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wool felts hybrid laminates
Materials and Design 50 (2013) 309–321
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Mechanical behaviour of jute cloth/wool felts hybrid laminates C. Santulli a, F. Sarasini a,⇑, J. Tirillò a, T. Valente a, M. Valente a, A.P. Caruso b,1, M. Infantino b, E. Nisini c, G. Minak c a
Department of Chemical Engineering Materials Environment, Sapienza – Università di Roma, Via Eudossiana 18, 00184 Roma, Italy IPSIA ‘‘G. Vallauri’’, Via B. Peruzzi 13, 41012 Carpi (MO), Italy c DIEM Alma Mater Studiorum – Università di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy b
a r t i c l e
i n f o
Article history: Received 30 January 2013 Accepted 26 February 2013 Available online 14 March 2013 Keywords: Polymer matrix composites Wool fibres Jute fibres Mechanical characterization Acoustic emission
a b s t r a c t This experimental work is aimed at the characterization of new fibre reinforced composites based on epoxy resin with both protein (wool) and lignocellulosic (jute) natural fibres. Wool-based and hybrid (wool/jute) composites with two different stacking sequences (intercalated and sandwich) were developed. Their microstructure has been investigated through optical and scanning electron microscopy, whereas their quasi-static mechanical behaviour has been evaluated in tension and bending. In addition, the impact behaviour under low-velocity impact at three different impact energies, namely 6 J, 8 J and 9 J has been addressed. The tensile and flexural tests have been monitored using acoustic emission (AE) in order to elicit further information about failure mechanisms. AE monitoring showed that development of damage was due to nucleation of matrix microcracks and subsequent debonding and pull-out phenomena in wool fibre composites and that only in hybrid composites a sufficient stress transfer across the jute fibre/matrix interface was achieved. The results confirmed the positive role of hybridization with jute fibres in enhancing both the tensile and flexural behaviour of wool-based composites, though highlighting the need for an improved adhesion between wool fibres and epoxy matrix. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With growing number of applications and mass volume uses, disposal of composites after their intended life is becoming progressively more critical, as well as expensive. In practice, most composites end up in landfills, while some are incinerated after use, although substantial efforts are devoted to recycling and/or reusing them. Both these disposal alternatives are expensive and may contribute to pollution. A number of factors contributed to triggering a sounder interest for innovative products and processes that are more compatible with the environment: these include global environmental awareness, high rate of depletion of petroleum resources, together with the broadening of the sustainability concept, leading to stricter environmental regulations. The use of biodegradable and environmentally friendly plant-extracted ‘lignocellulosic’ fibres has been a natural choice for reinforcing (or simply filling) polymers to make them ‘greener’. The availability of inexpensive lignocellulosic fibres in every part of the world has, in part, fuelled their use in the past few years [1]. The majority of plant fibres, which are being considered as reinforcements for polymeric materials, are bast fibres, such as jute, hemp and flax [2,3]. Many researchers have investigated the mechanical properties of such composites (both ⇑ Corresponding author. Tel.: +39 0644585408. 1
E-mail address: [email protected] (F. Sarasini). Present address: ITI ‘‘Leonardo Da Vinci’’, via B. Peruzzi 9, 41012 Carpi (MO), Italy.
0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.02.079
thermoplastics and thermosets), highlighting in particular the poor interfacial bonding between the hydrophilic natural fibres and the hydrophobic polymer matrices [4,5]. In spite of their favourable properties, natural fibres possess disadvantages like limited thermal stability, non-negligible water absorption, leading to significant swelling of the laminate and poor impact properties. In order to improve their properties, researchers turned their focus towards the hybridization of natural fibres with synthetic ones. The possible combinations of hybrid composites include synthetic–synthetic, natural–synthetic and natural–natural fibre types [6–10]. Hybrid composite materials have wide applications in the field of engineering due to low cost, high strength-to-weight ratio and ease of manufacturing, while providing the option of achieving a combination of properties such as stiffness, ductility and strength. This cannot be attained in composites reinforced by using a single type of fibres: in comparison with them, hybrid composites also showed increased fatigue life, higher fracture toughness and lower notch sensitivity [11]. In literature, few studies dealing with composites made from protein fibres are reported [12–23]. Most of them [12–16] deal with the use of keratin feather fibres as reinforcement in various polymer matrices mainly coming from the poultry processing, which accounts for about 1.5 billion kilograms of dry feathers in the US annually available for other uses [13]. Moreover, very few papers investigated the use of wool fibres as potential reinforcement in composite materials [17–23]. Wool fibre, which is the
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Fig. 3. Typical flexural force–displacement behaviour of the wool and hybrid composites. Fig. 1. Optical micrograph showing the microstructure of a J2W composite.
Table 2 Impact tests results for the different configurations and impact energies. Laminate
Impact energy (J)
Max. contact force (N)
Absorbed energy (J)
W-40
6 8 9 6 8 9 6 8 9
1223 ± 81 1246 ± 75 1340 ± 86 1114 ± 59 1146 ± 91 1237 ± 83 1131 ± 74 1157 ± 61 1446 ± 83
2.81 ± 0.14 3.46 ± 0.34 3.88 ± 0.15 2.52 ± 0.12 3.81 ± 0.24 4.24 ± 0.17 3.03 ± 0.21 4.14 ± 0.31 4.35 ± 0.18
J2W
J3W
Fig. 2. Typical tensile stress–strain behaviour of the wool and hybrid composites.
most popular natural material based on keratin, is a complex multi-cell system composed of inanimate cells, which differ in composition, morphology and properties. The principal component of wool fibres is keratin and there is also a small amount of lipids (0.1%) and mineral salts (0.5%). It has been estimated that wool contains more than 170 different proteins, which are not uniformly distributed throughout the fibre. The proteins in wool are composed of amino acids which are joined together to form long polymer chains: in practice, amino acid monomers are joined together into polypeptide chains to form proteins by a variety of covalent chemical bonds, referred to as crosslinks, and non-covalent physical interactions. The most important crosslinks are the sulphur-containing disulphide bonds, which are formed during fibre growth by a process called ‘‘keratinisation’’ [24]. These make keratin fibres
insoluble in water and more stable to chemical and physical attack than other types of proteins. In addition to its chemical complexity, wool also has a very complex physical structure. Wool fibres are composed of two types of cell: the internal cells of the cortex and the external cuticle cells that form a sheath around the fibres. The cortex of wool comprises approximately 90% of the fibre volume. It consists of overlapping spindle-shaped cortical cells which are held together by the cell membrane complex, which also separates cortical cells from those of the cuticle. The cell membrane complex (CMC) is a continuous region, containing relatively lightly-crosslinked proteins and waxy lipids, that extends throughout the whole fibre [24]. In the last few decades, wool production has become almost unprofitable because of the increased use of synthetic fibres and the difficulties and environmental problems involved in wool processing phases, like scouring and cleaning. In addition, the decreasing marketing importance of wool felts has suggested some possible alternative use of this valuable fibrous protein-based material, regarded as a significantly strong and durable material for a number of uses e.g., as damping materials in piano hammers [25]. A possibility in this regard is exploiting its potential to produce bio-degradable composite materials suitable for non-structural applications.
