Effect of basalt fibre hybridisation on post-impact mechanical behaviour of hemp fibre

reinforced composites

Hom Nath Dhakal*1, Fabrizio Sarasini2, Carlo Santulli3, Jacopo Tirillò2, Zhongyi Zhang1, Vellayaraj Arumugam 4

1Advanced Polymer and Composites (APC) Research Group, University of Portsmouth, School of Engineering, Anglesea Road, Anglesea Building, PO1 3DJ, UK

2Department of Chemical Engineering Materials Environment, Sapienza-Università di Roma, Via Eudossiana 18, 00184 Roma, Italy

3School of Architecture and Design, University of Camerino, Viale della Rimembranza,

63100 Ascoli Piceno, Italy

4 Department of Aerospace Engineering, Madras Institute of Technology Campus, Chromepet, Anna University, Chennai-44,Tamil Nadu, India.

Abstract

A major limitation to the spreading of natural fibre reinforced composites in semi-structural components is their

unsatisfactory impact performance. As a potential solution, the production of synthetic/plant fibre hybrid laminates has

been explored, trying to obtain materials with sufficient impact properties, while retaining a reduced cost and a

substantial environmental gain. This study explores the effects of hybridisation of basalt fibre on post-impact behaviour

and damage tolerance capability of hemp fibre reinforced composites. All reinforced laminates were impacted in a

range of energies (3, 6, and 9 J) and subjected to both quasi-static and cyclic flexural tests with a step loading

procedure. The tests have also been monitored by acoustic emission (AE), which has confirmed the existence of severe

limitations to the use of natural fibre reinforced composites even when impacted at energies not so close to penetration

and the enhanced damage tolerance offered by the hybridisation with basalt fibers.

Keywords: A. Hybrid; B. Impact behaviour; D. Acoustic emission; E. Compression moulding

* Corresponding author. Tel: + 44 (0) 23 9284 2582; fax: + 44 (0) 23 9284 2351.

E-mail: hom.dhakal@port.ac.uk

1. Introduction

The quest of engineering materials able to meet the ever more stringent environmental regulations has definitely

identified the biocomposites reinforced with natural fibres as materials of choice at least for non-structural applications

[1,2]. What is still hindering a more widespread use is a combination of drawbacks which can be briefly summarized as

follows: inhomogeneous quality and supply cycles, poor water resistance and dimensional stability (swelling), as well

as susceptibility to rotting [1,2]. All these factors tend to affect the mechanical response of both natural fibres and of the

resulting biocomposites. As a result, in most cases, commercial use of biocomposites has been limited to non-structural

or semi-structural applications due to their low stiffness and low impact properties. Hybrid composites are made from a

combination of more than one type of fibre reinforced in the same matrix. At present, there is significant interest in

enhancing the mechanical properties of natural fibre reinforced composites with the use of hybrid materials in order to

extend their use in structural applications. In this regard, a successful approach consists in the production of synthetic

(mostly glass)/plant fibre hybrid laminates with a view to obtaining materials with a balanced combination of

mechanical properties whilst retaining a reduced cost and a substantial environmentally friendly character [3–8]. In

particular, a significant concern is represented by the low impact resistance and damage tolerance of natural fibre

reinforced composites [9]. Shahzad [10] compared the impact damage tolerance of hemp fibre and chopped strand mat

(CSM) glass fibre composites with the same thickness (2.5 mm). He found that impact damage tolerance of hemp fibre

composites was significantly low compared to glass fibre ones. Hemp composites lost almost half of their strength and

stiffness after an impact of 2 J. Compared to this, glass fibre laminates were able to endure a low velocity impact up to

20 J before a reduction of 70 % in strength and stiffness occurred, while hemp composites exhibited a similar

percentage reduction after an impact of only 4 J. Fracture toughness is another parameter that is used to investigate the

damage tolerance of composite materials. Hughes et al. [11] performed an interesting study aimed at using linear elastic

fracture mechanics (LEFM) principles to assess and compare the fracture toughness of natural fibre composites (hemp

and jute fibres) and CSM glass fibre ones. It was found that KIc for the natural fibre reinforced composites was around 3

times lower than the volume equivalent (Vf = 0.2) glass fibre reinforced materials. When converted to Gc values, this

difference approximated to an order of magnitude. The lower fracture toughness observed in the natural fibre reinforced

material compared with the glass fibre material may, in part at least, be attributed to the lower tensile strengths reported

for these fibres and to the lack of fibre pull-out exhibited by these materials, when compared to the extensive fibre pull-

out observed in glass fibre reinforced material. Fibre pullout is a mechanism which can account for significant energy

absorption in fibre reinforced composites. Hybridization can lead to a significant improvement in impact resistance and

damage tolerance of natural fibre composites. Santulli et al. [12] reported that introducing some flax fibres (in

