Ballistic and electromagnetic shielding behaviour of multifunctional Kevlar fiber reinforced epoxy composites modified by carbon nanotubes

Carbon 104 (2016) 141e156

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Ballistic and electromagnetic shielding behaviour of multifunctional Kevlar fiber reinforced epoxy composites modified by carbon nanotubes D. Micheli a, *, A. Vricella a, R. Pastore a, A. Delfini a, A. Giusti a, M. Albano a, M. Marchetti a, F. Moglie b, V. Mariani Primiani b a b

Sapienza University of Rome, Department of Astronautic Electric and Energy Engineering, Via Salaria 851, 00138 Rome, Italy
Politecnica delle Marche, Dipartimento di Ingegneria dell' Informazione, Via Brecce Bianche, 60125 Ancona, Italy Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 January 2016 Received in revised form 15 March 2016 Accepted 28 March 2016 Available online 30 March 2016

Overall goal of the research is to develop a material simultaneously able to absorb mechanical shocks and shield from electromagnetic interferences. Several layered composite materials aimed at such multifunctionality have been conceived and realized: the characterization has been carried out in terms of electromagnetic shielding effectiveness in the range 0.8e8.0 GHz and of energy absorbing capability upon impact of metallic bullets fired at about 400 m/s and 1000 m/s (such velocity allows to explore the low energy range of potential mechanical shocks in aerospace structures). The composite specimens under test are ~3.5 mm thick tiles made of hybrid multi-scale material: the manufacturing has been performed by integrating several layers of Kevlar fabric and carbon fiber plies within a polymeric matrix reinforced by carbon nanotubes. The electromagnetic shielding effectiveness has been measured by means of a reverberation chamber; the performances of the layered composites approach the metallic behavior with values of shielding up to 80 dB. The impact tests have been carried out by using an inhouse built linear electromagnetic accelerator, known as railgun; the results show that a thin and light tile of the designed composite material is able to absorb high energy impacts with local delamination of the layered structure. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction In the aerospace field there is often the necessity of equipment box or even larger containers able to accomplish at the same time the electromagnetic (EM) shielding of the electronic payload and its protection from mechanical shock. Simultaneously, lightness and easy manufacturability are required too. Carbon-based ceramic composites (e.g. C/C, C/SiC) are often considered as promising materials for multifunctional applications in general, due to their impressive thermal stability, mechanical and electromagnetic performances and lightweight [1e3]. Nevertheless, the manufacturing of high-standard C/C- or C/SiC-based structures still represents a serious technological concern, let alone the lack of an effective time/cost saving production process. Thus, it is worth investigating the possibility to exploit the properties of more cost-effective and

* Corresponding author. E-mail address: [email protected] (D. Micheli). http://dx.doi.org/10.1016/j.carbon.2016.03.059 0008-6223/© 2016 Elsevier Ltd. All rights reserved.

easily realizable carbon-based polymeric composites for multifunctional employment. The objective of the present research is to develop a composite material with significant performances in terms of both ballistic resistance and electromagnetic shielding, while keeping suitable lightweight and low thickness characteristics. In this context, fiber-reinforced composites represent good candidates for the realization of such packages: in particular, carbon fiber (CF) grants EM shielding effectiveness (SE) at certain frequencies as well as good mechanical strength [4e6], whereas Kevlar aramid fiber is widely known for its antiballistic application [7]. Besides, several works report the beneficial effect of carbon nanofilaments as filler in composite materials. In particular, carbon nanotube (CNT) powders dispersed in different kind of matrix may enhance the composites mechanical strength and EM shielding capabilities [8e20]. In this work a layered composite material realized by integrating several layers of CF and Kevlar fabric within an epoxy resin is proposed; moreover, the polymeric matrix is further enriched by the inclusion of CNTs. In order to measure the

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EM SE, a reverberation chamber (RC) system operating in the range 0.8e8.0 GHz has been employed [21]. This equipment is often adopted in electromagnetic compatibility (EMC) issues [22e24] and mainly consists of a closed volume with reflecting walls allowing a random EM field distribution within. Thanks to such EM chaotic behavior, the SE can be evaluated as reliably as possible by taking into account for all the possible incidence directions of the EM field, as it happens in a real EM propagation environment. About the ballistic testing, an in-house built EM powered projectile launcher called ‘railgun’ has been adopted. This device is basically made up of a pair of parallel conducting boards (the rails), which a sliding metallic armature is accelerated along via the EM effect (Lorentz force) produced by high current pulses [25e28]. The paper is organized as follows: in Section 2 the materials and methods employed are described (with four subsections for the EM SE, the electric conductivity and the mechanical strain & stress measurement setup, and for the description of the railgun), in Section 3 the results of the measurements are reported and discussed, while in Section 4 the analysis of the SE by finite element method (FEM) simulations related to the experimental findings are finally presented. 2. Materials and methods 2.1. Materials The composite material specimens under testing are large layered tile structures manufactured by integrating several layers of Kevlar fabrics and CF plies within a polymeric matrix. This latter is the bi-component epoxy resin Sika Biresin CR82 composite resin system, and has been used either neat or filled with multi walled carbon nanotube (MWCNT). The MWCNTs are the NC7000™ series (average diameter around 9.5 nm, average length 1.5 mm, purity 90%, surface area 250e300 m2/g, volume resistivity 10
4 U cm) supplied by NANOCYL. Fig. 1 shows a scanning electron micrograph of the employed MWCNT powder, revealing the highly entangled morphology of the as-received material. The layered CF reinforced polymer (CFRP)þKevlar structures are composed of 6 layers of CF