Table 1 Summary of mechanical properties of wool and jute/wool hybrid composites. Sample
Tensile strength (MPa)
Young’s modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
W-33 W-40 J2W J3W
26.85 ± 8.26 31.05 ± 7.48 40.24 ± 6.75 50.51 ± 1.36
1.85 ± 0.52 2.10 ± 0.41 3.50 ± 0.38 4.97 ± 0.16
54.05 ± 3.35 50.21 ± 0.20 72.70 ± 1.15 76.01 ± 5.42
2.60 ± 0.28 3.00 ± 0.56 5.65 ± 0.12 6.10 ± 0.70
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In this work, biocomposites based on epoxy containing different amounts of wool felts (up to 40 vol.%) were manufactured and their morphological and mechanical properties (tensile, flexural and impact resistance) investigated. Moreover, the role played by the hybridization with jute fibres, according to two different stacking sequences, on the mechanical behaviour of the resulting composites has been clarified. The mechanical tests have been monitored by acoustic emission (AE) in order to get more information about the failure mechanisms. 2. Materials and methods 2.1. Materials and composite manufacturing In this study, two different types of jute/wool felt hybrid composites, produced by hand lay-up technique in a closed mould of dimensions 260 160 mm at room temperature using an epoxy resin (Mates SX10 ver.2 with hardener SX14), were compared. Each laminate was cured under a slight pressure of 0.02 bars for 2 h. The laminate was then removed from the mould and further cured at room temperature for at least 24 h before use. In particular, one (sandwich hybrid, labelled as J2W) was obtained by sandwiching two wool felt layers of areal weight 700 g/m2 between two skins made of jute plain weave tissue (hessian cloth) of areal weight 320 g/m2, while the other (intercalated hybrid, labelled as J3W) was obtained by alternating jute (J) plain weave tissue and wool (W) felts in the sequence J/W/J/W/J. Both hybrid configurations were characterized by a total volume of reinforcement of 45%. In addition, two wool-based composites containing different amounts of wool fibres, namely 33 vol.% (W-33) and 40 vol.% (W-40), were manufactured and tested.
Fig. 4. Impact hysteresis cycles (force vs. deflection) for composite W-40.
2.2. Composite testing From the laminates were cut the specimens for the mechanical characterization. Three-point bending tests were performed in accordance with ASTM: D790. Five specimens for each composite type were tested, having the following dimensions: 170 mm 25 mm 4 mm (L W t). A span-to-depth ratio of 25:1 and a cross-head speed of 2.5 mm/min were used. The strain needed for the evaluation of flexural modulus was measured by bonding strain gauges (K-LY41-3/120 by HBM GmbH, nominal resistance = 120 X, gauge factor = 2.1) to the tensile surface of the beam. The composites were also subjected to tensile testing according to ASTM: D3039. Here too, five flat composite specimens of dimensions 170 mm 25 mm 4 mm (L W t) were tested for each type of laminate. The tensile tests were performed at a cross-head speed of 2.5 mm/min. The specimens were gripped between flat mechanical wedge grips. The longitudinal strain was measured using an extensometer with 20 mm gauge length. The mechanical characterization was performed on a Zwick/Roell Z010 universal testing machine equipped with a 10 kN load cell. Low velocity impact tests were carried out using an in-house built drop-weight impact tower at DIEM – Università di Bologna. The impactor diameter was 12.7 mm and its mass was 1.25 kg, which was dropped from three different heights, being equal to 0.5, 0.65 and 0.75 m, so to give three impact energies of approximately 6, 8 and 9 J, respectively. Sampling frequency of the signals was 100 kHz with no external filtering. The variables measured on the drop-weight tower were: Contact force, measured using a piezoelectric load-cell for dynamical loading. Velocity, measured through elaboration of the signal supplied by a Laser sensor, placed at a height exceeding by approximately 50 mm that of the sample.
Fig. 5. Impact hysteresis cycles (force vs. deflection) for composite J2W.
Fig. 6. Impact hysteresis cycles (force vs. deflection) for composite J3W.
Impactor speed and position as a function of time obtained by double numerical integration of force signal (as suggested by ASTM: D7136 standard).
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Samples are supported between two steel plates with a central circular opening (diameter 76 mm), realised in accordance with ASTM: D3763 standard, at the centre of which the impact event takes place.
2.3. Acoustic emission (AE) The mechanical tests were monitored by acoustic emission until final fracture occurred using an AMSY-5 AE system by Vallen Systeme GmbH (Germany). The AE acquisition settings used throughout this experimental work were as follows: threshold = 35.1 dB, Rearm Time (RT) = 0.4 ms, Duration Discrimination Time (DDT) = 0.2 ms and total gain = 34 dB. Two broad-band (100– 1500 kHz) PZT AE sensors were used. The sensors were placed on the surface of the specimens at both ends to allow linear localization with silicone grease for coupling.
2.4. Water absorption tests Water absorption tests were performed in accordance with ASTM: D570. Three specimens were oven dried at 105 °C for 24 h. The conditioned specimens were immersed in distilled water for 2 h, 24 h and 48 h at a temperature of 23 ± 2 °C. At the end of the preset time, the specimens were removed from the water, all surface water was wiped off using a cloth, then were weighed. After 2 h immersion, the specimens were replaced in water and weighed again after 24 h and 48 h. The percentage increase in weight (W) during immersion was calculated as follows (Eq. (1)):
Wð%Þ ¼
mt m0 100 m0
ð1Þ
where m0 and mt are the conditioned and wet weight (at time t), respectively.
Fig. 7. Front and rear surfaces of impacted W-40 composites.
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2.5. Optical and Scanning Electron Microscopy (SEM) The fracture surfaces of composites were investigated using a scanning electron microscope (Philips XL40). All specimens were sputter coated with gold prior to examination. The optical micrographs of the composites were taken by Nikon Eclipse 150 L on specimens polished to 1 lm diamond finish. The porosity content of the composites was determined by processing micrograph images of the laminates. The composite samples were mounted using the EpoThin castable mounting system by Buehler. The mounted samples were mechanically polished up to 1 lm diamond finish. The polished mounts were then placed on a microscope Nikon Eclipse 150 L where digital micrographs were captured and analysed by two-dimensional image processing method to determine the porosity. The volume porosity was determined from a series of measurements taken on different areas of the laminates. To get sufficiently accurate results, 20 optical areas were analysed. The purpose of the image processing by grey level thresholding is to filter out the fibre and resin rich areas from the image while isolating the porosity. Lucia measurements soft-
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ware was used to process the images from their original state to a binary version where black and white pixels could be counted to obtain an area of porosity measurement.
3. Results and discussion 3.1. Tensile and flexural behaviour Fig. 1 shows the typical microstructure of a J2W composite. From the optical micrograph it is evident that wool fibres are randomly distributed and that voids are present particularly in those regions, where wool felts are concentrated. As expected, the void content was higher in wool-based composites (about 7%) than in hybrid ones (about 4%). This behaviour can be ascribed to the difficulty in achieving an effective impregnation of wool felts through a simple and inexpensive process like hand lay-up. Typical stress–strain curves of wool and hybrid composites are presented in Fig. 2. The different composites show a similar tensile behaviour. An observed continuous improvement in the composite
Fig. 8. Front and rear surfaces of impacted J2W composites.
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Fig. 9. Front and rear surfaces of impacted J3W composites.