proportions up to 1/3) in glass/epoxy composites resulted in a moderate reduction of impact properties (~ 25 %), but

still in a considerable weight gain (~ 12 %). In addition, hybrid composites showed penetration energy (~ 56 J)

significantly higher than the one for flax composites (~ 18 J). Also Ahmed et al. [13] confirmed that jute laminates had

better impact energy absorption capacity than jute–glass hybrid laminates, however their damage tolerance capability

was lower than for jute–glass hybrid ones. Shahzad [14] studied hemp/glass hybrid composites subjected to low

velocity impact and reported that replacement of 11 % by volume of hemp fibres with glass ones in hybrid composites

increased their impact damage tolerance considerably. After a 4 J-impact, hybrid composites lost only about 30% of

their strength and stiffness, compared to 70% loss in properties for hemp fibre composites at same impact energy level.

The authors reported also a positive hybridisation effect on the fatigue strength. A critical parameter that influences the

impact response of hybrid composites is the stacking sequence [14–18] and an optimal configuration is usually the one

with a natural fibre core sandwiched between two synthetic fibre skins [12,14].

The hybridisation of hemp fibres with glass ones has been widely studied in literature for both thermoplastic [19–21]

and thermosetting matrices [10,14,22]: in this study the replacement of glass fibres with basalt ones to hybridise hemp

composites is investigated. The use of mineral basalt fibres in combination with natural ones is not entirely new [23–25]

due to the growing interest that basalt has attracted in recent years as reinforcement for composite materials [26–29].

However, hardly any literature exists on hemp/basalt hybrid composites [25,30] and, in particular, on their damage

tolerance behaviour. In this work, the effect of basalt fibre hybridization on the impact tolerance of hemp/polyester

composites is addressed through the assessment of their residual post-impact properties using cyclic flexural tests after

falling weight impacts at 3, 6 and 9 J [31]. Acoustic emission (AE), a non-destructive technique widely used to monitor

a structure/component while under loading, has been used during post-impact flexural tests to gain insight into the

damage development. Recognizing, as discussed above, that impact performance of plant fibre composites is a

particular weakness suggests the usefulness of the methods presented with a view to establishing the suitability of these

materials for semi-structural applications.

2. Experimental

2.1Materials

Needle punched randomly oriented non-woven hemp fibre mat (330 g/m2) was used as reinforcement for this study and

was supplied by Hemcore Company Ltd., UK. The basalt fabric (BAS 220.1270.P) hybridised into hemp reinforced

composites was a plain weave fabric (220 g/m2) supplied by Basaltex-Flocart NV (Belgium). Figure 1 shows the

pictures of non-woven hemp mat and woven basalt fabric. The matrix material used in this study was based on

unsaturated polyester, purchased locally from Cathy Composites Portsmouth and commercially coded as AME 6000 T

35. The matrix was mixed with curing catalyst, methyl ethyl ketone peroxide (MEKP) at a concentration of 0.01 w/w of

the matrix for curing.

2.2 Preparation of laminates

All composite laminates were fabricated by the combination of hand lay-up and compression moulding techniques over

a mould sized 150 × 200 coated with Freekote Release Agent FRP90-NC. The laminates were then placed in a Bypel

press under a hydraulic pressure of 10 bars and left for cure for a duration of 60 min at 50 °C. A detailed description of

the manufacturing process is reported elsewhere [32]. Two different configurations of composites have been fabricated

and tested, which are schematically reported in Figure 2, with a final thickness of 4±0.1 mm and 4.2±0.1 for hemp and

hybrid composites, respectively. The purpose of using this type of configuration is to maintain the advantages of both

fibres and minimise the inherent disadvantages of natural fibre composites. In this case placing basalt fibres on the top

and bottom of non-woven hemp is expected to enhance the overall mechanical properties of the laminates without

effectively compromising their weight. For instance the basalt fibres would bear the majority of flexural loading in

much the same way as conventional sandwich composites. To assess this performance, the fibre volume fraction of the

hemp composites was 0.26 while for hybrid samples was 0.28

2.3 Mechanical testing

In this work, an impact test was used to introduce damage within composite materials and assess the damage tolerance

of composites during the post impact mechanical characterization. Low-velocity impact tests were performed on a

custom built, manually operated impact tube. The impactor had a hemispherical steel head with a diameter of 20 mm

and was captured after impact to prevent multiple strikes. Three different impact energies were used, namely 3 J, 6 J

and 9 J, with the idea of these being around the energy value for visual appearance of damage (VID) without incurring

in penetration of laminates. The impacted laminates were then subjected to post-impact four-point bending tests having

care to keep the impact point at the centre of the specimens loaded in flexure. The impacted surface was consistently in

compression during bending loading of all laminates. Flexural tests were carried out in accordance with ASTM D 6272,

with a support span of 80 mm while the distance between the loading noses (the load span) was one half of the support

span. The dimensions of the specimens were selected in order to allocate enough space for two acoustic emission

sensors and also to contain the whole area visually damaged by impact. These tests were performed in a Zwick/Roell

Z010 universal testing machine with a cross-head speed of 2.5 mm/min. Five specimens were tested for each impact

energy, including non impacted specimens, which served as reference materials, having the following in plane

dimensions: 165 mm × 40 mm. To further investigate the post-impact behaviour of laminates, tests were carried out

using a step loading procedure that consisted of four consecutive cycles from 0 to 20, 40, 60 and 80 % of the fracture

stress of the corresponding composite obtained in a static way. The cyclic procedure used is schematically shown in

Figure 3.

