Experimental Study on the Flexural Performance of Parallel Strand Bamboo Beams

The Scientific World Journal
Volume 2014 (2014), Article ID 181627, 6 pages
http://dx.doi.org/10.1155/2014/181627

Author

Aiping Zhou1 and Yuling Bian1,2

1School of Civil Engineering, Nanjing Forestry University, Nanjing, Jiangsu 210037, China
2Wuxi Institute of Commerce, Wuxi, Jiangsu 214153, China

Received 8 November 2013; Accepted 17 December 2013; Published 18 February 2014

Academic Editors: F. J. Belzunce and F. Pacheco-Torgal

Copyright © 2014 Aiping Zhou and Yuling Bian. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Searching for materials to provide proper housing with less emission and low energy becomes an urgent demand with the ever-growing population. Bamboo has gained a reputation as an ecofriendly, highly renewable source of material. Parallel Strand Bamboo (PSB) is a new biocomposite made of bamboo strips which has superiority performances than wood products. It has attracted considerable interests as a sustainable alternative for more traditional building materials. But the mechanical performance study of PSB as construction materials is still inadequate. Also, the structural behavior of PSB is not quite understood as conventional construction materials, which results in the difficulties to predict the performances of PSB structural members. To achieve this purpose, 4-point bending experiments for PSB beams were carried out. The flexural performances, mode of failure in bending, and the damage mechanism of PSB beams were investigated in this paper.

1. Introduction

In recent years, with the ever-growing population, searching for materials to provide proper housing with less emission and low energy becomes a challenge. Bamboo has gained a reputation as an ecofriendly, highly renewable source of material. Bamboo grows much faster than wood. Usually could reached maturity in 5 years. But raw bamboo can not meet the requirements as modern building materials because of their many varieties in mechanical properties and in geometrical shapes and sizes. To meet the requirements of current structural applications, many efforts have been made to bamboo to have stable properties and more uniform. Parallel Strand Bamboo (PSB) belongs to one of the engineered materials made with bamboo, which is fabricated by cutting bamboo into strips along parallel-to-grain direction by using adhesive in the lamination process parallel to each other into a prism often with a rectangular cross section [1]. It has attracted considerable interests as a sustainable alternative for more traditional building materials due to its outstanding mechanical performances. Currently, the compressive strength and Young’s modulus of PSB had reached more than 60 MPa and 12000 MPa, respectively [2], which could meet the standard as structure materials for buildings. But the mechanical performance study of PSB as construction materials is still inadequate. Also, the structural behavior of PSB is not quite understood as conventional construction materials, which results in difficulty to predict the mechanical performances of PSB. The parameters used in the design of some demohouses in China were obtained by testing of the structural members [3–5]. As a result the use of PSB as construction material is not as common. Originally PSB were only used as floor boards or decoration materials. Its physical and mechanical properties have been studies largely as decoration materials [6]. Hence, the criteria in approaching to use large scale in PSB as construction materials are to establish the design philosophy for PSB structure. This paper reported part of these works which aimed to study the flexural performance of PSB beams. The damage mechanism of PSB beams was investigated by using 4-point bending test.

2. Brief of Experiments for Beams

2.1. Preparation of PSB Beams

Bamboo for the experiment came from Anhui province in China, which had been grown for 5 years. Bamboo was cut into many segments with a length of 2 m parallel to its grain. To study the mechanical performance of its different parts, three groups of samples were divided into the bottom, the middle, and the top part of the culm. Then every segment was split into strips (3 mm thickness, 10 mm width, and 2 m length). These strips gotten rid of the bamboo green and the bamboo yellow were grinded to be linked together in parallel-to-grain directions and unlinked in transverse-to-grain directions. In order to reduce the starch content, bamboo strips were carbonized in the steam oven with a pressure of 0.3 MPa for 90 min., as bamboo with low humidity is less prone to mould attacks. For its conservation, bamboo was air-dried in a drying house, and then these bamboo strips were soaked in a glue water pond for 10 min., air-dried again in the drying house to a humidity moisture content of about 12%, and loaded into an iron mould for thermoforming in the stove with a temperature of 120°C, at 150 MPa pressure for 8 hours. PSB beams (160 mm × 110 mm × 1880 mm) were fabricated by this method.

