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flax twill laminates
Composites Part B 153 (2018) 17–25
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Impact behaviour of hybrid basalt/flax twill laminates I. Papa a b c
a,1
, M.R. Ricciardi
b,1
, V. Antonucci
b,∗
c
, V. Pagliarulo , V. Lopresto
T a
Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy Institute for Polymer, Composites and Biomaterials, National Research Council, Piazzale Enrico Fermi, 1, 80055, Portici, Italy National Council of Research - Institute of Applied Sciences & Intelligent Systems (ISASI) “E. Caianiello”, Pozzuoli, Naples, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid laminates Low velocity impact Basalt
The purpose of hybridization is to obtain a new material preserving advantages from all of its constituents. Hybridization offers intermediate properties respect to the original materials, by creating a balance effect within the fibres incorporated in the composite materials and leading to a composite with more tailored behaviour The increasing need to mitigate the environmental impact of synthetic fibres and polymers is promoting the use and application of natural materials orienting the research toward the development of biodegradable systems. In this framework, hybrid reinforced laminates with flax and basalt twill layers alternatively stacked, were manufactured by resin infusion fabrication technology and impacted at low velocity to investigate their dynamic behaviour, in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. For comparison purposes, the same experimental characterization has been performed on laminates reinforced with only basalt or flax fibres. The experimental results confirmed the positive role played by fibre hybridization in terms of damage. The Electronic Speckle Pattern Interferometry technique was adopted to analyze the internal damage and to provide information on the shape and the extent of the delamination, that was found concentrated under the impactor-material contact point for the basalt and flax/basalt laminates.
1. Introduction
(layers). Thus, there are several types of hybrid composites. In line with Kretsis [10], hybrid composites include: sandwich structures, where one material is sandwiched between two layers of another; interply (layer-by-layer) hybrids, in which layers of two (or more) fibers are stacked alternately in a regular manner; intraply (yarn-by-yarn) hybrids, where tows of two (or more) fiber types are mixed in a regular or random manner, intimately (fiber-by-fiber) mixed hybrids, in which the constituent fibers are mixed as randomly as possible [11]. Hybridization offers intermediate or better properties respect to the original materials, by creating a balance effect within the fibres incorporated in the composite materials [12,13] and taking advantages from the different properties of different stacked fibres and leading to a composite with more tailored behaviour in order to meet the requirements of the final structure. Moreover, it is possible to lower the costs since some reinforcements are more expensive than others [14–16]. The increasing need to mitigate the environmental impact of synthetic fibres and polymers is promoting the use and application of natural materials orienting the research toward the development of biodegradable systems.
During their service life, composites are subjected to various loading conditions of which low velocity impact is one of the most critical, especially for aerospace composite structures [1–6]. Due to their high specific stiffness and strength, carbon fibre reinforced polymer composite is the preferred material in aerospace industry. However, the toughness of carbon fibre is quite low and the resulting damage resistance is poor. In this regard, several approaches have been successfully exploited to enhance the impact damage resistance of composite laminates. One approach is known as hybridization, usually with high strain to failure fibres to improve the damage resistance of composites to both low velocity and instrumented Charpy impact [7–9] and to lower the materials costs. The purpose of hybridization is to obtain a new material preserving advantages from all of its constituents. Composites are themselves hybrid materials, but the term “hybrid composites” relates to composites including more than one type of fiber into one single matrix with a mixing level being on a small scale (fibers, tows) or on a large scale
∗
Corresponding author. E-mail addresses: [email protected] (I. Papa), [email protected] (M.R. Ricciardi), [email protected] (V. Antonucci), [email protected] (V. Pagliarulo), [email protected] (V. Lopresto). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.compositesb.2018.07.025 Received 24 April 2018; Received in revised form 5 July 2018; Accepted 19 July 2018 Available online 19 July 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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The inherent low resistance to low velocity impacts of natural fibre reinforced composites is still limiting their applications in semi-structural applications, as impact damage is well known to reduce significantly the mechanical properties of composite laminates [17]. The impact behaviour of these materials depends on many factors the most important of which are the stacking sequence and the architecture along with the thickness [17,18]. In the field of fibrous reinforcement, basalt fibres have gained an increasing attention in recent years as possible replacement of the conventional glass fibres [19] due to their advantages in terms of environmental cost and chemical–physical properties [20]. The mechanical properties of basalt fibre reinforced composite laminates have been thoroughly investigated for both thermoset [21,22] and thermoplastic matrices [24–27], but only limited attention has been devoted to the low-velocity impact behaviour of these class of composites [28–32]. Lopresto et al. [27] provided a thorough investigation of the mechanical properties of basalt/epoxy composites, studying also the impact resistance. A more detailed investigation of the effect of hybridization of basalt fibres on low velocity impact response of glass fabric reinforced epoxy composites was performed in Refs. [23,33]. Recently research has been also performed on hybrid composites made of basalt and organic and ductile fibres [13,29,31,34–37]. In particular, bio-sourced materials reinforced with vegetal fibres, such as flax, hemp, jute and sisal have gained popularity due to sustainable development requirements and cost-effectiveness [38]. Yan et al. [38] suggested that, when considering mechanical performance, cost and yield, flax, hemp and jute are the most promising bio-fibres that can be used instead of glass fibres in composite materials. Recent studies [34] have also confirmed the high potential of vegetal fibre reinforced composites. Flax fiber reinforced composites exhibited the highest impact energy absorption among natural fiber reinforced composites. In addition, some authors [13,37] have shown that the combination of a flax reinforcement with basalt fibers lowers the brittleness of the basalt and has a significant effect on the propagation of damage during the impact. Recently, two very interesting papers [37,51] were published about the impact behaviour of basalt/flax hybrid composite laminates immersed in vinylester resin. In particular, Fragassa et al. [37] demonstrated an improvement of impact performance by composites having a flax core between basalt fibre skins and evidenced the requirement to explore more complex stacking sequences with intercalation of flax and basalt layers inside the laminate. Thus, to explore their mutual influence on the impact behaviour, in this study flax and basalt fiber reinforcements in the twill form have been hybridized and intercalated: core of flax thick four layers and two external layers of basalt fibre instead of eight basalt and flax layers alternatively stacked here. However, similar results were obtained about the shape of the load curves as hereafter will be highlighted. In addition to these papers, beside the different resin, in the present research morphological investigations as well as a deep analysis and comparison on the different laminates investigated of the main impact parameters involved in the dynamic response of the laminates (laminate thickness, maximum load, impact, absorbed and penetration energy), were done. And most important, it was demonstrated the efficiency of an innovative NDT, ESPI, to investigate the onset and propagation of the damage where the most common US technique was revealed not able to highlight the internal damage since the signal absorption by flax fibers. The latter technique was not applied before on composite laminates. In particular, hybrid laminates, reinforced with flax and basalt twill layers alternatively stacked, were manufactured by vacuum resin infusion and impacted at low velocity to investigate their dynamic behaviour in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. For comparison purposes, the same experimental characterization has been performed on laminates reinforced with the single type fibers, i.e. only basalt or
Table 1 Characteristics of the reinforcements. Reinforcement 3
Density (g/cm ) Specific surface weight (g/m2) Weave type Thickness (mm)
Flax
Basalt
1,27 222,1 Twill 2/2 0,225