2.4 Acoustic emission

Both monotonic and cyclic flexural tests were monitored by acoustic emission until final fracture occurred, using an

AMSY-5 AE system by Vallen Systeme GmbH. The AE acquisition settings used throughout this experimental work

were as follows: threshold = 35 dB, RT (Rearm Time) = 0.4 ms, DDT (Duration Discrimination Time) = 0.2 ms, and

total gain = 34 dB. Two broad-band (100–1500 kHz, Fujicera 1045S) PZT AE sensors were used. The sensors were

placed on the surface of the specimens at both ends to allow linear localization: only events localised in the region

between the sensors were used for analysis, Silicone grease was used as coupling agent.

2.5 Scanning electron microscopy (SEM)

Morphological characterization of the fracture surfaces of the specimens was performed by scanning electron

microscopy (SEM) (SEM Philips XL40, FESEM Zeiss, Auriga). All specimens were sputter coated with gold prior to

examination.

3. Results and discussion

The flexural properties of hemp/basalt hybrid (HB) composites compared to hemp composites (H) are shown in Table

1. These are the mean values of five samples tested for each type of composite. The presence of basalt fibres in a

sandwich configuration improved in a significant way both the average flexural strength (> 120 %) and modulus (> 65

%) of non impacted hemp fibre reinforced laminates. Figure 3 shows the comparison of stress-strain curves of hemp and

hemp/basalt hybrid composites. It can be clearly observed that both laminates exhibited a brittle behaviour, failing by a

single, and abrupt, load drop after reaching the maximum stress but the hybridization of a low strain-to-failure fibre

with a high strain-to-failure one resulted in improved failure strain of the resulting composites. This hybrid effect has

been well documented by other studies [14,33]. The increase in strain to failure of hemp/basalt composites can be

ascribed to this effect as the elongation at break of hemp fibres (~ 1.6 %) is lower than that of basalt ones (~ 3 %). The

presence of basalt skins resulted in delaying the premature collapse of the hemp core by abrupt tearing off of the fibres

from the matrix as reported in other studies on glass/plant fibre hybrid laminates [12,18]. The curve for hybrid

composites showed a much more discernible knee compared to hemp composites. The knee in randomly oriented

composites could be ascribed to premature failure of weak fibres positioned at right angles to the direction of load

application even though this behaviour was not as evident in the present work as will be confirmed by the acoustic

emission analysis. In the case of hybrid composites, the knee can be ascribed to damage patterns occurring in the

compression side of the laminates involving matrix cracks and basalt fibre fractures. From the observation of Figure 4 it

is also worth noting the threefold increase in area under the stress-strain curves for hybrid composites compared to

hemp composites, which suggests that enhanced impact damage tolerance of the former can be expected.

From the results shown in Table 1, the main effect of impact appears to be a very significant reduction both of flexural

strength and modulus, depending on impact energy. This was found true for hemp and hybrid composites and, in order

to compare the effect of impact on the residual flexural properties, normalized residual strength and stiffness were

determined and are shown in Figure 5. These normalized values were obtained as the ratio between the mean flexural

strength (or stiffness) of the impacted specimens and the mean flexural strength (or stiffness) of the undamaged

specimens. It is worth noting that the hybridization had a positive role also on the damage tolerance and on the post-

impact residual properties, which were found to be not satisfactory for hemp fibre reinforced composites with an almost

45 % loss of intrinsic strength and 35 % loss of stiffness for a 9 J impact, respectively. Similar results were found by

Shahzad [14] for hemp fibre reinforced composites, where 4 J impact resulted in almost 70 % loss of strength and

stiffness for thinner laminates (2.5 mm). At increasing impact energies, the enhanced impact damage tolerance of basalt

skin-hemp core composites is likely to be ascribed to the stronger and tougher basalt fibres. The impact properties of

hemp composites can be explained in terms of the properties of the constituents and of the different energy absorbing

mechanisms. Maybe the most important factor is the brittle nature of both hemp and polyester matrix that does not

allow for energy absorption through plastic deformation. In this case, the only available mechanisms for dissipating the

impact energy are represented by initiation and growth of cracks in the matrix, an event whose occurrence is strongly

linked to the fracture toughness of the material. As reported by Hughes et al. [11], the energy release rate and crack tip

plastic zone radii of hemp fibre composites were significantly lower than the ones of chopped strand mat glass fibres

and it is reasonable to suggest that this applies also in the present case, when comparing them with basalt fibre

composites.

