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The self-reinforcing effect of Nylon 6,6 nano-fibres on CFRP laminates subjected to low velocity impact
Composite Structures 106 (2013) 661–671
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
The self-reinforcing effect of Nylon 6,6 nano-fibres on CFRP laminates subjected to low velocity impact R. Palazzetti a,⇑, A. Zucchelli a, I. Trendafilova b a b
DIN, Department of Industrial Engineering, University of Bologna, Italy MAE, Department of Mechanical and Aeronautic Engineering, Strathclyde University, Glasgow, UK
a r t i c l e
i n f o
Article history: Available online 16 July 2013 Keywords: Electrospinning Nano composite Impact behaviour Vibration Fibre-bridging
a b s t r a c t This work investigates the mechanical properties of CFR-epoxy laminates interleaved with electrospun Nylon 6,6 nano-fibres. The main goal is to investigate the interaction between the nanofibrous mats interleaved into a laminate and their influence on the property of the whole body and in particular their reinforcing effect. To achieve the purpose, an experimental programme is developed and carried out. Two different nanomodified configurations are suggested and tested together with virgin specimens. All the specimens are subjected to static and dynamic tests to assess their stiffness, harmonic frequencies and damping. The experiments are repeated before and after low velocity impacts in order to investigate the effect of nano-fibres to static and dynamic properties when the laminates are impacted. SEM images of fractured surfaces and the mechanical results were used to attest the benefits brought by the presence of the nanointerleave. Results show that the interaction between the resin and the nano-fibre is the key feature of the reinforcement mechanism. When the resin is undamaged the friction with Nylon increases the damping ratio of nanomodified specimens with respect to that of virgin ones. When the matrix is damaged a fibre-bridging mechanism is revealed, and the nano-fibres increase the damage tolerance of laminates. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Carbon Fibre Reinforced Plastic materials are widely used in many modern applications due to their high Young’s modulus and strength combined with low specific weight: it makes them ideal materials for aerospace, automotive, shipbuilding and many more applications [1–4]. Due to the laminate nature, interlayers are weak areas: physical discontinuities and mismatch in mechanical properties between adjacent layers are often the starts of failures like delamination, buckling and so on. Delamination is one of the most common failure modes for composite laminates structures and it is a crack that forms between adjacent plies. During the years many techniques have been developed to mitigate the delamination and to arrest the crack propagation involving 3D reinforcements [5–8], optimized stacking sequence [9–13], braiding technique [14,8,15], and reinforcing interleaves [16,17]. The present work deals with the use of nano-fibres into epoxybased composite laminate material following a patent owned by Dzenis and Reneker [18]. They proposed to use a nano-interlayer in the form of polymeric nanofibrous non-woven mat fabricated
⇑ Corresponding author. Address: Via del Risorgimento 2, 40135 Bologna, Italy. Tel.: +39 051 2090474. E-mail address: [email protected] (R. Palazzetti). 0263-8223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compstruct.2013.07.021
through the electrospinning process into ply-to-ply interfaces of composite laminates. The first papers on this topic are dated 1999 [19] and 2001 [20]. In this work Nylon 6,6 was chosen because of its chemical compatibility with the epoxy matrix [21] as well as because of its physical and mechanical properties. One of these is its high melting temperature (262 °C), which allows the nano-fibres to maintain their morphology during the curing process of the laminate. Among other polymers that can be used to prepare solutions for electrospinning process, Nylon 6,6 is characterized by superior properties and it can be used to manufacture electrospun nano-fibres characterized by good mechanical properties [22]. During the last decade more than twenty papers have been devoted to the study of composite laminates modified by integrating electrospun nanofibrous [23] and among them few works are available concerning the use of Nylon 6,6 including one from the present authors [24]. The two most important ones are those by Shivakumar [25] and Akangah et al. [26]. In the first work researchers performed DMA, impact and Mode I fracture mechanics tests on virgin and nano-modified specimens. Results showed that by a negligible increase of the laminate thickness and weight, nanomodified specimens exhibit significant increases in damping, fracture toughness, delamination strength, and a reduced impact damage size. Akangah et al. [26] developed a deeper investigation
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on the impact behaviour. They manufactured and impacted sixteen-ply quasi-isotropic composite laminates to assess the improvement in impact resistance by interleaving all the interfaces with electrospun Nylon 6,6 nano-fabric. Results showed that polymer nano-fabric interleaving marginally increased the laminate thickness while it increased the threshold impact force by about 60%, reduced the rate of impact damage growth rate to one-half with impact height and reduced the impact damage growth rate from 0.115 mm2/N to 0.105 mm2/N with impact force. In the mentioned literature there are still some unsolved issues that this work aims to address. The authors’ attention is focused on the damage propagation and the damage resistance in composite laminates interleaved with nano-fibres, due to an impact and on the reinforcement mechanism played by the nano-reinforce. Vibration and static flexural tests are performed to assess the harmonic frequencies, the damping and the stiffness of the specimens before and after the LVI. This information, together with the SEM images of the fractured surface, provide understanding into the process of strengthening the laminate by using Nylon 6,6 nano-fibres interleaves. Force history during impacts is also recorded: the peak force and the absorbed energy are measured in order to assess the energy absorbed during the impact and the properties of the damaged laminates. This is considered an important contribution since it aids towards the understanding of the dynamic behaviour of such materials, while most structures made of composites are subjected to dynamic and vibration loadings. It is worth noting that in the above mentioned works [25,26] nano-fibres are laid in all the interlayers. Since their manufacturing process is still a time consuming process and it can affect the final cost, it is suggested here to interleave only some specific interfaces. In particular, in this work two different configurations of nanomodified specimens, with different nano-reinforced interfaces are tested in order to understand the best position for the nanointerlayers. The rest of the paper is organised as follows. The next paragraph (Section 2) provides an overview of the preparation of the virgin and the nano-modified specimens. The experiments and the testing procedures performed before and after the impacts are introduced in Section 3. Section 4 presents the results from all the tests: it starts with a micrograph examination and analysis of the nano-modified interfaces (Section 4.1) and is followed by the pre-impact results of the static and the dynamic tests (Section 4.2). Section 4.3 introduces the results from the LVI tests and (Section 4.4) presents the post-impact results from the static and the dynamic experiments. All the presented results are discussed and commented in detail, including the Scanning Electron Microscope (SEM) images. The last Section 5 offers the conclusions. 2. Materials and preparation Composite laminates were manufactured by overlapping ten layers of GG205PIMP503-42 woven prepreg carbon fibre and epoxy resin, which was kindly provided by Impregnatex Compositi S.r.l. (Milan, Italy). 2.1. Polymeric nanofibrous mats Nano-fibres were manufactured by means of the electrospinning process, which is one of the most versatile techniques for mass-production of nano-fibres. Non-woven mats are fabricated by using a SPINBOW S.r.l. electrospinning semi-automatic machine (see Fig. 1) composed of: a high voltage power supply which provide tensions from cable (1), a double syringe pump (2), two chambers (3) containing the polymeric solution (each one equipped with four stainless-steel blunt-ended needles and connected with
Fig. 1. SPINBOW S.r.l. electrospinning semi-automatic machine.