2,67 220 Twill 2/2 0,13
flax fibres. The experimental results confirmed the positive role played by fibre hybridization. In fact, the hybrid flax composites showed an intermediate behaviour the single basalt and flax composites, resulting more resistant to impact than flax laminate and more capable than the basalt laminate to absorb the impact energy through not elastic mode and the deflect the impact progression. 2. Materials and experimental methods Basalt twill woven fabric with fibre areal weight of 220 g/m2 (BAS220,1270,T), supplied by Basaltex NV and a flax twill woven fabric (FlaxPly BL200) with fibre areal weight of 200 g/m2 supplied by Lineo, were used as reinforcements. Table 1 shows the characteristics of both reinforcement as reprted by the datasheet. The investigated reinforcements have been impregnated by the two-component commercial resin epoxy PRIME™ 20LV (100:26 resin/hardener weight ratio) formulated for infusion system by Gurit. PRIME™ 20LV has reduced viscosity (600–640 cP) and longer working time, which makes it ideal to impregnate very large parts with complex reinforcements in one-shot operation. It maintains the exceptionally low exothermic characteristic, which allows thick sections to be manufactured without risk of premature gelation due to the heat of exothermic reaction. Three different laminates, reinforced with basalt, flax and hybrid basalt/flax fabric, have been manufactured by stacking 16 plies of reinforcement. In particular, the hybrid one was realized by alternating 8 basalt and 8 flax fabric layers according with a symmetric and balanced configuration and placing externally the basalt layers in order to ensure higher impact resistance. The final stacking sequence was [B, F]8,s for the hybrid laminate. All composite panels have been produced by vacuum infusion process that basically involves three steps [39]: lay up of a fiber preform, vacuum application and fiber impregnation by a thermoset resin, cure of the resin. The reinforcement is placed between one-sided rigid mold and a formable vacuum bag material. The resin is injected from an input channel. Vacuum is applied through a single or multiple vents in order to remove the air from the fiber preform and to drive the fiber impregnation by resin. A resin distribution net medium is placed onto the reinforcement to promote the resin flow allowing the complete wetout of the preform and eliminating voids and dry spots. After infusion, the panels were cured for 16 h at 50 °C according with the technical datasheet. The manufactured composite panels showed different characteristics in terms of thickness and fiber volume fraction as reported in Table 2. The different thickness is due to the different resin absorption. In low velocity impact phenomenon the content of resin does not influence the response of the laminates. On the other hand, in literature it is largely reported that the fibre content is the fundamental parameter governing the impact behaviour. Thus, the investigated laminates were Table 2 Characteristics of the manufactured composites.
18
Property
Basalt
Flax
Flax/Basalt
Thickness, mm Fiber volume fraction Density, g/cm3
2.7 0.69 1.87
8.1 0.47 1.05
5.7 0.61 1.34
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coupled into an optical fiber reaching the camera sensor where also the light scattered from the object is addressed. Before reaching the object, the object beam passes through an electronically tilted etalon to shift the probing beam laterally in a parallel direction. By such way, it is possible to obtain four equivalent images of the speckle pattern, which are used to perform an average to reduce the speckle noise. Upon an external perturbation, the object is deformed and the reflected wavefront is slightly changed respect to the initial state, while reference beam remains unperturbed. Thus, the camera records a new speckle pattern and the subtraction of the two registered speckle patterns (deformed and non-deformed states) provides the correlation fringes. When, the subtraction of the fringes patterns is computer-aided, the technique is called electronic. The correlation fringes allow obtaining the phase-contrast maps in grayscale (at 640 × 480), from which it is possible to measure the displacement field quantitatively. The calculation of phase-contrast map is obtained by using well-known numerical processing, consisting into noise reduction and demodulation of the fringe pattern image, which provides a wrapped phase image, and the subsequent unwrapping step [43].
obtained by stacking the same number of layers of different fabrics that have been selected with the same areal weight in order to obtain comparable conditions in terms of fiber content by weight. However, due to the different reinforcement porosity and compatibility with the epoxy resin, the final composites resulted with different resin content and fiber volume fraction. The morphology and fracture surface of the investigated composites were studied by using a Quanta 200 FEG (FEI company, Eindhoven, the Netherland) Scanning Electron Microscopy (SEM) and the sputter coater Emitech K575X for the sample preparation. The results were, then, processed by an INCA software energy. Impact tests were carried out by a falling weight machine, Ceast Instron, at complete penetration to obtain and study the whole load displacement curve, interesting for giving useful information about the response of the laminates and for investigating the effect of the varied impact parameters. Then different increasing energy levels corresponding to the Fmax at the 35 and 70% of the linear increasing stretch of the curve, were chosen to carry out tests useful to study the damage start and evolution. At least three tests were performed in each test conditions and the data reported in the graphs are the mean values of the obtained results. The rectangular specimens, 100 mm × 150 mm, cut by a diamond saw from the original panels, were supported by the clamping device suggested by the ASTM D7136 Standard [40]. Standard and were centrally loaded by an instrumented cylindrical impactor with a hemispherical nose, 19.8 mm in diameter. The total minimum mass of 3.640 kg, that combined with the drop heights allowed to obtain the selected impact energies, was considered. After the impact tests, the specimens were observed by ESPI technique to investigate the internal damage. A holographic method was used to investigate the internal impact damage. Electronic speckle pattern interferometry (ESPI) technique allows to measure the out of plane displacements of specimen after a stress event and in such way enables to identify cracks, strain and flaws on rough surfaces with high sensitivity in real time and full field modality without contact [41,42]. The detection of micro-deformations is obtained illuminating the sample surface by a visible laser. A ChargeCoupled Device (CCD) camera records the deformation under correlation fringes form, deformation due to a slight warming imposed to the material. The direction of object displacement measurement lies along the bisector of the angle between the illumination beam and the direction of observation by the camera lens. In this configuration the illumination aperture and camera lens are quite close and if the object does not subtend too large an angle, the measurement direction can be considered nearly constant and perpendicular to the sample surface. A typical scheme of the system for recording speckle interferometry measurements is illustrated in Fig. 1. The laser beam is split into a reference beam and an object beam by means of a Beam Splitter (BS), which enables control of the relation between reference beam and object beam. The reference beam is
SEM images have been acquired for each type of composite laminate. Figs. 2–4 show the SEM micrograph for the flax, basalt and hybrid composite respectively. The morphology of flax composite (Fig. 2) is characterized by the presence of fibrils and fibers gropus with not homogenous size that determine some resin accumulation zones. On the other hand, a more homogenous morphology is noticed for the basalt reinforced composite (Fig. 3). In Fig. 4, showing the micrograph of the hybrid composite, it is possible to observe the proper alternation between the two types of layers, the flax reinforcement (yellow label) and the basalt reinforcement (green label) and good adhesion both between the fibers and the matrix. The impact response of the three investigated different composite laminates is shown in Fig. 5, where it is possible to observe that the shape of the three curves at penetration are completely different to each other and, more interestingly, that the curve obtained for the hybrid laminate show intermediate characteristics between the basalt and the flax laminates (Fig. 5). Of course, since at least three tests were carried out in each condition, for clarity and to avoid confusion, since the three curves are completely overlapped, only one of them was reported in the graph. In particular, the response of the hybrid composite does not seem an average of the behaviour of the two single type composites, but rather a superposition of effects. Specifically, with reference to Fig. 5, it is evident that hybrid composite shows the identical trend and linear stiffness of flax composite during the first part of load application, while on
Fig. 1. Optical setup of ESPI.
