Impact response of Kevlar composites with filled epoxy matrix

Composite Structures 94 (2012) 3520–3528

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Impact response of Kevlar composites with filled epoxy matrix P.N.B. Reis a,⇑, J.A.M. Ferreira b, P. Santos a, M.O.W. Richardson c, J.B. Santos d a

Department of Electromechanical Engineering, University of Beira Interior, Covilhã, Portugal CEMUC, Department of Mechanical Engineering, University of Coimbra, Coimbra, Portugal c GTG/CTA Ltd., 1 Well Holme Mead, New Farnley, Leeds, W/Yorkshire LS12 5RF, England, United Kingdom d CEMUC, Department of Electrical and Computers Engineering, University of Coimbra, Coimbra, Portugal b

a r t i c l e

i n f o

Article history: Available online 28 May 2012 Keywords: A. Polymer–matrix composites (PMCs) A. Hybrid B. Impact behaviour D. Mechanical testing

a b s t r a c t Kevlar fibres have been widely used as impact-resistant reinforcement in composite materials. The paper studies the impact behaviour as well as damage tolerance of Kevlar/filled epoxy matrix. Two different fillers, cork powder and nanoclays Cloisite 30B, were used in order to improve the impact response of these laminates. For better dispersion and interface adhesion matrix/clay nanoclays were previously subjected to a silane treatment appropriate to the epoxy resin. The fillers adding increases the maximum impact load but the opposite tendency was observed for the displacement. Nanoclays promote higher maximum impact loads, lower displacements, the best performance in terms of elastic recuperation and the maximum residual tensile strength. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Laminated composites offer an attractive potential for reducing the weight of high-performance structures as consequence of their high specific strength and stiffness. Although these materials offer excellent in-plane performance they have inferior through-thickness properties and, in case of impact loads, the damage resultant can range from matrix cracking, fibre failure and/or delaminations. This is particularly true for carbon fibre composites, due to the brittle nature of the fibre, with consequent reduction of the residual strength [1–9]. In fact the failure mechanisms are the same in most composites but their mode of occurrence vary with type of loading and properties of the constituents. The shear strength at the fibre matrix interface proved to be important in determining the various failure mechanisms. Aramid fibres are very important reinforcement for advanced composites, which were developed during the 1960s and first introduced commercially by DuPont in the 1970s under the trade name KevlarÒ. Their high degree of toughness, associated with the failure mechanism of aramids, and damage tolerance promotes good impact/ballistic performance. When aramid fibres break, they do not fail by brittle cracking, as do glass or carbon fibres. Instead, the aramid fibres fail by a series of small fibril failures, where the fibrils are molecular strands that make up each aramid fibre and are oriented in the same direction as the fibre itself. These many

⇑ Corresponding author. Tel.: +351 968066552. E-mail address: [email protected] (P.N.B. Reis). 0263-8223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compstruct.2012.05.025

small failures absorb much energy and, therefore, result in very high toughness. There are a really interest in these fibres, for military and civil systems, and several studies can be found in literature. Due to the low surface energy and chemically inert surface of the Kevlar fibre, and consequent poor interfacial adhesion between fibre/matrix, extensive works have been performed in order to improve the interfacial adhesion [10–18]. A review of the effect of stitching on the in-plane mechanical properties of fibre–reinforced polymer composites was made by Mouritz et al. [19] and, when this technique was applied to ballistic impact and explosive blast resistance, significant improvements were found [20,21]. On the other hand, for thin-to-medium thickness structures it can be used multiaxial warp knit fabric (MWK). Studies developed in Kevlar multiaxial warp-knit fabric composites by Kang and Kim [22] shows that these materials have the capability of absorbing high impact energy while constraining the damaged area in a relatively small region compared with those of Kevlar woven laminates. Sometimes, and for specific applications, Kevlar fibres can be also associated with other ones in form of hybrid laminates [23,24] or in sandwich composites [25–29]. In particular, for body armours applications, Lee et al. [30] investigated the ballistic properties of woven aramid fabrics impregnated with a colloidal, discontinuous shear thickening fluid (STF). These studies showed that, under some conditions, STF-fabric composite offers ballistic properties higher than no impregnated fabrics. For example, 8 ml of STF coated onto 4-layers of Kevlar is found to have the same energy absorbed as 14-layers of uncoated Kevlar layers. The performance enhancement provided by the STF was suspected have increased frictional interaction and energy transfer between the yarns [31].