(biaxial woven roving 0 e90 ) type HS CF k e dtex 450 g/m2 6 k 4/4 twill, and 2 layers of Kevlar fabric type 49 Saati, style 103 density 635 g/m2, tensile strength 320 Da N/cm, twill 4/4. The manufacturing is performed by taking care to overlap eight layers: three CF layers by following the scheme (0 ÷90 ), two layers of biaxial Kevlar fabric, and again three layers of CF ply as above, with fiber volume fraction in the range 0.82e0.85 and fiber weight fraction in the range 0.75e0.77. In Fig. 2 several steps of the layered CFRP þ Kevlar structures manufacturing are shown. Before the fabrics impregnation, for the multiscale composite items the resin is filled with a fixed amount of CNT powder. The CNTs are homogeneously dispersed within the epoxy resin by means of a wellestablished procedure [11,14,29]. A critical issue of the nano-reinforced composite manufacturing is to perform the nanoparticles mixing within the matrix in such a way to obtain an homogeneous and isotropic distribution. Such requirement, needful for any kind of application, is critically hindered by the Van Der Walls forces that tend to aggregate the nanoparticles to each other. Before mixing within the polymer matrix the carbon nanomaterial is treated by sonication at room temperature in excess of ethanol. The sonication is carried out at 20 kHz for about 6 h by means of an ultrasonic processor (Sonics Ultrasonicator VCX750 model, setting 20% amplitude with respect to the full-scale oscillation magnitude). After this preliminary step, the resin is added to the alcoholic solution in such amount to have the desired MWCNT concentration in the final composite; the composite mixtures realized consist in epoxy resin with inclusion of MWCNTs at 1wt% versus the matrix. The liquid compound is stirred for about 1 h at room temperature, and then put in oven at ~60  C till the total evaporation of the solvent (typically it takes ~48 h), finally an aminic hardener is added and mixed. The layers are resin impregnated by two-side brushing, and positioned on each other within a mold by following the designed sequence; then a pressure loading of about 7 bar is applied over a rectangular zone (30  20 cm2) of the aspackaged multilayer, finally the curing process is carried on in oven (50  C for 16 h þ 80  C for 2 h). 2.2. Electric conductivity measurement set-up The realized composites have been electrically characterized in the frequency range 20 Hze2 MHz by means of the Agilent precision LCR meter model E498A and the four terminal technique measurement system [30,31]. The samples have been obtained from the main plates by water-jet technique; such precise cutting method ensures a very fine tolerance in the samples dimension. Some pictures of the measurement setup are shown in Fig. 3. In order to reduce as much as possible any source of errors, the surface of the samples and of the copper electrodes used for the connection to the LCR meter have been brushed and treated with an electrically conductive silver paint. The samples have been electrically characterized by both series and parallel circuit measurements scheme. In the series scheme the electrical useful dimensions of the samples are ly45 mm, ay11.5 mm, by3.5 mm; in particular, L is the effective length and does not take into account of the silver painted part of the sample where the copper electrodes are placed (see Fig. 3a). The electric conductivity ss of the samples in the series scheme has been computed from the measured value of the resistance Rs by

ss ¼

Fig. 1. SEM picture of MWCNT pristine material.

1 l Rs ab

(1)

where the resistance-inductance series circuit (RseLs) is set in the LCR meter configuration (suggested for small value of Rs and Ls).

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Fig. 2. (a) Bleeder, Film PTFE, Peel-ply, CF and Kevlar fabrics used to manufacture a CNT-reinforced polymeric composite tile, (b, c, d) assembling by layers impregnation with CNTfilled resin, preform lid and pressure application, (e) final hybrid composite 3.5 mm thick multilayer tiles manufactured by fabrics impregnation of neat (left) and CNT-enriched resin (right). (A colour version of this figure can be viewed online.)

About the parallel scheme, squared samples having dimensions a’y26 mm, b’y26 mm, l’y3.5 mm, dy1 mm (d is the Kevlar thickness separating two CF layered conductive plates) have been cut from the main large plates in order to measure the parallel resistance Rp and capacitance Cp, and to compute the relative electric permittivity εr of the middle Kevlar layers. A clamp has been applied on the electrically isolated structures in order to ensure the

contact between the electrodes and the sample. The value of the relative electric permittivity has been computed by

εr ¼

Cp d ε0 a0 b0

(2)

where ε0 is the vacuum dielectric permittivity. In Fig. 3d the

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Fig. 3. Measurement of electric conductivity. (a) Materials sample schematic, (b) Samples with external parts coated by silver-paint; parts of copper electrodes, holes and screws are coated too. (c) Connection to the LCR meter wiring in the 4-terminals configuration for the measurements of the Ls-Rs series circuit, (d) measurement of the parallel RpeCp, and of the dielectric constant εr of the equivalent parallel plate capacitor made of external CFRP layers and inner Kevlar-based part. (A colour version of this figure can be viewed online.)

configuration employed for the measurement is shown. All the electric conductivity data have been subsequently adopted for the SE numerical simulations reported in the discussion section.