Figures 6-7 show the pictures of damage progression in the composite panels as the impact energy grows. From the

observation of front and rear surfaces it is noted that, for hemp fibre reinforced laminates, impacts up to 6 J did not

cause marked damage on the impacted surface apart from a contact-induced indentation that grows with increasing

impact energy. The surface opposed to impact suffered from severe matrix cracking and fibre fracture starting from the

lowest impact energy, namely 3 J. This damage appeared to be visible also on the impacted surface for the highest

impact energy applied (9 J). which is however not sufficient to produce spalling i.e., loss of material at rear, which is a

typical occurrence in natural fibre composites when subjected to high strains [34]. Spalling has been observed in other

studies on hemp mat reinforced laminates for energies quite close to the penetration energy [35]. These characteristic

damage patterns are a further evidence of the brittle nature of this type of laminates. As a general comment, the

evolution of damage with impact energy in these materials appears to be gradual, so that a small increase of impact

energy results, as expected, in further degradation, but not in a sudden change of damage characteristics. This involves

the formation at rear of an elongated crack that subsequently propagates along different directions due to the random

orientation of hemp fibres [31]. A completely different scenario was observed for hybrid composites, where the

presence of hemp skins promoted the occurrence of debonding phenomena and delaminations on both the front and rear

surfaces, with delaminated area that increases with the increase in the impact energy. The delamination pattern was

cross-like shaped on the front face and approximately circular on the rear face, which is a typical occurrence in hybrid

composites not necessarily made of natural and synthetic fibres [13,15,36]. These phenomena absorb a lot of energy,

thus protecting the hemp core from undergoing extensive damage. This can explain the higher residual properties of

hybrid composites compared to hemp ones, the bottleneck being represented by the brittle nature of hemp fibre

reinforced laminates. In addition, the enhanced ductility of hybrid composites has definitely enhanced the absorption of

impact energy.

As a general comment from acoustic emission monitoring, hybrid composites exhibited a most intense acoustic

emission activity in terms of localized events compared to hemp reinforced laminates. The most likely explanation of

this outcome appears to be that in the former a larger number of AE sources are present, i.e., areas where damage is

progressing (multiple sources of debonding phenomena and small delaminations). It is then possible to suggest that

damage is more diffused in hybrids and more concentrated in hemp laminates. A further confirmation can be obtained

from localization plots shown in Figure 8 for non impacted and 9 J-impacted hemp composites where the damage

appears to be relatively concentrated due to the presence of impact. The impact caused also a slightly anticipated onset

of acoustic emission signals (Figure 9) even though most signals occurred close to the failure of composites irrespective

of the level of impact energy, thus confirming the catastrophic and localized failure of such laminates. Hybrid

composites, on the contrary, exhibited a localization of AE signals much more diffused also for non impacted

specimens (Figure 10) and the presence of signals characterized by high amplitudes (> 80 dB) close to the occurrence of

the knee in the stress-strain curve (Figure 11). These signals, usually ascribed to fibre fractures [16,37], were detected

for all the specimens tested i.e., from 0 J to 9 J, on the compression side: this suggests that the damage mechanisms are

likely to be very similar for all hybrid laminates, at least for the range of impact energies investigated. This would

explain that the presence of impact did not cause any significant localization of the damage and that the flexural loading

is prevailing for such composites. Further analysis of AE data is then possible by considering AE events amplitude and

duration distributions in the whole test for samples impacted at different energies, as shown in Tables 2 and 3,

respectively. On hemp laminates, the amplitude of the largest majority of AE signals is lower than 60 dB, which is

likely to suggest the onset of matrix cracking and non-critical debonding on the laminates. Signals related to fibre

fractures or delaminations were hardly detected. The presence of impact promotes the initiation of cracks in the matrix.

The higher the number of microcracks, the higher the probability of crack coalescence to generate critical cracks

responsible of composite’s catastrophic failure. The lack of fibre fractures is a further confirmation that this kind of

damage occurs during the impact event, thus explaining the low residual properties of such composites. Hybrid

composites showed signals characterized by higher amplitudes and durations, related to fibre fractures in the

compression side and delaminations located mainly at the basalt skin/hemp core interface. Also in this case the impact

seems to increase the matrix microcraking and the AE signal distributions with increasing impact energy are similar,

due to the similar damage mechanisms involved.

To better investigate the damage mechanisms, specimens were subjected to a complex loading scheme (Figure 3) while

being monitored by acoustic emission. The results for the two kinds of laminates are summarized in Figures 12 and 13.