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Thermal-sprayed hard ceramic coatings have been also applied directly to aramid fabrics with success; however, in this case, the weight increases significantly [32]. Then it is possible to conclude that the Kevlar fibres have several application fields but, in all of them, the final target is increase the impact performance at lower weight. In this context the aim of this work is study the low velocity impact response of a Kevlar/ epoxy composite with filled matrix. Two different fillers, cork powder and nanoclays Cloisite 30B, were used in order to improve the impact resistance of these laminates. Cork presents an alveolar structure and it is characterized by high specific strength and stiffness, near zero Poisson coefficient, high damage tolerance to impact loads, impervious to liquid and gases, resistance to reactive agents and microorganisms, resistance to wear and fire, very low thermal conductivity, good acoustic insulation capacity and excellent damping characteristics [33–36]. On the other hand nanoparticles materials, with typical dimensions in the range between 1 and 100 nm [37], have been widely studied and applied in numerous engineering and commercial areas due to its unique surface effect, increased chemical activity and particular physical properties [38,39]. Many researchers have shown great performances, such as mechanical (strength and stiffness) and thermal properties, with adding low concentrations of nanoparticles into polymers without compromising on density, toughness or manufacturing process [37,40,41]. For this contributes the larger surface/interface area per unit volume and, consequently, the chemical and physical interactions increasing with the matrix. The results of the present paper are discussed in terms of load– time, load–displacement, energy–time diagrams and evaluation of the damage. The impacted plates were inspected by ultrasonic techniques to evaluate the size, shape and position of the damage in-plane and through the thickness of the plate. Finally residual strength was obtained by tensile tests. The open literature reports essentially the CAI tests to evaluate the residual strength after impact in composite laminates, because their use in primary structures can be seriously affected, however this work will focus on residual tensile strength. In fact damage tolerance studies show that the reduction in tensile strength of the laminates depends mainly of the extent of fibre breakage and, in this conditions, the impact properties can be significantly affected (the amount of energy absorbed depends of the fibre strength) [42,43]. On the other hand the delaminations and its size cause little effect on the tensile strength of the laminates [44]. 2. Material and experimental procedure Twelve ply laminates, all in the same direction, of woven bi-directional Kevlar 170-1000P (170 g/m2), were prepared by hand lay-up and the overall dimensions of the plates were 330  330  3 (mm). SR 1500 epoxy resin and a SD 2503 hardener, supplied by Sicomin, were used. The system was placed inside a vacuum bag and a load of 2.5 kN was applied during 24 h in order to maintain a constant fibre volume fraction and uniform laminate thickness. During the first 10 h the bag remained attached to a vacuum pump to eliminate any air bubbles existing in the composite. The postcure was followed according to manufacturer datasheet (epoxy resin) in an oven at 40 °C during 24 h. With the same manufacturing process was produced composite laminates with filled epoxy matrix by cork powder and organoclays Cloisite 30B. The bulk density of the cork powder is 0.10 95 g cm3 and the particles’ size, in terms of percentile, is: d(0.1) = 18.6 lm, d(0.5) = 78.9 lm and d(0.9) = 208.3 lm. More details about the cork powder can be found in [45]. Before being added to the resin the cork powder was dried in an oven-dried (Heraus, model UT 6060) at about 120 °C during 2 h. When dried it was placed in a desiccators, cooled to room temperature, and