Table 3 Water absorption of wool and jute/wool hybrid composites. Composite sample code
Water absorption (%) 2h
W-33 W-40 J2W J3W
0.86 1.95 2.03 2.93
24 h a
(0.03 ) (0.09) (0.62) (0.47)
1.77 4.19 4.93 6.70
48 h (0.08) (0.18) (0.48) (0.49)
2.17 5.37 5.70 7.80
(0.11) (0.21) (0.44) (0.57)
a Values are average of three replicates and values in parentheses are standard deviations.
stiffness and strength is measured with the increase in fibre volume fraction of jute fibres, as summarized in Table 1. This behaviour can be ascribed to both the higher mechanical properties of lignocellulosic fibres (jute) compared to those of protein-based ones (wool) [2,26] and to the higher void content of wool-based composites. Moreover, it is well known that tensile properties of
composites are strongly dependent on the strength and modulus of fibres, on their length and orientation, on the fibre volume fraction as well as on the interfacial adhesion between fibre and matrix. Interfacial adhesion plays a fundamental role in determining the mechanical properties of composites and it depends on three main factors: mechanical interlocking, physical attractive forces (hydrogen bonds, van der Waals forces) and chemical bonds between fibre and matrix. The surface of natural fibres has many hydroxylic groups which enable the formation of hydrogen bonds with the hydroxylic groups of the backbone chain of the matrix (epoxy resin). In this case, it is to be expected some degree of compatibility between jute fibres and epoxy matrix [27] and some mechanical interlocking due to the fact that jute technical fibres i.e., with diameter and stiffness sufficient to use them for textile purposes, are actually bundles of elementary fibres, and therefore with a quite irregular and rough surface. Wool fibres show a more uniform shape with a relatively smooth surface, which is not suitable for a high degree of mechanical interlocking. As regards their
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Fig. 10. SEM micrographs of fracture surface of composite W-40.
Fig. 11. SEM micrograph of fracture surface of composite W-40.
Fig. 12. SEM micrograph of fracture surface of composite J2W.
chemical composition, wool proteins are composed of amino acids, which contain basic amino (NH2) and acidic carboxyl (COOH) groups. Individual amino acids differ from each other in the nature of the side groups which vary in size and can be grouped, according to their chemical properties: hydrophobic, hydrophilic, acidic, basic and amino acids that contain sulphur [24]. On the grounds of their structure, wool fibres can show variable proportions of hydrophobic and hydrophilic character, as it is the case for other keratin fibres, such as those obtained from chicken feathers, which contain approximately 60% hydrophobic and 40% hydrophilic amino acids in their amino acid sequence [13].
A further parameter to be considered is the stacking sequence, which can significantly affect the mechanical behaviour of hybrid laminates [28]. The tensile strength tends to increase when the stronger material is used as skin in a hypothetical sandwich structure. The high tensile strength of jute fibres in the outer layers enables increasing stresses to be borne, whereas the wool-based core tends to take up the stresses and to distribute them evenly throughout the composite. The strength of laminates tends to increase with increasing volume fraction of jute fibres. A similar behaviour occurred for composites tested in bending, as can be clearly seen in Fig. 3, which show the typical flexural force vs. displacement curves for the different types of composites. Also in this case, the presence of jute fibres involves a considerable improvement in both strength and stiffness (Table 1). These properties are determined by the strength of the outer layers of reinforcement [29]. The presence of jute implies also a change in the failure mode which appears to be sharp and abrupt, while it proceeds gradually for wool fibre reinforced composites. All composites tested have shown failure in the tensile side without experiencing complete separation thus pointing out, as a whole, a brittle behaviour. The increase in wool content seems to positively affect only the modulus of elasticity, while it appears to be less influent on the flexural strength. This finding can be ascribed to the higher void content due to the difficulty in impregnating increasing amounts of wool fibres. Jute has been one of the most important natural fibres used as reinforcement in composites and therefore a great deal of literature is available on the mechanical characterization of jute reinforced composites. To allow for a comparison, it is worth noting nonetheless that results are significantly influenced e.g., by the type of jute fabric used, the manufacturing process and fibre content. In a recent work, woven jute (400 g/m2)/epoxy composites, fabricated by hand lay-up with a fibre volume fraction of 0.45, have been characterized both in tensile and in bending [30]. Composite tensile strength was 55 MPa, Young’s modulus was 10 GPa, flexural strength was 102 MPa and flexural modulus was 4.7 GPa. Venkateshwaran and ElayaPerumal prepared by hand lay-up composites with a total weight fraction of 0.25, finding a tensile strength of 25 MPa for jute fibre composites and of 40 MPa for all banana fibre composites [31]. The trilayer banana/jute/banana composite showed better tensile strength and modulus of 54.60 MPa and 13.69 GPa, respectively, and higher flexural strength and modulus of 91.66 MPa and 9.44 GPa, respectively. Abdallah et al. investigated jute/epoxy laminate manufactured by the infusion process with three lay-up sequences, namely [0]S for the laminate, [90]S for warp, and [+45/45]S for weft. They obtained tensile strengths in the range 38–51 MPa and Young’s moduli in the range 4.5–5 GPa, while the flexural properties varied in the ranges 80–83 MPa and 3.2 GPa for strength and modulus, respectively [32]. Unfortunately, no information was provided on the jute
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Fig. 13. SEM micrographs of fracture surface of composite J2W at different magnifications.
Fig. 14. SEM micrographs of fracture surface of composite J3W at different magnifications.
content in the composites. Santulli tested plain woven jute fabric/ polyester laminates manufactured by RTM with a fibre fraction of 0.63 by volume [33]. The resulting laminates showed a tensile strength and modulus of 61.13 MPa and 5.61 GPa, respectively. De Rosa et al. manufactured by hand lay-up jute (300 g/m2)/epoxy composites with a fibre weight fraction of 0.35 [34]. The composites yielded a tensile strength and modulus of 70 MPa and 6.5 GPa, respectively, while the flexural properties in three point
bending were 95 MPa and 7 GPa for strength and modulus, respectively. These results appear to indicate that the materials investigated in the present study exhibit mechanical properties which are comparable with those of competing material systems, even though they need to be improved by selecting suitable surface treatments and optimized manufacturing techniques, as confirmed by other authors for both thermosetting [22] and thermoplastic [23] matrices.
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Fig. 15. Stress-time- AE amplitude plot for the different composites.
Fig. 16. Typical localization plot of AE signals for a tensile test (left) and a three-point bending test (right).
3.2. Low velocity impact behaviour Low velocity impact is a useful test to investigate the effect produced by accidental damage during service of polymer composites, which may result in a significant reduction of the local resistance of the laminate and in a consequent shorter lifespan for the component [35]. In Table 2, the maximum contact force and the unrecoverable absorbed energy, which is the maximum energy the material can transform, are reported [36]. These variables are obviously growing with nominal impact energy applied. While the highest maximum contact force is offered by the wool-based composite W-40 except for 9-Joule impact, the largest energy is ab-
sorbed during impact on the intercalated hybrid J3W, most likely due to the presence of more interfaces, as previously suggested [37]. When looking at the respective hysteresis cycles for the various laminates (Figs. 4–6) it can be noticed that the ascendant part of the curve presents very different patterns between the different samples and is very far from linearity. This can be attributed to the variable void content in the laminates, which results in different local properties and in unpredictable vibration behaviour under impact loading. As regards the damage patterns, these appear to be very similar among the different laminates (Figs. 7–9). On the impacted surface only a local indentation due to the impactor
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was revealed, while on the opposite surface the presence of cracks with a cross-like pattern can be observed with no obvious signs of delamination or fibre fracture. 3.3. Water absorption behaviour Results of water absorption are given in Table 3. Water absorption increases with higher fibre content in the composites, a trend that is observed for both types of reinforcement and for 2 h, 24 h and 48 h water immersion tests. With the increase in natural fibre content there are more sites for water absorption (free hydroxyl groups in cellulose and hemicelluloses). Water absorption in composites can be mainly ascribed to the presence of lumens and hydrogen bonding sites in the jute fibres and wool felts, the gaps at the interface between matrix and reinforcement and porosity [38,39]. This is a well known behaviour and to reduce the moisture absorption, the fibre has to be modified chemically and physically. Several studies have shown that moisture uptake can be reduced by the use of suitable coupling agents and fibre surface treatments, such as alkali treatment, acetylation, permanganate treatment, maleic anhydride treatment, acrylation and so forth [40–43]. An additional solution to this problem can be represented by the reduction of porosity obtained improving the manufacturing process. 3.4. Morphological analysis The mechanical characterization has been supported by a morphological analysis of fracture surfaces through scanning electron microscopy. Fig. 10 shows a SEM micrograph and related detail of the fracture surface of a wool fibre reinforced composite
(W-40). The fracture surface appearance indicates a macroscopically brittle behaviour with a significant presence of debonding and pull-out phenomena (Fig. 11). All the observations confirm a weak adhesion between fibre and matrix. The lack of compatibility between fibre and epoxy matrix is also evident from the observation of the surface of fibres pulled out from the matrix: the surface appears clean without any resin particles sticking to it (Figs. 10 and 11). Another feature to be noted is the fracture surface of wool fibres, which is sharp without occurrence of significant fibrillation, often related to effective stress transfer across the interface. All these considerations explain the low mechanical properties of wool fibre reinforced composites. The hybridization with jute fibres involved a significant improvement in mechanical properties. This behaviour can be ascribed to the better degree of interfacial adhesion between jute and epoxy resin, as confirmed by the observation of Figs. 12–14, which are related to the composites J2W and J3W. The fracture appearance is still brittle, as pointed out by the river lines in epoxy matrix (Fig. 14c) [44]. Compared to wool fibres, jute fibres seem to show a better compatibility with the matrix. Even though pull-out phenomena are present, these appear to be less significant, as shown in Figs. 12 and 13a–c and 14a and b. The length of pull-out is shorter for jute fibres and there is no direct evidence of voids related to the complete pull-out of warp or weft filaments of the fabric. The surface of jute fibres appears less clean than that of wool fibres (Fig. 14d) and the fracture surface of jute fibres is also different. In fact, in Fig. 15d, the occurrence of fibrillation phenomena it noteworthy and the fracture surface of fibres is not flat, as was the case for wool fibres. In this regard, Fig. 13d shows a clear axial splitting of the fibre, while Fig. 14d shows the cross section of a jute fibre, in which different parts are on staggered planes. All these observations confirm that a
Fig. 17. AE amplitude vs. duration plot, for the different composites.