Very few signals were detected for non impacted hemp composites (Figure 12a) to confirm the extremely localized and

catastrophic failure of such composites. Impact tends to increase the number of AE signals, even if up to 6 J they are

mainly concentrated during the fourth cycle while, at 9 J, a sudden and consistent increase in the number of AE signals

is detected during the third one. This increase anticipated the imminent failure of the composite which occurred as soon

as the maximum stress in the fourth cycle was reached. The acoustic emission in the third cycle did not take place only

during the load rise but also during the load hold, which is likely to indicate structurally significant defects. This

emission can be ascribed in part to crack face friction or rubbing at rough spots at the crack surface (secondary

emission) caused by impact and in part to the formation and growth of new cracks (primary emission). With regard to

the signal distributions (Table 4), it is to be noted, with increasing impact energy, a gradual shift from signals of fibre

fractures (amplitudes > 80 dB) and debonding phenomena (amplitudes ~ 50-65 dB) to signals of matrix cracks

(amplitudes ~35-50 dB) triggered by impact events with a decrease, for a 9 J-impact, of fibre fracture related signals

(already fractured during the absorption of impact energy). Hybrid composites showed a completely different behaviour

(Figure 13). In fact, a considerable acoustic emission activity was detected soon after the second cycle for non impacted

specimens. This condition held out with increasing impact energy even though the emissivity decreased passing from 0

J to 9 J, which is likely to suggest a high absorption of energy through irreversible damage mechanisms. Despite

acoustic activity was recorded both during load rise and during load hold, the less brittle nature of hybrid composites

prevented the failure before the completion of the fourth and last cycle. The basalt skins assisted in delaying the failure

of hemp core. In this regard it is clear and significant the increase in signals of high duration (> 1 ms) (Table 5) related

to delaminations at the hemp/basalt interface compared to composites reinforced with hemp fibres. This effect i.e,

delamination growth, was emphasised by the impact thus identifying it as the main mechanism responsible for energy

absorption. Most signals are caused by early debonding phenomena occurring on the compression side and transversal

cracks that triggered basalt fibre fractures since the very first cycles. The brittle nature of hemp fibre composites is

clearly seen form SEM micrographs of Figure 14 where, in addition, extensive interface failures (pull-out and

debonding) can be appreciated. Micrographs of Figure 15 report for hybrid composites evidence of delaminations at

hemp/basalt interface with occurrence of matrix cracks and debonding, thus validating what inferred from the AE

analysis.

Conclusions

Natural fibre hybrid composites can be viable alternatives to conventional fibre reinforced composites as structural or

semi-structural components. This area of research continues to be of interest for both industry and academia. The effect

of hybridization of basalt fibres on damage resistance and damage tolerance capability of hemp fibre reinforced

composites has been experimentally investigated through both quasi-static and step loading post-impact flexural testing.

As a conclusion, this work confirms concerns about the low velocity impact resistance of natural fibre laminates. Impact

produces a significant level of damage mainly in the form of early fibre fracture and matrix cracking, so that post-

impact residual strength can be remarkably low. The hybridisation with a higher strain to failure fibre (basalt) in a

sandwich configuration markedly improved both post-impact residual properties and damage tolerance of resulting

composites, even though the sudden failure of the hemp fibre core may represent a limiting factor for their use,

especially at high impact energies. In this regard, hemp fibre composites lost for a 9J-impact around 46% and 34% in

strength and stiffness, respectively, compared to around 23% of hybrid composites for both mechanical properties, An

optimization of the stacking sequence is therefore the required next step: a possibility can be the interruption of the

hemp fibre core by the addition of interlayer basalt fibre configuration.

References

[1] Fuqua MA, Huo S, Ulven CA. Natural Fiber Reinforced Composites. Polym Rev 2012;52:259–320.

[2] Faruk O, Bledzki AK, Fink H-P, Sain M. Biocomposites reinforced with natural fibers: 2000–2010. Prog Polym Sci 2012;37:1552–96.

[3] Jawaid M, Abdul Khalil HPS. Cellulosic/synthetic fibre reinforced polymer hybrid composites: a review. Carbohydr Polym 2011;86:1–18.

[4] Akil HM, De Rosa IM, Santulli C, Sarasini F. Flexural behaviour of pultruded jute/glass and kenaf/glass hybrid composites monitored using acoustic emission. Mater Sci Eng A 2010;527:2942–50.

[5] Almeida Júnior JHS, Ornaghi Júnior HL, Amico SC, Amado FDR. Study of hybrid intralaminate curaua/glass composites. Mater Des 2012;42:111–7.

[6] Devi L, Bhagawan S, Nair K, Thomas S. Water absorption behavior of PALF/GF hybrid polyester composites. Polym Compos 2011;32:335–46.