the power supply electrode) and a grounded plane collector (4) positioned 10 cm away from the tip of the needles. The polymer was Nylon 6,6 ZytelÒ E53 NC010 kindly provided by DuPont dissolved in a solution made of Formic Acid and Chloroform (50:50 v/v) purchased from Sigma Aldrich, used without further purification. The polymer was dissolved at a concentration of 14% w/v. The electrospinning process was carried out under the following conditions: applied voltage 22–26 kV, feed rate 0.3 mL/h per nozzle, at room temperature and relative humidity RH = 40–50%. Electrospun non-woven mats were 25 ± 8 lm thick and kept under vacuum at room temperature overnight to remove residual solvents before the lay-up inside laminates. Fibres diameter distribution (150 ± 20 nm) was determined by measuring 200 samples with image acquisition software (EDAX Genesis). Thermal properties of Nylon 6,6 electrospun mat were investigated by means of Differential Scanning Calorimetry (DSC) using a TA Instrument Q100 DSC equipped with the LNCS low-temperature accessory. Nano-fibres show excellent impregnation to epoxy matrix and they are characterized by a high-melting crystal phase (Tm = 262 °C, DHm 65 J/g). The prepreg curing treatment, at a temperature of 130 °C, is therefore below the Nylon 6,6 melting temperature: the autoclave process does not cause any modification to mat shape or fibre morphology. 2.2. Specimen fabrication Composite laminate panels were fabricated by hand lay-up of ten prepreg plies and nylon nanofibrous layers were placed in specific interfaces. Three panels configurations were considered: a virgin configuration and two nano-modified configurations, as summarized in Fig. 2. For each configuration nine panels were manufactured. Nano1 configuration (see Fig. 2(b)) is symmetric and the nanofibrous mats are placed into the two most external interfaces of the laminate. The Nano2 configuration is non-symmetric (see Fig. 2(c)) and the nanofibrous mats are placed only in the three lowest interfaces. This configuration became popular since Abrate [27] reported that for low velocity impact in thin laminates the bending
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Fig. 2. Tested configurations.
stress acting in layers, which are opposite to the impacted side, induces matrix cracks which initiate a pattern of cracks and delaminations leading to the so called ‘‘reversed pine tree pattern’’. For this reason nanofibrous layers were placed on the interfaces which are more prone to delamination. After lay-up, the panels were cured in an autoclave according to process specifications provided by the supplier. The final thickness was 2.27 ± 0.13 mm. All the specimens were manufactured in the same production batch. The presence of nano-fibres did not cause a significant increase in thickness with respect to the virgin ones.
rier Transform. The damping ratio n was calculated for the first mode by a logarithmic decrement method [29]. The measurement chain (see Fig. 4) consisted of a strain gauge powered by a P-3550 Strain Indicator (1); the signal was then digitalized by an analogue-to-digital acquisition device (2). The signal was acquired in a personal computer by a Labview executable program (3). A sampling frequency of 10 kHz and 6 s of acquisition time were chosen. The experiment was performed five times per specimen and the results are given in terms of the average of all the tests for each configuration.
3. Mechanical testing
3.3. Low velocity impact tests
Mechanical tests were performed to assess some mechanical properties of the nano-modified laminates. The tests were done before and after the impact in order to assess the effect of the nanointerleaves on the mechanical behaviour of laminates. First microscope investigation of the specimens is done in order to assess their structure. This is followed by flexural static tests performed on cantilevered panels to collect direct information about their stiffness. Following that, vibration experiments are performed on the same cantilevered panels to determine the first harmonic frequencies and to evaluate the damping ratio. Impact tests were then performed according to ASTM D7136 [28] on the same plates. In what follows mechanical tests and then the low velocity impact test procedures are introduced.
LVI tests were carried out in a drop-weight machine (see Fig. 5) equipped with a laser device to determine the position of the impactor and with a piezoelectric load cell inside the tip of the impactor which is able to measure the contact force history. The load cell was equipped with a hemispherical head with a diameter of 12.7 mm, the impactor mass is 1.22 ± 0.01 kg. Multiple collisions were avoided by means of an electromagnetic braking system. The load cell was switched on 30 min before the tests to ensure the full charge of the piezoelectric transducer. A detailed description of the machine can be found in [30]. Three different drop heights of 0.25, 0.5 and 1 m were chosen, corresponding to a nominal potential energy of 3 J, 6 J, and 12 J respectively. Three specimens were tested for each energy level. The laminates were placed in a clamping fixture equipped with four rubber pins. Data acquisition was made using the same NI9215 acquisition devices that were used for vibration tests. Load cell and laser signals were acquired at the sampling frequency of 100 kHz without any filtering except the intrinsic one due to the measurement chain. The maximum force and the absorbed energy during impact were than chosen to investigate the nano-layers effects. Peak forces were directly determined by the load cell, whereas the absorbed energy is calculated by the integration of the force–displacement values. The rebound velocities were calculated by the derivation of the displacement signals obtained by the laser.
3.1. Static flexural test Laminates were cantilevered on one edge and subjected to a quasi-static load (see Fig. 3(a)), while two strain gauges (Vishay Micro-Measurement N2A-13-T004R-350) measured the longitudinal deformation of the external skin (see Fig. 3(b)). The flexural test measures the force and the deflection to evaluate the bending stiffness. It was performed before and after the impact in order to evaluate the effect of the Nylon 6,6 interleaves. For each laminate type the experiments were repeated 5 times and the results were averaged for all repetitions.
4. Results and discussion 3.2. Dynamic/vibration tests 4.1. Micrographic analysis pre-impacted specimens Free decay vibration tests were carried out under one edge cantilever condition as presented in Fig. 4. External excitation was provided by a steel hammer, after which the free vibration response of the plates was measured by the two strain gauges. The natural frequencies of the plate were determined using the Discrete Fou-
The micrographic analysis is performed on the pre-impacted specimens and it aims to get more information about the ply-to-ply interfaces and in particular the influence of the nanofibrous mat on their formation. Fig. 6 shows some micrographic
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Fig. 3. Free decay test and equipment.
Fig. 4. Dynamic free decay experiment.
Fig. 5. Impact tests and equipment.
images taken from the through-thickness section of the virgin and the nano-modified laminates. From a global comparison of the transversal sections, it is possible to observe the matrix enrichment of the nano-modified interfaces.
Fig. 6. Through-thickness microscopy of the 3 laminate configurations. Nano-layers are clearly visible.
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The horizontal lines are fibres seen from aside, while the black and grey dots are the sections of perpendicular fibres. The dark areas represent the matrix. The upper image shows the section of a virgin panel, while the two lower ones depict the nano-modified specimens: the Nano1 configuration is on the left hand and the Nano2 on the right hand. In the nano-modified interfaces it is possible to observe the typical matrix enrichment due to the presence of the layer of nano-fibres and the reinforcement itself. The nanointer-layers are the slightly darker grey areas particularly visible into the dark areas of the matrix (see Fig. 6). They extend from one side of the images over to the other. Four such interlayers are detectable in the Nano1 configuration, and three in the Nano2. This shows that the nano-layers are not melted after the cycle cure into the autoclave and that the nano-fibres kept their morphology. Fig. 7 shows a detailed comparison of the various types of nanomodified interfaces. The two types of interfaces were classified according to the fibre orientations: 0° (the horizontal direction of fibres in the plane of the photos) and the 90° (the perpendicular direction with respect to the plane of the photos). The interface type 1 is where the 0° fibres couple with the 90° fibres of the adjacent ply; interface type 2 is the 0°/0° pair of fibres and plies. For both types of interfaces it can be noted that the presence of the nanofibrous mat induces the formation of interlayers characterized by a matrix enrichment that influence the mechanical behaviour of the laminates as it will be shown later. 4.2. Pre-impact test results Table 1 shows the elastic moduli, the harmonic frequencies and the damping ratios determined before the impact test. From the flexural test results (Table 1) it can be noted that the nano-modified panels have a lower stiffness value with respect to the virgin ones.
Table 1 Pre impact results. l: mean value and r: standard deviation. Flexural stiffness (GPa)
Free decay vibration test f1 (Hz)
Virgin Nano1 Nano2
Damping ratio
l
r/l%
l
r/l%
l
r/l%
59.3 52.5 51.5
1.0 1.5 1.2
127.1 108.2 108.4
0.5 1.5 3.9
4.0 E 3 9.9 E 3 6.5 E 3
6.3 7.4 14.9
This behaviour can be attributed to the fact that in the nanomodified laminates the nanofibrous mats keep part of the resin that would be squeezed out from the prepreg during the curing phase. This causes the formation of the enriched matrix interfaces (Section 4.1), which in turn reduce the global stiffness of the laminate. By performing a percentage comparison of the flexural moduli of the tested panels it can be observed that the flexural moduli of the Nano1 and the Nano2 modified specimens are 11% and 13% less than those of the virgin configuration, respectively. Since the Nano2 configuration is not symmetric, the flexural stiffness is determined by taking the average of the stiffnesses measured testing the specimen with nano-fibres on the top and on the bottom. Free decay test results on the pre-impacted specimens show that the virgin plates have a higher first natural frequency as compared to the nano-modified ones. These results agree with the flexural test results: the presence of matrix enriched interfaces reduces the global stiffness and consequently the first natural frequency of the plate. No significant differences can be found between the two nano-modified configurations. On the other hand, the analysis of vibration test results revealed that the damping of the nano-modified laminates is higher than the damping of the virgin specimens. In particular, the first frequency of both nano-modified laminates is about 15% less than
Fig. 7. Details of interfaces with (right) and without (left) nano-fibres (the bar scale is the same for all images).