2.2. Experimental Design of PSB Beams

The 13 PSB beams were tested to investigate their bending performances. The cross sections of the beams were 80 × 110 mm2, and the lengths of them were 1560 mm. 5 PSB beams were fabricated from the bottom bamboo, 4 PSB beams were fabricated from the middle bamboo, and 4 PSB beams were fabricated from the top bamboo. Because there is no standard test method for the structural members of bamboo composites, the dimensions of specimens and the test setup were designed referring to ASTM D198-02 [7]. All spans of the beams were taken as 1400 mm. 450 mm long half-shear-span was taken into account, a beam simply supported subjected to a two-point load at the third span, referring to Figure 1. No lateral support was needed to prevent the beams from lateral instability because the depth-to-width ratio was less than 3. Five electrical strain gauges parallel to the longitudinal direction of the beam were uniformly and symmetrically adhered over the side surface in the middle span. The sixth was at the bottom. Three deformation sensors were installed under the middle span of the beam and the spot of load to measure the deflection there. Total load on the beam was symmetrically and monotonously applied at two points equidistant from the reactions. The details of the test setup are shown in Figure 1. Loading speed is controlled by force at the rate of 2 kN/min while it is controlled by deflection in the middle span at the rate of 1 mm/min at a later stage till damage occurred. The total load of the beam, the strains in longitudinal direction over the side surface in middle span, and the deflection in middle span were simultaneously recorded.

Figure 1: Test setup and photo of test.

2.3. The Damage Mechanism of PSB Beams

2.3.1. Phenomena of the Damage

The phenomena of the damage process could be summarized as follows. In the earlier stage of loading, the deflections in the middle span were linearly associated with the augmentation of load. When the load exceeded 1/3 to 1/2 of the ultimate load, some fine cracks within the moment span emerged and expanded along the parallel-to-grain direction below the neutral axis as the load increased, and bulking could be observed at the top surface of the beams in some cases. Finally the break occurred at the bottom of the beams when the loading reached the ultimate value (referring to Figure 2). The damage was almost presented in the pure moment span, which was due to bending damage.

Figure 2: Failure modes of specimens.

2.3.2. Mode of Failure in Bending

The bending strength of PSB is a complex problem. Bending strength depends on three main factors: the ratio of tension to compression strength of the material, nonlinear ductile behavior in the compression zone, and size-dependent brittle fracture in the tension zone. The tension strength of PSB was far greater than compression strength according to preliminary work of authors of this paper [8]. In this mode, maximum moment was associated with a brittle tension failure, but after some compression yielding had occurred. As compression yielding occurred, the neutral axis shifted toward the tension face, and tension stresses continue to increase until failure occurred as a rupture in the tension zone. The load-deflection relationship became curved after compression stresses exceed the proportional limit.

In summary, the bending damage mechanism of the beams could assume that (i) the beams were in the linear stage and could be idealized as a perfect elastic element at the beginning of loading; (ii) with the augmentation of loading, beams went into the nonlinear stage; the fibers in the top of the compressive zone, which were over the neutral axis of the cross section in the moment span, first reached the plastic state and the stress on them was gradually coming to the ultimate compressive strength from the outside extending to the inside of the beam; and (iii) the stress of the fibers in the outside of the tensile zone, which was below the neutral axis in the moment span, finally reached its ultimate (maximum) tensile strength and then broke.

3. Analysis of Experiments for PSB Beams

3.1. Strain of the Cross Section

Figure 3 showed the longitudinal strains over the bending section of a beam obtained from the experiments. It could be found that the envelope of longitudinal strains at the 5 measuring points remained approximately linear from the beginning of loading to damage. This implied that the longitudinal strain at the various points across the section was proportional to the distance from the neutral axis. Thus the plane assumption, that is, plane sections before bending remained plane after bending, was nearly corrected for the flexure of PSB beams. Therefore, the influence of the shearing’s effects on the bending of the beam may be neglected. Figure 3 also showed that the neutral axis of the bending section was offsetting towards its lower part during loading process. This indicated that the compressive zone was expanding towards the lower part of the beam during the loading process because the fibers were gradually bulking from the top surface to the inside of the beam.

Figure 3: Strain distribution in parallel-to-grain direction over the depth of section.

3.2. Analysis of Load-Strain of Beams

The relationship of strain changing with load was shown in Figure 4 that tensile strain was positive and compressive strain was negative. Line 1 was the strain of the bottom of beam. The approximate linear relationship between strain and load in the beginning stage of loading could be seen in the figure. In the subsequent stage of loading, strain augmented faster than load. Strain was released suddenly in the last damage stage, so the strain of the measuring points decreased. The maximum tensile strain approaching destruction was commonly 10000 με~12000 με, as only a few specimens achieved 15000 με~16000 ; the maximum compressive strain of bamboo fiber was commonly 10000 με~14000 με, and a few specimens achieved 16000 με. Compressive strain was higher than the tensile strain and further showed the difference between tension and compression elastic modulus, as the tension modulus of PSB was more than the compression modulus. The change of the ultimate strain range was wide, also showing the large variability in performances of PSB.