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Fig. 18. Typical amplitude distribution for a (a) tensile test and for a (b) flexural test of a W-40 composite.
sufficient stress transfer at the interface occurred, and that the jute fibres have carried out their reinforcing action. 3.5. Acoustic emission monitoring The time evolution and nature of different failure modes of composites have been studied through the analysis of acoustic emission signals recorded in real time during mechanical tests. Fig. 15 shows the evolution of AE amplitude as a function of applied stress during tensile tests. As a whole, AE events are associated with damage modes of the material and therefore the presence of acoustic activity during loading indicates the occurrence of damage. Fig. 15 indicates also that AE events take place immediately after the onset of loading. This behaviour suggests that composites suffer from early damage, as the number of signals increases with increasing applied load. This trend is common to all types of composites investigated. A difference between wool-based composites and the hybrids is due to the type of AE signals. In wool-based composites (Fig. 15a and b) AE signals are characterized by amplitudes in the range 35–70 dB. In literature, the amplitude range 35–55 dB is usually associated to damage modes
occurring in the matrix (microcracks) and to friction among the new created surfaces, while amplitudes belonging to the range 55–70 dB are ascribed to interfacial damage [45–47]. However, interface failure is never isolated as a damage mechanism, being very often linked to pull-out mechanisms instead. It is worth noting that these two amplitude ranges are not well separated and often overlapping occurs. Despite that, it is clear that matrix cracking and interface failure are present during tensile loading of wool reinforced laminates. This analysis confirms the results yielded from the morphological analysis of fracture surfaces, where the presence of extensive pull-out was clearly apparent. The microcracks in the matrix can be also triggered by high void contents. Moreover, few signals of high amplitude (75–100 dB) and long duration are present, which suggests that the final failure of the composite is not likely to be due to fibre breakages, but to the accumulation of other phenomena. AE activity for hybrid composites shows also an increased number of signals and localized events. Technical jute fibres represent complex systems and the amplitude distribution of events associated to their failure can be described as follows: longitudinal splitting of the non-cellulosic boundary layer among the elementary fibres (AE amplitude < 40 dB); transverse
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cracking of the elementary fibre (40–60 dB); fracture of elementary fibres and their microfibrils (>60 dB) [48]. These phenomena occur during the test and are superimposed to those due to the fibre/matrix interface. This can explain the larger number of AE events in the case of hybrid composites. AE allows also for the localization of the irreversible damage sources. An example of such localization is reported in Fig. 16a for a tensile test. It is to be noted that the AE sources are distributed over the whole gauge length of the specimens, thus conforming that a large volume of material is stressed during a tensile test. In contrast, during flexural loading, a higher number of localized AE events were recorded for flexural tests. This outcome is to be ascribed to the fact that the damage zone is much more localized in a three-point bending test, thus causing less AE signal attenuation. In addition, during a bending test, the material is subjected to a complex state of stress involving tensile, compressive and interlaminar stresses. A confirmation of the narrow damage zone can be obtained from Fig. 16b, which reports a typical localization plot for a flexural test. The characteristics of AE signals (Fig. 17) in bending are similar to those obtained for tensile tests, even though an increase of signals showing medium–high amplitudes (60–70 dB) and long durations (>400 ls), especially for hybrids, was recorded. These signals are associated to interface failures (delaminations), which begin to play an important role in determining the final failure of composites in bending. The energy of signals in bending was higher than that of signals in tensile test. The importance of interface failures in bending can also be inferred from the comparison between amplitude distributions of tensile and bending tests (Fig. 18). Here, a shift of signal amplitudes towards higher values (60–70 dB), attributed to interface failures, can be noted, when compared to tensile tests, where most signals lie in the ranges typical of debonding and matrix microcraks (35–50 dB). In general, the results demonstrate that these hybrids can offer some potential, in principle allowing the application of quite significant stresses on the skins, being then distributed through the core. This is justified by the fact that mechanical performance, in spite of the simple method employed for laminate fabrication, is not much lower than what found on similar materials, as discussed above. However, this potential can prove valuable for semi-structural applications, only in case problems linked to the not very effective fibre/matrix interface link are solved, and a substantial reduction of void content is achieved during their production.
4. Conclusions This study revealed that hybridization with jute fibres can significantly enhance both the tensile and flexural properties (strength and stiffness) of wool fibre reinforced epoxy composites. In fact, the use of wool fibres in the form of felts does not allow an effective impregnation through a simple and inexpensive process such as hand lay-up. AE monitoring showed that development of damage was due to nucleation of matrix microcracks and subsequent debonding and pull-out phenomena: only in hybrid composites a sufficient stress transfer across the jute fibre/matrix interface was achieved. To get significant exploitation of wool fibre reinforcing efficiency, compatibility between wool fibres and polymeric matrices has to be improved through the application of suitable physico-chemical treatments. References [1] Faruk O, Bledzki AK, Fink H-P, Sain M. Biocomposites reinforced with natural fibers: 2000–2010. Prog. Polym. Sci. 2012;37:1552–96. [2] Summerscales J, Dissanayake NPJ, Virk AS, Hall W. A review of bast fibres and their composites. Part 1 – Fibres as reinforcements. Compos Part A 2010;41:1329–35.