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the one of the virgin specimens, whereas the damping of the Nano1 and the Nano2 plates is 149% and 62% higher than the damping of the virgin panels, respectively. Some previous studies performed mainly with carbon nanotubes found the same trend. Zhou et al. [31] explained the higher damping by the interfacial friction between the nano-reinforce and the polymer resin and introduced the concept of interfacial ‘‘stick–slip’’: a frictional motion between the nano-reinforce and the resin. Liang et al. in [32] also relates the increased energy dissipation to the fiction between the fibres and the resin. Gou et al. [33] and Koratkar [34] confirm that the energy dissipation is due to the cross-linkages within the nano-fibres. 4.3. LVI test results and analysis The tests were performed with three levels of impact energy: 3, 6 and 12 J. In these tests force versus the time and the force versus the displacement are measured for different impact energies. Fig. 8 presents examples of curves related to the impact process.
The maximum impact forces and the absorbed energies are summarized in Table 2. From the force–time diagrams shown in Fig. 8(a) left, it can be noted that the 3 J impacts in nano-modified specimens take longer time but they have lower peak forces, as compared to the virgin panels. In particular (see Table 2) the maximum forces of the nano-modified specimens are about 8% less than the forces measured for the virgin ones. Moreover, from Fig. 8(a) right it can be seen that the loop area under the force–displacement curve is approximately zero for all specimens. Hence, no significant damage is induced to panels by the lowest impact energy. Such a conclusion is supported by the absorbed energy data reported in Table 2(b). The absorbed energy is equal to the kinetic energy lost by the impactor after the impact (friction between the impactor and its guide is considered negligible [15]). Thus the energy lost by the impactor is totally transferred to the panel. The very low values of the absorbed energy (less than 0.4 J) determined for the 3 J impact, compared to those shown for bigger energies, support the assumption that they are only due to the elastic deformation of the plates during the impact.
Fig. 8. LVI curves: force vs. time (left) and force vs. displacement (right).
R. Palazzetti et al. / Composite Structures 106 (2013) 661–671 Table 2 Impact results. l: mean value and r: standard deviation. Energy of impact (J)
Virgin
Nano1
l
r/l%
l
(a) Max force (N) 3 6 12
1905 2338 2703
1.2 1.1 1.6
1757 2288 2820
(b) Absorbed energy (J) 3 6 12
0.34 2.09 5.82
8.8 4.3 1.4
0.39 2.01 6.26
Nano2
r/l%
l
r/l%
0.5 1.5 3.9
1762 2329 2730
6.3 7.4 14.9
51.3 2.4 2.4
0.2 1.79 6.57
25.0 3.9 0.8
The force–time curves corresponding to the 6 J impacts are presented in Fig. 8(b) left. The curves do not show any notable differences between virgin and nano-modified specimens. Nevertheless it can be noted that for the 6 J impact tests the absorbed energy by the nano-modified specimens is about 10% less as compared to the energy absorbed by the virgin ones. In particular the Nano1 and the Nano2 configurations absorbed 3.8% and 14.3% less energy than the virgin specimens, respectively.
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Such behaviour can be related to the fact that during the impact the nano-fibres play an important role in strengthening the interfaces. Since the delamination area starts narrow in the impact zone and grows wider down through the thickness, the nano-fibres placed in the bottom layers are responsible for the lower energy absorption in the specimens. The 3 J impacted specimens (see Table 2(b)) present a similar trend: the Nano2 energy absorption is lower than the energies absorbed for the other 2 configuration, although the specimens are not damaged at all. For the 6 J energy impact the only detectable damage is matrix breakage, as shown by Fig. 10 left. The surfaces of the virgin specimens appear clearly more damaged than the others, in which only few signs of the impact can be observed. Fig. 9 shows images, produced by a SEM, of the nano-modified surfaces after the 6 J and 12 J impacts. From Fig. 9(a) it can be observed that nano-fibres stitch the two layers they are inserted between and many of the fibres still bridge the two layers with a net. This aspect will be further discussed later. Fig. 10 also shows images of the laminate surface after the 6 J and the 12 J impacts. Form Fig. 10(a) left it is clear that the matrix of the bottom layer of the virgin laminates is damaged: fibres appear to be still unbroken, while the matrix exhibits some fails. After
Fig. 9. SEM investigation of nano-modified interfaces after impact.
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6J impact
12J impact
(a) Impact on virgin specimens
(b) Impact on Nano1 specimens
(c) Impact on Nano2 specimens Fig. 10. Post impact images.
the same impact the nano-modified specimen surfaces (Fig. 10(b) right and (c) right) present a smoother and more intact surface. And also the damaged area is much smaller in the nano-modified specimens. From the 12 J energy force–time diagrams (Fig. 8(c) left) no significant differences between the virgin and the nano-modified specimens can be observed. In fact, for the virgin and the nanomodified laminates the maximum force values as well as the impact durations are quite similar. Nevertheless, the analysis of the absorbed energy (see Fig. 8(c) right and Table 2) shows that nano-modified specimens have energy absorption of about 8% higher than that of the virgin panels. In particular the Nano1 and the Nano2 configurations absorbed 7.5% and 12.9% more energy, compared to the virgin specimens, respectively. For this energy level, the trend for nano-modified specimens is inverted-they appear to absorb more energy than virgin specimens, and Nano2 register the highest value. The authors explain this behaviour with the fact that at 12 J impact the failure mode changes significantly and both the matrix and the fibres are cracked (see Fig. 10, right pictures). The nano-fibres are no longer able to resist such a strong impact. But it can be
seen from Fig. 9(b) that a few nano-fibres still link the two adjacent layers. 4.4. Post-impact results Flexural tests on the impacted plates were performed to measure the variation of the laminate flexural stiffness modulus after the impacts. Table 3 summarizes the bending test results before and after the different impacts. Comparing the results of the flexural tests obtained before and after the impacts, it can be observed that the virgin and the nanomodified specimens exhibit two different types of behaviour. The virgin panels demonstrate, as expected, a stiffness reduction proportional to the impact energy, while the flexural modulus of the nano-modified panels increases. In particular, for the cases of 6 J and 12 J impacts the flexural stiffnesses of the impacted panels are 6% and 8% less than the stiffnesses of undamaged ones, respectively. On the contrary, the Nano1 panels impacted with 6 J and 12 J exhibit flexural stiffnesses that are 14% and 8% higher, respectively, as compared to the undamaged ones. In the case of the Nano2 panels impacted at 6 J and 12 J the flexural moduli are 9%
R. Palazzetti et al. / Composite Structures 106 (2013) 661–671 Table 3 Flexural stiffness moduli (GPa). l: mean value and r: standard deviation. Energy of impact (J)
Virgin
0 3 6 12
Nano1
Nano2
l
r/l%
l
r/l%
l
r/l%
59.3 57.9 56.0 54.4
1.0 0.4 0.3 0.9
52.5 54.2 60.1 56.8
1.5 3.7 5.9 5.4
51.5 52.8 56.4 53.5
1.2 5.5 2.2 2.8
Table 4 First harmonic frequencies after impacts (Hz). l: mean value and r: standard deviation. Impact energy (J)
0 3 6 12
Virgin
Nano1
Nano2
l
r/l%
l
r/l%
l
r/l%
127.1 127.3 126.0 120.5
0.5 2.28 2.48 0.91
108.2 110.1 121.4 121.2
1.5 0.82 0.48 1.91
108.4 110.0 122.5 120.7
3.9 3.00 3.33 1.99
and 3% higher than the undamaged ones, respectively. The modulus of the Nano1 configuration appears to be higher, because the modulus on the Nano2 is calculated by the average of two tests as was explained above. Nonetheless, the trend for both configurations is very similar. Similar results are observed from the analysis of the dynamic test results. In the virgin panels the first harmonic frequency decreases with the increase of the energy of impact. This is in agreement with the results from the flexural tests, which show a stiffness decrease. On the contrary for the nano-modified panels, the harmonic frequencies were found to increase with the increase of the energy of the impact. Table 4 presents the measured first harmonic frequencies for each configuration before and after the impacts. It can be observed that the results are in accordance with those presented in Table 3 for the stiffness. The first frequency of the virgin laminates decreases with up to 5% for highest energy impact as compared to the undamaged ones. In the case of the impacted nano-modified panels, the first harmonic increases by 10% for Nano1 and by 11% for Nano2 panels as compared to the undamaged ones. These results again confirm the stiffness increase observed for the nano-modified specimens after the impact with respect to the undamaged ones. In Fig. 11 both trends of the first harmonic frequencies with the impact energies (left) and the flexural stiffnesses (right) are plotted.
