Elsevier

Materials & Design

Volume 71, 15 April 2015, Pages 17-25
Materials & Design

Effect of UV and water spraying on the mechanical properties of flax fabric reinforced polymer composites used for civil engineering applications

https://doi.org/10.1016/j.matdes.2015.01.003Get rights and content

Highlights

  • UV weathering degraded mechanical properties of flax/epoxy composites.

  • SEM confirmed degradation in fibre/matrix interfacial bonding.

  • UV weathering caused discolouration, matrix erosion, microcracking.

Abstract

The lack of data related to durability is one major challenge that needed to be addressed prior to the widespread acceptance of natural fibre reinforced polymer composites for engineering applications. In this work, the combined effect of ultraviolet (UV) radiation and water spraying on the mechanical properties of flax fabric reinforced epoxy composite was investigated to assess the durability performance of this composite used for civil engineering applications. Specimens fabricated by hand lay-up process were exposed in an accelerated weathering chamber for 1500 h. Tensile and three-point bending tests were performed to evaluate the mechanical properties. Scanning electron microscope (SEM) was used to analyse the microstructures of the composites. In addition, the durability performance of flax/epoxy composite was compared with synthetic (glass and carbon) and hybrid fibre reinforced composites. The test results show that the tensile strength/modulus of the weathered composites decreased 29.9% and 34.9%, respectively. The flexural strength/modulus reduced 10.0% and 10.2%, respectively. SEM study confirmed the degradation in fibre/matrix interfacial bonding after exposure. Comparisons with other composites implies that flax fabric/epoxy composite has potential to be used for civil engineering applications when taking its structural and durability performance into account. Proper treatments to enhance its durability performance will make it more comparable to synthetic fibre reinforced composites when considering as construction building materials.

Introduction

Synthetic carbon/glass fibre reinforced polymer (G/CFRP) composites with concrete have been used as bridge piers, pole and marine fender pile structures because of high strength-to-mass ratio, design flexibility and increased deformability [1]. The corrosive-resistant characteristics also enable G/CFRP composites to be an alternative to steel reinforcement for civil engineering applications [2]. Nowadays, high initial cost of G/CFRP composites is probably one of the most influential factors which limits their wider application in civil infrastructure [3]. Recently, because of increasing environmental concern and the high demand for environmentally-friendly materials, the use of natural fibres to replace glass fibres in FRP composites has gain popularity [4]. In 2010, European market consumed approximately 315,000 tonnes natural fibres, which was 13% of the total reinforcement materials (glass, carbon and natural fibres) for composites. It is forecasted that the amount will increase to 830,000 tonnes in 2020, with a share of 28% for the total reinforcement materials [5]. Natural fibres come from renewable resources. They are non-abrasive, less energy consumption and health risk, recyclable and bio-degradable, which being regarded as the representative of highly “sustainable” materials [6].

Among various natural fibres, flax is one of the fibres has favourable mechanical properties which could be used to replace glass fibres in FRP composites. Dittenber and GangaRao [7] compared more than 20 commonly used natural fibres and concluded that flax fibre offers the best potential combination of low cost, light weight, and high strength/stiffness for structural applications. Yan et al. [8] stated that when taking cost, mechanical performance and production yielding into account, flax, hemp and jute are the three most promising candidates that can be used to replace glass fibres. Assarar et al. [9] showed that flax fabric/epoxy composite has a tensile strength of 300 MPa, which is close to GFRP composite.

Nowadays, one large demand market of natural FRP composites is from automotive engineering. According to European Guideline 2000/53/EG, 95% of the weight of a vehicle have to be recyclable by 2015 [10]. Natural FRP composites not only reduce mass of components but also lower the energy needed for production by 80% [11]. For structural application, one potential of natural FRP composites is as crashworthy structures for vehicle design. Most recently, Yan and Chouw [12] examined the crashworthiness characteristics of flax fabric/epoxy tubes from the viewpoint of energy absorption. It was found that flax fabric/epoxy composite tubes have potential to be useful energy absorber devices. Flax fabric/epoxy composite tubes with foam-filler exhibited better energy absorption capability than conventional metallic or G/CFRP composite tubes during axial and lateral crushing [13], [14]. Meredith et al. [15] recommended flax/epoxy composites to be used as energy absorbers for vehicle design, after compared the energy absorption capacities of composites reinforced by unwoven hemp, woven flax and jute. Study by Oshkovr et al. [16] also showed the potential of silk/epoxy composite tubes as energy absorber device.