2.3. SE measurements equipment The RC has dimensions 4.0  6.0  2.5 m3 and it is equipped with two stirrers. The vertical stirrer has a Z-folded shape, width of 1.2 m and height of 2.4 m. The horizontal stirrer consisted originally of four 1.0  0.5 m2 separated panels successively joined by aluminum sheets to improve its performance. The mechanical engine, associated to each stirrer axis, allows to move them separately and in both stirring and stepped modes with 1 of resolution. A vector network analyzer (VNA e Agilent E5071) is used to acquire the scattering parameter (Sij, i, j ¼ 1,2) between the two or four antennas setting the maximum frequency resolution (1601 points in each investigated sub-range); the data are acquired in tuned mode. The experimental set-up includes a nested reverberation chamber environment (see Fig. 4) based on the use of a smaller RC located inside the larger one. The smaller RC has dimensions 1.2  0.9  0.8 m3 and has an electrically large aperture of dimensions 270  170 mm2, allowing for the coupling between the two RCs, where the sample under test can be fixed. An EM source in the outer chamber produces the incident field, while an antenna inside the inner one collects the field transmitted from the loaded/ unloaded aperture. The material sample is fitted in a metallic frame, thus realizing conductive boundaries, and fixed above the aperture through sealing system. Both the outer and the inner chambers host a second antenna. By this way, thanks to the VNA 4-port operation it is possible to measure also the Quality (Q) of the single RC before and after the sample arrangement: the induced

variation is of considerable importance for the sample SE computation. Referring to the outer RC, the transmitting and receiving antennas are log-periodic Schwarzbeck USLP 9143 operating between 300 MHz and 8 GHz. The resonance of the fundamental mode of the inner cavity is f0 ¼ 205 MHz, so the theoretical lowest usable frequency (LUF) is 6f0 ¼ 1.230 GHz. Inside the small RC two wide-band double ridge antennas (EMCO 3115 and AH Systems SAS-571) are located. For these measurements, the frequencystirring (FS) procedure has been applied in order to investigate the composites samples shielding performance. The dimensions of the sample holder are 300  200 mm2, while the thickness varies in the range 1e20 mm. The mechanical mode-stirring (MS) procedure has been solely adopted in the outer chamber. The stirrer operates in tuning mode as a static diffractor, moving step-by-step among a few independent positions. Further details on both the MS and FS procedures can be found in literature [32]. As widely investigated [33], the SE measurements in highly resonant/reverberating environment use four antennas to capture the Q factor through scattering parameter measurements: the SE is thus expressed as

ED ED E1 0D jS21 j2ns jS41 j2s jS43 j2ns ED ED EA SE ¼
10log10 @ D jS21 j2s jS41 j2ns jS43 j2s

(3)

where the bracket operator takes the usual meaning of joint MS and FS ensemble average, the subscript s or ns means that the measure has been performed with or without the sample respectively, while S21 and S43 are the transmission coefficients of outer and inner chamber respectively and S41 is the transmission coefficient between the outer and the inner chamber. The scattering parameters are measured by VNA 4-port operation: the ports 1 and 2 are

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Outer Chamber Z−Folded Stirrer Rx (Logperiodic) − Port 2

Tile Rx (Double Ridge) − Port 4

Tx (Logperiodic) − Port 1

Tx (Double Ridge) − Port 3

z Inner (Nested) Chamber y

x

(a)

(b)

(c)

(d)

Fig. 4. (a) Reverberation chamber schematic, (b) inner view of the external chamber, (c) removal of the aluminum foil on the sample after the measurement of the floor level, (d) sample mounted on the aperture of the nested reverberation chamber: in the inset, sample edges brushing to enhance the electric contact with the sample holder. (A colour version of this figure can be viewed online.)

connected to the two log-periodic antennas in the outer chamber while the ports 3 and 4 are connected to the two double ridge antennas in the inner chamber. Equation (3) is applied assuming a negligible antenna mismatching [23]. This full scenario should give enhanced results at low frequencies (even if higher than LUF), where the losses are due to both receiving processes and cavity wall dissipations. The external log-periodic antennas limit the working frequency range as they are efficient between 0.3 GHz and 8 GHz. Moreover, the VNA is able to give the maximum output power (10 mW) up to 4 GHz: above this frequency the power is reduced to 1 mW, thus limiting the SE measurement dynamic range. The whole band ranges from 0.8 GHz to 8.0 GHz and is partitioned in 8 sub-bandwidths of 400 MHz. The SE has been measured in the band 0.8e8.4 GHz, with a 250 kHz frequency step. The large number of frequency points (30400) is important to correctly apply the FS averaging procedure. An in-house software takes over VNA acquisition and stepper motor movement, thus the computation is performed averaging by FS procedure and over the 360 uncorrelated angular positions of the metallic stirrer operating in tuning mode. Fig. 4 reports the schematic and some pictures of the experimental setup. The picture inside the external chamber shows part of the nested chamber, the vertical stirrer and the transmitting antenna. The two pictures of the aperture shows how the sample is mounted. In order to avoid electromagnetic leakages around the edges, the superficial resin of the sample is removed with a sandpaper, a large number of screws is used, and proper gaskets are inserted between the sample and the border on the aperture. The

inaccuracy due to the mounting operation has been minimized by covering the sample with an aluminum foil during the measurement of the floor level and by removing the foil up the sample without unmounting it for the SE subsequent measurement. The same chamber set-up is adopted to measure the absorption cross section (ACS) of the sample under test. Such quantity essentially shows the capability of the sample to absorb energy, since it is the ratio between the averaged power absorbed by the sample and the incident scalar power density in the outer chamber Si

ACS ¼

hPs i Si

(4)

The ACS can be derived by measuring the Q factor of the chamber [22].