[7] Dhakal HN. Zhang ZY, Guthrie R, Bennett N. Development of flax/carbon fibre hybrid composites for enhanced properties. Carbohydr polym 2013; 96(1),1–8.

[8] Thakur VK, Thakur MK. Processing and characterisation of natural cellulose fibres/tehrmoset polymer composites. Carbohydr Polym 2014; 109:102-117.

[9] Santulli C. Impact properties of glass/plant fibre hybrid laminates. J Mater Sci 2007;42:3699–707.

[10] Shahzad A. Comparison of tensile properties and impact damage tolerance of hemp and glass fiber composites. J Reinf Plast Compos 2011;30:1877–93.

[11] Hughes M, Hill CAS, Hague JRB. The fracture toughness of bast fibre reinforced polyester composites Part 1 Evaluation and analysis. J Mater Sci 2002;37:4669–76.

[12] Santulli C, Janssen M, Jeronimidis G. Partial replacement of E-glass fibers with flax fibers in composites and effect on falling weight impact performance. J Mater Sci 2005;40:3581–5.

[13] Ahmed KS, Vijayarangan S, Kumar A. Low Velocity Impact Damage Characterization of Woven Jute Glass Fabric Reinforced Isothalic Polyester Hybrid Composites. J Reinf Plast Compos 2007;26:959–76.

[14] Shahzad A. Impact and fatigue properties of hemp-glass fiber hybrid biocomposites. J Reinf Plast Compos 2011;30:1389–98.

[15] Sarasini F, Tirillò J, Valente M, Valente T, Cioffi S, Iannace S, et al. Effect of basalt fiber hybridization on the impact behavior under low impact velocity of glass/basalt woven fabric/epoxy resin composites. Compos Part A Appl Sci Manuf 2013;47:109–23.

[16] Aktaş A, Aktaş M, Turan F. The effect of stacking sequence on the impact and post-impact behavior of woven/knit fabric glass/epoxy hybrid composites. Compos Struct 2013;103:119–35.

[17] Sevkat E, Liaw B, Delale F, Raju BB. Drop-weight impact of plain-woven hybrid glass–graphite/toughened epoxy composites. Compos Part A Appl Sci Manuf 2009;40:1090–110.

[18] De Rosa IM, Santulli C, Sarasini F, Valente M. Post-impact damage characterization of hybrid configurations of jute/glass polyester laminates using acoustic emission and IR thermography. Compos Sci Technol 2009;69:1142–50.

[19] Panthapulakkal S, Sain M. Studies on the Water Absorption Properties of Short Hemp-Glass Fiber Hybrid Polypropylene Composites. J Compos Mater 2007;41:1871–83.

[20] Panthapulakkal S, Sain M. Injection-molded short hemp fiber/glass fiber-reinforced polypropylene hybrid composites—Mechanical, water absorption and thermal properties. J Appl Polym Sci 2007;103:2432–41.

[21] Reis PNB, Ferreira JAM, Antunes FV, Costa JDM. Flexural behaviour of hybrid laminated composites. Compos Part A Appl Sci Manuf 2007;38:1612–20.

[22] Dhakal HN, Zhang ZY, Bennett N. Influence of fibre treatment and glass fibre hybridisation on thermal degradation and surface energy characteristics of hemp/unsaturated polyester composites. Compos Part B Eng 2012;43:2757–61.

[23] Mokhtar I, Yahya MY, Kadir MRA, Kambali MF. Effect on Mechanical Performance of UHMWPE/HDPE-Blend Reinforced with Kenaf, Basalt and Hybrid Kenaf/Basalt Fiber. Polym Plast Technol Eng 2013;52:1140–6.

[24] Amuthakkannan P, Manikandan V, Jappes JTW, Uthayakumar M. Influence of stacking sequence on mechanical properties of basalt-jute fiber-reinforced polymer hybrid composites. J Polym Eng 2012;32:547–54.

[25] Petrucci R, Santulli C, Puglia D, Sarasini F, Torre L, Kenny JM. Mechanical characterisation of hybrid composite laminates based on basalt fibres in combination with flax, hemp and glass fibres manufactured by vacuum infusion. Mater Des 2013; 49:728-735.

[26] Czigány T. Trends in fiber reinforcements – the future belongs to basalt fiber. eXPRESS Polym Lett 2007;1:59.

[27] Deak T, Czigany T. Chemical Composition and Mechanical Properties of Basalt and Glass Fibers: A Comparison. Text Res J 2009;79:645–51.

[28] Carmisciano S, Rosa IM De, Sarasini F, Tamburrano A, Valente M. Basalt woven fiber reinforced vinylester composites: Flexural and electrical properties. Mater Des 2011;32:337–42.

[29] Dorigato A, Pegoretti A. Fatigue resistance of basalt fibers-reinforced laminates. J Compos Mater 2012;46:1773–85.

