact. This second case will just show the scale of destruction. If more bundles are broken, the maximal resistance in the cycle becomes greater. Although the bundle’s cracks were appearing always close to any of internal contacts and apparently should have a similar nature, the destruction process was different for each specimen. Visual inspection of specimens and the level of maximal resistance in a cycle tells us about this phenomenon. The average values of maximal resistance in a cycle between adjacent internal contacts (25 mm range) in compressed layers for 10 specimens equals 0.56 X and standard deviation in the population equals 0.4 X. When the specimen is maximally bent, i.e. ends of broken fibers are in contact, the average resistance measured in the same area is 2.8 times smaller, and equals 0.2 X. The standard deviation in the population equals 0.14 X. 3.3. Quality of the contacts test The residual peel strength of the connections between the contact and the CFRP was tested. The test was performed on the specimen that was removed from the experimental stand after the fatigue test. The wire that was soldered to the contact was pulled by an electronic dynamometer and, at the same time, resistance was measured using the four-wire method. The wire that was soldered to the contact was broken off in each contact test, and the contacts remained intact. The average value of the force needed to brake off the wires equals 8.5 N. No changes in resistance were observed during this test. 4. Conclusions The experiments described in the paper showed that the method of measuring the change of resistance in CFRP allows one to gather information about the damage of the
laminate. The results indicate that small structural defects can change the resistance of the composite, despite the fact that they cannot be detected even using other traditional methods, like measuring of the stiffness of the laminate. This is particularly important when the composite element is assessed. Deformation of CFRP i.e. the change in the length of fibers affects the resistance change, which is easier to measure than the decrease in stiffness. It is meaningful that the method can be used to monitor the load level in structures made of CFRP. This procedure may be used in static and fatigue load. In both cases, the measurable change in resistance precedes visible changes in the stiffness. The obtained results confirm the findings of other researchers. The use of hybrid structures of specimens with a glass core allows for the study of both tension and compression of CFRP at the same time. Additionally, the structure of the specimen allows the large load on longitudinal layers of CFRP what means that the stress can reach high values. The special, novel experimental set-up enabled us to obtain pure bending of the specimen, regardless of the level of the curvature and deflection of the specimen. Durability and strength of connection between the proposed new type of contacts used in the test and the composite is very high, also in the case when the composite element reaches a large curvature. The use of deflection controlled load enables precise examination of changes in stiffness and analysis of resistance changes after the destruction of the carbon layer in the fatigue test. Future research should lead to a better understanding of the relationship between the changes in the structure and the changes in resistance, and should increase the sensitivity of the method. In particular, it is important to develop a method for determining an optimal location of contacts attachment points on the structure depending on its shape, structure and expected loads. Further work should also allow one to build appropriate numerical models, useful for determining the location, size and nature of the damage on the basis of the measured change in resistance.
Acknowledgements This work has been supported by the Scientific Research Communities of Poland, Project No. 4034/B/T02/2008/35.
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