Figure 4: Load-strain typical curve of PSB beam.

3.3. Curves of the Load-Deflection of Beams

The direct results obtained from the bending tests are the load versus midspan deflection curves. Figures 5(a) and 5(b) present the curves obtained for PSB beams The linear relationship between deflections and load was shown only at the beginning of loading; the deflection-load curve began to deviate from the straight line after 1/3~1/2 of the ultimate load. The bending stiffness of the beam gradually reduced because there were some flaws and gaps distributing randomly in the bamboo fibers of the PSB beam. In the process of loading, stress in the flaws appeared as distortion, leading to its surrounding stress more than the stress limit of the fiber or agglutination; at the same time some compression yielding has occurred, then the crack was extended constantly, and stiffness decayed simultaneously. After the maximum load, bearing capacity dropped rapidly. Large nonlinear deformations were achieved and the ultimate deflections of PSB beams could reach about 1/35 span before breaking. PSB beams made from different parts of raw bamboo had different performance, referring to Figure 5(a). The mechanical performances of the PSB beams made from the bottom bamboo were inferior to that of the upper.

Figure 5: (a) Load versus midspan deflection typical curves of PSB beams the tip.mid.bottom, respectively. (b) Load versus midspan deflection curves of most specimens.

3.4. The Mechanical Performance of PSB Beams

The test result showed that tension modulus was slightly superior to the compression modulus. The load-deflection information derived from the 4-point bending tests can be analyzed using formulae for structural analysis. In this case, formulae for beams under bending loads are used. Assuming a perfect adhesion between PSB, in the proposed tests  = 13.2, in this case shear effects could be neglected referring to ASTM D198-02. The deflection of straight beams that are elastically stressed and have a constant cross section throughout their length could be given by the theory of mechanics of materials. And then the maximum bending stress, deflection of load, is given bywhere  is the ultimate strength and  and  are deflection of midspan and Young’s modulus in parallel-to-grain directions, respectively.  is the total beam load acting perpendicular to beam neutral axis.  and  are width and height of beam, respectively.  and  is 1400 mm and 450 mm respectively referring to Figure 1. The formula was only suitable for the linear range of load-deflection curves. Young’s modulus of PSB can be given using the Hooke’s law assuming a uniaxial stress state in the grain direction:where  is maximum strain of midspan beam in the elastic stage; compression modulus was obtained by maximum strain in the compressive zone of the beam; tension modulus was acquired from maximum strain in the tensile zone of the beam. According to (2) and (3), Young’s modulus was calculated by the strain collected continuously through strain gauge. The elastic stage was confirmed when the Young’s modulus began to decline. Although (2) and (3) are valid for linear elastic behavior, they are also used beyond the limits of Hooke’s law. When using these equations beyond elastic limit, stresses and strains can be considered as pseudostresses and pseudostrains, respectively. Table 1 showed the mean, standard, and deviations of variation of the maximum bending stress, the Young’s modulus for the tested specimens. These properties corresponded to the longitudinal direction.

Table 1: Test results of specimens.

The bending strength and elastic modulus of upper PSB was about 10% higher than that of the bottom referring to Table 1. According to the early mesoscopic research about bamboo by the authors in this paper, bamboo is a kind of composite composed of a vascular bundle and matrix (thin-walled cells). The mechanical performances of bamboo depended on its vascular bundle; the content of the vascular bundle in the bottom was less than that of the upper [9]. So the mechanical performances of the bottom were less than that of the upper.

4. Conclusions

An overall analysis of the experimental results revealed the following.(1)Mode of failure in bending was nonlinear ductile behavior in the compression zone and size-dependent brittle fracture in the tension zone. As compression yielding occurred, the neutral axis shifted toward the tension face, and tension stresses continued to increase until failure occurred as a rupture in the tension zone. The bending damage mechanism of the PSB beams could assume that (i) the beams were in the linear stage and could be idealized as a perfect elastic element at the beginning of loading; (ii) with the augmentation of loading, beams went into the nonlinear stage, as the fibers in the top of the compressive zone firstly reached the plastic state; and (iii) the stress of the fibers in the outside of the tensile zone finally reached its ultimate tensile strength before breaking.(2)The PSB beams were accorded with the plane assumption. Therefore, the influence of the shearing effect on the bending of the beam may be ignored. Large nonlinear deformations were achieved and the ultimate deflections of PSB beams could reach about 1/35 span before breaking.(3)The tension modulus was slightly superior to the compression modulus. The mechanical performances of PSB made from the bottom raw bamboo were inferior to that of the upper. The bending strength and Young’s modulus of PSB could be about 89 MPa, and 12000 MPa respectively.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The research was supported by the National Natural Science Fund of China (51378263), the Provincial Fund of Science of Jiangsu province (no. BK2012820), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Unidirectional Cordenka Fibre-Reinforced Furan Resin Full Biocomposite: Properties and Influence of High Fibre Mass Fraction