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stored until use. On other hand, in order to improve the dispersion and interface adhesion matrix/clay, nanoclays were previously subjected to a special treatment appropriate to the epoxy resin. Surface treated clays were custom formulated by GTG/CTA Ltd using chemical treatments and high shear mixing techniques subject to patent applications and may be defined as an organ ically modified layered silicate with a tetrahedral–octahedral– tetrahedral T–O–T basic structure with additional surface layer treatment to give it enhanced dispersive characteristics (in a resin) compared to more traditionally available commercial nanoclay (e.g. with granule size <50 lm, heavy metal content <100 ppm, and volatile content <1%). More details about the effect of the surface treatments on the mechanical properties can be found in [46]. The epoxy resin, in a glass beaker, was heated at 75 °C to decrease the viscosity and the fillers were added. For the cork powder filler the mixture was conducted at 900 rpm for 2 h and, at same time, subjected to ultrasonic bath sonicator. The filler content employed was 3 wt.% of the epoxy resin–hardener mixture. The mixture was degassed in a vacuum oven, while it cools, followed by addition of hardener agent. Special care occurs in this process to avoid the formation of bubbles. In respect to nanoclyas filler the mixture was conducted at a shear rate of 2500 rpm for 1 h, using a high speed shear mixer, followed by sonication (using an ultrasonicator) for 3 h to further disperse the clay, while maintaining the resin temperature at 75 °C using a hot water bath. After sonication, the translucent colour of the epoxy/clay mixture indicates uniform distribution of nanoclays. Once more was employed 3 wt.% of the epoxy resin-hardener mixture and the degassing process followed by addition of hardener agent occurs. Finally when both fillers was added, 1.5 wt.% of cork powder and 1.5 wt.% of nanoclays, it was used the last procedure (similar to that used with nanoclays), however, only after 1 h at high shear mixture was added the cork powder. In this case the mixture was conducted at 900 rpm for over 1 h. The samples used in the experiments were cut from those thin plates to square specimens with 100 mm side and 3 mm thickness (100  100  3 mm). Low-velocity impact tests were performed using a drop weight-testing machine IMATEK-IM10. More details of the impact machine can be found in [47]. Impactor diameter of 20 mm with masses of 3.005 kg was used. The tests were performed on square section samples of 75  75 mm and the impactor stroke at the centre of the samples obtained by centrally supporting the 100  100 mm specimens. The impact energies used in the tests were 6, 12 and 21 J. These energies were previously selected in order to enable the measuring of the damage area, but without promote perforation of the specimens. For each condition, five specimens were tested at room temperature. After impact tests, the specimens were inspected by C-Scan technique to evaluate the position and the size of the damage. Finally, the residual strength was obtained by tensile tests, performed in an electromechanical Instron Universal Testing machine (model 4206), with a strain rate of 5 mm/s at room temperature. 3. Results and discussion Impact tests were carried out considering as variables, not only the material of the sample, but also the impact energy. Figs. 1 and 2 shows the force–time and force–displacement curves for laminates with epoxy resin and epoxy filled by cork. These diagrams represent a typical behaviour occurred for all laminates and agree with the bibliography [22,25,48–51]. The curves contain oscillations that, according Schoeppner and Abrate [52], result from the elastic wave and are created by the vibrations of the samples. It depends on the stiffness and the mass of the specimen and impactor being excited by the rapid variation of the cinematic magnitudes at the collision moment [53].

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3

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Load [kN]

Fig. 1. Diagrams load versus time for composites with: (a) epoxy resin; (b) epoxy filled by cork.

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Fig. 2. Diagrams load versus displacement for composites with: (a) epoxy resin; (b) epoxy filled by cork.

Energy [J]

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Fig. 3. Diagrams energy versus time for composites with: (a) epoxy resin; (b) epoxy filled by cork.