[30] Öztürk S. The effect of fibre content on the mechanical properties of hemp and basalt fibre reinforced phenol formaldehyde composites. J Mater Sci 2005;40:4585–92.

[31] De Rosa IM, Dhakal HN, Santulli C, Sarasini F, Zhang ZY. Post-impact static and cyclic flexural characterisation of hemp fibre reinforced laminates. Compos Part B Eng 2012;43:1382–96.

[32] Dhakal H, Zhang Z, Richardson M. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos Sci Technol 2007;67:1674–83.

[33] Kretsis G. A review of the tensile, compressive, flexural and shear properties of hybrid fibre-reinforced plastics. Composites 1987;18:13–23.

[34] Wambua P, Vangrimde B, Lomov S, Verpoest I. The response of natural fibre composites to ballistic impact by fragment simulating projectiles. Compos Struct 2007;77:232–40.

[35] Santulli C, Caruso AP. Effect of Fibre Architecture on the Falling Weight Impact Properties of Hemp/Epoxy Composites. J Biobased Mater Bioenergy 2009;3:291–7.

[36] Sarasini F, Tirillò J, Ferrante L, Valente M, Valente T, Lampani L, et al. Drop-weight impact behaviour of woven hybrid basalt–carbon/epoxy composites. Compos Part B Eng 2014;59:204–20.

[37] Barré S, Benzeggagh ML. On the use of acoustic emission to investigate damage mechanisms in glass-fibre-reinforced polypropylene. Compos Sci Technol 1994;52:369–76.

Figure captions

Figure 1: Pictures of non-woven hemp mat and woven basalt fabric

Figure 2. Schematic description of hemp and hemp/basalt hybrid laminates

Figure 3. Schematic plot of the cyclic loading scheme used for testing laminates

Figure 4. Typical stress-strain curves of non impacted hemp and hemp/basalt hybrid composites

Figure 5. Residual normalized flexural strength (a) and stiffness (b) variation with impact energy

Figure 6. Photos of damage progression on front and rear faces of hemp composites impacted in the range 3–9 J

Figure 7. Photos of damage progression on front and rear faces of hemp/basalt hybrid composites impacted in the range

3–9 J

Figure 8. Typical localization plots of AE signals for hemp fibre reinforced composites: (a) non impacted and (b)

impacted at 9 J

Figure 9. Typical amplitude of AE signals vs time plot for hemp fibre reinforced composites: (a) non impacted and (b)

impacted at 9 J

Figure 10. Typical localization plots of AE signals for hybrid composites: (a) non impacted and (b) impacted at 9 J

Figure 11. Typical AE amplitude and stress vs time plots for hybrid composites: (a) non impacted and (b) impacted at 9

J

Figure 12. Amplitude of AE signals vs time during cyclic loading of hemp fibre reinforced composites as a function of

impact energy

Figure 13. Amplitude of AE signals vs time during cyclic loading of hemp/basalt reinforced composites as a function of

impact energy

Figure 14. SEM micrographs of fracture surfaces for hemp reinforced composites

Figure 15. SEM micrographs of fracture surfaces for hemp/basalt reinforced composites

Figures

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

(a)

(b)

Figure 6

Figure 7

Figure 8

(a)

(b)

Figure 9

(a)

(b)

Figure 10

(a)

(b)

Figure 11

(a)

(b)

Figure 12

(a) (b)

(c) (d)

Figure 13

(a) (b)

(c) (d)

Figure 14

(a) (b)

Figure 15

(a) (b)

(c)

Table captions

Table 1. Summary of flexural properties for hemp and hemp/basalt hybrid composites

Table 2a. Relative distribution (%) of amplitudes in hemp fibre reinforced composites

Table 2b. Relative distribution (%) of amplitudes in hemp/basalt reinforced composites

Table 3a. Relative distribution (%) of durations in hemp fibre reinforced composites

Table 3b. Relative distribution (%) of durations in hemp/basalt reinforced composites

Table 4a. Relative distribution (%) of amplitudes in hemp fibre reinforced composites during cyclic loading

Table 4b. Relative distribution (%) of amplitudes in hemp/basalt fibre reinforced composites during cyclic loading

Table 5a. Relative distribution (%) of durations in hemp fibre reinforced composites during cyclic loading

Table 5b. Relative distribution (%) of durations in hemp/basalt fibre reinforced composites during cyclic loading

Table 1. Summary of flexural properties for hemp and hemp/basalt hybrid composites

Specimen Flexural strength

(MPa)

Flexural modulus

(GPa)

Non-impacted

H 75.86 (8.58)* 5.64 (1.64)

HB 169.04 (4.90) 9.52 (1.75)

Impact Energy: 3 J

H 61.07 (10.97) 5.07 (0.27)

HB 163.19 (0.82) 8.36 (0.11)