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From the plots in Fig. 11(a) and (b) one can visually observe the previously described trends for the stiffness and for the first harmonic frequencies. Moreover it can be seen that by increasing the impact energy the stiffnesses as well as the first frequencies of both types of panels, virgin and nano-modified, tend towards the same values. The limit value for natural frequencies being about 121 Hz and the one for the stiffness around 56 GPa. In the case of the virgin panels the reduction of both, the stiffness and the first frequency, observed by increasing the impact energy are due to the increase of the damaged area. In the case of the nano-modified panels the increase of the stiffness and the first natural frequency can be explained by the crack bridging and the matrix reinforcing effects due to the presence of nano-fibres between the adjacent plies. Nylon is more ductile than the resin, and when the resin brakes, nano-fibres can slip over the matrix and then they bridge the cracks and reinforce the interface, even if the matrix is damaged. Once the impactor falls on the panel, the epoxy macrostructure is damaged, the matrix breaks and delamination propagates. After the impacts, nano-fibres are free from the matrix and then they can play the role of a net-like reinforcing web, thus enabling a ply-to-ply bridging effect. But when the energy of the impact increases (to 12 J) most of the nano-fibres are broken and the plyto-ply bridging effect is reduced as it is shown in Fig. 9(b). The results for the damping ratios are summarized in Table 5 and in Fig. 12. The damping ratios are plotted with respect to the impact energy values. From the results in Table 5 it can be seen that by increasing the energy of impact the damping ratio goes down for both the virgin and the nano-modified laminates and tends toward the same limit value of 2.9 10 3 (see Fig. 12). It is interesting to compare the damping ratios of damaged and the undamaged panels from a physical point of view. As shown in Fig. 12, when the panels are undamaged the damping ratio of the nano-modified laminates is higher than the damping ratio of the virgin ones. This can be explained by the fact that when nano-fibres are embedded into the matrix, they are able to dissipate energy, thus giving the nano-modified laminate a higher damping ratio with respect to the virgin ones. So, in the case of undamaged laminates, nano-fibres play the role of interlayer dampers. When the panels are damaged some of the nano-fibres are free from the matrix (Fig. 9), and then they can contribute to the increase of the flexural stiffness and the first harmonic frequency by working as ‘‘ropes in a Tibetan bridge’’, but in the same time the nanofibres are not able to dissipate energy like they did when embedded into the matrix. When the nano-modified panels are damaged some of the nano-fibres are free from the matrix and then they work as internal
Fig. 11. Post-impact harmonic frequencies and flexural stiffnesses.
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Table 5 Post-impact damping ratios. l: mean value and r: standard deviation. Energy of impact (J)
Virgin
Nano1
l 3 6 12
3.24 10 3.03 10 2.92 10
3 3 3
r/l%
l
4.63 7.92 12.33
4.27 10 2.94 10 2.64 10
Fig. 12. Diagrams of damping ratio; experimental points refers to the damping ratio of undamaged (0 J) and impact-damaged panels (3 J, 6 J, 12 J).
links that contribute to the increase of the flexural stiffness and the first harmonic frequency, whereas their contribution as internal dampers is reduced with respect to the condition when they were embedded into the matrix.
Nano2
3 3 3
r/l%
l
3.98 8.16 4.17
3.50 10 3.22 10 3.01 10
r/l% 3 3 3
3.14 4.35 6.31
3. impact tests on nano-modified specimens reveal an 8% lower peak force (for the lowest energy impact) and 14.3% less energy absorbed compared to the virgin ones. These results are enhanced for the case of non-symmetric specimens, which have nanofibres where larger delamination is expected; 4. post-impact analysis of virgin panels shows the expected decrease in stiffness and in the harmonic frequencies, proportionally to the impact energy. Nano-modified panels present unexpected reinforcement effect: both the stiffness and the first harmonic frequency increase with up to 14% and 12%, respectively after the 6 J impact; 5. SEM pictures of the damaged condition show that when the matrix is broken the nano-fibres still link the layers they are inserted in between. The images (see Fig. 9) show that the nano-fibres continue to keep the layers together, even after that the macrostructure is irreversibly damaged. This mechanism is a self-stiffening and a self-repair effect that the nanofibrous interlayers are capable to provide to the whole panel once delaminated.
References 5. Conclusions and discussion This is an experimental study which aims to investigate the effect of Nylon 6,6 electrospun nanofibrous interlayers on the behaviour of carbon fibre reinforced flat laminate panels. By means of electrospinning technology 25 lm thick Nylon 6,6 nanofibrous mats were manufactured. Two nano-modified laminates configurations were manufactured, one symmetric (nanofibres in the two uppermost and in the two lowest interfaces) and the other one non-symmetric (nanofibres in the 3 lowest interfaces). The focus of the research is on global/macro mechanical properties and structural behaviour of nano-modified panels compared to the virgin ones. In particular, the aim is to investigate the influence of nanofibrous interleave into composite laminate and the interaction mechanism between the two. In particular the experimental program is based on static (flexural) and dynamic (free vibration) tests which are performed on undamaged laminates and on laminates that have been damaged by low velocity impact performed at three energy levels (3 J, 6 J and 12 J). Based on this experimental activities the following conclusions are made: 1. the presence of the nanofibrous mats induce a matrix enriched volume at the nanomodified interfaces; 2. the static flexural tests on pre-damaged laminates showed that the stiffness of nanomodified laminates is about 10% less than virgin ones; the free decay vibration tests showed that the nanomodified laminates have a first harmonic frequency 10% less and a damping ratio 160% higher than the virgin ones; so it was possible to conclude that the undamaged nanomodified laminates are less stiff but significantly more damped respect virgin ones;
[1] Mouritz AP, Gellert E, Burchill P, Challis K. Review of advanced composite structures for naval ships and submarines. Compos Struct 2001;53(1):21–42. [2] Hollaway LC. A review of the present and future utilisation of FRP composites in the civil infrastructure with reference to their important in-service properties. Constr Build Mater 2010;24(12):2419–45. [3] Ramakrishna S, Mayer J, Wintermantel E, Leong KW. Biomedical applications of polymer-composite materials: a review. Compos Sci Technol 2001;61(9):1189–224. [4] Soutis C. Fibre reinforced composites in aircraft construction. Prog Aerosp Sci 2005;41(2):143–51. [5] Howard WE, Gossard T, Jones RM, Robert MJ. Composite laminate free-edge reinforcement with U-Shapes caps. Part I: stress analysis. AIAA Pape 1986;27(5):610–6. [6] Partridge IK, Cartie DDR. Delamination resistant laminates by Z- Fiber pinning: Part I manufacture and fracture performance. Compos A Appl Sci Manuf 2005;36(1):55–64. [7] Mattheij P, Gliesche K, Feltin D. 3D reinforced stitched carbon/epoxy laminatesmade by tailored fibre placement. Compos A Appl Sci Manuf 2000;31(6):571–81. [8] Chen L. Analysis of mode I and mode II tests for composites with translaminar reinforcements. J Compos Mater 2005;39(15):1311–33. [9] Fuoss E, Straznicky PV, Poon C. Effects of stacking sequence on the impact resistance in composite laminates. Part 1: parametric study. Compos Struct 1998;41:67–77. [10] Fuoss E, Straznicky PV, Poon C. Effect of stacking sequence on the impact resistance in composite laminates. Part 2: prediction method. Compos Struct 1998;41:177–86. [11] Kere P, Koski J. Multicriterion stacking sequence optimization scheme for composite laminates subjected to multiple loading conditions. Compos Struct 2001;54(2–3):225–9. [12] Ghiasi H, Pasini D, Lessard L. Optimum stacking sequence design of composite materials Part I: constant stiffness design. Compos Struct 2009;90(1):1–11. [13] Ghiasi H, Fayazbakhsh K, Pasini D, Lessard L. Optimum stacking sequence design of composite materials Part II: variable stiffness design. Compos Struct 2010;93(1):1–13. [14] Tang G, Yan Y, Chen X, Zhang J, Xu B, Feng Z. Dynamic damage and fracture mechanism of three-dimensional braided carbon fibre epoxy resin composites. Mater Des 2001;22(1):21–5. [15] Mouritz AP, Baini C, Herszberg I. Mode I interlaminar fracture toughness properties of advanced textile fibreglass composites. Compos A Appl Sci Manuf 1999;30(7):859–70.