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Maximum average displacement [mm]

12 Impact energy 6 J

11

Impact energy 12 J Imopact energy 21 J

10 9 8 7 6 5 4

E

E+Ck

E+Cl

E+Ck+Cl Material

Fig. 5. The effect of the resin filled on maximum average displacement (E = only epoxy resin; E + Ck = epoxy filled by cork; E + Cl = epoxy filled by nanoclays; E + Ck + Cl = epoxy filled by cork and clays).

60 55 50

Elastic recuperation [%]

In detail, it is possible to observe that the force increases up to a maximum value, Pmax, followed by a drop after the peak load. The value of Pmax is very dependent of the impact energy and represents the peak load value that the composite laminate can tolerate, under a particular impact level, before undergoing major damage. The impact energy was not high enough to infiltrate full penetration, because the impactor sticks into specimens and rebound always. In this context non-perforating impact occurred for all laminates, however for control laminates (Kevlar/epoxy), when tested at 21 J, after Pmax the force decrease while the displacement increases, which means that major damage occurs. Some authors found that the maximum load increased with increasing impact energy [51] and similar tendency was observed in this work. Increase, between 6 and 21 J, were observed around 39.5%, 40%, 47.1% and 44.3% for, respectively, control laminates (Kevlar/epoxy), laminates with epoxy filled by cork, epoxy filled by clays and epoxy filled by cork plus clays. For 21 J the addition of clays promote maximum loads around 16.1% highest than occur in control laminates. The average impact time was also measured and it varied within the range of 7.7–8.4 ms for 6 J, 7.4–8.2 for 12 J and 7.5–8.3 ms for 21 J. In all cases the minimum average time was observed for laminates with resin filled by nanoclays and the time of maximum contact occurred for laminates with pure resin. According with the values collected, it is possible to conclude that the average impact time is very similar, around 7.4–8.4 ms, independently of the laminate. Fig. 3 represents typical energy versus time curves. It is possible to observe that the increase in impact energy promotes a decrease of the elastic recovery and, consequently, the damage increase. The beginning of the plateau of the curve coincides with the loss of contact between the striker and the specimen, so, this energy coincides with that absorbed by the specimen [54]. The average load values shown in Fig. 4 characterize how the maximum impact forces changed when Kevlar laminates are filled by cork, nanoclays or both fillers. For all energies studied it is possible to observe that the lower impact forces occur for Kevlar laminates with pure epoxy resin. When the fillers were added an increase around 3.5% and 4% was found, respectively, for impact energies of 6 J and 12 J. In this energy range the fillers type was not relevant because all of them increase the impact load in the same amount. However, for impact energy of 21 J this deference increase substantially with values around 4.5% for laminates filled by cork, 10.4% for laminates filled by cork/clays and 16.1% for laminates filled by clays. According with Gustin et al. [25] the difference in maximum forces result of the different failure modes, because each sample type had different tensile and shear

Impact energy 6 J

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Impact energy 12 J

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Impact energy 21 J

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Fig. 6. The effect of the resin filled on elastic recuperation (E = only epoxy resin; E + Ck = epoxy filled by cork; E + Cl = epoxy filled by nanoclays; E + Ck + Cl = epoxy filled by cork and clays).

properties. For these authors, Kevlar laminates have a tensile failure mode, which is shown by the bending at the striker perimeter and tearing at the centre of impact [25]. Fig. 5 presents the effect of the fillers type and the impact energy in the laminates displacement during impact loads. The average values represented show, independently of the impact energy,

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Impact energy 6 J Impact energy 12 J Impact energy 21 J

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Fig. 4. The effect of the resin filled on maximum average peak load (E = only epoxy resin; E + Ck = epoxy filled by cork; E + Cl = epoxy filled by nanoclays; E + Ck + Cl = epoxy filled by cork and clays).

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Impact energy [J] Fig. 7. Energy profile diagram of the laminates tested.

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Laminates

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0.0184 0.0164 0.0113 0.0177

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0.3747 0.446 0.8767 0.1627

Damage area [%]

Table 1 Constants of the equation DE ¼ a  E20 þ bE0 þ e for different laminates.