Impact Energy: 6 J

H 50.47 (8.97) 4.69 (0.31)

HB 140.59 (11.00) 7.77 (0.73)

Impact Energy: 9 J

H 40.72 (2.86) 3.73 (0.42)

HB 129.89 (8.82) 7.40 (0.18)*The values in parentheses are standard deviations

Table 2a. Relative distribution (%) of amplitudes in hemp fibre reinforced composites

Impact energy (J)

Amplitude interval (dB)

35-50 50-60 60-70 70-80 80-90 90-100

0 42.76 29.11 19.41 7.40 0.99 0.33

3 53.98 29.49 11.94 3.78 0.82 -

6 60.14 28.58 9.38 1.77 0.14 -

9 53.31 30.02 12.58 3.31 0.72 0.05

Table 2b. Relative distribution (%) of amplitudes in hemp/basalt reinforced composites

Impact energy (J)

Amplitude interval (dB)

35-50 50-60 60-70 70-80 80-90 90-100

0 49.77 29.01 10.75 3.79 2.42 4.26

3 55.24 28.89 8.86 2.48 0.64 3.88

6 58.76 27.95 6.33 1.49 0.43 5.03

9 51.17 29.89 9.81 3.14 0.68 5.32

Table 3a. Relative distribution (%) of durations in hemp fibre reinforced composites

Impact energy (J)

Duration interval (ms)

0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1 1-1.2 1.2-1.4 1.4-1.6 1.6-1.8 1.8-2 > 2

0 79.28 16.45 2.30 0.66 0.66 0.16 - - 0.49 - -

3 80.92 14.59 2.45 1.02 0.31 0.20 0.10 - 0.10 - 0.31

6 86.41 11.91 1.18 0.32 0.14 - - - - - -

9 82.80 13.78 2.16 0.96 0.10 0.10 0.05 - - 0.05 -

Table 3b. Relative distribution (%) of durations in hemp/basalt reinforced composites

Impact energy (J)

Duration interval (ms)

0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1 1-1.2 1.2-1.4 1.4-1.6 1.6-1.8 1.8-2 > 2

0 76.53 13.11 2.49 1.23 0.82 0.67 0.47 0.42 0.47 0.75 3.03

3 80.94 12.20 1.63 0.44 0.23 0.11 0.21 0.13 0.21 0.27 3.64

6 79.36 12.96 1.57 0.54 0.16 0.09 0.09 0.14 0.16 0.35 4.60

9 77.14 14.22 2.09 0.62 0.23 0.21 0.09 0.08 0.24 0.24 4.85

Table 4a. Relative distribution (%) of amplitudes in hemp fibre reinforced composites during cyclic loading

Impact energy (J)

Amplitude interval (dB)

35-50 50-60 60-70 70-80 80-90 90-100

0 - - - - - -

3 51.82 7.41 4.94 9.88 14.81 11.14

6 34.69 23.47 6.12 12.24 21.43 2.05

9 59.46 27.15 10.52 2.48 0.39 -

Table 4b. Relative distribution (%) of amplitudes in hemp/basalt fibre reinforced composites during cyclic

loading

Impact energy (J)

Amplitude interval (dB)

35-50 50-60 60-70 70-80 80-90 90-100

0 62.70 22.66 5.95 3.28 2.04 3.37

3 69.52 18.41 4.40 1.39 1.29 4.99

6 69.35 19.70 3.20 1.35 1.62 4.78

9 66.72 19.37 3.27 0.54 1.01 9.09

Table 5a. Relative distribution (%) of durations in hemp fibre reinforced composites during cyclic loading

Impact energy (J)

Duration interval (ms)

0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1 1-1.2 1.2-1.4 1.4-1.6 1.6-1.8 1.8-2 > 2

0 - - - - - - - - - - -

3 48.15 14.82 - 20.99 12.34 1.23 1.23 1.24 - - -

6 24.49 21.43 23.47 10.20 9.18 6.12 4.08 1.03 - - -

9 83.36 14.15 1.72 0.58 - 0.19 - - - - -

Table 5b. Relative distribution (%) of durations in hemp/basalt fibre reinforced composites during cyclic

loading

Impact energy (J)

Duration interval (ms)

0-0.2 0.2-0.4 0.4-0.6 0.6-0.8 0.8-1 1-1.2 1.2-1.4 1.4-1.6 1.6-1.8 1.8-2 > 2

0 84.38 6.90 2.21 0.72 0.57 0.58 0.43 0.43 0.43 0.63 2.72

3 84.99 6.88 1.19 0.55 0.15 0.42 0.27 0.40 0.60 0.94 3.61

6 87.70 4.43 0.62 0.46 0.58 0.19 0.54 0.50 0.54 0.74 3.70

9 83.36 5.65 0.47 0.23 0.31 0.19 - 0.39 0.62 0.51 8.27