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
The self-reinforcing effect of Nylon 6,6 nano-fibres on CFRP laminates subjected to low velocity impact R. Palazzetti a,⇑, A. Zucchelli a, I. Trendafilova b a b
DIN, Department of Industrial Engineering, University of Bologna, Italy MAE, Department of Mechanical and Aeronautic Engineering, Strathclyde University, Glasgow, UK
a r t i c l e
i n f o
Article history: Available online 16 July 2013 Keywords: Electrospinning Nano composite Impact behaviour Vibration Fibre-bridging
a b s t r a c t This work investigates the mechanical properties of CFR-epoxy laminates interleaved with electrospun Nylon 6,6 nano-fibres. The main goal is to investigate the interaction between the nanofibrous mats interleaved into a laminate and their influence on the property of the whole body and in particular their reinforcing effect. To achieve the purpose, an experimental programme is developed and carried out. Two different nanomodified configurations are suggested and tested together with virgin specimens. All the specimens are subjected to static and dynamic tests to assess their stiffness, harmonic frequencies and damping. The experiments are repeated before and after low velocity impacts in order to investigate the effect of nano-fibres to static and dynamic properties when the laminates are impacted. SEM images of fractured surfaces and the mechanical results were used to attest the benefits brought by the presence of the nanointerleave. Results show that the interaction between the resin and the nano-fibre is the key feature of the reinforcement mechanism. When the resin is undamaged the friction with Nylon increases the damping ratio of nanomodified specimens with respect to that of virgin ones. When the matrix is damaged a fibre-bridging mechanism is revealed, and the nano-fibres increase the damage tolerance of laminates. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Carbon Fibre Reinforced Plastic materials are widely used in many modern applications due to their high Young’s modulus and strength combined with low specific weight: it makes them ideal materials for aerospace, automotive, shipbuilding and many more applications [1–4]. Due to the laminate nature, interlayers are weak areas: physical discontinuities and mismatch in mechanical properties between adjacent layers are often the starts of failures like delamination, buckling and so on. Delamination is one of the most common failure modes for composite laminates structures and it is a crack that forms between adjacent plies. During the years many techniques have been developed to mitigate the delamination and to arrest the crack propagation involving 3D reinforcements [5–8], optimized stacking sequence [9–13], braiding technique [14,8,15], and reinforcing interleaves [16,17]. The present work deals with the use of nano-fibres into epoxybased composite laminate material following a patent owned by Dzenis and Reneker [18]. They proposed to use a nano-interlayer in the form of polymeric nanofibrous non-woven mat fabricated
⇑ Corresponding author. Address: Via del Risorgimento 2, 40135 Bologna, Italy. Tel.: +39 051 2090474. E-mail address: [email protected] (R. Palazzetti). 0263-8223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compstruct.2013.07.021
through the electrospinning process into ply-to-ply interfaces of composite laminates. The first papers on this topic are dated 1999 [19] and 2001 [20]. In this work Nylon 6,6 was chosen because of its chemical compatibility with the epoxy matrix [21] as well as because of its physical and mechanical properties. One of these is its high melting temperature (262 °C), which allows the nano-fibres to maintain their morphology during the curing process of the laminate. Among other polymers that can be used to prepare solutions for electrospinning process, Nylon 6,6 is characterized by superior properties and it can be used to manufacture electrospun nano-fibres characterized by good mechanical properties [22]. During the last decade more than twenty papers have been devoted to the study of composite laminates modified by integrating electrospun nanofibrous [23] and among them few works are available concerning the use of Nylon 6,6 including one from the present authors [24]. The two most important ones are those by Shivakumar [25] and Akangah et al. [26]. In the first work researchers performed DMA, impact and Mode I fracture mechanics tests on virgin and nano-modified specimens. Results showed that by a negligible increase of the laminate thickness and weight, nanomodified specimens exhibit significant increases in damping, fracture toughness, delamination strength, and a reduced impact damage size. Akangah et al. [26] developed a deeper investigation
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on the impact behaviour. They manufactured and impacted sixteen-ply quasi-isotropic composite laminates to assess the improvement in impact resistance by interleaving all the interfaces with electrospun Nylon 6,6 nano-fabric. Results showed that polymer nano-fabric interleaving marginally increased the laminate thickness while it increased the threshold impact force by about 60%, reduced the rate of impact damage growth rate to one-half with impact height and reduced the impact damage growth rate from 0.115 mm2/N to 0.105 mm2/N with impact force. In the mentioned literature there are still some unsolved issues that this work aims to address. The authors’ attention is focused on the damage propagation and the damage resistance in composite laminates interleaved with nano-fibres, due to an impact and on the reinforcement mechanism played by the nano-reinforce. Vibration and static flexural tests are performed to assess the harmonic frequencies, the damping and the stiffness of the specimens before and after the LVI. This information, together with the SEM images of the fractured surface, provide understanding into the process of strengthening the laminate by using Nylon 6,6 nano-fibres interleaves. Force history during impacts is also recorded: the peak force and the absorbed energy are measured in order to assess the energy absorbed during the impact and the properties of the damaged laminates. This is considered an important contribution since it aids towards the understanding of the dynamic behaviour of such materials, while most structures made of composites are subjected to dynamic and vibration loadings. It is worth noting that in the above mentioned works [25,26] nano-fibres are laid in all the interlayers. Since their manufacturing process is still a time consuming process and it can affect the final cost, it is suggested here to interleave only some specific interfaces. In particular, in this work two different configurations of nanomodified specimens, with different nano-reinforced interfaces are tested in order to understand the best position for the nanointerlayers. The rest of the paper is organised as follows. The next paragraph (Section 2) provides an overview of the preparation of the virgin and the nano-modified specimens. The experiments and the testing procedures performed before and after the impacts are introduced in Section 3. Section 4 presents the results from all the tests: it starts with a micrograph examination and analysis of the nano-modified interfaces (Section 4.1) and is followed by the pre-impact results of the static and the dynamic tests (Section 4.2). Section 4.3 introduces the results from the LVI tests and (Section 4.4) presents the post-impact results from the static and the dynamic experiments. All the presented results are discussed and commented in detail, including the Scanning Electron Microscope (SEM) images. The last Section 5 offers the conclusions. 2. Materials and preparation Composite laminates were manufactured by overlapping ten layers of GG205PIMP503-42 woven prepreg carbon fibre and epoxy resin, which was kindly provided by Impregnatex Compositi S.r.l. (Milan, Italy). 2.1. Polymeric nanofibrous mats Nano-fibres were manufactured by means of the electrospinning process, which is one of the most versatile techniques for mass-production of nano-fibres. Non-woven mats are fabricated by using a SPINBOW S.r.l. electrospinning semi-automatic machine (see Fig. 1) composed of: a high voltage power supply which provide tensions from cable (1), a double syringe pump (2), two chambers (3) containing the polymeric solution (each one equipped with four stainless-steel blunt-ended needles and connected with
Fig. 1. SPINBOW S.r.l. electrospinning semi-automatic machine.