Journal of Composites
Volume 2015 (2015), Article ID 707151, 8 pages
http://dx.doi.org/10.1155/2015/707151

Author

Talent Malaba and Jiajun Wang

College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, China

Received 23 June 2015; Revised 14 August 2015; Accepted 30 August 2015

Academic Editor: Laurent Orgéas

Copyright © 2015 Talent Malaba and Jiajun Wang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A full biocomposite was fabricated from Cordenka CR fibre and furan resin. High fibre mass fractions (FMF) were achieved by pressing the CR fibres into unidirectional sheets prior to incorporation into the resin. Results of testing indicated that the tensile properties of the biocomposite were improved by the initial increase of FMF from 51 to 64%, with a subsequent increase of FMF to 75% resulting in a deterioration of those properties. Examination of the tensile fracture surfaces with a scanning electron microscope (SEM) revealed moderate deterioration in fibre-matrix adhesion after the initial increase of FMF. Further increase of the FMF to 75% was shown by SEM to result in worse fibre-matrix adhesion. On the other hand, the flexural, interlaminar-shear, and dynamic mechanical properties were adversely affected by the increase in fibre-mass fraction from 51 through 75%. These effects were mainly attributed to reduced fibre wetting that resulted in weakened fibre-matrix interfacial bonding and subsequent poor stress exchange at the fibre-matrix interface. Observations made with a digital microscope revealed normal crack behaviour in the laminated composite, and the shear fracture modes were I and II. This biocomposite has mechanical properties comparable to those of flax and glass fibre-reinforced furan resin biocomposites.

1. Introduction

Biocomposites are composite materials that consist of either a bio-derived matrix or biofibre reinforcement or both. Companies and research institutions are developing biocomposites mainly because of heightening environmental conscience amongst the public, government legislation, the depletion of raw materials for synthetic polymers, and price increases [1].

A bio-derived matrix is known as a bioresin [2]. One type of thermosetting bioresin is called furan resin. At room temperature, furan resin is a reddish-brown liquid consisting of prepolymers of furfuryl alcohol (2-furanmethanol), which can cross-link into a solid thermoset polymer called polyfurfuryl alcohol (PFA). This cross-linking is accelerated by heat and acid catalysts [3]. To produce furfuryl alcohol, plant-sourced hemicellulose is hydrolysed in acidic conditions into pentose sugars [4]. The five-carbon sugars are then hydrolysed under the same conditions into the aldehyde furfural (furan-2-carbaldehyde), which is then reduced into furfuryl alcohol [5]. Furan resins are used in the manufacture of corrosion resistant fibre-reinforced plastics (FRPs) because the polymer has excellent chemical resistance and is particularly good at handling solvents and aggressive service environments [6]. Some FRP manufacturers also select furan resins due to their high temperature performance, which has been found to exceed 200°C in some furan based laminates [6].

The adoption of long unidirectional natural fibres as reinforcement in bioresins can result in “full biocomposites” with enhanced mechanical properties and the potential for use in more structural applications [7]. Natural fibres, however, tend to possess poorer mechanical properties when compared to their synthetic counterparts. They are also prone to large variations in measurable parameters, such as length, diameter, and strength, even within the same batch [2]. These naturally occurring variations are almost inevitable and can make the design and quality control of natural fibre-reinforced biocomposites challenging.

To continue relying on biomass as raw material, rayon fibre is a viable option to overcome some challenges posed by natural fibres [8]. Rayon fibre is made from purified cellulose sourced from plant material, particularly wood pulp. A variant of the manufacturing process yields high tenacity rayon (HTR) which, as its name suggests, exhibits high tensile strength. Cordenka GmbH & Co. KG (Obernburg, Germany) make a type of HTR called CR fibre for composite applications [9]. The CR fibre has tensile strength of 850 MPa, an elastic modulus of 19.5 GPa, and an elongation at break of 12% [9, 10]. Due to these properties, thermoplastics reinforced with this fibre possess a significant energy absorption capacity, which translates to a high degree of impact resistance [10]. The research into the application of CR fibres in thermoset based composites is still ongoing [10].