Fig. 2. Micrograph of flax composite.
3. Results and discussion
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hybrid laminates is larger than that observed for the basalt laminate. This characteristic belongs also to flax composites even if the maximum force value is lower than the others and is related to the capability of the plant reinforced composites to absorb the impact energy through not elastic mode and deflect the impact progression [29]. The basalt composite attains its maximum load showing a linear behaviour with almost constant slope, denoting a small internal damage like matrix cracks, that do not affect its strength. This has been deduced by considering that in classical carbon fibre reinforced plastics, CFRP, and glass fibre reinforced plastics, GFRP, laminates [42–49], the significant and not negligible slope changing or sudden load drops in the first increasing part of the load-displacement curve have been found to correspond to delamination start, propagation and fibre failures [50]. Thus, by comparing the same part of the curve with the impact curve of carbon or glass fibre composites, it is possible to assert that basalt composites were poorly damaged during the impact event. As shown hereafter, this was confirmed by ESPI investigation that evidenced low delamination extensions limited to the impactor material contact point. As expected, flax laminates showed a lower performance in terms of load. However, as it is clear from Fig. 5, the load reaches a first peak with initial rigidity higher than basalt laminates. It decreases a little with a noisy shape and then increases again up to a second peak higher than the first one. Then, it sudden decreases up to zero almost vertically denoting a lack in strength due to a catastrophic damage. The curve recorded for the hybrid laminate showed an intermediate behaviour: looking at Fig. 5, the shape of the load curve is similar to the one recorded on flax specimens even if the forces are higher and the maximum load peak is very close to the one recorded on basalt laminates, and it happens at the same deflection. In other terms, according to diagrams, flax fibers seem to represent a very good solution as impact absorber, ensuring to reduce the peak of stress and adsorbing energy by an inelastic modulated response, conversely basalt reinforcements are also able to resist to higher impact forces. Thus, the hybrid flax/basalt is more proper than the other laminates in the case of short recurring impacts reducing cumulative failures. In Figs. 6 and 7, the comparison between maximum load and penetration energy obtained for the three different material systems, is reported. Very interestingly, both of these two important characteristics are higher for the hybrid laminate. It means that flax/basalt laminate needs more energy to be completely perforated. However, as just observed in Fig. 5, even if the maximum load, Fmax, of flax composite is sensibly lower than that of basalt fibre laminate, the values of penetration energy are very close each other. In Figs. 8–10, the load curves obtained on flax, basalt and flax/basalt fibre laminates respectively at increasing energy levels are reported. The different increasing energy levels corresponding to the Fmax at the 35 and 70% of the linear increasing stretch of the curve, were chosen to carry out tests useful to study the damage start and evolution. In Table 3, the corresponding energy value in [J], for each composite
Fig. 3. Micrograph of basalt composite.
Fig. 4. Micrograph of hybrid basalt flax composite.
Fig. 5. Force-displacement curves: comparison.
the other hand are characterized by a similar behaviour to basalt composites during the long unloaded phase, after attaining the maximum load with a value close to the basalt composite peak. However, the range of displacements in correspondence of the force peak for the
Fig. 6. Comparison about maximum load, Fmax. 20
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Table 3 Impact energy levels, U. Composite system
U [%]
U [J]
Flax
35 70 35 70 35 70
8 16 20 35 30 40
Basalt Flax/Basalt
Fig. 7. Comparison about penetration energy, Upen.
Fig. 11. Maximum load, Fmax, vs impact energy, U.
system, is reported. A closed type curve is obtained for all cases: the samples are not penetrated/perforated by the impactor that rebounds and the area enclosed in the loop of the loading/unloading part of the curve, is just the energy absorbed by the laminate to create damage or to bend, or both, depending on the thickness. As expected, as the impact energy increases, an increase of the maximum load (Fig. 11) and of the absorbed energy, Ua (Fig. 12) is recorded. However, in the case of the hybrid composites, lower differences are noticed (Fig. 10). In Fig. 13, the comparison among all tested samples (U = 35%) is shown: flax and flax/basalt laminates exhibit several load drops before attaining the maximum force, this could represent a higher internal damage in terms of fibre failures respect to the basalt composite. In addition, they have a similar rigidity higher than that of basalt fibre laminates whereas the latter showed the higher deflection in correspondence of the maximum load. In order to get more information on the effect of the impact on the different laminates, Figs. 14–22 show its impacted and rear surfaces after the penetration and the indentation tests. A concentrated damage around the impact point is clear for both the basalt and the hybrid composite on the front surface (Figs. 14a and 16a), while the flax composite (Fig. 15) is characterized by a larger penetrated area with elongated fibre tearing and a more brittle damage. Similarly, tearing is
Fig. 8. Load displacement curves of Flax samples impacted at U = [35, 70]% Up .
Fig. 9. Load displacement curves of Basalt samples impacted at U = [35, 70]% Up .
Fig. 10. Load displacement curves of Flax/Basalt samples impacted at U = [35, 70]% Up. Fig. 12. Absorbed energy, Ua, vs impact energy, U. 21
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Fig. 13. Comparison of load displacement curves for all impacted samples; U = [35]%.
Fig. 16. Photographs of basalt/flax composite after impact penetration: a) impacted surface, b) rear surface.
Fig. 14. Photographs of basalt composite after impact penetration: a) impacted surface, b) rear surface.
Fig. 17. Photographs of flax composite after impact at U = 8 J: a) impacted surface, b) rear surface.
Fig. 15. Photographs of flax composite after impact penetration: a) impacted surface, b) rear surface.
evident on the rear surfaces of flax and hybrid composites. Further, due to the flax reinforcement and its waviness, capable to propagate the crack on larger areas respect to the basalt fibers, different failure modes have been occurred trough the composites, inducing a cross-like damage for the flax laminate, a diamond shape damage for the basalt laminate and a mixed shape for the damage for the basalt laminate and a mixed shape for the hybrid composite. The described damage
Fig. 18. Photographs of flax composite after impact at U = 16 J: a) impacted surface, b) rear surface.
mechanisms on the hybrid configuration are due to the particular stacking sequence where basalt layers were alternated to the flax ones along the thickness, in which basalt fibres behave like a reinforcement, limiting the brittle failures of the flax layers evidenced in Fig. 15 by 22
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Fig. 19. Photographs of basalt composite after impact at U = 20 J: a) impacted surface, b) rear surface.