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Laminates with resin filled by cork and clays

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Fig. 10. Damage area in function of the different laminates.

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Impact energy [J] Fig. 8. Identification of penetration threshold.

that the largest displacements occur with laminates manufactured exclusively with pure epoxy resin. In the energy range studied the displacements increase around 26.9%, between 6 and 12 J, and 45.7%, between 6 and 21 J, for Kevlar/epoxy laminates. However, when the fillers are added to the resin the displacements decrease

and this is more evident for laminates filled by clays. This decrease can be around 5.4% and 6.7% when we are talking in energies of 6 and 12 J, respectively, or 11.8% at 21 J. In fact this behaviour can be interesting because some system must absorb the energy of the projectile, but cannot be allowed to deform so extensively that the wearer of the armour is crushed in the process. Fig. 6 compares the elastic recuperation for each laminate. The elastic energy was calculated as the difference between the absorbed energy and the energy at peak load from the diagrams presented in Fig. 3. The average values represented, in percentage, show that higher energies present lower elastic recuperation and, consequently, major damages. For Kevlar/epoxy laminates it is possible to observe a decrease of the elastic recuperation around

Fig. 9. C-Scan images of the damages for laminates, tested at 12 J, with: (a) epoxy resin, (b) epoxy filled by cork, (c) epoxy filled by nanoclays, and (d) epoxy filled by cork and clays.

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(a)

Front face

Back face

(b)

Front face

Back face

Fig. 11. Pictures of the front and back face of the damaged laminates with: (a) epoxy resin, and (b) epoxy filled by nanoclays.

24.8%, between 6 and 12 J, but this value drops dramatically to 69.8% between 6 and 21 J. On the other hand when the fillers are added better results can be found. The laminates manufactured with epoxy resin filled by nanoclays present best performance in terms of elastic recuperation. For impact energies of 6 J, nanoclays improve the elastic recuperation around 7.5% while for 12 J this value increase to 13.8%. For impact energies of 21 J, and comparing with control material, we obtain elastic recuperation around 40.1% higher when nanoclays are added. In fact, for low velocity impacts, when the impactor strikes the specimen surface, the given impact energy (Ei) by the impactor could be classified into two quantities: elastic energy (Ee), which stored elastically in the specimen and transferred back to the impactor, and the absorbed energy (Ea) [22]. More details about each mechanism that contributes to the energy absorbed can be found in Kang and Kim [22], however, according with Aktas et al. [55] it is possible characterize some impact properties such as pure elastic limit, penetration and perforation thresholds using a diagram, called ‘‘energy profile diagram (EPD)’’, where is represented the relationship between Ei and Ea. Fig. 7 shows the energy profile diagram of the laminates considered in this study. The data points of each laminates are close and all of them below of the equal curve. The absorbed energy never equals the impact energy, then, the penetration threshold was not reached yet. In his region the extent of damage is dependent on the impact energy and the excessive energy is used to rebound the impactor. The data plotted in Fig. 7 can be fitted, for each laminate type, by the equation DE ¼ a  E20 þ bE0 þ e, where DE is the energy absorbed, Eo is the impact energy and a, b, d constants presented in Table 1. This equation is more reasonable than the linear relationship between impact energy and the energy absorbed suggested by Shim et al. [56] and agrees with Aktas et al. [55]. The good correlation obtained for these impact energies promotes to extrapolate results with great reliability. The penetration thresholds can be found if the data points Ee–Ei were represented in a diagram and fitted by polynomial equations. According Aktas et al. [55] the roots of these