In most previous studies of furan based natural fibre-reinforced composites, the proportion of the fibre reinforcement has been below 50% [7, 11, 12]. However, it has been suggested that to attain high mechanical strength levels (>200 MPa), the fibre proportion must exceed 50% [12]. One proposed technique for achieving a high fibre proportion in these laminates is to compact the natural fibres into unidirectional sheets before their incorporation into the resin [12]. Thus, in the current study, a full biocomposite has been fabricated through wet lay-up and compression moulding using CR fibre sheets and furan resin. Composite specimens with high fibre proportions are obtained and tested to evaluate the properties of the composite, as well as the influence of the high fibre proportion on the tensile, flexural, and interlaminar shear strengths of the composites. Digital microscope and scanning electron microscope (SEM) are employed to analyse the fractured specimens. In addition, the viscoelastic behaviour of the composite is studied using dynamic mechanical analysis (DMA), whilst thermogravimetric analysis (TGA) is conducted for an evaluation of the thermostability of the biocomposite. The results of these tests will also serve to assess the feasibility of applying the CR fibre-reinforced furan resin full biocomposite in the relevant industries.

2. Materials and Methods

2.1. Materials

High tenacity rayon of the type (Cordenka) CR fibre with a linear density of 1840 dtex and zero twist was obtained in wound package form from Cordenka GmbH & Co. KG (Obernburg, Germany). Furan resin was sourced from Hangzhou Tianyu Chemicals Co. Ltd. (Xiaoshan, Hangzhou, China). The resin had the molecular formula (C5H6O2)n, where n ≥ 2; the purity was 80% with about 20% moisture, 30 mPa·s viscosity (20°C), 1.16 g/cm3 density, and a pH of 7. The resin manufacturer also provided the resin’s curing agent: p-toluenesulphonic acid (4-methylbenzenesulphonic acid).

2.2. Fibre Preparation

The CR fibres were unwound from the package, cut, and evenly laid into 36 individual sheets with dimensions of 280 × 70 mm2; each sheet had a mass of approximately 10 g. The sheets were separately wrapped in aluminium foil and compacted in a hot press at 10 MPa and 80°C for 2 hours to form flat and firm sheets consisting of unidirectional fibres. The compacted fibre sheets were finally arranged into three discrete sets, where in the first set the sheets were layered into double plies, triple plies in the second set, and quadruple plies in the third set. Each set had four stacks of the sheets, with the mass of a stack in set 1 being 20 g, 30 g in set 2, and 40 g in set 3.

2.3. Resin Preparation

350 mL of the furan resin was poured into a 500 mL glass beaker and stirred without heating using a magnetic stirrer. The curing agent was then gradually added into the beaker in five 3 g partitions to avoid the potentially violent cross-linking reaction. The mixture was continuously stirred for approximately 20 minutes, or until its temperature reached 45°C, as indicated by a mercury thermometer.

2.4. Composite Laminates Fabrication

The composite laminates were prepared inside a 2 mm thick aluminium mould with dimensions 300 × 300 × 20 mm3. The mould had first been sprayed with mould release agent (Daifree GA 7550, Daikin Fluorochemicals China Co. Ltd.). The four fibre stacks from set 1 were laid side by side inside the mould. The hot resin/curing agent mixture was then poured into the mould to thoroughly wet the fibres. The moulding of furan resin is done in two steps: drying and then curing. Thus, prior to curing, the wet laminates were placed inside a vacuum oven at 45°C for 1 hour to remove the moisture from the resin [7]. The aluminium mould was then placed inside a compression moulding machine to cure the laminates.

2.5. Curing the Laminates

Curing was done at 10 MPa pressure using an adapted multistep sequence [13]. The temperature was kept at 40°C for 12 hours, then raised to 60°C to remain there for another 12 hours, and then increased to 80°C and kept there for 6 hours. The temperature was finally increased to 110°C and the pressure released to 0 MPa to postcure the laminates for 1 hour. The temperature was increased gradually in order to slowdown the polymerisation process to facilitate the release of water produced during the cross-linking of the prepolymers [11]. If curing is too rapid the trapped water may cause matrix cracking and delamination [11].

The above resin preparation, laminate fabrication, and curing procedures were repeated with the stacks of fibre sheets from the second and third sets, respectively.

2.6. Preparation of Test Specimens

After cooling to room temperature, each of the three cured composite plaques was cut into four smaller strips; each strip was approximately 2 mm thick and had dimensions of 280 × 70 mm2 in order to correspond with the initial fibre sheet stacks of the same dimensions. The densities of the fibre and cured resin were obtained from their manufacturers as 1.5 and 1.8 g/cm3, respectively. The composite strips were weighed to determine the fibre mass fraction (FMF) and to calculate the fibre volume fraction (FVF). Table 1 displays the outcomes.

Table 1: Fibre mass and volume proportions in the composite strips.

