Effect of neat and reinforced polyacrylonitrile nanofibers incorporation on interlaminar fracture toughness of carbon/epoxy composite

S.M.J. Rzvi*, R. Esmeely Neisiny , S. Nouri Khorsni, S. Rmkrishn F. Berto

a Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology (NTNU), Richard Birkelands vei 2b, 7491, Trondheim, Norway

b Center for Nanofibers and Nanotechnology, Department of Mechanical Engineering, National University of Singapore (NUS),Singapore 117576, Singapore

c Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

Laminated polymer matrix composites (PMCs) reinforced with carbon, glass, aramids etc fibers have been extensively employed in variety of applications due to their superior properties as well as facile and economical methods of fabrication [1-3].The in-plane mechanical properties of these types of materials were dictated by the reinforcement phase (fibers) mechanical properties subsequently, the in-plane properties of PCMs are powerful enough for many applications including aerospace,ground vehicles and wind turbines. In contrast, the out-of-plane mechanical properties of the laminated composite were dominated by mechanical properties of the matrix [4, 5]. It is worth mentioning that the matrix mechanical properties are significantly less than of the mechanical properties of reinforcement phase resulting inadequate out-of-plane properties. Moreover,ply-by-ply temperament of the laminated PMCs makes them susceptible to delamination owning to creation of microcracks and subsequently, microcracks propagation through the weak resin-rich layer. Due to, this significant disadvantage laminated PMCs have not been used in critical applications [4]. Therefore,a series of researches have been carried out for enhancing the matrix mechanical properties by incorporation of a second phase such as carbon nanotubes [6, 7], micro/nanofillers [8-12]in the matrix or even improving the curing system [13, 14].However, the above mentioned methods have not been employed in fabrication of composite materials due to some limitations such as troubles due to significant increase in the viscosity of resins which adversely affect the manufacturing process and subsequently, decreases the mechanical properties of final fabricated composites [15].

A method of composite strengthening was proposed by Dzenis and Reneker with the use of nanothechnology and has gathered much research interest. In this method un-oriented electrospun nanofiber mats were incorporated between layers of the reinforcement phase leading to significant improvement in the mechanical properties of the weak resin-rich area [16, 17].The method provides an inexpensive and facile technique for enhancing mechanical properties of resin-rich area with minimum influences on the fabrication processes [4]. The improvement in mechanical properties of resin-rich area, via incorporation polymeric nanofibers, has been extensively studied during recent years [18-21] as well as improvement in fracture toughness of epoxy adhesives [22-28] and neat resins [29, 30]. The researchers reported that the nanofiber breakage, nanofiber pull out, stress distribution by the nanofibers, and stress transform from weak phase to reinforcing phase are the mechanisms of mechanical properties improvement [20, 21]. Therefore, the nanofibers diameter, the type of nanofiber (mechanical properties of the nanofibers), thickness of the nanofiber mat, and the nanofibers and matrix interaction have a significant impact on the amount of mechanical properties improvement [5].

In the present research, the effect of reinforcing nanofiber,via using of the nanoparticles, on the improvement of fracture toughness of a carbon/epoxy composite was investigated. Improving mechanical properties of nanofibers by incorporating nanofillers was reported in previous works [31, 32]. Consequently, the approach is reinforcing the polyacrylonitrile(PAN) nanofibers by adding Al2O3nanoparticles [33]. Therefore,the neat PAN and PAN-Al2O3nanofibers were deposited on carbon fabrics. Three types of the carbon/epoxy composites namely control samples, PAN reinforced the carbon fiber reinforced polymer (CFRP) and PAN-Al2O3reinforced CFRP panels were fabricates. The composite panels were then evaluated via mode I fracture energy assessment tests to investigate the influence of reinforcing nanofibers on the fracture toughness of a conventional carbon/epoxy composite.

PAN (Mw=150000 g/gmol) and N, N-dimethylformamide(DMF, 99.8%) were provided from Sigma-Aldrich. The epoxy resin (EPONTMResin 828), and its curing agent (EPIKURETMCuring Agent F205) were supplied by Hexion Inc. The conventional carbon fiber fabric (300 g·m-2) was purchased from Jinsor-Tech Industrial Co. Al2O3nanoparticles (99%, diameter average=20 nm)were purchased from nanosany Co.