ACS ¼

uV c



1 1
Qs Qns

 ¼

2pV

l



1 1
Qs Qns

 (5)

where V is the outer chamber volume and l is the wavelength at the measurement frequency and subscripts s and ns stay for “with sample” and without sample respectively. The capability to absorb energy provided by the sample can be evaluated by comparing the ACS with its geometrical area. The advantages in using the RC also for the characterization of the absorbing properties of materials must be remarked. In fact, the concept of ensemble averaged quantities, denoted by < >, has very

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Fig. 5. Experimental set-up for the materials flexural properties evaluation: (a) set of sample extracted from a CFRP þ Kevlar plate, (b) flexural test by Schenck Trebel Testing facility and HBM Z4A load cell (class0.5, 100 KN, sensitivity 2 mV/V), (c) measurement schematic. (A colour version of this figure can be viewed online.)

Fig. 7. Ballistic experimental set-up: (a) Railgun connected to high voltage capacitors, (b) picture of the entire bank of capacitors, (c) break-wire system used to measure the projectile velocity. (A colour version of this figure can be viewed online.) Fig. 6. (a) Railgun scheme, with the bank of high voltage capacitors for impulse discharge electrically connected to the rails. (b) Break-wire system for the bullet velocity measurement. (A colour version of this figure can be viewed online.)

important physical implications. As it happens for equipment and devices [34,35], also materials exhibit a response that depends on dimensions, angle of incidence and polarization of the impinging

waves. Traditional measurements by waveguide fixtures provide only one field polarization (typically TE) and one angle of incidence (normal incidence). On the contrary, the RC provides all polarizations and all angles of incidence to excite the sample, during a complete rotation cycle of the stirrers, having the same probability

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Fig. 8. LCR measurements: (a) series resistance Rs, (b) series inductance Ls, (c) series electric conductivity ss at 20 Hz, (d) parallel resistance Rp, (e) parallel capacitance Cp and relative electric permittivity εr, and (f) parallel electric conductivity sp at 20 Hz. (A colour version of this figure can be viewed online.)

of occurrence. Therefore, this measurement environment is closer to real life situations where the electromagnetic excitation behaves as a random quantity [36]. 2.4. Mechanical properties measurement set-up The ASTM C1341 ‘Standard Test Method for flexural properties of fiber-reinforced composites’ has been used to test samples extracted from the main plates by water jet cutting technique: in particular, by following the standard recommendation the width

and the length of the samples are 12.4 mm and 75 mm respectively, while the distance between the two supports is 60 mm. The test facility is a Schenck Trebel Testing with load cell HBM Z4A (class 0.5) of 100 KN shown in Fig. 5. The measurements allow the computation of the stress s and the strain ε defined as

s¼3

PL 2bd2

ε¼6

Dd L2

(6)

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elasticity by tangent E is concerned, the computation is performed as reported in the reference standard by linear regression on the loading diagram points, where the curves are approximately linear with slope m and the modulus (expressed in GPa) is calculated by

E ¼ 0:25

mL3 bd3

(7)

2.5. Ballistic test equipment An electromagnetic launcher called railgun has been designed and realized in order to perform the composites ballistic characterization. In Fig. 6a the basic scheme of the railgun is shown. There are two parallel barrels (the rails), a moving armature (the bullet) and the electrical assembly of electrodes and capacitors deputed to energy storing and supply. The railgun is 1 m length, the rail bars are 5 mm thick and 15 mm spaced, and are electrically connected to a bank of 160 high voltage capacitors (6000 V, 80 mF) for an overall equivalent capacitance of about 12,000 mF. Each capacitor is about 50 kg weighing and has been supplied by ICAR S.p.A. INDUSTRIA CONDENSATORI (such capacitors were used in the past for the ‘HotShot’ system to test and study materials in plasma wind tunnel under high temperature plasma wind, with the aim to simulate the Earth's atmosphere reentry conditions). The capacitors can be charged up to 6000 V for a theoretical overall stored energy around 200 kJ. A tunable power supplier is used to set the capacitors charging voltage at the desired level: by this way the energy of the railgun can be easily tuned as a function of the desired bullet velocity. A great effort has been provided in order to achieve a reasonable high level of ballistic test reproducibility, mainly for what concerns the control of the railgun bias parameters and their influence on both values and statistical dispersion of the output energy. In Fig. 6b the break wire system used to measure the projectile velocity is schematically shown. It consists of two thin copper strings stretched along the trajectory of the bullet and connected to a double power supply with two oscilloscope channels. When the projectile breaks up these copper conductors, the oscilloscope shows the voltage exponential decay at the two channels: the projectile velocity can be computed by taking into account of the elapsed time and the traveled distance. The pictures in Fig. 7 show the railgun connected to the high voltage capacitors and the break wire system connected to the oscilloscope. The rails are mounted on a dielectric support mechanically resistant to the strong solicitations during the firing phase. In particular, the material supporting the projectile course is Teflon 15 mm thick while the rest of mechanical support is made of Vetronite type G11. Stainless steel screws of diameter 15 mm are used for the railgun assembly, while the electrical connection to the bank of capacitors is achieved by means of copper bars having section of 15  50 mm2, held together by copper screws. These apparently over dimensioned electric conductors are required to withstand the sharp high current pulses (hundreds of thousand A), able to completely destroy conventional flexible cable connections, as experienced.