2.7. Evaluation of Mechanical Properties

The composite strips were cut into the appropriate shapes for tensile, flexural, and interlaminar shear strength (ILSS) testing. For each test, a minimum of 5 specimens were tested for each category of FMF shown in Table 1. Specimen dimensions were measured to the nearest 0.05 mm with a sliding vernier. The specimens were conditioned at room temperature for at least 24 hours prior to testing, which was then carried out under the same conditions using a Universal Testing Machine (Shenzhen Reger Instrument Co. Ltd., Shenzhen, China). The tensile strength test was done in accordance with ASTM D3039 at a cross-head speed of 2 mm/min on specimens with dimensions of 250 × 15 mm2. The flexural strength test was done in accordance with ASTM D790 at a cross-head speed and was 2 mm/min on specimens with dimensions of 100 × 13 mm2. The short-beam shear method specified in ISO 14130 was used for the ILSS test. The specimens had dimensions of 20 × 10 mm2 and the cross-head speed was 1 mm/min.

2.8. Tensile Fracture Surface Analysis with Scanning Electron Microscope (SEM)

The tensile fracture surfaces of specimens that attained tensile strength values closest to the mean were gold-sputter coated for 20 minutes in a JEOL JFC-1600 Auto Fine Coater (JEOL Ltd., Japan). The fracture surfaces were then observed in a JEOL JSM-5610LV SEM (JEOL Ltd., Japan) with acceleration voltage of 5 kV.

2.9. Interlaminar Fracture Analysis with Digital Microscope

A Hirox KH-7700 digital microscope (Hirox Co. Ltd., Japan) was used to observe the damaged specimens of 51% FMF after the ILSS test. The cracked specimens with values of ILSS close to the mean were cut cross-wise 5 mm below the cracks and placed on the microscope slide to observe the cracks and to determine the shear mode(s).

2.10. Study of Viscoelasticity with Dynamic Mechanical Analysis (DMA)

DMA was carried out to evaluate the storage and loss moduli and the damping properties () of the composites using a DMA Q800 instrument (TA Instruments Inc., United States). In addition to the specimens from the three categories of FMF in Table 1, a similarly cured control specimen with FMF of 0% was included in the analysis. All the specimens had dimensions of 50 × 13 mm2 and they were clamped in the 3-point bend mode. The strain amplitude and frequency were 15 μm and 1.0 Hz, respectively. The temperature was increased from 0 to 240°C at a heating rate of 5°C/min.

2.11. Evaluation of Thermostability with Thermogravimetric Analysis (TGA)

The thermostability of the composites was assessed using a Perkin Pyris 1 TGA (PerkinElmer Inc., United States) instrument. Composite specimens from the three categories of FMF in Table 1 and a similarly cured control specimen with FMF of 0% were heated from 25 to 600°C at 20°C/min under nitrogen flow rate of 20 mL/min.

3. Results and Discussion

Table 2 displays the results of the mechanical tests carried out on the composites. The strength and moduli values are shown plotted in Figures 1(a) and 1(b), respectively, with the error bars inserted at the respective standard deviations.

Table 2: Mean values of the mechanical properties of the composites.

Figure 1: (a) Tensile, flexural, and interlaminar shear strengths. (b) Tensile and flexural moduli.

3.1. Tensile Properties

The mean tensile strength and modulus were significantly improved by the increase of FMF from 51 to 64%. The tensile strength of 121 MPa at 64% FMF is almost twice that of a unidirectionally stitched flax fabric-reinforced furan biocomposite, which attained tensile strength of 64 MPa at 60% FMF [14]. However, the tensile modulus of the flax-furan composite was much higher at 8.5 GPa, compared to just 5.6 GPa in the current study. This can be attributed to the greater stiffness of the flax fibre, which has just one quarter (3.3%) of the elongation at break of the CR fibre [15]. In PLA based composites, it has also been indicated that Cordenka fibre improves tensile strength, whilst flax fibre improves the tensile modulus [16]. Similar findings were obtained using Cordenka and flax fabrics in epoxy composites [17]. In the current study, further increase of the FMF from 64 to 75% resulted in a decrease of about 34 MPa in the mean tensile strength. In another study, unidirectionally aligned kenaf fibre-reinforced furan biocomposite attained tensile strength and modulus of approximately 40 MPa and 1.3 GPa, respectively [18]. In that study, the FMF was restricted to a considerably low 20%, above which the tensile properties were observed to decrease sharply. The deterioration of the tensile properties above a certain FMF in that study correlates with the results of the current study. The decrease in tensile strength is caused by poor dispersion, which is a consequence of the high degree of proximity of the fibres to each other [19]. Poor dispersion leads to unreinforced matrix-only areas through which the stress of testing concentrates, leading to rapid crack propagation and ultimately rupture [20]. The tensile strength is also affected by the alignment of the fibres during fabrication [21]. In the current study, the fibres were manually aligned and stacked unidirectionally, a technique that becomes less reliable as the quantity of the fibres increases and may result in misalignment of some fibres. The misaligned fibres do not contribute enough support during testing [21].