Fig. 22. Photographs of Flax/basalt composite after impact at U = 40 J: a) impacted surface, b) rear surface.
levels (Figs. 17–22) it is possible to note a different branching of damage on the impacted side at the same impact energy, U. On other hand, even a different involvement of the whole thickness of tested specimen can be imagined by observing the opposite side to impacted one.
the the the the
3.1. Delamination In Fig. 23, the results from the non-destructive investigations by ESPI technique adopted in the present research, obtained on the same specimens impacted at the two different energy levels, were compared. The figure shows the phase-contrast maps for all the types of impacted samples. The phase is directly linked to the out of plane displacement of the samples after a thermal perturbation. The discontinuities of the shape/direction of the fringes allow to highlight discontinuities of the deformations (i.e. areas with an unexpected displacement) thus damaged or delaminated areas. Looking at the flax\basalt it is possible to recognize the shape of the impactor and this means that there is not damage propagation but is concentrated under the impactor-material contact point. On the other side looking at basalt the damaged area appears to be larger and with a lobe shape, showing that the impact energy caused more damages to the laminate. Finally, in the flax case no discontinuities into the fringes pattern can be noticed that means there is no damages or ESPI is unable to detect them. Looking at the latter image in Fig. 23, it may seem that the central spot is referred to damage. However, since these images are not simple to be understood, they are deeply analysed and elaborated to measure the extension of the delamination. In Table 4, the measured delamination areas are reported. Very interestingly, the hybrid laminates showed a lower damage extension respect to the basalt fibre composite materials. Moreover, the two values at 35% and 70% Up are very close to each other (272 mm2 and 290 mm2). This could be due to the effect of flax fibre within the basalt ones. By visually observing the flax fibres laminates loaded by the increasing energy levels not at penetration, it seems that no damage was caused by the impact. The images at penetration reported in Fig. 15 showed a damage on flax specimens apparently larger than the one on hybrid laminates (Fig. 16). However, the damage showed in Fig. 15 is clearly a brittle damage that could explain the influence of the entity of the load in determining the different response of the material. The latter would be insensitive to the low energy values and would show a brittle behaviour when the energy is high enough to cause the catastrophic final failure and it could also explain the highest energy absorbed at penetration, used to create brittle damages in the matrix.
Fig. 20. Photographs of basalt composite after impact at U = 35 J: a) impacted surface, b) rear surface.
Fig. 21. Photographs of flax/basalt composite after impact at U = 30 J: a) impacted surface, b) rear surface.
large fractures on both the front and back side of the laminate. In this way, the influence of both the reinforcements were highlighted leading to a behaviour in the middle between the one evidenced in Fig. 14 and the one reported in Fig. 15. Further, by the analysis of impacted specimens at different energy 23
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Fig. 23. ESPI techniques for all type of impacted samples; U = [35,70]% Up.
hybrid composite seems a superposition of the effects belonging to the basalt and flax single laminates. Thus, the hybrid flax/basalt is more efficient than the other laminates in the case of short recurring impacts, reducing cumulative failures.
Table 4 Delaminated areas measured by ESPI technique.
Basalt Flax Flax/Basalt
A [mm2] U = 35% Up
A [mm2] U = 70% Up
300 – 272
490 30 290
Acknowledgements The authors gratefully acknowledge the ONR Solid Mechanics Program (12174953), in the person of Dr. Yapa D.S. Rajapakse, Program Manager, for the financial support provided to this research. The authors are very gratefully to Eng. Fabrizio Sarasini and its work team for the basalt textile used in this research.
4. Conclusion Basalt, flax and hybrid laminates, obtained by alternatively stacking flax and basalt twill layers, were manufactured by vacuum resin infusion and impacted at low velocity to investigate their dynamic behaviour in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. The results of the experimental tests evidenced the advantages of the fibre hybridization. As a matter of fact, hybrid basalt/flax composite showed better impact performances than those of pure basalt and pure flax laminates, being characterized by the maximum impact stress value due to the external layer of basalt fibers, the highest energy absorption correlated to a lower delamination respect to the one measured on the basalt laminate and the highest penetration energy. Moreover, from the comparison between the load curves at penetration obtained impacting all the different laminates under study, a large plateau around the maximum load due to the presence of the flax layers and their capability to absorb the impact energy through non elastic mode and deflect the impact progression, was observed. By looking at that curves, the response of the
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Compos B Eng 2001;32:565–74. [10] Kretsis G. A review of the tensile, compressive, flexural and shear properties of hybrid fibre-reinforced plastics. Composites 1987;18:13–23. https://doi.org/10. 1016/0010-4361(87)90003-6]. [11] Sarasini Fabrizio. Low-velocity impact behaviour of hybrid composites. In: Thakur Vijay Kumar, Thakur Manju Kumari, editors. Hybrid polymer composite materials: properties and characterisation. Asokan Pappu Woodhead Publishing; 2017. 151+18. [12] Atiqah A, Maleque MA, Jawaid M, Iqbal M. Composites: Part B Development of kenaf-glass reinforced unsaturated polyester hybrid composite for structural applications. vol.56. 2014. p. 68–73. [13] Nisini E, Santulli C, Liverani A. Composites Part B Mechanical and impact characterization of hybrid composite laminates with carbon, basalt and flax fibers. vol.127. 2017. p. 92–9. [14] Safri Syafiqah Nur Azrie, Sultan Mohamed Thariq Hameed, Jawaid Mohammad, Jayakrishna Kandasamy. Impact behaviour of hybrid composites for structural applications: a Review Composites Part B. vol.133. 2018. p. 112–21. [15] Petrucci P, Santulli C, Puglia D, Sarasini F, Torre L, Kenny JM. Materials and Design Mechanical characterization of hybrid composite laminates based on basalt fibres in combination with flax, hemp and glass fibres manufactured by vacuum infusion. vol.49. 2013. p. 728–35. [16] Dehkordi MT, Nosraty H, Shokrieh MM, Minak G, Ghelli D. Low velocity impact properties of intra-ply hybrid composites based on basalt and nylon woven fabrics. Mater Des 2010;31:3835–44. [17] Caprino G, Lopresto V. Composites science and technology. The significance of indentation in the inspection of carbon fibre reinforced plastic panels damaged by low velocity impact. vol.60. 