The 10 wt% of PAN and 10 wt% of PAN containing 1 wt% of Al2O3nanoparticles were dissolved in adequate DMF by employing magnetic stirrer for 24 h at room temperature. In order to make sure excellent dispersion of Al2O3nanoparticles, the solution (containing Al2O3nanoparticles) was solicited for 20 min using high intensity ultrasonic liquid processor (UP 400S, 40 KHz Sonics Vibra Cell, Hielscher, Inc). The obtained solutions were filled in the syringes with needle gauge 21 (inner diameter=0.51 mm). Before the electrospinning, the conventional unidirectional carbon fiber fabrics were placed around a 15 cm diameter grounded drum. The nanofibers were directly electrospun on the carbon fabric surfaces. The electrospinning process parameters were set the same as our previous work for both neat PAN nanofibers and Al2O3-PAN nanofibers [5] leading to a stable electrospinning process for preparation of the uniform nanofibers without any beads. Electrospinning process was hold until 1 g of nanofibers was gathered on a square meter of carbon fiber fabrics. Figure 1 schematically shows the nanofiber deposition on the surfaces of carbon fiber fabrics.

Carbon/epoxy composite panels were prepared using hand lay-up method proceed by the vacuum assist resin transfer molding (VARTM) method. The epoxy resin and its hardener were mixed in the mass ratio 100:58 in accordance with the manufacturer’s recommendation. Two composite panels were fabricated containing 12 layers of uniaxial carbon fabric (parallel aligned) and epoxy matrix. The nanofiber reinforced panels compromised 11 layers of the neat PAN and Al2O3-PAN nanofibers in the resin-rich area, whereas the control composite did not contain any nanofibers. A 30 μm thick polyethylene film was also inserted in the mid-interface of all the specimens during the lay-up to create an initial artificial crack. Vacuum pressure was hold for 18 h in order to complete cure of epoxy resin at room temperature. Subsequently, three composite panels were separated from vacuum system and post cured in an oven at 60 °C for 30 min.

The morphology of Al2O3nanofiber and the fractured surfaces of the composites were analyzed by employing a field emission scanning electron microscope (FE-SEM), Hitachi S-4300, Japan. Before morphological investigations, the surfaces of the nanofibers and fractured composites were coated with a fine layer of gold. As mentioned in our previous work the average diameter of the neat PAN nanofiber and its standard deviation were obtained 380 and 70 nm, respectively [5]. Figure 2 shows the FE-SEM micrograph of Al2O3-PAN nanofibers and diameter distribution of the nanofibers for 50 diameter measurement using Image J software. From Fig. 2, it can be observed that the nanofibers containing Al2O3nanoparticles were fabricated uniformly without any signs of beads. The average nanofiber diameter and its standard deviation were measured 417 and 112 nm, respectively. Therefore, the average of Al2O3-PAN nanofibers is 10% more than average of neat PAN nanofibers diameter. This increasing in nanofiber diameter can be attributed to growing of solution viscosity due to addition of the Al2O3nanoparticles.

The double cantilever beam (DCB) samples have been prepared with 130 mm length, 25 mm width, initial crack of 45 mm.DCB samples were cut from composite panels using water jet.Steel hinges were glued on both tips of the samples to apply the load. DCB fracture tests have been performed to calculate the mode I energy release rate GIc, using Eq. (1), from the modified beam theory (MBT) according to ASTM D 5528 [34]:where P is the load, δ is the load point displacement, b is the specimens width, and a is the delamination length.

Fig. 1. Schematic illustration of the nanofiber deposition on the surfaces of carbon fiber fabric.

Fig. 2. a FE-SEM image of Al2O3-PAN nanofiber. b distribution of the nanofiber diameter.

For each specimen configuration, at least four samples were manufactured and tested. The DCB samples were tested under quasi-static loading with a constant displacement rate of 3 mm/min using an MTS Criterion? Series 40 Electromechanical Universal Test System (Minnesota, United States) with a 5 kN load cell to obtain the load-displacement (P - δ) curves. All the experiments were carried out at temperature of approximately 20 °C and at 60% relative humidity. The crack propagation was tracked at 5 s intervals during testing using a digital camera(Canon EOS 600D with an EF 100 mm f/2.8 Macro Lens, Tokyo-Japan).