3.2. Flexural Properties

The flexural properties deteriorated with the increase of FMF from 51 through 75%. The greatest mean flexural strength and modulus were approximately 95 MPa and 7.0 GPa, respectively, attained at 51% FMF. The flexural strength in this case (where the FVF is 34%) compares well with a value of 99 MPa obtained with unidirectionally stitched flax fabric-reinforced furan resin at 41% FVF [7]. The flexural modulus in that study, however, was significantly higher at 9.6 GPa. For reference, a glass plain weave fabric-reinforced furan resin composite with 47% FVF attained flexural strength and modulus of 105 MPa and 9.0 GPa, respectively [7]. In the current study, the flexural properties decrease at increasing fibre proportions because the flexure test has a compressive component in it, and the compressive strength of a composite is determined by the properties of the matrix and the fibre-matrix interface [17]. Furthermore, there was an increase in weak spaces in the composite due to reduced fibre wetting that occurs at higher FMF, which consequently reduced the flexural strength [22]. The weak areas consist of fibre sheet regions that have the least amount of resin wetting [11]. This effect is apparent in Table 1, where resin uptake is observed to decline as the fibre loading is increased. Thus, the furan matrix clearly dominates the flexural properties of this composite. However, it appears that the CR fibre of the current study could still compete well with the already established glass and flax fibres in furan composites, especially where the flexural strength is of prime interest. The optimum FMF that results in the best flexural strength and modulus has to be found through further research.

3.3. Interlaminar Properties

The ILSS decreased with the increase of FMF from 51 through 75%. The greatest mean value was 11.2 MPa at 51% FMF. For reference, a commercially available flax twill fabric-reinforced epoxy prepreg was found to have ILSS of 10.7 MPa [17]. In the short-beam shear test, the ratio (support span length/specimen thickness) is much smaller (10/2 = 5), compared to 80/2 = 40 in the flexural strength test. The smaller ratio in the ILSS test causes the specimen to fail through shearing of its layers, as intended. These results indicate that the fibre-matrix interfacial adhesion deteriorated steadily with increased FMF.

3.4. Results of Analysis with SEM

Figure 2 shows SEM images of the tensile fracture surfaces. Figure 2(a) shows a fracture surface from a specimen of 51% FMF. The matrix appears prominently and exhibits signs of porosity, some of which can be attributed to water that got trapped during curing. Some fibres have been pulled out from the matrix and they appear to be smeared with matrix material, which is indicative of good fibre-matrix adhesion [8]. Figure 2(b) shows a fracture surface from a specimen that had 64% FMF. The matrix shows a high level of porosity and signs of brittle fracture, revealing some fibres still embedded within the matrix, whilst many were pulled out. Some fibres were also misaligned. The tensile properties were still improved, however, which means these adverse effects were outweighed by the reinforcing effect of the CR fibres. A high level of porosity was also observed in a unidirectional flax-furan resin biocomposite with 60% FMF, which was attributed to excessive resin bleed-out [14]. Figure 2(c) shows a fracture surface from a specimen that had 75% FMF. The fibre-matrix interaction was not optimal, as there are many short pulled-out fibres in proximity to each other. For comparison, a Cordenka fibre-reinforced PLA composite with 30% FMF was found to have poor fibre-matrix adhesion due to long and clean pulled-out fibres [16]. In Figure 2(c) the fibres appear to have huddled together into bundles that led to dry spots and became interfacial weak areas; the huddling of the fibres can be attributed to hydrogen bonds that formed between them [17, 23]. This is one major consequence of the high FMF. The large gaps between the fibres and the matrix are also the result of poor dispersion during fabrication [16].

Figure 2: (a) SEM image of tensile fracture surface (51% FMF). (b) SEM image of tensile fracture surface (64% FMF). (c) SEM image of tensile fracture surface (75% FMF).

3.5. Results of Analysis with Digital Microscope

Figure 3 shows cross section images of some specimens of 51% FMF from the short-beam shear test. In Figure 3(a), an intralaminar or matrix crack is seen extending from the edge of the specimen towards the interlaminar crack. The crack propagation was transferred from the matrix or the fibre-matrix interface towards the fibre plies, causing delamination [24, 25]. This type of crack behaviour signals satisfactory fibre-matrix compatibility [26]. Matrix cracks are one of the main causes of the commencement of delamination [27]. The cracks may initiate in the matrix due to its lower failure strain. The release of cure pressure can also result in fractures within the matrix [17]. Despite the extensive cracking, the specimens exhibited minimal deformation to their overall shapes, which is typical of a brittle material [28]. This can be seen in Figure 3(b), where the delamination crack runs in a fairly straight line across the specimen, and the outer edges of the specimen remained parallel to each other. The shear fracture modes are II and III.