2000. p. 1003–12. [18] Hosur MV, Vaidya UK, Ulven C, Jeelani S. Experimental investigations on the response of stitched/unstitched woven S2-glass/SC15 epoxy composites under single and repeated low velocity impact loading. Compos Struct 2003;61:89–102. [19] Deak T, Czigany T. Chemical composition and mechanical properties of basalt and glass fibers: a comparison. Textil Res J 2009;79:645–51. [20] Fiore V, Scalici T, Di Bella G, Valenza A. A review on basalt fibre and its composites. Compos B Eng 2015;74:74. [21] Manikandan V, Winowlin Jappes JT, Suresh Kumar SM, Amuthakkannan P. Investigation of the effect of surface modifications on the mechanical properties of basalt fibre reinforced polymer composites. Compos B Eng 2012;43:812–8. [22] Szabó J, Czigány T. Static fracture and failure behaviour of aligned discontinuous mineral fiber reinforced polypropylene composites. Polym Test 2003;22:711–9. [23] Gning PB, Tarfaoui M, Collombet F, Riou L, Davies P. Damage development in thick composite tubes under impact loading and influence on implosion pressure: experimental observations. Compos Part B-Engineering 2005;36:306–18. [24] Karger-Kocsis J, Zhang Z, Czigány T. Special manufacturing and characteristics of basalt fiber reinforced hybrid polypropylene composites: mechanical properties and acoustic emission study. Compos Sci Technol 2006;66:3210–20. [25] Zhang Y, Yu C, Chu PK, Lv F, Zhang C, Ji J, et al. Mechanical and thermal properties of basalt fiber reinforced poly(butylene succinate) composites. Mater Chem Phys 2012;133:845–9. [26] Akinci A, Yilmaz S, Sen U. Wear behaviour of basalt filled low density polyethylene composites. Appl Compos Mater 2011;19:499–511. [27] Lopresto V, Leone C, De Iorio I. Mechanical characterisation of basalt fibre reinforced plastic. Compos B Eng 2011;42:717–23. [28] Dehkordi MT, Nosraty H, Shokrieh MM, Minak G, Ghelli D. Low velocity impact properties of intra-ply hybrid composites based on basalt and nylon woven fabrics. Mater Des 2010;31:3835–44. [29] De Rosa IM, Marra F, Pulci G, Santulli C, Sarasini F, Tirillò J, et al. Post-impact mechanical characterisation of E-glass/basalt woven fabric interply hybrid laminates. eXPRESS. Polym Lett 2011;5:449–59. [30] Rosa IM, Marra F, Pulci G, Santulli C, Sarasini F, Tirillò J, et al. Post-impact mechanical characterisation of glass and basalt woven fabric laminates. Appl Compos
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Impact behaviour of hybrid basalt/flax twill laminates I. Papa a b c
a,1
, M.R. Ricciardi
b,1
, V. Antonucci
b,∗
c
, V. Pagliarulo , V. Lopresto
T a
Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy Institute for Polymer, Composites and Biomaterials, National Research Council, Piazzale Enrico Fermi, 1, 80055, Portici, Italy National Council of Research - Institute of Applied Sciences & Intelligent Systems (ISASI) “E. Caianiello”, Pozzuoli, Naples, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid laminates Low velocity impact Basalt
The purpose of hybridization is to obtain a new material preserving advantages from all of its constituents. Hybridization offers intermediate properties respect to the original materials, by creating a balance effect within the fibres incorporated in the composite materials and leading to a composite with more tailored behaviour The increasing need to mitigate the environmental impact of synthetic fibres and polymers is promoting the use and application of natural materials orienting the research toward the development of biodegradable systems. In this framework, hybrid reinforced laminates with flax and basalt twill layers alternatively stacked, were manufactured by resin infusion fabrication technology and impacted at low velocity to investigate their dynamic behaviour, in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. For comparison purposes, the same experimental characterization has been performed on laminates reinforced with only basalt or flax fibres. The experimental results confirmed the positive role played by fibre hybridization in terms of damage. The Electronic Speckle Pattern Interferometry technique was adopted to analyze the internal damage and to provide information on the shape and the extent of the delamination, that was found concentrated under the impactor-material contact point for the basalt and flax/basalt laminates.
1. Introduction
(layers). Thus, there are several types of hybrid composites. In line with Kretsis [10], hybrid composites include: sandwich structures, where one material is sandwiched between two layers of another; interply (layer-by-layer) hybrids, in which layers of two (or more) fibers are stacked alternately in a regular manner; intraply (yarn-by-yarn) hybrids, where tows of two (or more) fiber types are mixed in a regular or random manner, intimately (fiber-by-fiber) mixed hybrids, in which the constituent fibers are mixed as randomly as possible [11]. Hybridization offers intermediate or better properties respect to the original materials, by creating a balance effect within the fibres incorporated in the composite materials [12,13] and taking advantages from the different properties of different stacked fibres and leading to a composite with more tailored behaviour in order to meet the requirements of the final structure. Moreover, it is possible to lower the costs since some reinforcements are more expensive than others [14–16]. The increasing need to mitigate the environmental impact of synthetic fibres and polymers is promoting the use and application of natural materials orienting the research toward the development of biodegradable systems.
During their service life, composites are subjected to various loading conditions of which low velocity impact is one of the most critical, especially for aerospace composite structures [1–6]. Due to their high specific stiffness and strength, carbon fibre reinforced polymer composite is the preferred material in aerospace industry. However, the toughness of carbon fibre is quite low and the resulting damage resistance is poor. In this regard, several approaches have been successfully exploited to enhance the impact damage resistance of composite laminates. One approach is known as hybridization, usually with high strain to failure fibres to improve the damage resistance of composites to both low velocity and instrumented Charpy impact [7–9] and to lower the materials costs. The purpose of hybridization is to obtain a new material preserving advantages from all of its constituents. Composites are themselves hybrid materials, but the term “hybrid composites” relates to composites including more than one type of fiber into one single matrix with a mixing level being on a small scale (fibers, tows) or on a large scale
∗
Corresponding author. E-mail addresses: [email protected] (I. Papa), [email protected] (M.R. Ricciardi), [email protected] (V. Antonucci), [email protected] (V. Pagliarulo), [email protected] (V. Lopresto). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.compositesb.2018.07.025 Received 24 April 2018; Received in revised form 5 July 2018; Accepted 19 July 2018 Available online 19 July 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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The inherent low resistance to low velocity impacts of natural fibre reinforced composites is still limiting their applications in semi-structural applications, as impact damage is well known to reduce significantly the mechanical properties of composite laminates [17]. The impact behaviour of these materials depends on many factors the most important of which are the stacking sequence and the architecture along with the thickness [17,18]. In the field of fibrous reinforcement, basalt fibres have gained an increasing attention in recent years as possible replacement of the conventional glass fibres [19] due to their advantages in terms of environmental cost and chemical–physical properties [20]. The mechanical properties of basalt fibre reinforced composite laminates have been thoroughly investigated for both thermoset [21,22] and thermoplastic matrices [24–27], but only limited attention has been devoted to the low-velocity impact behaviour of these class of composites [28–32]. Lopresto et al. [27] provided a thorough investigation of the mechanical properties of basalt/epoxy composites, studying also the impact resistance. A more detailed investigation of the effect of hybridization of