Fig. 9. RC experimental results: (a) comparison between four measurements of the floor level, (b) SE and ACS of a CFRP þ Kevlar tile, (c) SE and ACS of a CFRP þ Kevlar þ CNT tile (the differences between floor level and samples SE are reported on the top graphs). (A colour version of this figure can be viewed online.)

where P is the load (expressed in N), while L, b, d and D are support distance, sample width, sample thickness, and sample deflection (all expressed in mm) respectively. As far as the modulus of

3. Results 3.1. Electric conductivity The measure accuracy has been assessed by testing a copper reference specimen having dimension l ¼ 80 mm, a ¼ 15 mm, b ¼ 0.5 mm. Values of electric conductivity of 5.80  107 S/m delivered at 20 Hz are close to that of copper in dc (5.8  107 S/m),

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Fig. 10. 3-Point Bending Test results: the displacement load and the flexural stress properties are plotted for (a, b) CFRP þ Kevlar and (c, d) CFRP þ Kevlar þ CNT samples. (A colour version of this figure can be viewed online.)

which is assumed as standard value in normal conditions. In Fig. 8a, b the average measurements of lumped Rs and Ls series elements are shown, while in Fig. 8c the computed average electric conductivity is reported. The value of electric conductivity s has been reported only at 20 Hz, since the expression (1) is valid close to the DC scenario where skin-depth effects can be neglected. It can be noticed that the presence of the CNTs greatly enhances the electric conductivity of the samples; the effect of the conductive nanofilaments becomes more evident at higher frequencies, where they are able to speed-up the increase of measured resistance due to skindepth and losses effect [11,29,37]. The LCR meter is able to display a stable value of Ls at frequencies greater than 20 Hz, thus only the results at 500 kHz, 1 MHz and 2 MHz are reported: the increase of inductance in the CNT-filled material is likely due to electric transport chaotic effects, resulting in magnetic energy storage within the disordered agglomerates of the CNT network [38e40]. As far as the parallel configuration is concerned, some data are shown in Fig. 8def. The CNT filler reduces drastically the Rp values of about two order of magnitude (the value discovered for the unreinforced material at 20 Hz approaches 20 MU and is not displayed in the plot). The values of the dielectric permittivity εr have been computed from the results obtained for the capacitance Cp. It can be assessed that the CNTs increase both permittivity and capacitance of the composite, as well as the resistive losses due to the reduced Rp. The higher value of Cp and εr can be attributed to the increased interfacial polarization effects enhanced by the nanofiller: isolated CNTs interfaced to each other act as nanocapacitors able to increase the overall charge accumulation within the material's bulk. At the same time, the carbon nanopowder constitutes an electric conductive network, where EM losses typically occur. The former effect takes place locally - i.e. where CNTs are not in electric contact to each other - whereas the latter is due to electric currents between CNTs close to each other, where tunneling phenomena take place as well [11]: both phenomena can simultaneously occur within composites where

dielectric fibers like Kevlar are reinforced by conductive nanofiller like CNT-powder. 3.2. SE measurements Fig. 9 reports the measurements of the EM SE of several composite tiles. Values of shielding up to 80 dB have been discovered, thus approaching the metallic behavior of the floor level (this latter is reached when an aluminum foil covers the aperture on the borders and the upside of the sample mounted on the aperture). In other words, the hybrid composite material tiles provide the maximum measurable level of SE. That is clearly due to the presence of carbon-based elements, which greatly enhance the microwave reflection and the absorption properties of dielectric matrix composites. The perfect matching between the reported plots suggests that the manufactured materials have even higher values of SE within the microwave range explored, which cannot be appreciable in the adopted set up. The RC-measurable maximum values threshold is due to unavoidable cable and connection losses, which are greater at higher frequencies, and to the VNA maximum output power, that above 4 GHz decreases from 10 mW to 1 mW thus reducing the dynamic range of the SE measurements. Such results are quite interesting and clearly show that the proposed hybrid composites can be effectively used to produce lightweight and thin EM shielding materials to build parts of aerospace vehicles EMI immune in the frequency band considered. The use of CNT as further reinforcement of conventional CFRP is believed to improve the material's EM SE [41], as discussed in Section 4.1. To investigate the reliability of the floor level definition, the measurements have been repeated four times by mounting/unmounting the aluminum foil. Fig. 9a reports the four measurements showing discrepancies lower than 5 dB that can be ascribed to little differences of screws pressure, sample positioning, and electrical contact of the sample with the gasket. The samples have been covered on the top side and along the edges in contact with the border of the aperture with an