equations represent the points where Ei/Ea = 1(Ee = 0) and the higher roots give the penetration thresholds. Then Fig. 8 shows the penetration threshold for all laminates and values of 30.9 J, 34.8 J, 43.5 J and 35.1 J were found, respectively, for control laminates, laminates filled by cork, clays and both fillers. In fact the polynomial fit was done only with three values of impact energies, which can promote some imprecision in terms of penetration threshold. However, it is evident that the fillers increase the penetration threshold giving better performances, around 29% higher, when the clays are added. For all laminates tested the damages were analysed and Fig. 9 shows, for example, the C-Scan images obtained. The blue1 colour represents the main damage, in this case the delaminations that occurred, and the area (blue area) was measured by Image-Pro Plus software. All samples were inspected in a square area of 40  40 mm, containing the impact zone, and Fig. 10 represents the percentage of damaged area. These values were obtained by the quotient between damaged area (blue colour) and the square area inspected (1600 mm2). It is evident that the clays increase de damaged area, relatively to the control samples, for all energies used. This difference is, in terms of average values, around 29% higher. On the other hand the cork powder promotes small damage area, for all energies, and is around 20% less when compared with control laminates. Iqbal et al. [51] found an opposite tendency when nanoclays were added to the carbon/epoxy laminates. The improvement in impact damage tolerance arose mainly from the increases in shear strength and stiffness of the matrix material, which in turn gave rise to improved resistance to fibre buckling under compression, due to the presence of nanoclay in the matrix. However the mechanical properties of the carbon fibres are very different from the Kevlar fibres. In fact the aramid fibres do not fail by brittle cracking, as do glass or carbon fibres, but they fail essentially by 1 For interpretation of colour in Figs. 9–11 and 13, the reader is referred to the web version of this article.

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Residual tensile strength [MPa]

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Impact energy [J] Fig. 12. Residual tensile strength versus impact energy.

a series of small fibril failures. According with Aktas et al. [55] the main energy absorption mechanism for glass and carbon–fibre reinforced composite is fibre breakage mode, while for Kevlar composites it is essentially by delaminations. On the other hand, for a woven laminate, delamination was initiated at the centre of impact, and propagated to the directions of warp and fill fibres. Also, matrix cracking could be occurring, but many researchers reported that matrix cracking plays an important role at the onset of delamination [22]. No attempt is made to divide delamination and matrix cracking, since either matrix cracking is always associated with delamination and that the ratio of matrix cracking area and delamination area is constant for a given system, or that energy absorption by matrix cracking is negligible [22]. Caprino [57] and Hirai et al. [58] reported that the delamination area increases with the increase of absorbed energy, and they suggested that there is an exponential relationship. Finally Fig. 11 shows some pictures of the front and back face of the damaged laminates with pure resin (control laminates) and with epoxy filled by nanoclays. For control laminates it is notorious

(a)

that occurs a severe damage in the region of the impact point and it is characterized by big deformation in the thickness direction (see back face of Fig. 1a). On the other hand for laminates with resin filled by clays this behaviour it is not observed and the brittle aspect of the resin appears visible by the bright tonalities observed in Fig. 11b for front face. It is evident that the damage mode is different, because the matrix filled by clays present higher stiffness and, at same time, its ductile behaviour decreases. So the damage mechanisms are different and, consequently, explain the better impact strength of the hybrid laminates with nanoclays. After impact, residual tensile strength was obtained for each energy level as shows in Fig. 12. It is evident that the increasing of the impact energy decreases the residual strength for all laminates and is consequence of the damage occurred. The control laminates present the lower residual tensile strength, for all impact energies, and the higher values are obtained with epoxy filled by nanoclays. If the difference observed for 6 J is around 3.9%, this value increases to 27.1% when the impact energy is 21 J. In fact for the energy of 6 J the residual strength is very close, because the damage effect is very similar independently of the laminates. After this value the fillers presents a significant influence in residual strength of the laminates and, its value, shows to be very dependent of the fillers type. The cork powder filler increases the residual strength, compared with the control laminates, but when nanoclays are added better results can be obtained. For example, compared with the ultimate strength, the laminates after impacted with energy of 21 J presents a residual strength around 57.7% and 40.6% lower for control samples and laminates filled by nanoclays, respectively. The damage was observed for all samples and Fig. 13 presents the principal damages observed in laminates impacted at 21 J. It is possible to conclude that occur two different damages. Fig. 13a shows the typical damage for laminates filled by nanoclays, where it is possible to observe that the failure occurs along the matrix cracking promoted by the impact load. As shown in Fig. 11 this is more evident for laminates filled by nanoclays (compared with control laminates) as consequence of the matrix embrittlement promoted by the fillers. However, in back face, the