basalt fibres on low velocity impact response of glass fabric reinforced epoxy composites was performed in Refs. [23,33]. Recently research has been also performed on hybrid composites made of basalt and organic and ductile fibres [13,29,31,34–37]. In particular, bio-sourced materials reinforced with vegetal fibres, such as flax, hemp, jute and sisal have gained popularity due to sustainable development requirements and cost-effectiveness [38]. Yan et al. [38] suggested that, when considering mechanical performance, cost and yield, flax, hemp and jute are the most promising bio-fibres that can be used instead of glass fibres in composite materials. Recent studies [34] have also confirmed the high potential of vegetal fibre reinforced composites. Flax fiber reinforced composites exhibited the highest impact energy absorption among natural fiber reinforced composites. In addition, some authors [13,37] have shown that the combination of a flax reinforcement with basalt fibers lowers the brittleness of the basalt and has a significant effect on the propagation of damage during the impact. Recently, two very interesting papers [37,51] were published about the impact behaviour of basalt/flax hybrid composite laminates immersed in vinylester resin. In particular, Fragassa et al. [37] demonstrated an improvement of impact performance by composites having a flax core between basalt fibre skins and evidenced the requirement to explore more complex stacking sequences with intercalation of flax and basalt layers inside the laminate. Thus, to explore their mutual influence on the impact behaviour, in this study flax and basalt fiber reinforcements in the twill form have been hybridized and intercalated: core of flax thick four layers and two external layers of basalt fibre instead of eight basalt and flax layers alternatively stacked here. However, similar results were obtained about the shape of the load curves as hereafter will be highlighted. In addition to these papers, beside the different resin, in the present research morphological investigations as well as a deep analysis and comparison on the different laminates investigated of the main impact parameters involved in the dynamic response of the laminates (laminate thickness, maximum load, impact, absorbed and penetration energy), were done. And most important, it was demonstrated the efficiency of an innovative NDT, ESPI, to investigate the onset and propagation of the damage where the most common US technique was revealed not able to highlight the internal damage since the signal absorption by flax fibers. The latter technique was not applied before on composite laminates. In particular, hybrid laminates, reinforced with flax and basalt twill layers alternatively stacked, were manufactured by vacuum resin infusion and impacted at low velocity to investigate their dynamic behaviour in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. For comparison purposes, the same experimental characterization has been performed on laminates reinforced with the single type fibers, i.e. only basalt or
Table 1 Characteristics of the reinforcements. Reinforcement 3
Density (g/cm ) Specific surface weight (g/m2) Weave type Thickness (mm)
Flax
Basalt
1,27 222,1 Twill 2/2 0,225
2,67 220 Twill 2/2 0,13
flax fibres. The experimental results confirmed the positive role played by fibre hybridization. In fact, the hybrid flax composites showed an intermediate behaviour the single basalt and flax composites, resulting more resistant to impact than flax laminate and more capable than the basalt laminate to absorb the impact energy through not elastic mode and the deflect the impact progression. 2. Materials and experimental methods Basalt twill woven fabric with fibre areal weight of 220 g/m2 (BAS220,1270,T), supplied by Basaltex NV and a flax twill woven fabric (FlaxPly BL200) with fibre areal weight of 200 g/m2 supplied by Lineo, were used as reinforcements. Table 1 shows the characteristics of both reinforcement as reprted by the datasheet. The investigated reinforcements have been impregnated by the two-component commercial resin epoxy PRIME™ 20LV (100:26 resin/hardener weight ratio) formulated for infusion system by Gurit. PRIME™ 20LV has reduced viscosity (600–640 cP) and longer working time, which makes it ideal to impregnate very large parts with complex reinforcements in one-shot operation. It maintains the exceptionally low exothermic characteristic, which allows thick sections to be manufactured without risk of premature gelation due to the heat of exothermic reaction. Three different laminates, reinforced with basalt, flax and hybrid basalt/flax fabric, have been manufactured by stacking 16 plies of reinforcement. In particular, the hybrid one was realized by alternating 8 basalt and 8 flax fabric layers according with a symmetric and balanced configuration and placing externally the basalt layers in order to ensure higher impact resistance. The final stacking sequence was [B, F]8,s for the hybrid laminate. All composite panels have been produced by vacuum infusion process that basically involves three steps [39]: lay up of a fiber preform, vacuum application and fiber impregnation by a thermoset resin, cure of the resin. The reinforcement is placed between one-sided rigid mold and a formable vacuum bag material. The resin is injected from an input channel. Vacuum is applied through a single or multiple vents in order to remove the air from the fiber preform and to drive the fiber impregnation by resin. A resin distribution net medium is placed onto the reinforcement to promote the resin flow allowing the complete wetout of the preform and eliminating voids and dry spots. After infusion, the panels were cured for 16 h at 50 °C according with the technical datasheet. The manufactured composite panels showed different characteristics in terms of thickness and fiber volume fraction as reported in Table 2. The different thickness is due to the different resin absorption. In low velocity impact phenomenon the content of resin does not influence the response of the laminates. On the other hand, in literature it is largely reported that the fibre content is the fundamental parameter governing the impact behaviour. Thus, the investigated laminates were Table 2 Characteristics of the manufactured composites.
18
Property
Basalt
Flax
Flax/Basalt
Thickness, mm Fiber volume fraction Density, g/cm3
2.7 0.69 1.87
8.1 0.47 1.05
5.7 0.61 1.34
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coupled into an optical fiber reaching the camera sensor where also the light scattered from the object is addressed. Before reaching the object, the object beam passes through an electronically tilted etalon to shift the probing beam laterally in a parallel direction. By such way, it is possible to obtain four equivalent images of the speckle pattern, which are used to perform an average to reduce the speckle noise. Upon an external perturbation, the object is deformed and the reflected wavefront is slightly changed respect to the initial state, while reference beam remains unperturbed. Thus, the camera records a new speckle pattern and the subtraction of the two registered speckle patterns (deformed and non-deformed states) provides the correlation fringes. When, the subtraction of the fringes patterns is computer-aided, the technique is called electronic. The correlation fringes allow obtaining the phase-contrast maps in grayscale (at 640 × 480), from which it is possible to measure the displacement field quantitatively. The calculation of phase-contrast map is obtained by using well-known numerical processing, consisting into noise reduction and demodulation of the fringe pattern image, which provides a wrapped phase image, and the subsequent unwrapping step [43].