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Fig. 12. Ballistic test. Oscilloscope display of the break-wire system measuring the bullet's velocity. (A colour version of this figure can be viewed online.)

graph of the figures, where negative and positive deviations can be noticed. It can be concluded that the SE of the tiles is equal or greater than the measured floor level. Two main mechanisms contribute to enhance the SE of a material: the reflection mechanism due to the wave impedance step and the absorption mechanisms where the EM energy is transformed into other energy, such as heat energy. In order to quantify this aspect, the Fig. 9b and c reports also the measurement of the ACS of the two materials, obtained according to the procedure described in Section 2.3. The ACS values are much lower than the sample geometrical area (about 0.046 m2): that means that the very high SE values are mainly due to the reflection capability of the investigated materials, while the absorption phenomena can be neglected [22,32,33,42].

3.3. Mechanical properties

Fig. 11. (a, b, c) Sequence of railgun pictures at firing instants, (d) shot-side camera view. (A colour version of this figure can be viewed online.)

aluminum foil in order to reduce the uncertainty due to the mounting procedure, leaving uncovered only the lower side above the aperture of the nested RC: by this way the aluminum foil covers all the aperture thus allowing the floor level evaluation. The sample SE is then measured by gently removing only the part of aluminum foil on the top of the sample, leaving all else unchanged with respect to the previous configuration: by this way the intrinsic uncertainty is limited and the two measurements can be rightly compared. Fig. 9b and c report the floor level and the SE for the CFRP þ Kevlar and the CFRP þ Kevlar CNT case respectively. The floor level and the samples SE are even closer than the measurements of Fig. 9a; the discrepancies (DSE) are reported in the upper

In Fig. 10 the results of the stress/strain tests are shown. The laminates appear compact and uniform, no discontinuity caused by the presence of voids or poor adhesion between the fabric sheets being found during the test. The samples thickness remains practically constant over the entire length with deviations in the order of hundredths of millimeter: such issue must be taken into strong consideration, since it can highly affect the conversion of the load (kN) e deflection (mm) measured data. The fractures have always occurred in the center of the samples, at the point of load application in the upper part of the specimen e i.e. in the laminate of CF. It can be concluded that the breakage is due to the CF compression, according to the load diagram shown in Fig. 5c. The plots of Fig. 10 confirm the samples break for crashing at maximum load; moreover, a brittle behavior is highlighted by the linear trend typical of CF-reinforced polymeric materials [6] and by rather low strain values around 1.2% at the break point. The slight deviations from brittleness properties are due to the presence of the Kevlar laminate which provides a more plastic behavior compared to CF-made laminates, thus presenting delamination induced by structural deformations. In such framework, the introduction of CNTs within the matrix enhances the fragility of the composite structure, resulting in a maximum averaged resistance lowering from about 500 MPa down to 475 Mpa, and in a greater stiffness, since an increasing of the elasticity modulus from about 51 GPa up to 60 GPa on average is detected.

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Fig. 13. Visual inspection of nano-reinforced composite multilayer tiles within the sample holder after the ballistic test: (a) front and (b) back face view after firing at 400 m/s, (c) front and (d) back face view after two fires at 1000 m/s, (e) bullet mass before and (f) after a firing test at 1000 m/s, with more than 70% of mass reduction. (A colour version of this figure can be viewed online.)

3.4. Ballistic test A sequence of four pictures showing a railgun firing is reported in Fig. 11. In the first one the rail gun is at rest and ready to fire. The second one is completely white - but it's not a printing error! In fact, in all the tests the same phenomena has been observed: the electromagnetic impulse (EMP) generated by the current at firing instant is able to blind the camera located about 2 m distant from the railgun for about a time frame. The other pictures show the effect of the current spikes in subsequent time frames around railgun, connection bars and capacitors. It is also clearly visible the flow of plasma outgoing the railgun and directed to the target. Fig. 12 shows the measurement of the elapsed time by means of the break wire system. In Fig. 13 some results of the ballistic tests performed on the manufactured composite tiles made of CFRP þ Kevlar þ CNT are shown. In the first case the time measured by the oscilloscope is around 2 ms: since the distance between the wires is 0.8 m, the resulting bullet velocity is evaluated around

400 m/s. At 400 m/s a strong delamination of the composite material takes place, but the bullet is not able to penetrate into the layered structure. The situation is completely different when the velocity of the bullet is around 1000 m/s, since the sample is completely holed, as well visible in the center of the tile. In this test the bullet has pierced the material and hit the second aluminum plate located about 20 cm far from the sample holder. Moreover, in a second fire test performed at 1000 m/s on the same tile the bullet has hit the sample holder melting the aluminum itself and penetrating into materials as well. The bullet is made of aluminum and its mass strongly reduces, passing from 2 g to 0.6 g after firing. A qualitative comparison between the performance supplied by the naked and the CNT-reinforced laminates can be assessed from Fig. 14, where the bullet exit wound dimension is emphasized in the two cases after a firing at 1000 m/s. It can be easily appreciable how the presence of the nano-filler in the matrix results in an increasing of the hole's surface, thus suggesting a tougher interaction with the bullet during its piercing run along the tile