The most important application of natural FRP composites is probably as construction building materials. Using these “sustainable” materials will reduce cost and increase energy efficiency which provides a solution to immediate infrastructure needs while promoting the concept of sustainability [17]. For infrastructure application, natural fibres and/or natural FRP composites could be combined with conventional building materials as hybrid system, e.g. flax fibre reinforced polymer (FFRP) tube encased coir fibre reinforced concrete (CFRC) structure (FFRP-CFRC) [18]. In a FFRP-CFRC system, the pre-fabricated flax fabric/epoxy composite tube acts as permanent formwork for fresh concrete and also provides confinement to concrete thus increases load carrying capacity and ductility. Coir fibre, another kind of natural fibre, as the reinforcement within concrete, is used to reduce concrete cracks and thus modify the concrete failure to be ductile because of fibre bridging effect. Previous study showed the potential of FFRP-CFRC hybrid structure as axial and flexural structural members [19]. In addition, flax FRP tube and coir fibre can increase the damping of the hybrid system in vibration, thus reduces the effect of dynamic loadings on the structural response [20]. FFRP-CFRC hybrid system also exhibited structural performance (i.e. energy dissipation and load carrying capabilities) better than conventional steel reinforced concrete [21]. Comparisons between flax FRP tube confined concrete and G/CFRP tube confined concrete indicated that the confinement performance of flax FRP tube on concrete is close to or comparable to G/CFRP tubes [22].

Although natural FRP composites exhibited potential for automotive and civil engineering applications, several challenges for practical applications are still exist [23]. One major obstacle is their durability issue [8], [24]. The lack of data related to durability is a major challenge that needed to be addressed prior to the widespread acceptance of natural FRP composites for practical engineering application. If being used in practice, FFRP-CFRC hybrid structure will be exposed to various harsh environments such as lighting, raining and temperature-variance, resulting in degradation of the composite material and thus raising safety concerns. Therefore, understanding the durability performance of flax FRP composites has potential industrial significance.

For durability investigation of natural FRP composites, Joseph et al. [25] considered the mechanical properties of sisal fibre reinforced polymer composites exposed to UV light and water spray weathering. Azwa and Yousif [26] studied the characteristics of kenaf/epoxy composites subjected to thermal degradation. The composites were exposed to high temperature up to 600 °C. Shahzad [27] investigated the effect of UV radiation on hemp/polyester composites. Other scholar had reported on the reduction in tensile properties for wood fibre reinforced composites after UV exposure [28]. In these studies, the reinforcement fibres were monofilament short fibres, rather than fabric fibre. To date, studies on durability investigation of flax FRP composites subject to UV radiation and water spraying weathering are very rare, especially for composite reinforced with flax fabrics [23], [24]. Therefore, in this work, the combined effect of UV radiation and water spraying on the mechanical properties of bidirectional flax fabric/epoxy composites used for civil engineering application is investigated.

Section snippets

Materials

In this study, commercial bidirectional woven flax fabric (550 g/m2) was used because initially it was used to make flax fabric/epoxy composite tubes for FFRP-CFRC hybrid structures [19], [20], [21]. The fabric has count of 7.4 threads/cm in warp and 7.4 threads/cm in the weft direction. The tensile strength, modulus of elasticity and elongation at break of the single-strand flax yarn extracted from fabric is 136.3–154.8 MPa, 15.2–17.3 GPa and 2.8–3.5%, respectively. The epoxy used was SP High

Visual observations during exposure

Fig. 1 shows the surface of the flax fabric/epoxy composite at different exposure stages: 0 h, 500 h, 1000 h and 1500 h. It can be seen that the composite underwent severe discoloration during the weathering. The specimen surface became fuzzier and fuzzier with an increase in exposure time. The change in colour occurred immediately at the first 500 h (Fig. 1(a) and (b)). From 500 h to 1000 h (see Fig. 1(c)), less colour change occurred in the specimen surface. From 1000 h to 1500 h, the change in the

Conclusion

In this study, the durability of flax fabric reinforced epoxy composites used for civil engineering applications was evaluated by accelerated UV weathering. Scanning electron microscope study was performed to analyse the microstructure of the composites. The durability performance of flax fabric/epoxy composites was compared with synthetic and hybrid fibre reinforced composites. Some conclusions drawn from the investigation are listed as follows:

  • 1.

    Both tensile and flexural properties of flax

References (38)

Cited by (134)

View all citing articles on Scopus
View full text