the power supply electrode) and a grounded plane collector (4) positioned 10 cm away from the tip of the needles. The polymer was Nylon 6,6 ZytelÒ E53 NC010 kindly provided by DuPont dissolved in a solution made of Formic Acid and Chloroform (50:50 v/v) purchased from Sigma Aldrich, used without further purification. The polymer was dissolved at a concentration of 14% w/v. The electrospinning process was carried out under the following conditions: applied voltage 22–26 kV, feed rate 0.3 mL/h per nozzle, at room temperature and relative humidity RH = 40–50%. Electrospun non-woven mats were 25 ± 8 lm thick and kept under vacuum at room temperature overnight to remove residual solvents before the lay-up inside laminates. Fibres diameter distribution (150 ± 20 nm) was determined by measuring 200 samples with image acquisition software (EDAX Genesis). Thermal properties of Nylon 6,6 electrospun mat were investigated by means of Differential Scanning Calorimetry (DSC) using a TA Instrument Q100 DSC equipped with the LNCS low-temperature accessory. Nano-fibres show excellent impregnation to epoxy matrix and they are characterized by a high-melting crystal phase (Tm = 262 °C, DHm 65 J/g). The prepreg curing treatment, at a temperature of 130 °C, is therefore below the Nylon 6,6 melting temperature: the autoclave process does not cause any modification to mat shape or fibre morphology. 2.2. Specimen fabrication Composite laminate panels were fabricated by hand lay-up of ten prepreg plies and nylon nanofibrous layers were placed in specific interfaces. Three panels configurations were considered: a virgin configuration and two nano-modified configurations, as summarized in Fig. 2. For each configuration nine panels were manufactured. Nano1 configuration (see Fig. 2(b)) is symmetric and the nanofibrous mats are placed into the two most external interfaces of the laminate. The Nano2 configuration is non-symmetric (see Fig. 2(c)) and the nanofibrous mats are placed only in the three lowest interfaces. This configuration became popular since Abrate [27] reported that for low velocity impact in thin laminates the bending
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Fig. 2. Tested configurations.
stress acting in layers, which are opposite to the impacted side, induces matrix cracks which initiate a pattern of cracks and delaminations leading to the so called ‘‘reversed pine tree pattern’’. For this reason nanofibrous layers were placed on the interfaces which are more prone to delamination. After lay-up, the panels were cured in an autoclave according to process specifications provided by the supplier. The final thickness was 2.27 ± 0.13 mm. All the specimens were manufactured in the same production batch. The presence of nano-fibres did not cause a significant increase in thickness with respect to the virgin ones.
rier Transform. The damping ratio n was calculated for the first mode by a logarithmic decrement method [29]. The measurement chain (see Fig. 4) consisted of a strain gauge powered by a P-3550 Strain Indicator (1); the signal was then digitalized by an analogue-to-digital acquisition device (2). The signal was acquired in a personal computer by a Labview executable program (3). A sampling frequency of 10 kHz and 6 s of acquisition time were chosen. The experiment was performed five times per specimen and the results are given in terms of the average of all the tests for each configuration.
3. Mechanical testing
3.3. Low velocity impact tests
Mechanical tests were performed to assess some mechanical properties of the nano-modified laminates. The tests were done before and after the impact in order to assess the effect of the nanointerleaves on the mechanical behaviour of laminates. First microscope investigation of the specimens is done in order to assess their structure. This is followed by flexural static tests performed on cantilevered panels to collect direct information about their stiffness. Following that, vibration experiments are performed on the same cantilevered panels to determine the first harmonic frequencies and to evaluate the damping ratio. Impact tests were then performed according to ASTM D7136 [28] on the same plates. In what follows mechanical tests and then the low velocity impact test procedures are introduced.
LVI tests were carried out in a drop-weight machine (see Fig. 5) equipped with a laser device to determine the position of the impactor and with a piezoelectric load cell inside the tip of the impactor which is able to measure the contact force history. The load cell was equipped with a hemispherical head with a diameter of 12.7 mm, the impactor mass is 1.22 ± 0.01 kg. Multiple collisions were avoided by means of an electromagnetic braking system. The load cell was switched on 30 min before the tests to ensure the full charge of the piezoelectric transducer. A detailed description of the machine can be found in [30]. Three different drop heights of 0.25, 0.5 and 1 m were chosen, corresponding to a nominal potential energy of 3 J, 6 J, and 12 J respectively. Three specimens were tested for each energy level. The laminates were placed in a clamping fixture equipped with four rubber pins. Data acquisition was made using the same NI9215 acquisition devices that were used for vibration tests. Load cell and laser signals were acquired at the sampling frequency of 100 kHz without any filtering except the intrinsic one due to the measurement chain. The maximum force and the absorbed energy during impact were than chosen to investigate the nano-layers effects. Peak forces were directly determined by the load cell, whereas the absorbed energy is calculated by the integration of the force–displacement values. The rebound velocities were calculated by the derivation of the displacement signals obtained by the laser.
3.1. Static flexural test Laminates were cantilevered on one edge and subjected to a quasi-static load (see Fig. 3(a)), while two strain gauges (Vishay Micro-Measurement N2A-13-T004R-350) measured the longitudinal deformation of the external skin (see Fig. 3(b)). The flexural test measures the force and the deflection to evaluate the bending stiffness. It was performed before and after the impact in order to evaluate the effect of the Nylon 6,6 interleaves. For each laminate type the experiments were repeated 5 times and the results were averaged for all repetitions.
4. Results and discussion 3.2. Dynamic/vibration tests 4.1. Micrographic analysis pre-impacted specimens Free decay vibration tests were carried out under one edge cantilever condition as presented in Fig. 4. External excitation was provided by a steel hammer, after which the free vibration response of the plates was measured by the two strain gauges. The natural frequencies of the plate were determined using the Discrete Fou-
The micrographic analysis is performed on the pre-impacted specimens and it aims to get more information about the ply-to-ply interfaces and in particular the influence of the nanofibrous mat on their formation. Fig. 6 shows some micrographic
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Fig. 3. Free decay test and equipment.
Fig. 4. Dynamic free decay experiment.
Fig. 5. Impact tests and equipment.
images taken from the through-thickness section of the virgin and the nano-modified laminates. From a global comparison of the transversal sections, it is possible to observe the matrix enrichment of the nano-modified interfaces.
Fig. 6. Through-thickness microscopy of the 3 laminate configurations. Nano-layers are clearly visible.
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The horizontal lines are fibres seen from aside, while the black and grey dots are the sections of perpendicular fibres. The dark areas represent the matrix. The upper image shows the section of a virgin panel, while the two lower ones depict the nano-modified specimens: the Nano1 configuration is on the left hand and the Nano2 on the right hand. In the nano-modified interfaces it is possible to observe the typical matrix enrichment due to the presence of the layer of nano-fibres and the reinforcement itself. The nanointer-layers are the slightly darker grey areas particularly visible into the dark areas of the matrix (see Fig. 6). They extend from one side of the images over to the other. Four such interlayers are detectable in the Nano1 configuration, and three in the Nano2. This shows that the nano-layers are not melted after the cycle cure into the autoclave and that the nano-fibres kept their morphology. Fig. 7 shows a detailed comparison of the various types of nanomodified interfaces. The two types of interfaces were classified according to the fibre orientations: 0° (the horizontal direction of fibres in the plane of the photos) and the 90° (the perpendicular direction with respect to the plane of the photos). The interface type 1 is where the 0° fibres couple with the 90° fibres of the adjacent ply; interface type 2 is the 0°/0° pair of fibres and plies. For both types of interfaces it can be noted that the presence of the nanofibrous mat induces the formation of interlayers characterized by a matrix enrichment that influence the mechanical behaviour of the laminates as it will be shown later. 4.2. Pre-impact test results Table 1 shows the elastic moduli, the harmonic frequencies and the damping ratios determined before the impact test. From the flexural test results (Table 1) it can be noted that the nano-modified panels have a lower stiffness value with respect to the virgin ones.