Figure 3: (a) Matrix cracking. (b) Delamination.

3.6. Results of Viscoelastic Behaviour Study

Table 3 displays the data outputted by the DMA instrument. The viscoelastic behaviours of the composites and control specimen are shown in Figure 4.

Table 3: Storage modulus, damping coefficient and glass transition temperature values of the composites and control specimen.

Figure 4: (a) Storage moduli of the composites and control specimen. (b) Loss moduli of the composites and control specimen.

In Figure 4(a), the storage moduli () decreased with increasing temperature due to the deterioration of the elasticity of the furan matrix. The highest storage modulus at room temperature (25°C) was approximately 4.6 GPa, which was attained by the specimens with 51% FMF. For the three tested composites, the value of  is observed to decrease with increasing FMF, a trend that is consistent with the flexural and interlaminar shear strengths. The three composites, however, had much higher values of  than the control specimen of 0% FMF, which is an indication of the increased stiffness of the composites due to the reinforcing properties of the CR fibres. In addition, the curves of  do not exhibit any increase in the value of  in the tested temperature range, suggesting that the resin was cured sufficiently [29]. In Figure 4(b), the loss moduli () are observed to reach maximum values corresponding with the respective glass transition temperatures . Incorporating the fibre reinforcement elevated the  of the resin. Amongst the three composites, however, the  decreased with increasing FMF. At higher FMF, it has been noted that the furan matrix becomes less reinforced as the fibres tend to huddle together. The data in Table 3 indicate that the three composites had similar damping coefficients () at 25°C. Above 25°C, the specimen with 51% FMF maintained the highest values of , which means it was the most efficient at absorbing and dissipating energy. This is due to the adequate wetting of the fibres by the resin, which enhanced the damping effect at the fibre-matrix interface during cyclic loading [30]. This is an important consideration for automotive applications, where the biocomposite is expected to absorb and dissipate vibrational energy safely and efficiently.

3.7. Results of Thermostability Study

Figure 5 shows the TGA curves of the specimens. The three composites and the control specimen each lost about 7% mass at 200°C. Above 200°C, the thermostability of the three composites appears to be improved over that of unreinforced furan resin. A mass loss of 10% corresponds with a temperature of approximately 260°C for the composites, whilst for unreinforced furan resin it is lower at approximately 220°C. The three composites show rapid mass loss at approximately 275°C, which can be attributed to the degradation of the cellulosic structures of the CR fibres [31]. This degradation became worse with the increase of FMF since although the fibres are completely enclosed within the furan matrix, at higher FMF there is less matrix material to protect the fibres from degrading [12]. Between 280 and 320°C, the specimen with 51% FMF lost 5% less mass compared to the other two specimens with the higher FMFs. The specimen with 51% FMF also retained the greatest mass residue (41.1%) at 600°C, whilst the specimens with 64% FMF and 75% FMF retained mass residues of 34.0% and 13.5%, respectively.

Figure 5: Thermogravimetric curves of the composites and control specimen.

4. Conclusions

A full biocomposite has been successfully fabricated from high tenacity rayon fibre and furan resin. Study of the laminate cracking behaviour revealed the two materials to be compatible as a composite system. The comparisons made with related furan resin-based biocomposites indicated that the CR fibres are a viable reinforcement in such composites. In particular, the CR fibres will be a suitable candidate to replace flax fibres if the tensile strength is of prime interest. Furthermore, the tensile properties benefit from a fibre mass fraction that is just above 60%; for additional improvements, it will be necessary to find means to stall the loss of fibre-matrix adhesion. One way to achieve this could be the incorporation of coupling agents to enhance the fibre-matrix adhesion. Since the CR fibres provided similar flexural strength as glass fibres in furan resin biocomposites, the CR fibres may be a suitable substitute for glass fibres in these biocomposites. This would add the advantage of reduced weight, which is necessary in the automotive sector. More research is required to determine the optimum fibre mass fraction corresponding with the best flexural and dynamic mechanical properties. For better control of resin bleed-out to improve fibre wetting and reduce porosity, alternative fabrication techniques have to be investigated, for example, vacuum bagging. The fibres can also be stitched into a unidirectional fabric for better alignment.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors are very grateful to the management and staff at the Cordenka Shanghai Representative Office for their assistance in procuring the Cordenka CR fibre packages from Germany. Sales personnel at Hangzhou Tianyu Chemicals Company Ltd. are also acknowledged for providing the furan resin at a discounted price.

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