obtained by stacking the same number of layers of different fabrics that have been selected with the same areal weight in order to obtain comparable conditions in terms of fiber content by weight. However, due to the different reinforcement porosity and compatibility with the epoxy resin, the final composites resulted with different resin content and fiber volume fraction. The morphology and fracture surface of the investigated composites were studied by using a Quanta 200 FEG (FEI company, Eindhoven, the Netherland) Scanning Electron Microscopy (SEM) and the sputter coater Emitech K575X for the sample preparation. The results were, then, processed by an INCA software energy. Impact tests were carried out by a falling weight machine, Ceast Instron, at complete penetration to obtain and study the whole load displacement curve, interesting for giving useful information about the response of the laminates and for investigating the effect of the varied impact parameters. Then different increasing energy levels corresponding to the Fmax at the 35 and 70% of the linear increasing stretch of the curve, were chosen to carry out tests useful to study the damage start and evolution. At least three tests were performed in each test conditions and the data reported in the graphs are the mean values of the obtained results. The rectangular specimens, 100 mm × 150 mm, cut by a diamond saw from the original panels, were supported by the clamping device suggested by the ASTM D7136 Standard [40]. Standard and were centrally loaded by an instrumented cylindrical impactor with a hemispherical nose, 19.8 mm in diameter. The total minimum mass of 3.640 kg, that combined with the drop heights allowed to obtain the selected impact energies, was considered. After the impact tests, the specimens were observed by ESPI technique to investigate the internal damage. A holographic method was used to investigate the internal impact damage. Electronic speckle pattern interferometry (ESPI) technique allows to measure the out of plane displacements of specimen after a stress event and in such way enables to identify cracks, strain and flaws on rough surfaces with high sensitivity in real time and full field modality without contact [41,42]. The detection of micro-deformations is obtained illuminating the sample surface by a visible laser. A ChargeCoupled Device (CCD) camera records the deformation under correlation fringes form, deformation due to a slight warming imposed to the material. The direction of object displacement measurement lies along the bisector of the angle between the illumination beam and the direction of observation by the camera lens. In this configuration the illumination aperture and camera lens are quite close and if the object does not subtend too large an angle, the measurement direction can be considered nearly constant and perpendicular to the sample surface. A typical scheme of the system for recording speckle interferometry measurements is illustrated in Fig. 1. The laser beam is split into a reference beam and an object beam by means of a Beam Splitter (BS), which enables control of the relation between reference beam and object beam. The reference beam is
SEM images have been acquired for each type of composite laminate. Figs. 2–4 show the SEM micrograph for the flax, basalt and hybrid composite respectively. The morphology of flax composite (Fig. 2) is characterized by the presence of fibrils and fibers gropus with not homogenous size that determine some resin accumulation zones. On the other hand, a more homogenous morphology is noticed for the basalt reinforced composite (Fig. 3). In Fig. 4, showing the micrograph of the hybrid composite, it is possible to observe the proper alternation between the two types of layers, the flax reinforcement (yellow label) and the basalt reinforcement (green label) and good adhesion both between the fibers and the matrix. The impact response of the three investigated different composite laminates is shown in Fig. 5, where it is possible to observe that the shape of the three curves at penetration are completely different to each other and, more interestingly, that the curve obtained for the hybrid laminate show intermediate characteristics between the basalt and the flax laminates (Fig. 5). Of course, since at least three tests were carried out in each condition, for clarity and to avoid confusion, since the three curves are completely overlapped, only one of them was reported in the graph. In particular, the response of the hybrid composite does not seem an average of the behaviour of the two single type composites, but rather a superposition of effects. Specifically, with reference to Fig. 5, it is evident that hybrid composite shows the identical trend and linear stiffness of flax composite during the first part of load application, while on
Fig. 1. Optical setup of ESPI.
Fig. 2. Micrograph of flax composite.
3. Results and discussion
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hybrid laminates is larger than that observed for the basalt laminate. This characteristic belongs also to flax composites even if the maximum force value is lower than the others and is related to the capability of the plant reinforced composites to absorb the impact energy through not elastic mode and deflect the impact progression [29]. The basalt composite attains its maximum load showing a linear behaviour with almost constant slope, denoting a small internal damage like matrix cracks, that do not affect its strength. This has been deduced by considering that in classical carbon fibre reinforced plastics, CFRP, and glass fibre reinforced plastics, GFRP, laminates [42–49], the significant and not negligible slope changing or sudden load drops in the first increasing part of the load-displacement curve have been found to correspond to delamination start, propagation and fibre failures [50]. Thus, by comparing the same part of the curve with the impact curve of carbon or glass fibre composites, it is possible to assert that basalt composites were poorly damaged during the impact event. As shown hereafter, this was confirmed by ESPI investigation that evidenced low delamination extensions limited to the impactor material contact point. As expected, flax laminates showed a lower performance in terms of load. However, as it is clear from Fig. 5, the load reaches a first peak with initial rigidity higher than basalt laminates. It decreases a little with a noisy shape and then increases again up to a second peak higher than the first one. Then, it sudden decreases up to zero almost vertically denoting a lack in strength due to a catastrophic damage. The curve recorded for the hybrid laminate showed an intermediate behaviour: looking at Fig. 5, the shape of the load curve is similar to the one recorded on flax specimens even if the forces are higher and the maximum load peak is very close to the one recorded on basalt laminates, and it happens at the same deflection. In other terms, according to diagrams, flax fibers seem to represent a very good solution as impact absorber, ensuring to reduce the peak of stress and adsorbing energy by an inelastic modulated response, conversely basalt reinforcements are also able to resist to higher impact forces. Thus, the hybrid flax/basalt is more proper than the other laminates in the case of short recurring impacts reducing cumulative failures. In Figs. 6 and 7, the comparison between maximum load and penetration energy obtained for the three different material systems, is reported. Very interestingly, both of these two important characteristics are higher for the hybrid laminate. It means that flax/basalt laminate needs more energy to be completely perforated. However, as just observed in Fig. 5, even if the maximum load, Fmax, of flax composite is sensibly lower than that of basalt fibre laminate, the values of penetration energy are very close each other. In Figs. 8–10, the load curves obtained on flax, basalt and flax/basalt fibre laminates respectively at increasing energy levels are reported. The different increasing energy levels corresponding to the Fmax at the 35 and 70% of the linear increasing stretch of the curve, were chosen to carry out tests useful to study the damage start and evolution. In Table 3, the corresponding energy value in [J], for each composite
Fig. 3. Micrograph of basalt composite.
Fig. 4. Micrograph of hybrid basalt flax composite.
Fig. 5. Force-displacement curves: comparison.
the other hand are characterized by a similar behaviour to basalt composites during the long unloaded phase, after attaining the maximum load with a value close to the basalt composite peak. However, the range of displacements in correspondence of the force peak for the
Fig. 6. Comparison about maximum load, Fmax. 20
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Table 3 Impact energy levels, U. Composite system
U [%]
U [J]
Flax
35 70 35 70 35 70
8 16 20 35 30 40
Basalt Flax/Basalt
Fig. 7. Comparison about penetration energy, Upen.