[3] Summerscales J, Dissanayake NPJ, Virk AS, Hall W. A review of bast fibres and their composites. Part 2. Compos Part A 2010;41:1336–44. [4] Ku H, Wang H, Pattarachaiyakoop N, Trada M. A review on the tensile properties of natural fiber reinforced polymer composites. Compos Part B 2011;42:856–73. [5] Malkapuram R, Kumar V, Negi YS. Recent development in natural fiber reinforced polypropylene composites. J Reinf Plast Compos 2009;28:1169–89. [6] Jawaid M, Abdul Khalil HPS. Cellulosic/synthetic fibre reinforced polymer hybrid composites: A review. Carbohyd Polym 2011;86:1–18. [7] Ashori A. Hybrid composites from waste materials. J Polym Environ 2010;18:65–70. [8] Kiani H, Ashori A, Ahmad Mozaffari S. Water resistance and thermal stability of hybrid lignocellulosic filler–PVC composites. Polym Bull 2011;66:797–802. [9] Ho M-P, Lau K-T. Design of an impact resistant glass fibre/epoxy composites using short silk fibres. Mater Des 2012;35:664–9. 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Stacking sequence effect of aramid-UHMPE hybrid composites by flexural test method. Polym Test 1998;16:549–62. [29] Munikenche Gowda T, Naidu ACB, Chhaya R. Some mechanical properties of untreated jute fabric-reinforced polyester composites. Compos Part A 1999;30:277–84. [30] Sabeel Ahmed K, Mallinatha V, Amith SJ. Effect of ceramic fillers on mechanical properties of woven jute fabric reinforced epoxy composites. J Reinf Plast Compos 2011;30:1315–26. [31] Venkateshwaran N, ElayaPerumal A. Mechanical and water absorption properties of woven jute/banana hybrid composites. Fiber Polym 2012;13:907–14. [32] Abdallah MIR, Zitoune R, Collombet F, Bezzazi B. Study of mechanical and thermomechanical properties of jute/epoxy composite laminate. J Reinf Plast Compos 2010;29:1669. [33] Santulli C. Post-impact damage characterisation on natural fibre reinforced composites using acoustic emission. NDT & E Int 2001;34:531–6. [34] De Rosa IM, Santulli C, Sarasini F. 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Mechanical behaviour of jute cloth/wool felts hybrid laminates C. Santulli a, F. Sarasini a,⇑, J. Tirillò a, T. Valente a, M. Valente a, A.P. Caruso b,1, M. Infantino b, E. Nisini c, G. Minak c a
Department of Chemical Engineering Materials Environment, Sapienza – Università di Roma, Via Eudossiana 18, 00184 Roma, Italy IPSIA ‘‘G. Vallauri’’, Via B. Peruzzi 13, 41012 Carpi (MO), Italy c DIEM Alma Mater Studiorum – Università di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy b
a r t i c l e
i n f o
Article history: Received 30 January 2013 Accepted 26 February 2013 Available online 14 March 2013 Keywords: Polymer matrix composites Wool fibres Jute fibres Mechanical characterization Acoustic emission
a b s t r a c t This experimental work is aimed at the characterization of new fibre reinforced composites based on epoxy resin with both protein (wool) and lignocellulosic (jute) natural fibres. Wool-based and hybrid (wool/jute) composites with two different stacking sequences (intercalated and sandwich) were developed. Their microstructure has been investigated through optical and scanning electron microscopy, whereas their quasi-static mechanical behaviour has been evaluated in tension and bending. In addition, the impact behaviour under low-velocity impact at three different impact energies, namely 6 J, 8 J and 9 J has been addressed. The tensile and flexural tests have been monitored using acoustic emission (AE) in order to elicit further information about failure mechanisms. AE monitoring showed that development of damage was due to nucleation of matrix microcracks and subsequent debonding and pull-out phenomena in wool fibre composites and that only in hybrid composites a sufficient stress transfer across the jute fibre/matrix interface was achieved. The results confirmed the positive role of hybridization with jute fibres in enhancing both the tensile and flexural behaviour of wool-based composites, though highlighting the need for an improved adhesion between wool fibres and epoxy matrix. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With growing number of applications and mass volume uses, disposal of composites after their intended life is becoming progressively more critical, as well as expensive. In practice, most composites end up in landfills, while some are incinerated after use, although substantial efforts are devoted to recycling and/or reusing them. Both these disposal alternatives are expensive and may contribute to pollution. A number of factors contributed to triggering a sounder interest for innovative products and processes that are more compatible with the environment: these include global environmental awareness, high rate of depletion of petroleum resources, together with the broadening of the sustainability concept, leading to stricter environmental regulations. The use of biodegradable and environmentally friendly plant-extracted ‘lignocellulosic’ fibres has been a natural choice for reinforcing (or simply filling) polymers to make them ‘greener’. The availability of inexpensive lignocellulosic fibres in every part of the world has, in part, fuelled their use in the past few years [1]. The majority of plant fibres, which are being considered as reinforcements for polymeric materials, are bast fibres, such as jute, hemp and flax [2,3]. Many researchers have investigated the mechanical properties of such composites (both ⇑ Corresponding author. Tel.: +39 0644585408. 1
E-mail address: [email protected] (F. Sarasini). Present address: ITI ‘‘Leonardo Da Vinci’’, via B. Peruzzi 9, 41012 Carpi (MO), Italy.
0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.02.079
thermoplastics and thermosets), highlighting in particular the poor interfacial bonding between the hydrophilic natural fibres and the hydrophobic polymer matrices [4,5]. In spite of their favourable properties, natural fibres possess disadvantages like limited thermal stability, non-negligible water absorption, leading to significant swelling of the laminate and poor impact properties. In order to improve their properties, researchers turned their focus towards the hybridization of natural fibres with synthetic ones. The possible combinations of hybrid composites include synthetic–synthetic, natural–synthetic and natural–natural fibre types [6–10]. Hybrid composite materials have wide applications in the field of engineering due to low cost, high strength-to-weight ratio and ease of manufacturing, while providing the option of achieving a combination of properties such as stiffness, ductility and strength. This cannot be attained in composites reinforced by using a single type of fibres: in comparison with them, hybrid composites also showed increased fatigue life, higher fracture toughness and lower notch sensitivity [11]. In literature, few studies dealing with composites made from protein fibres are reported [12–23]. Most of them [12–16] deal with the use of keratin feather fibres as reinforcement in various polymer matrices mainly coming from the poultry processing, which accounts for about 1.5 billion kilograms of dry feathers in the US annually available for other uses [13]. Moreover, very few papers investigated the use of wool fibres as potential reinforcement in composite materials [17–23]. Wool fibre, which is the
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Fig. 3. Typical flexural force–displacement behaviour of the wool and hybrid composites. Fig. 1. Optical micrograph showing the microstructure of a J2W composite.
Table 2 Impact tests results for the different configurations and impact energies. Laminate
Impact energy (J)
Max. contact force (N)
Absorbed energy (J)
W-40
6 8 9 6 8 9 6 8 9
1223 ± 81 1246 ± 75 1340 ± 86 1114 ± 59 1146 ± 91 1237 ± 83 1131 ± 74 1157 ± 61 1446 ± 83
2.81 ± 0.14 3.46 ± 0.34 3.88 ± 0.15 2.52 ± 0.12 3.81 ± 0.24 4.24 ± 0.17 3.03 ± 0.21 4.14 ± 0.31 4.35 ± 0.18
J2W
J3W
Fig. 2. Typical tensile stress–strain behaviour of the wool and hybrid composites.
most popular natural material based on keratin, is a complex multi-cell system composed of inanimate cells, which differ in composition, morphology and properties. The principal component of wool fibres is keratin and there is also a small amount of lipids (0.1%) and mineral salts (0.5%). It has been estimated that wool contains more than 170 different proteins, which are not uniformly distributed throughout the fibre. The proteins in wool are composed of amino acids which are joined together to form long polymer chains: in practice, amino acid monomers are joined together into polypeptide chains to form proteins by a variety of covalent chemical bonds, referred to as crosslinks, and non-covalent physical interactions. The most important crosslinks are the sulphur-containing disulphide bonds, which are formed during fibre growth by a process called ‘‘keratinisation’’ [24]. These make keratin fibres
insoluble in water and more stable to chemical and physical attack than other types of proteins. In addition to its chemical complexity, wool also has a very complex physical structure. Wool fibres are composed of two types of cell: the internal cells of the cortex and the external cuticle cells that form a sheath around the fibres. The cortex of wool comprises approximately 90% of the fibre volume. It consists of overlapping spindle-shaped cortical cells which are held together by the cell membrane complex, which also separates cortical cells from those of the cuticle. The cell membrane complex (CMC) is a continuous region, containing relatively lightly-crosslinked proteins and waxy lipids, that extends throughout the whole fibre [24]. In the last few decades, wool production has become almost unprofitable because of the increased use of synthetic fibres and the difficulties and environmental problems involved in wool processing phases, like scouring and cleaning. In addition, the decreasing marketing importance of wool felts has suggested some possible alternative use of this valuable fibrous protein-based material, regarded as a significantly strong and durable material for a number of uses e.g., as damping materials in piano hammers [25]. A possibility in this regard is exploiting its potential to produce bio-degradable composite materials suitable for non-structural applications.