Table 1 Pre impact results. l: mean value and r: standard deviation. Flexural stiffness (GPa)
Free decay vibration test f1 (Hz)
Virgin Nano1 Nano2
Damping ratio
l
r/l%
l
r/l%
l
r/l%
59.3 52.5 51.5
1.0 1.5 1.2
127.1 108.2 108.4
0.5 1.5 3.9
4.0 E 3 9.9 E 3 6.5 E 3
6.3 7.4 14.9
This behaviour can be attributed to the fact that in the nanomodified laminates the nanofibrous mats keep part of the resin that would be squeezed out from the prepreg during the curing phase. This causes the formation of the enriched matrix interfaces (Section 4.1), which in turn reduce the global stiffness of the laminate. By performing a percentage comparison of the flexural moduli of the tested panels it can be observed that the flexural moduli of the Nano1 and the Nano2 modified specimens are 11% and 13% less than those of the virgin configuration, respectively. Since the Nano2 configuration is not symmetric, the flexural stiffness is determined by taking the average of the stiffnesses measured testing the specimen with nano-fibres on the top and on the bottom. Free decay test results on the pre-impacted specimens show that the virgin plates have a higher first natural frequency as compared to the nano-modified ones. These results agree with the flexural test results: the presence of matrix enriched interfaces reduces the global stiffness and consequently the first natural frequency of the plate. No significant differences can be found between the two nano-modified configurations. On the other hand, the analysis of vibration test results revealed that the damping of the nano-modified laminates is higher than the damping of the virgin specimens. In particular, the first frequency of both nano-modified laminates is about 15% less than
Fig. 7. Details of interfaces with (right) and without (left) nano-fibres (the bar scale is the same for all images).
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the one of the virgin specimens, whereas the damping of the Nano1 and the Nano2 plates is 149% and 62% higher than the damping of the virgin panels, respectively. Some previous studies performed mainly with carbon nanotubes found the same trend. Zhou et al. [31] explained the higher damping by the interfacial friction between the nano-reinforce and the polymer resin and introduced the concept of interfacial ‘‘stick–slip’’: a frictional motion between the nano-reinforce and the resin. Liang et al. in [32] also relates the increased energy dissipation to the fiction between the fibres and the resin. Gou et al. [33] and Koratkar [34] confirm that the energy dissipation is due to the cross-linkages within the nano-fibres. 4.3. LVI test results and analysis The tests were performed with three levels of impact energy: 3, 6 and 12 J. In these tests force versus the time and the force versus the displacement are measured for different impact energies. Fig. 8 presents examples of curves related to the impact process.
The maximum impact forces and the absorbed energies are summarized in Table 2. From the force–time diagrams shown in Fig. 8(a) left, it can be noted that the 3 J impacts in nano-modified specimens take longer time but they have lower peak forces, as compared to the virgin panels. In particular (see Table 2) the maximum forces of the nano-modified specimens are about 8% less than the forces measured for the virgin ones. Moreover, from Fig. 8(a) right it can be seen that the loop area under the force–displacement curve is approximately zero for all specimens. Hence, no significant damage is induced to panels by the lowest impact energy. Such a conclusion is supported by the absorbed energy data reported in Table 2(b). The absorbed energy is equal to the kinetic energy lost by the impactor after the impact (friction between the impactor and its guide is considered negligible [15]). Thus the energy lost by the impactor is totally transferred to the panel. The very low values of the absorbed energy (less than 0.4 J) determined for the 3 J impact, compared to those shown for bigger energies, support the assumption that they are only due to the elastic deformation of the plates during the impact.
Fig. 8. LVI curves: force vs. time (left) and force vs. displacement (right).
R. Palazzetti et al. / Composite Structures 106 (2013) 661–671 Table 2 Impact results. l: mean value and r: standard deviation. Energy of impact (J)
Virgin
Nano1
l
r/l%
l
(a) Max force (N) 3 6 12
1905 2338 2703
1.2 1.1 1.6
1757 2288 2820
(b) Absorbed energy (J) 3 6 12
0.34 2.09 5.82
8.8 4.3 1.4
0.39 2.01 6.26
Nano2
r/l%
l
r/l%
0.5 1.5 3.9
1762 2329 2730
6.3 7.4 14.9
51.3 2.4 2.4
0.2 1.79 6.57
25.0 3.9 0.8
The force–time curves corresponding to the 6 J impacts are presented in Fig. 8(b) left. The curves do not show any notable differences between virgin and nano-modified specimens. Nevertheless it can be noted that for the 6 J impact tests the absorbed energy by the nano-modified specimens is about 10% less as compared to the energy absorbed by the virgin ones. In particular the Nano1 and the Nano2 configurations absorbed 3.8% and 14.3% less energy than the virgin specimens, respectively.
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Such behaviour can be related to the fact that during the impact the nano-fibres play an important role in strengthening the interfaces. Since the delamination area starts narrow in the impact zone and grows wider down through the thickness, the nano-fibres placed in the bottom layers are responsible for the lower energy absorption in the specimens. The 3 J impacted specimens (see Table 2(b)) present a similar trend: the Nano2 energy absorption is lower than the energies absorbed for the other 2 configuration, although the specimens are not damaged at all. For the 6 J energy impact the only detectable damage is matrix breakage, as shown by Fig. 10 left. The surfaces of the virgin specimens appear clearly more damaged than the others, in which only few signs of the impact can be observed. Fig. 9 shows images, produced by a SEM, of the nano-modified surfaces after the 6 J and 12 J impacts. From Fig. 9(a) it can be observed that nano-fibres stitch the two layers they are inserted between and many of the fibres still bridge the two layers with a net. This aspect will be further discussed later. Fig. 10 also shows images of the laminate surface after the 6 J and the 12 J impacts. Form Fig. 10(a) left it is clear that the matrix of the bottom layer of the virgin laminates is damaged: fibres appear to be still unbroken, while the matrix exhibits some fails. After
Fig. 9. SEM investigation of nano-modified interfaces after impact.
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6J impact
12J impact
(a) Impact on virgin specimens
(b) Impact on Nano1 specimens
(c) Impact on Nano2 specimens Fig. 10. Post impact images.