Fig. 11. Maximum load, Fmax, vs impact energy, U.
system, is reported. A closed type curve is obtained for all cases: the samples are not penetrated/perforated by the impactor that rebounds and the area enclosed in the loop of the loading/unloading part of the curve, is just the energy absorbed by the laminate to create damage or to bend, or both, depending on the thickness. As expected, as the impact energy increases, an increase of the maximum load (Fig. 11) and of the absorbed energy, Ua (Fig. 12) is recorded. However, in the case of the hybrid composites, lower differences are noticed (Fig. 10). In Fig. 13, the comparison among all tested samples (U = 35%) is shown: flax and flax/basalt laminates exhibit several load drops before attaining the maximum force, this could represent a higher internal damage in terms of fibre failures respect to the basalt composite. In addition, they have a similar rigidity higher than that of basalt fibre laminates whereas the latter showed the higher deflection in correspondence of the maximum load. In order to get more information on the effect of the impact on the different laminates, Figs. 14–22 show its impacted and rear surfaces after the penetration and the indentation tests. A concentrated damage around the impact point is clear for both the basalt and the hybrid composite on the front surface (Figs. 14a and 16a), while the flax composite (Fig. 15) is characterized by a larger penetrated area with elongated fibre tearing and a more brittle damage. Similarly, tearing is
Fig. 8. Load displacement curves of Flax samples impacted at U = [35, 70]% Up .
Fig. 9. Load displacement curves of Basalt samples impacted at U = [35, 70]% Up .
Fig. 10. Load displacement curves of Flax/Basalt samples impacted at U = [35, 70]% Up. Fig. 12. Absorbed energy, Ua, vs impact energy, U. 21
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Fig. 13. Comparison of load displacement curves for all impacted samples; U = [35]%.
Fig. 16. Photographs of basalt/flax composite after impact penetration: a) impacted surface, b) rear surface.
Fig. 14. Photographs of basalt composite after impact penetration: a) impacted surface, b) rear surface.
Fig. 17. Photographs of flax composite after impact at U = 8 J: a) impacted surface, b) rear surface.
Fig. 15. Photographs of flax composite after impact penetration: a) impacted surface, b) rear surface.
evident on the rear surfaces of flax and hybrid composites. Further, due to the flax reinforcement and its waviness, capable to propagate the crack on larger areas respect to the basalt fibers, different failure modes have been occurred trough the composites, inducing a cross-like damage for the flax laminate, a diamond shape damage for the basalt laminate and a mixed shape for the damage for the basalt laminate and a mixed shape for the hybrid composite. The described damage
Fig. 18. Photographs of flax composite after impact at U = 16 J: a) impacted surface, b) rear surface.
mechanisms on the hybrid configuration are due to the particular stacking sequence where basalt layers were alternated to the flax ones along the thickness, in which basalt fibres behave like a reinforcement, limiting the brittle failures of the flax layers evidenced in Fig. 15 by 22
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Fig. 19. Photographs of basalt composite after impact at U = 20 J: a) impacted surface, b) rear surface.
Fig. 22. Photographs of Flax/basalt composite after impact at U = 40 J: a) impacted surface, b) rear surface.
levels (Figs. 17–22) it is possible to note a different branching of damage on the impacted side at the same impact energy, U. On other hand, even a different involvement of the whole thickness of tested specimen can be imagined by observing the opposite side to impacted one.
the the the the
3.1. Delamination In Fig. 23, the results from the non-destructive investigations by ESPI technique adopted in the present research, obtained on the same specimens impacted at the two different energy levels, were compared. The figure shows the phase-contrast maps for all the types of impacted samples. The phase is directly linked to the out of plane displacement of the samples after a thermal perturbation. The discontinuities of the shape/direction of the fringes allow to highlight discontinuities of the deformations (i.e. areas with an unexpected displacement) thus damaged or delaminated areas. Looking at the flax\basalt it is possible to recognize the shape of the impactor and this means that there is not damage propagation but is concentrated under the impactor-material contact point. On the other side looking at basalt the damaged area appears to be larger and with a lobe shape, showing that the impact energy caused more damages to the laminate. Finally, in the flax case no discontinuities into the fringes pattern can be noticed that means there is no damages or ESPI is unable to detect them. Looking at the latter image in Fig. 23, it may seem that the central spot is referred to damage. However, since these images are not simple to be understood, they are deeply analysed and elaborated to measure the extension of the delamination. In Table 4, the measured delamination areas are reported. Very interestingly, the hybrid laminates showed a lower damage extension respect to the basalt fibre composite materials. Moreover, the two values at 35% and 70% Up are very close to each other (272 mm2 and 290 mm2). This could be due to the effect of flax fibre within the basalt ones. By visually observing the flax fibres laminates loaded by the increasing energy levels not at penetration, it seems that no damage was caused by the impact. The images at penetration reported in Fig. 15 showed a damage on flax specimens apparently larger than the one on hybrid laminates (Fig. 16). However, the damage showed in Fig. 15 is clearly a brittle damage that could explain the influence of the entity of the load in determining the different response of the material. The latter would be insensitive to the low energy values and would show a brittle behaviour when the energy is high enough to cause the catastrophic final failure and it could also explain the highest energy absorbed at penetration, used to create brittle damages in the matrix.
Fig. 20. Photographs of basalt composite after impact at U = 35 J: a) impacted surface, b) rear surface.
Fig. 21. Photographs of flax/basalt composite after impact at U = 30 J: a) impacted surface, b) rear surface.
large fractures on both the front and back side of the laminate. In this way, the influence of both the reinforcements were highlighted leading to a behaviour in the middle between the one evidenced in Fig. 14 and the one reported in Fig. 15. Further, by the analysis of impacted specimens at different energy 23
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Fig. 23. ESPI techniques for all type of impacted samples; U = [35,70]% Up.
hybrid composite seems a superposition of the effects belonging to the basalt and flax single laminates. Thus, the hybrid flax/basalt is more efficient than the other laminates in the case of short recurring impacts, reducing cumulative failures.
Table 4 Delaminated areas measured by ESPI technique.
Basalt Flax Flax/Basalt
A [mm2] U = 35% Up
A [mm2] U = 70% Up
300 – 272
490 30 290
Acknowledgements The authors gratefully acknowledge the ONR Solid Mechanics Program (12174953), in the person of Dr. Yapa D.S. Rajapakse, Program Manager, for the financial support provided to this research. The authors are very gratefully to Eng. Fabrizio Sarasini and its work team for the basalt textile used in this research.
4. Conclusion Basalt, flax and hybrid laminates, obtained by alternatively stacking flax and basalt twill layers, were manufactured by vacuum resin infusion and impacted at low velocity to investigate their dynamic behaviour in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. The results of the experimental tests evidenced the advantages of the fibre hybridization. As a matter of fact, hybrid basalt/flax composite showed better impact performances than those of pure basalt and pure flax laminates, being characterized by the maximum impact stress value due to the external layer of basalt fibers, the highest energy absorption correlated to a lower delamination respect to the one measured on the basalt laminate and the highest penetration energy. Moreover, from the comparison between the load curves at penetration obtained impacting all the different laminates under study, a large plateau around the maximum load due to the presence of the flax layers and their capability to absorb the impact energy through non elastic mode and deflect the impact progression, was observed. By looking at that curves, the response of the
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