Table 1 Summary of mechanical properties of wool and jute/wool hybrid composites. Sample
Tensile strength (MPa)
Young’s modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
W-33 W-40 J2W J3W
26.85 ± 8.26 31.05 ± 7.48 40.24 ± 6.75 50.51 ± 1.36
1.85 ± 0.52 2.10 ± 0.41 3.50 ± 0.38 4.97 ± 0.16
54.05 ± 3.35 50.21 ± 0.20 72.70 ± 1.15 76.01 ± 5.42
2.60 ± 0.28 3.00 ± 0.56 5.65 ± 0.12 6.10 ± 0.70
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In this work, biocomposites based on epoxy containing different amounts of wool felts (up to 40 vol.%) were manufactured and their morphological and mechanical properties (tensile, flexural and impact resistance) investigated. Moreover, the role played by the hybridization with jute fibres, according to two different stacking sequences, on the mechanical behaviour of the resulting composites has been clarified. The mechanical tests have been monitored by acoustic emission (AE) in order to get more information about the failure mechanisms. 2. Materials and methods 2.1. Materials and composite manufacturing In this study, two different types of jute/wool felt hybrid composites, produced by hand lay-up technique in a closed mould of dimensions 260 160 mm at room temperature using an epoxy resin (Mates SX10 ver.2 with hardener SX14), were compared. Each laminate was cured under a slight pressure of 0.02 bars for 2 h. The laminate was then removed from the mould and further cured at room temperature for at least 24 h before use. In particular, one (sandwich hybrid, labelled as J2W) was obtained by sandwiching two wool felt layers of areal weight 700 g/m2 between two skins made of jute plain weave tissue (hessian cloth) of areal weight 320 g/m2, while the other (intercalated hybrid, labelled as J3W) was obtained by alternating jute (J) plain weave tissue and wool (W) felts in the sequence J/W/J/W/J. Both hybrid configurations were characterized by a total volume of reinforcement of 45%. In addition, two wool-based composites containing different amounts of wool fibres, namely 33 vol.% (W-33) and 40 vol.% (W-40), were manufactured and tested.
Fig. 4. Impact hysteresis cycles (force vs. deflection) for composite W-40.
2.2. Composite testing From the laminates were cut the specimens for the mechanical characterization. Three-point bending tests were performed in accordance with ASTM: D790. Five specimens for each composite type were tested, having the following dimensions: 170 mm 25 mm 4 mm (L W t). A span-to-depth ratio of 25:1 and a cross-head speed of 2.5 mm/min were used. The strain needed for the evaluation of flexural modulus was measured by bonding strain gauges (K-LY41-3/120 by HBM GmbH, nominal resistance = 120 X, gauge factor = 2.1) to the tensile surface of the beam. The composites were also subjected to tensile testing according to ASTM: D3039. Here too, five flat composite specimens of dimensions 170 mm 25 mm 4 mm (L W t) were tested for each type of laminate. The tensile tests were performed at a cross-head speed of 2.5 mm/min. The specimens were gripped between flat mechanical wedge grips. The longitudinal strain was measured using an extensometer with 20 mm gauge length. The mechanical characterization was performed on a Zwick/Roell Z010 universal testing machine equipped with a 10 kN load cell. Low velocity impact tests were carried out using an in-house built drop-weight impact tower at DIEM – Università di Bologna. The impactor diameter was 12.7 mm and its mass was 1.25 kg, which was dropped from three different heights, being equal to 0.5, 0.65 and 0.75 m, so to give three impact energies of approximately 6, 8 and 9 J, respectively. Sampling frequency of the signals was 100 kHz with no external filtering. The variables measured on the drop-weight tower were: Contact force, measured using a piezoelectric load-cell for dynamical loading. Velocity, measured through elaboration of the signal supplied by a Laser sensor, placed at a height exceeding by approximately 50 mm that of the sample.
Fig. 5. Impact hysteresis cycles (force vs. deflection) for composite J2W.
Fig. 6. Impact hysteresis cycles (force vs. deflection) for composite J3W.
Impactor speed and position as a function of time obtained by double numerical integration of force signal (as suggested by ASTM: D7136 standard).
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Samples are supported between two steel plates with a central circular opening (diameter 76 mm), realised in accordance with ASTM: D3763 standard, at the centre of which the impact event takes place.
2.3. Acoustic emission (AE) The mechanical tests were monitored by acoustic emission until final fracture occurred using an AMSY-5 AE system by Vallen Systeme GmbH (Germany). The AE acquisition settings used throughout this experimental work were as follows: threshold = 35.1 dB, Rearm Time (RT) = 0.4 ms, Duration Discrimination Time (DDT) = 0.2 ms and total gain = 34 dB. Two broad-band (100– 1500 kHz) PZT AE sensors were used. The sensors were placed on the surface of the specimens at both ends to allow linear localization with silicone grease for coupling.
2.4. Water absorption tests Water absorption tests were performed in accordance with ASTM: D570. Three specimens were oven dried at 105 °C for 24 h. The conditioned specimens were immersed in distilled water for 2 h, 24 h and 48 h at a temperature of 23 ± 2 °C. At the end of the preset time, the specimens were removed from the water, all surface water was wiped off using a cloth, then were weighed. After 2 h immersion, the specimens were replaced in water and weighed again after 24 h and 48 h. The percentage increase in weight (W) during immersion was calculated as follows (Eq. (1)):
Wð%Þ ¼
mt m0 100 m0
ð1Þ
where m0 and mt are the conditioned and wet weight (at time t), respectively.
Fig. 7. Front and rear surfaces of impacted W-40 composites.
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2.5. Optical and Scanning Electron Microscopy (SEM) The fracture surfaces of composites were investigated using a scanning electron microscope (Philips XL40). All specimens were sputter coated with gold prior to examination. The optical micrographs of the composites were taken by Nikon Eclipse 150 L on specimens polished to 1 lm diamond finish. The porosity content of the composites was determined by processing micrograph images of the laminates. The composite samples were mounted using the EpoThin castable mounting system by Buehler. The mounted samples were mechanically polished up to 1 lm diamond finish. The polished mounts were then placed on a microscope Nikon Eclipse 150 L where digital micrographs were captured and analysed by two-dimensional image processing method to determine the porosity. The volume porosity was determined from a series of measurements taken on different areas of the laminates. To get sufficiently accurate results, 20 optical areas were analysed. The purpose of the image processing by grey level thresholding is to filter out the fibre and resin rich areas from the image while isolating the porosity. Lucia measurements soft-
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ware was used to process the images from their original state to a binary version where black and white pixels could be counted to obtain an area of porosity measurement.