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Fig. 14. Railgun firing test at 1000 m/s: exit wound dimension in CFRP þ Kevlar (a) and in CFRP þ Kevlar þ CNT (b) composite tiles. (A colour version of this figure can be viewed online.)

thickness. This intriguing response gives evidence of a real efficient ballistic behavior of the composite filled by carbon nanoparticles, since the energy absorption capability is increased by the matrixfiller load transfer optimization. Such mechanical behavior is in agreement with the well-known elastic properties of MWCNTs, which are capable to withstand tensional, torsional, buckling and compression stresses unsustainable by common materials [43,44]. When the contact between the projectile and the target occurs, shock weaves propagate through the material and an amount of energy is converted into vibration: well-dispersed MWCNTs create a network of spring-dampers increasing the damping capacity of the composite material and reducing the damages of the laminate. The CNT-reinforced polymer thus provides more resistance towards the impact by retarding the matrix cracking. In particular, the stiffer matrix compacts the fiber tows thus preventing their mobility and inducing the projectile to involve/break a major fibers number: that results in an effective energy absorption [45]. As further comparison, two other materials have been tested upon impact at 1000 m/s, a Kevlar tile 10 mm thick and an aluminum tile 25 mm thick. The former specimen has shown heavy damages, since the bullet has gone through the whole thickness, whereas the typical crater by material exfoliation has been formed on the surface of the latter one. In Fig. 15 the impacts on the Kevlar and aluminum tiles are shown.

Fig. 15. Visual inspection of a 10 mm thick Kevlar tile after the ballistic test firing at 1000 m/s: (a) front and (b) profile view. (c) Visual inspection of a 25 mm thick aluminum tile after the ballistic test firing at 1000 m/s: crater formation after the material exfoliation due to the impact (in the inset the bullet and the aluminum tile profiles are shown). (A colour version of this figure can be viewed online.)

4. Discussion 4.1. FEM analysis of SE In order to understand the root of the high SE performances

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when the number of plies is higher than 3, as shown in RC measurements of oriented CF-reinforced composite materials [4]. The EM interfaces of the code allow the modeling of EM fields and waves in the frequency domain, formulating and solving the differential form of Maxwell's equations with the aid of initial and boundary conditions. The SE of a material is related to the ratio between incident and transmitted power, as described by the parameter S21 [33,49]. It can be expressed as the sum of three main contributions, namely SEr, SEa and SEmr. SEr is mainly caused by the mismatch of impedance at the air-material interface: such quantity can be evaluated by taking into consideration the S11 scattering parameter, which represents the ratio between incident and reflected power. SEa results from energy attenuation and dissipation within the materials bulk, while SEmr is closely related to the multireflections occurring at the inner interfaces (it is usually assumed that such term is negligible once SE > 15 dB [50,51]). The EMI shielding properties of the material can be thus evaluated from the scattering parameters, accordingly to the following equations [49,50].

SEðdBÞ ¼ SEr ðdBÞ þ SEa ðdBÞ þ SEmr ðdBÞySEr ðdBÞ þ SEa ðdBÞ (8) SEðdBÞ ¼
10log10 jS21 j2

(9)

  SEr ðdBÞ ¼
10log10 1
jS11 j2

SEa ðdBÞ ¼
10log10 Fig. 16. Mesh of geometry for the SE FEM analysis. (a) Model with ports and materials placed in the middle part of the coaxial air-line 7 mm, (b) quality of the mesh: the lower values of the thin layers indicate a finer mesh for the EM field computation where a suitable accuracy is needed. (A colour version of this figure can be viewed online.)

reported in Section 3.2, a numerical simulation by finite element method (FEM) analysis has been carried out by using Comsol Multiphysics commercial code [46,47]. In particular, a coaxial airline hosting the material has been simulated in order to compute the scattering parameters S11 and S21 needed for the SE evaluation. In Fig. 16 the mesh of the coaxial airline is shown. The maximum size of the mesh element is 1/10 of the shortest EM wavelength (l ¼ 3$108/fmax y 3.7 cm), while the minimum one is of the order of 10
7 m: these dimensions enable an optimal growth of the mesh and guarantee the needed accuracy in the EM computation [48]. The quadrilateral sweep method has been used to approach the entire mesh by regular shaping. Port 1 and Port 2 are used to compute the scattering parameters in the coaxial 7 mm airline; the power set at the two ports is 3 dBm (2 mW). In Fig. 16a the geometry and the ports are depicted, while in Fig. 16b the mesh quality is plotted: the lower the elements size, the higher the quality of the mesh. The material under test is in the central part of the simulated coaxial airline; the mesh size is small enough to take into account for the accuracy required in the computation of the full EM field within the layered composite. The material dielectric permittivity and conductivity measured at 20 Hz (i.e. close to DC) have been taken into account to run the FEM simulation. The thin skin-depth of pure CF layers limits the penetration of an EM plane wave in the first fractions of mm, thus the presence of Kevlar (with or without CNT) has not any practical influence on the SE performance of the whole composite. In fact, from the EM propagation point of view, CF shall be regarded as homogeneous bulk material

jS21 j2

!