the same impact the nano-modified specimen surfaces (Fig. 10(b) right and (c) right) present a smoother and more intact surface. And also the damaged area is much smaller in the nano-modified specimens. From the 12 J energy force–time diagrams (Fig. 8(c) left) no significant differences between the virgin and the nano-modified specimens can be observed. In fact, for the virgin and the nanomodified laminates the maximum force values as well as the impact durations are quite similar. Nevertheless, the analysis of the absorbed energy (see Fig. 8(c) right and Table 2) shows that nano-modified specimens have energy absorption of about 8% higher than that of the virgin panels. In particular the Nano1 and the Nano2 configurations absorbed 7.5% and 12.9% more energy, compared to the virgin specimens, respectively. For this energy level, the trend for nano-modified specimens is inverted-they appear to absorb more energy than virgin specimens, and Nano2 register the highest value. The authors explain this behaviour with the fact that at 12 J impact the failure mode changes significantly and both the matrix and the fibres are cracked (see Fig. 10, right pictures). The nano-fibres are no longer able to resist such a strong impact. But it can be
seen from Fig. 9(b) that a few nano-fibres still link the two adjacent layers. 4.4. Post-impact results Flexural tests on the impacted plates were performed to measure the variation of the laminate flexural stiffness modulus after the impacts. Table 3 summarizes the bending test results before and after the different impacts. Comparing the results of the flexural tests obtained before and after the impacts, it can be observed that the virgin and the nanomodified specimens exhibit two different types of behaviour. The virgin panels demonstrate, as expected, a stiffness reduction proportional to the impact energy, while the flexural modulus of the nano-modified panels increases. In particular, for the cases of 6 J and 12 J impacts the flexural stiffnesses of the impacted panels are 6% and 8% less than the stiffnesses of undamaged ones, respectively. On the contrary, the Nano1 panels impacted with 6 J and 12 J exhibit flexural stiffnesses that are 14% and 8% higher, respectively, as compared to the undamaged ones. In the case of the Nano2 panels impacted at 6 J and 12 J the flexural moduli are 9%
R. Palazzetti et al. / Composite Structures 106 (2013) 661–671 Table 3 Flexural stiffness moduli (GPa). l: mean value and r: standard deviation. Energy of impact (J)
Virgin
0 3 6 12
Nano1
Nano2
l
r/l%
l
r/l%
l
r/l%
59.3 57.9 56.0 54.4
1.0 0.4 0.3 0.9
52.5 54.2 60.1 56.8
1.5 3.7 5.9 5.4
51.5 52.8 56.4 53.5
1.2 5.5 2.2 2.8
Table 4 First harmonic frequencies after impacts (Hz). l: mean value and r: standard deviation. Impact energy (J)
0 3 6 12
Virgin
Nano1
Nano2
l
r/l%
l
r/l%
l
r/l%
127.1 127.3 126.0 120.5
0.5 2.28 2.48 0.91
108.2 110.1 121.4 121.2
1.5 0.82 0.48 1.91
108.4 110.0 122.5 120.7
3.9 3.00 3.33 1.99
and 3% higher than the undamaged ones, respectively. The modulus of the Nano1 configuration appears to be higher, because the modulus on the Nano2 is calculated by the average of two tests as was explained above. Nonetheless, the trend for both configurations is very similar. Similar results are observed from the analysis of the dynamic test results. In the virgin panels the first harmonic frequency decreases with the increase of the energy of impact. This is in agreement with the results from the flexural tests, which show a stiffness decrease. On the contrary for the nano-modified panels, the harmonic frequencies were found to increase with the increase of the energy of the impact. Table 4 presents the measured first harmonic frequencies for each configuration before and after the impacts. It can be observed that the results are in accordance with those presented in Table 3 for the stiffness. The first frequency of the virgin laminates decreases with up to 5% for highest energy impact as compared to the undamaged ones. In the case of the impacted nano-modified panels, the first harmonic increases by 10% for Nano1 and by 11% for Nano2 panels as compared to the undamaged ones. These results again confirm the stiffness increase observed for the nano-modified specimens after the impact with respect to the undamaged ones. In Fig. 11 both trends of the first harmonic frequencies with the impact energies (left) and the flexural stiffnesses (right) are plotted.
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From the plots in Fig. 11(a) and (b) one can visually observe the previously described trends for the stiffness and for the first harmonic frequencies. Moreover it can be seen that by increasing the impact energy the stiffnesses as well as the first frequencies of both types of panels, virgin and nano-modified, tend towards the same values. The limit value for natural frequencies being about 121 Hz and the one for the stiffness around 56 GPa. In the case of the virgin panels the reduction of both, the stiffness and the first frequency, observed by increasing the impact energy are due to the increase of the damaged area. In the case of the nano-modified panels the increase of the stiffness and the first natural frequency can be explained by the crack bridging and the matrix reinforcing effects due to the presence of nano-fibres between the adjacent plies. Nylon is more ductile than the resin, and when the resin brakes, nano-fibres can slip over the matrix and then they bridge the cracks and reinforce the interface, even if the matrix is damaged. Once the impactor falls on the panel, the epoxy macrostructure is damaged, the matrix breaks and delamination propagates. After the impacts, nano-fibres are free from the matrix and then they can play the role of a net-like reinforcing web, thus enabling a ply-to-ply bridging effect. But when the energy of the impact increases (to 12 J) most of the nano-fibres are broken and the plyto-ply bridging effect is reduced as it is shown in Fig. 9(b). The results for the damping ratios are summarized in Table 5 and in Fig. 12. The damping ratios are plotted with respect to the impact energy values. From the results in Table 5 it can be seen that by increasing the energy of impact the damping ratio goes down for both the virgin and the nano-modified laminates and tends toward the same limit value of 2.9 10 3 (see Fig. 12). It is interesting to compare the damping ratios of damaged and the undamaged panels from a physical point of view. As shown in Fig. 12, when the panels are undamaged the damping ratio of the nano-modified laminates is higher than the damping ratio of the virgin ones. This can be explained by the fact that when nano-fibres are embedded into the matrix, they are able to dissipate energy, thus giving the nano-modified laminate a higher damping ratio with respect to the virgin ones. So, in the case of undamaged laminates, nano-fibres play the role of interlayer dampers. When the panels are damaged some of the nano-fibres are free from the matrix (Fig. 9), and then they can contribute to the increase of the flexural stiffness and the first harmonic frequency by working as ‘‘ropes in a Tibetan bridge’’, but in the same time the nanofibres are not able to dissipate energy like they did when embedded into the matrix. When the nano-modified panels are damaged some of the nano-fibres are free from the matrix and then they work as internal
Fig. 11. Post-impact harmonic frequencies and flexural stiffnesses.
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Table 5 Post-impact damping ratios. l: mean value and r: standard deviation. Energy of impact (J)
Virgin
Nano1
l 3 6 12
3.24 10 3.03 10 2.92 10
3 3 3
r/l%
l
4.63 7.92 12.33
4.27 10 2.94 10 2.64 10
Fig. 12. Diagrams of damping ratio; experimental points refers to the damping ratio of undamaged (0 J) and impact-damaged panels (3 J, 6 J, 12 J).
links that contribute to the increase of the flexural stiffness and the first harmonic frequency, whereas their contribution as internal dampers is reduced with respect to the condition when they were embedded into the matrix.
Nano2
3 3 3
r/l%
l
3.98 8.16 4.17
3.50 10 3.22 10 3.01 10
r/l% 3 3 3
3.14 4.35 6.31
3. impact tests on nano-modified specimens reveal an 8% lower peak force (for the lowest energy impact) and 14.3% less energy absorbed compared to the virgin ones. These results are enhanced for the case of non-symmetric specimens, which have nanofibres where larger delamination is expected; 4. post-impact analysis of virgin panels shows the expected decrease in stiffness and in the harmonic frequencies, proportionally to the impact energy. Nano-modified panels present unexpected reinforcement effect: both the stiffness and the first harmonic frequency increase with up to 14% and 12%, respectively after the 6 J impact; 5. SEM pictures of the damaged condition show that when the matrix is broken the nano-fibres still link the layers they are inserted in between. The images (see Fig. 9) show that the nano-fibres continue to keep the layers together, even after that the macrostructure is irreversibly damaged. This mechanism is a self-stiffening and a self-repair effect that the nanofibrous interlayers are capable to provide to the whole panel once delaminated.
References 5. Conclusions and discussion This is an experimental study which aims to investigate the effect of Nylon 6,6 electrospun nanofibrous interlayers on the behaviour of carbon fibre reinforced flat laminate panels. By means of electrospinning technology 25 lm thick Nylon 6,6 nanofibrous mats were manufactured. Two nano-modified laminates configurations were manufactured, one symmetric (nanofibres in the two uppermost and in the two lowest interfaces) and the other one non-symmetric (nanofibres in the 3 lowest interfaces). The focus of the research is on global/macro mechanical properties and structural behaviour of nano-modified panels compared to the virgin ones. In particular, the aim is to investigate the influence of nanofibrous interleave into composite laminate and the interaction mechanism between the two. In particular the experimental program is based on static (flexural) and dynamic (free vibration) tests which are performed on undamaged laminates and on laminates that have been damaged by low velocity impact performed at three energy levels (3 J, 6 J and 12 J). Based on this experimental activities the following conclusions are made: 1. the presence of the nanofibrous mats induce a matrix enriched volume at the nanomodified interfaces; 2. the static flexural tests on pre-damaged laminates showed that the stiffness of nanomodified laminates is about 10% less than virgin ones; the free decay vibration tests showed that the nanomodified laminates have a first harmonic frequency 10% less and a damping ratio 160% higher than the virgin ones; so it was possible to conclude that the undamaged nanomodified laminates are less stiff but significantly more damped respect virgin ones;
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