3. Results and discussion 3.1. Tensile and flexural behaviour Fig. 1 shows the typical microstructure of a J2W composite. From the optical micrograph it is evident that wool fibres are randomly distributed and that voids are present particularly in those regions, where wool felts are concentrated. As expected, the void content was higher in wool-based composites (about 7%) than in hybrid ones (about 4%). This behaviour can be ascribed to the difficulty in achieving an effective impregnation of wool felts through a simple and inexpensive process like hand lay-up. Typical stress–strain curves of wool and hybrid composites are presented in Fig. 2. The different composites show a similar tensile behaviour. An observed continuous improvement in the composite
Fig. 8. Front and rear surfaces of impacted J2W composites.
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Fig. 9. Front and rear surfaces of impacted J3W composites.
Table 3 Water absorption of wool and jute/wool hybrid composites. Composite sample code
Water absorption (%) 2h
W-33 W-40 J2W J3W
0.86 1.95 2.03 2.93
24 h a
(0.03 ) (0.09) (0.62) (0.47)
1.77 4.19 4.93 6.70
48 h (0.08) (0.18) (0.48) (0.49)
2.17 5.37 5.70 7.80
(0.11) (0.21) (0.44) (0.57)
a Values are average of three replicates and values in parentheses are standard deviations.
stiffness and strength is measured with the increase in fibre volume fraction of jute fibres, as summarized in Table 1. This behaviour can be ascribed to both the higher mechanical properties of lignocellulosic fibres (jute) compared to those of protein-based ones (wool) [2,26] and to the higher void content of wool-based composites. Moreover, it is well known that tensile properties of
composites are strongly dependent on the strength and modulus of fibres, on their length and orientation, on the fibre volume fraction as well as on the interfacial adhesion between fibre and matrix. Interfacial adhesion plays a fundamental role in determining the mechanical properties of composites and it depends on three main factors: mechanical interlocking, physical attractive forces (hydrogen bonds, van der Waals forces) and chemical bonds between fibre and matrix. The surface of natural fibres has many hydroxylic groups which enable the formation of hydrogen bonds with the hydroxylic groups of the backbone chain of the matrix (epoxy resin). In this case, it is to be expected some degree of compatibility between jute fibres and epoxy matrix [27] and some mechanical interlocking due to the fact that jute technical fibres i.e., with diameter and stiffness sufficient to use them for textile purposes, are actually bundles of elementary fibres, and therefore with a quite irregular and rough surface. Wool fibres show a more uniform shape with a relatively smooth surface, which is not suitable for a high degree of mechanical interlocking. As regards their
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Fig. 10. SEM micrographs of fracture surface of composite W-40.
Fig. 11. SEM micrograph of fracture surface of composite W-40.
Fig. 12. SEM micrograph of fracture surface of composite J2W.
chemical composition, wool proteins are composed of amino acids, which contain basic amino (NH2) and acidic carboxyl (COOH) groups. Individual amino acids differ from each other in the nature of the side groups which vary in size and can be grouped, according to their chemical properties: hydrophobic, hydrophilic, acidic, basic and amino acids that contain sulphur [24]. On the grounds of their structure, wool fibres can show variable proportions of hydrophobic and hydrophilic character, as it is the case for other keratin fibres, such as those obtained from chicken feathers, which contain approximately 60% hydrophobic and 40% hydrophilic amino acids in their amino acid sequence [13].
A further parameter to be considered is the stacking sequence, which can significantly affect the mechanical behaviour of hybrid laminates [28]. The tensile strength tends to increase when the stronger material is used as skin in a hypothetical sandwich structure. The high tensile strength of jute fibres in the outer layers enables increasing stresses to be borne, whereas the wool-based core tends to take up the stresses and to distribute them evenly throughout the composite. The strength of laminates tends to increase with increasing volume fraction of jute fibres. A similar behaviour occurred for composites tested in bending, as can be clearly seen in Fig. 3, which show the typical flexural force vs. displacement curves for the different types of composites. Also in this case, the presence of jute fibres involves a considerable improvement in both strength and stiffness (Table 1). These properties are determined by the strength of the outer layers of reinforcement [29]. The presence of jute implies also a change in the failure mode which appears to be sharp and abrupt, while it proceeds gradually for wool fibre reinforced composites. All composites tested have shown failure in the tensile side without experiencing complete separation thus pointing out, as a whole, a brittle behaviour. The increase in wool content seems to positively affect only the modulus of elasticity, while it appears to be less influent on the flexural strength. This finding can be ascribed to the higher void content due to the difficulty in impregnating increasing amounts of wool fibres. Jute has been one of the most important natural fibres used as reinforcement in composites and therefore a great deal of literature is available on the mechanical characterization of jute reinforced composites. To allow for a comparison, it is worth noting nonetheless that results are significantly influenced e.g., by the type of jute fabric used, the manufacturing process and fibre content. In a recent work, woven jute (400 g/m2)/epoxy composites, fabricated by hand lay-up with a fibre volume fraction of 0.45, have been characterized both in tensile and in bending [30]. Composite tensile strength was 55 MPa, Young’s modulus was 10 GPa, flexural strength was 102 MPa and flexural modulus was 4.7 GPa. Venkateshwaran and ElayaPerumal prepared by hand lay-up composites with a total weight fraction of 0.25, finding a tensile strength of 25 MPa for jute fibre composites and of 40 MPa for all banana fibre composites [31]. The trilayer banana/jute/banana composite showed better tensile strength and modulus of 54.60 MPa and 13.69 GPa, respectively, and higher flexural strength and modulus of 91.66 MPa and 9.44 GPa, respectively. Abdallah et al. investigated jute/epoxy laminate manufactured by the infusion process with three lay-up sequences, namely [0]S for the laminate, [90]S for warp, and [+45/45]S for weft. They obtained tensile strengths in the range 38–51 MPa and Young’s moduli in the range 4.5–5 GPa, while the flexural properties varied in the ranges 80–83 MPa and 3.2 GPa for strength and modulus, respectively [32]. Unfortunately, no information was provided on the jute
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Fig. 13. SEM micrographs of fracture surface of composite J2W at different magnifications.
Fig. 14. SEM micrographs of fracture surface of composite J3W at different magnifications.
content in the composites. Santulli tested plain woven jute fabric/ polyester laminates manufactured by RTM with a fibre fraction of 0.63 by volume [33]. The resulting laminates showed a tensile strength and modulus of 61.13 MPa and 5.61 GPa, respectively. De Rosa et al. manufactured by hand lay-up jute (300 g/m2)/epoxy composites with a fibre weight fraction of 0.35 [34]. The composites yielded a tensile strength and modulus of 70 MPa and 6.5 GPa, respectively, while the flexural properties in three point
bending were 95 MPa and 7 GPa for strength and modulus, respectively. These results appear to indicate that the materials investigated in the present study exhibit mechanical properties which are comparable with those of competing material systems, even though they need to be improved by selecting suitable surface treatments and optimized manufacturing techniques, as confirmed by other authors for both thermosetting [22] and thermoplastic [23] matrices.
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Fig. 15. Stress-time- AE amplitude plot for the different composites.
Fig. 16. Typical localization plot of AE signals for a tensile test (left) and a three-point bending test (right).