1
jS11 j2

         Lossesð%Þ ¼ 100 1
S211 
S221  1 Skin
depthðmÞ ¼ pffiffiffiffiffiffiffiffiffiffiffi pf ms

(10)

(11)

(12)

(13)

where S11 and S21 are the reflection and transmission coefficient obtained by the FEM simulation of 50 U coaxial airline 7 mm, f is the frequency, s is the electric conductivity and m is the magnetic permeability. In Fig. 17 the simulations of the SE and EM losses of unreinforced and CNT-filled composites are reported. A not great difference between the two materials can be appreciated;

Fig. 17. FEM simulation of SE and EM losses of the layered composite materials. The scenarios with and without CNT-filling are analyzed. (A colour version of this figure can be viewed online.)

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Fig. 18. Skin-depth simulation for the layered composite materials; the aluminum behavior is also shown. (A colour version of this figure can be viewed online.)

moreover, the SE shows very high values from 120 up to about 240 dB at higher frequencies. With respect to the experimental results, the computed SE increases with the frequency, as expected due to the skin depth effect (whereas in the ‘real’ RC measurements the cable and connection transition losses at higher frequencies reduce dynamic range and sensitivity of the system, thus producing a decreasing SE with the frequency, cfr. Fig. 9). In Fig. 18 the computation of the skin-depth is plotted and the comparison with aluminum is shown. As already observed, the bulk laminate material made of overlapped plies of CF behave as an homogeneous material having its own electric conductivity, which in turns determines the skin-depth effect. The comparison with the aluminum skin-depth is shown to highlight the very high SE performance of the composite material, which actually approaches a metallic-like behavior as confirmed by the experimental results. In Fig. 19, the SEa and SEr component of the SE are plotted. SEa provides the major contribution to the total SE and increases with the frequency due to the skin-depth effect, depending on both frequency and material electric conductivity. On the contrary, SEr decreases with the frequency due to material wave impedance increments [51]. Noticeably, the CNT-filled material shows higher values of both SEa and SEr due to its greater electric conductivity. Fig. 20 reports the computation of the electric field for a layered material made by

Fig. 20. FEM analysis of the EM field propagation within CFRP þ Kevlar þ CNT composite material inserted in coaxial airline 7 mm: the simulation is run at 0.8 GHz for (a) electric field (V/m) and (b) resistive power losses (W/m3). (A colour version of this figure can be viewed online.)

CF laminate, CNT and Kevlar plies in the middle. The SE mechanisms is quite evident: in the second half of the coaxial airline there is a strong attenuation of the electric field (Fig. 20a), thus the shielding is practically due to the first CF laminate only, where the whole incident power is absorbed and reflected (Fig. 20b). Finally, a comparison between composite laminate material and aluminum is reported. Fig. 21 shows the magnetic field by circular lines of different intensity and the current direction by arrows. In the aluminum both the magnetic field intensity and the resistive losses are lower, due to the higher electric conductivity of such material: the SE effect is quite evident since no propagation takes place in the rear part.

5. Conclusion

Fig. 19. FEM simulation of SEa and SEr of the layered composite materials. The scenarios with and without CNT-filling are analyzed. (A colour version of this figure can be viewed online.)

A multifunctional composite material has been realized and characterized in terms of electromagnetic shielding effectiveness and ballistic properties. The proposed multifunctional material is made of Kevlar fabric, CF layers and epoxy resin loaded with carbon nanotube powder. The tests have been performed by means of a reverberation chamber for the measurement of the shielding effectiveness and of a railgun for the shock resistance and ballistic characterization. It has been discovered that a 3.5 mm thick structure of such lightweight material could effectively be employed for multifunctional applications, since it resists and absorb impacts up to 400 m/s of energy up to 600 J, and provides at

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References

Fig. 21. FEM analysis of the EM field propagation within (a) CFRP þ Kevlar þ CNT composite material and (b) aluminum inserted in coaxial airline 7 mm. The arrows represent the currents direction perpendicular to the contour magnetic field line, evaluated in A/m (power at coaxial airline ports 2 mW, frequency 0.8 GHz). (A colour version of this figure can be viewed online.)

least 80 dB of shielding effectiveness in the microwave range 0.8e8.0 GHz. Both the electrical characterization and the FEM simulations show that the inclusion of a few weight percentage of carbon nanotubes within the composite matrix enhances the material EM shielding capabilities, thanks to the formation of conductive multi-paths in the bulk. Likewise, the CNT-filling increases the mechanical stiffness of the composite, resulting in an effective behavior at high velocity impacts via energy spreading effects. Such layered composite allows for lighter weight application when compared to the conventional use of metals; for instance, the obtained results suggest that the proposed material could be employed to assemble a box for electronic equipment requiring both EMI shielding and mechanical shock resistance properties. Further investigations are needed in order to achieve a better knowledge of these topics. Mainly, the complex mechanism that drives the physical/chemical interactions between the carbon nanostructures and the fiber/matrix substrate during EMI phenomena and/or sharp high energy external stress has to be fully analyzed. Nevertheless, the preliminary results presented in this work promote the carbon nanotubes as good candidates for employment as composite materials reinforcement for aerospace applications, where constraints as lightweight, mechanical resistance and electromagnetic shielding efficiency must be simultaneously satisfied.

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