Figure 3 shows the typical load-displacement curves of the tested DCB specimens. The presence of the nanofiber postponed the crack nucleation and increased the maximum force value on both PAN and Al2O3-PAN reinforced laminates. By calculating the fracture energy, GIcduring the test, R-curves were obtained for various crack lengths of the tested specimens. Figure 4 illustrates the variations of fracture energy during crack propagation. According to Fig. 4, fracture energies of all the tested samples are minimum at crack initiation step and then they are increased to a certain level followed by a plateau. This plateau shows a stable crack growth, which is not a function of crack length; hence, the plateau value was considered as fracture energy of the sample.

According to the experimental data, the fracture energies of control samples, PAN reinforced samples and Al2O3-PAN samples were 0.832 ± 0.07 N/mm, 0.981 ± 0.08 N/mm, and 1.219± 0.13 N/mm, respectively. Experimental results show a general improvement of fracture energy on both PAN and Al2O3-PAN reinforced samples, which are about 18% and 47% higher than the control samples. Nevertheless, the improvement values for Al2O3-PAN reinforced CFRP samples were higher than the samples reinforced with PAN nanofibers, which can be a result of different failure mechanisms in these samples.

Fig. 3. Typical load-displacement curves of the tested DCB specimens.

By means of an electron microscope, images of fracture surface have been captured to investigate the failure mechanisms in different reinforcing configurations. Figure 5 shows the FE-SEM images of the fracture surfaces of control composite (Fig. 5(a)),composites reinforced with PAN nanofibers (Fig. 5(b)) and Al2O3-PAN nanofibers (Fig. 5(c)). Comparing fracture surface of control composite with composites reinforced with thin layer of the nanofibers reveals that fracture surface of the control composites is comparatively smooth with oriented fracture features.This type of fracture was resulted from the propagation of microcracks that initiated at the place of stress concentration. In contrast, for reinforced composites with PAN and Al2O3-PAN nanofiber (Fig. 5(b) and (c)), it can be observed that the roughness of the fracture surface significantly increased. Higher roughness of fracture surfaces confirms that the attendance of nanofibers in the resin-rich area could avert the microcracks, conducting the cracks along more zigzag paths, and subsequently the resistance to crack propagation would be increased. Voids and holes were also observed on the fracture surfaces of the composite reinforced with the neat PAN and Al2O3nanofibers owning to nanofibers pullout and deboning from the matrix. It was reported that, the nanofiber breakage and pullout are the mechanisms attributed for improvement in fracture toughness of the composite [20, 35]. Furthermore, comparing Fig. 5(b) with 5(c) reveals that the fracture surfaces of the composite reinforced with Al2O3nanofibers is rougher than the fracture surfaces of composite containing neat PAN nanofibers. Therefore, it can be concluded that incorporating Al2O3nanoparticles increased the mechanical properties of the nanofibers, subsequently more energy needed for nanofiber breakage and better stress distribution(through the resin-rich area) and stress transform from resinrich area to reinforcing phase (carbon fiber) were occurred. Considering the samples reinforced by PAN nanofibers the local failure mode around the carbon fibers is a combination of adhesive and cohesive failures while a complete cohesive failure was observed for the samples reinforced by Al2O3-PAN nanofibers. This can be due to probable impact of Al2O3particles on improvement of the bonding energy between the enriched resin and the carbon fibers.

As reported by Brugo and Palazzetti [36], UD samples are less affected by Nylon 6.6 nanofibers due to the fact that the crack tends to cross plies and not to propagate in the same interface the nanolayer is laid. According to their results, higher fracture energy enhancement values were obtained for Nylon 6.6 nanofiber reinforced woven carbon plies. The same conclusion can be made in the case of PAN and Al2O3-PAN reinforced samples.Although the improvement of fracture behavior was considerable, however, it is expected to have higher improvements for woven plies, which can be examined by further studies.

This paper presented an experimental study on the effect of incorporating a thin film of PAN nanofiber and Al2O3-PAN nanofibers in the interface between carbon fiber plies on the delamination behavior of the composite panels. Al2O3nanoparticles were incorporated in PAN nanofibers to improve the mechanical behavior of them and consequently improve the delamination strength of the reinforced panel. A significant improvement was observed for the interlaminar fracture energy of both types of reinforced panels, however, the samples reinforced by Al2O3-PAN nanofibers had higher delamination strength compared to neat PAN nanofiber reinforced samples.The increased interlaminar fracture energy GIcproved that the presence of nanofibers postpone the crack initiation by reinforcing the matrix. In addition, FE-SEM micrographs revealed that during the propagation stage, the crack in Al2O3-PAN reinforced samples is forced to break a larger amount of matrix compared to virgin samples, requiring higher energy to extend.

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