3.2. Low velocity impact behaviour Low velocity impact is a useful test to investigate the effect produced by accidental damage during service of polymer composites, which may result in a significant reduction of the local resistance of the laminate and in a consequent shorter lifespan for the component [35]. In Table 2, the maximum contact force and the unrecoverable absorbed energy, which is the maximum energy the material can transform, are reported [36]. These variables are obviously growing with nominal impact energy applied. While the highest maximum contact force is offered by the wool-based composite W-40 except for 9-Joule impact, the largest energy is ab-
sorbed during impact on the intercalated hybrid J3W, most likely due to the presence of more interfaces, as previously suggested [37]. When looking at the respective hysteresis cycles for the various laminates (Figs. 4–6) it can be noticed that the ascendant part of the curve presents very different patterns between the different samples and is very far from linearity. This can be attributed to the variable void content in the laminates, which results in different local properties and in unpredictable vibration behaviour under impact loading. As regards the damage patterns, these appear to be very similar among the different laminates (Figs. 7–9). On the impacted surface only a local indentation due to the impactor
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was revealed, while on the opposite surface the presence of cracks with a cross-like pattern can be observed with no obvious signs of delamination or fibre fracture. 3.3. Water absorption behaviour Results of water absorption are given in Table 3. Water absorption increases with higher fibre content in the composites, a trend that is observed for both types of reinforcement and for 2 h, 24 h and 48 h water immersion tests. With the increase in natural fibre content there are more sites for water absorption (free hydroxyl groups in cellulose and hemicelluloses). Water absorption in composites can be mainly ascribed to the presence of lumens and hydrogen bonding sites in the jute fibres and wool felts, the gaps at the interface between matrix and reinforcement and porosity [38,39]. This is a well known behaviour and to reduce the moisture absorption, the fibre has to be modified chemically and physically. Several studies have shown that moisture uptake can be reduced by the use of suitable coupling agents and fibre surface treatments, such as alkali treatment, acetylation, permanganate treatment, maleic anhydride treatment, acrylation and so forth [40–43]. An additional solution to this problem can be represented by the reduction of porosity obtained improving the manufacturing process. 3.4. Morphological analysis The mechanical characterization has been supported by a morphological analysis of fracture surfaces through scanning electron microscopy. Fig. 10 shows a SEM micrograph and related detail of the fracture surface of a wool fibre reinforced composite
(W-40). The fracture surface appearance indicates a macroscopically brittle behaviour with a significant presence of debonding and pull-out phenomena (Fig. 11). All the observations confirm a weak adhesion between fibre and matrix. The lack of compatibility between fibre and epoxy matrix is also evident from the observation of the surface of fibres pulled out from the matrix: the surface appears clean without any resin particles sticking to it (Figs. 10 and 11). Another feature to be noted is the fracture surface of wool fibres, which is sharp without occurrence of significant fibrillation, often related to effective stress transfer across the interface. All these considerations explain the low mechanical properties of wool fibre reinforced composites. The hybridization with jute fibres involved a significant improvement in mechanical properties. This behaviour can be ascribed to the better degree of interfacial adhesion between jute and epoxy resin, as confirmed by the observation of Figs. 12–14, which are related to the composites J2W and J3W. The fracture appearance is still brittle, as pointed out by the river lines in epoxy matrix (Fig. 14c) [44]. Compared to wool fibres, jute fibres seem to show a better compatibility with the matrix. Even though pull-out phenomena are present, these appear to be less significant, as shown in Figs. 12 and 13a–c and 14a and b. The length of pull-out is shorter for jute fibres and there is no direct evidence of voids related to the complete pull-out of warp or weft filaments of the fabric. The surface of jute fibres appears less clean than that of wool fibres (Fig. 14d) and the fracture surface of jute fibres is also different. In fact, in Fig. 15d, the occurrence of fibrillation phenomena it noteworthy and the fracture surface of fibres is not flat, as was the case for wool fibres. In this regard, Fig. 13d shows a clear axial splitting of the fibre, while Fig. 14d shows the cross section of a jute fibre, in which different parts are on staggered planes. All these observations confirm that a
Fig. 17. AE amplitude vs. duration plot, for the different composites.
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Fig. 18. Typical amplitude distribution for a (a) tensile test and for a (b) flexural test of a W-40 composite.
sufficient stress transfer at the interface occurred, and that the jute fibres have carried out their reinforcing action. 3.5. Acoustic emission monitoring The time evolution and nature of different failure modes of composites have been studied through the analysis of acoustic emission signals recorded in real time during mechanical tests. Fig. 15 shows the evolution of AE amplitude as a function of applied stress during tensile tests. As a whole, AE events are associated with damage modes of the material and therefore the presence of acoustic activity during loading indicates the occurrence of damage. Fig. 15 indicates also that AE events take place immediately after the onset of loading. This behaviour suggests that composites suffer from early damage, as the number of signals increases with increasing applied load. This trend is common to all types of composites investigated. A difference between wool-based composites and the hybrids is due to the type of AE signals. In wool-based composites (Fig. 15a and b) AE signals are characterized by amplitudes in the range 35–70 dB. In literature, the amplitude range 35–55 dB is usually associated to damage modes
occurring in the matrix (microcracks) and to friction among the new created surfaces, while amplitudes belonging to the range 55–70 dB are ascribed to interfacial damage [45–47]. However, interface failure is never isolated as a damage mechanism, being very often linked to pull-out mechanisms instead. It is worth noting that these two amplitude ranges are not well separated and often overlapping occurs. Despite that, it is clear that matrix cracking and interface failure are present during tensile loading of wool reinforced laminates. This analysis confirms the results yielded from the morphological analysis of fracture surfaces, where the presence of extensive pull-out was clearly apparent. The microcracks in the matrix can be also triggered by high void contents. Moreover, few signals of high amplitude (75–100 dB) and long duration are present, which suggests that the final failure of the composite is not likely to be due to fibre breakages, but to the accumulation of other phenomena. AE activity for hybrid composites shows also an increased number of signals and localized events. Technical jute fibres represent complex systems and the amplitude distribution of events associated to their failure can be described as follows: longitudinal splitting of the non-cellulosic boundary layer among the elementary fibres (AE amplitude < 40 dB); transverse
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cracking of the elementary fibre (40–60 dB); fracture of elementary fibres and their microfibrils (>60 dB) [48]. These phenomena occur during the test and are superimposed to those due to the fibre/matrix interface. This can explain the larger number of AE events in the case of hybrid composites. AE allows also for the localization of the irreversible damage sources. An example of such localization is reported in Fig. 16a for a tensile test. It is to be noted that the AE sources are distributed over the whole gauge length of the specimens, thus conforming that a large volume of material is stressed during a tensile test. In contrast, during flexural loading, a higher number of localized AE events were recorded for flexural tests. This outcome is to be ascribed to the fact that the damage zone is much more localized in a three-point bending test, thus causing less AE signal attenuation. In addition, during a bending test, the material is subjected to a complex state of stress involving tensile, compressive and interlaminar stresses. A confirmation of the narrow damage zone can be obtained from Fig. 16b, which reports a typical localization plot for a flexural test. The characteristics of AE signals (Fig. 17) in bending are similar to those obtained for tensile tests, even though an increase of signals showing medium–high amplitudes (60–70 dB) and long durations (>400 ls), especially for hybrids, was recorded. These signals are associated to interface failures (delaminations), which begin to play an important role in determining the final failure of composites in bending. The energy of signals in bending was higher than that of signals in tensile test. The importance of interface failures in bending can also be inferred from the comparison between amplitude distributions of tensile and bending tests (Fig. 18). Here, a shift of signal amplitudes towards higher values (60–70 dB), attributed to interface failures, can be noted, when compared to tensile tests, where most signals lie in the ranges typical of debonding and matrix microcraks (35–50 dB). In general, the results demonstrate that these hybrids can offer some potential, in principle allowing the application of quite significant stresses on the skins, being then distributed through the core. This is justified by the fact that mechanical performance, in spite of the simple method employed for laminate fabrication, is not much lower than what found on similar materials, as discussed above. However, this potential can prove valuable for semi-structural applications, only in case problems linked to the not very effective fibre/matrix interface link are solved, and a substantial reduction of void content is achieved during their production.
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