Back face Front face

(b)

Front face

Back face

Fig. 13. Typical failures (front and back face) occurred in tensile tests for laminates with: (a) epoxy filled by nanoclays; (b) epoxy resin.

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damage presents some differences. One more time the failure occurs along the matrix cracking but there is a preferential direction. In fact matrix damage is the first type of failure induced by transverse low-velocity impact, and usually takes the form of matrix cracking but also debonding between fibre and matrix. The last damage occurs due to property mismatching between the fibre and matrix and are usually oriented in planes parallel to the fibre direction [59]. In this case there are evidences that the debonding between fibre and matrix occur in a preferential direction and coincide with the fibre direction. This can explain the preferential direction of the damage and it is associated with the failures caused by impact. On the other hand, as consequence of the matrix cracks appear delaminations [59], which propagate during the tensile test. After tensile tests large delaminations are visible in the laminates. All of these damages are responsible of the lower tensile strength and agrees with Reis et al. [44]. According with these authors significant reductions of ultimate strength occurs as consequence of the debonding between fibre/matrix and the stress concentration promoted by the delaminations. Fig. 13 shows the other damage observed, which is representative of the remaining laminates. In this case the failure occurs along the transverse planes. 4. Conclusions Low velocity impact response of a Kevlar/epoxy composite with filled matrix was studied. Cork powder and nanoclays Cloisite 30B, especially modified, were used in order to improve the impact resistance of these laminates. The fillers adding increases the maximum impact load, which is very dependent of the filler type for high impact energy. For 21 J the maximum load increase about 4.5% for laminates filled by cork, 10.4% for laminates filled by cork/clays and 16.1% for laminates filled by clays. The opposite tendency was observed for the displacement, where the nanoclays are the fillers that promote lower values. The best performance in terms of elastic recuperation was obtained for laminates manufactured with epoxy resin filled by nanoclays. For 21 J the elastic recuperation of nanoclay filled composite was around 40.1% higher than control material. Adding clays the damaged area increases relatively to the control samples around 29%. In opposite the cork powder promotes small damage area around 20%. The filler increases the residual strength and the better results were obtained when nanoclays were added. Acknowledgements The authors acknowledge financial support from Nato for this work, Project CBP.MD.CLG.983977, and Global Trading Group/CTA Ltd. by supply of the nanoclays. References [1] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44:155–90. [2] Caprino G. Residual strength prediction of impacted CFRP laminates. J Compos Mater 1984;18:508–18. [3] Cantwell WJ, Morton J. Comparison of the low and high velocity impact response of CFRP. Composites 1989;20:545–51. [4] Prichard JC, Hogg PJ. The role of impact damage in post-impact compression testing. Composites 1990;21:503–9. [5] Davies GAO, Hitchings D, Zhou G. Impact damage and residual strengths of woven fabric glass/polyester laminates. Compos Part A Appl S 1996;27A:1147–56. [6] Zheng S, Sun C. Delamination interaction in laminated structures. Eng Fract Mech 1998;59:225–40. [7] de Moura MFSF, Marques AT. Prediction of low velocity impact damage in carbon–epoxy laminates. Compos Part A Appl S 2002;33:361–8. [8] Amaro AM, de Moura MFSF, Reis PNB. Residual strength after low velocity impact in carbon–epoxy laminates. Mater Sci Forum 2006;514–516:624–8.

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