Flax fabric-reinforced epoxy pipes subjected to lateral compression

Highlights

• Flax fabric-reinforced epoxy (FFRE) showed the potential to be used for pipe manufacture.

• FFRE pipes was classified as flexible pipe.

• FFRE pipes showed greater strength, flexibility, and specific energy absorbed than other natural composite pipes.

• FFRE pipes showed comparable strength with synthetic fibre composite pipes.

• FFRE pipes showed comparable specfic energy absorbed with synthetic fibre composite pipes.

Abstract

Application of flax fibre as reinforcement in polymer composites used in the construction and manufacturing industries is a step towards achieving more sustainable construction practices. Flax fabric-reinforced epoxy (FFRE) was used to manufacture forty-two pipes with a variety of internal diameters and flax fabric layers, all with a length-to-internal diameter ratio of 1.5:1. Lateral compression loading was performed on the FFRE pipes to establish the effect of internal diameter and number of fabric layers on the strength, energy absorption capability, strains, and failure mechanism. When compared to previously reported test results for pipes manufactured from different natural fibre composites, the FFRE pipes manufactured in this study and subjected to lateral compression showed higher strength, flexibility, and specific energy absorbed while demonstrating comparable strength, specific strength, and specific energy absorbed with synthetic and hybrid fibre composite pipes.

Introduction

Natural fibres are readily available in many countries, and there is considerable potential for their application as a sustainable construction material [1], [2] due to increasing environmental concern [3], [4]. In order to achieve a more sustainable construction industry, the United States (US) Department of Agriculture and the US Department of Energy have set goals of having at least 10% of all basic chemical building blocks to be created from renewable and plant-based sources by 2020, increasing to 50% by 2050 [5], [6]. Additionally, the International Council for Research and Innovation in Building and Construction (CIB) published its Agenda 21 on Sustainable Construction (CIB Report Publication 237) in 1999, placing emphasis on the use of agricultural waste products and other biological materials as building products for further sustainable construction practices [7].

The substitution of synthetic fibres for natural fibres has recently become a focus in composite research [8], [9], [10], [11], [12], [13], [14], [15]. For example, Dittenber and Ganga Rao [12] compared the physical and mechanical properties of glass fibre to more than 20 commonly used natural fibres such as sisal, ramie, kenaf, jute, hemp, flax, coir, and cotton, and found that of the various natural fibres tested, flax fibre offered the best potential combination of low mass, low cost, high strength, and high stiffness.

The behaviour of structural members of various configurations made or strengthened with flax fabric-reinforced epoxy (FFRE) laminates has been widely studied previously [16], [17], [18], [19]. Such research shows close similarity in tensile strength between FFRE and glass fibre reinforced epoxy (GFRE) composites [18], indicating the substantial potential for FFRE composites to be used in structural components, especially in developing countries where flax fibres are often available in abundance [16], [17], [20].

GFRP composites used in pipes, tanks and corrosion-resistant applications represent the third largest group of glass fibre applications in Europe [2], [21], such that this industry sector provides many potential opportunities for the application of flax fibres in composite pipe manufacture. FFRE offers many advantages for manufacturing pipes when compared to conventional pipe materials such as polyvinyl chloride (PVC), glass fibre reinforced plastic (GRP), steel, and concrete. FFRE resists corrosion [13], [22] and no protective coating is required on the internal and external pipe surfaces when compared to their steel counterparts. PVC and GRP pipes are commonly used to address the corrosion issue, and these alternative pipe systems are manufactured from non-sustainable resources. The application of synthetic fibres for the manufacture of water pipes may be a health hazard if any synthetic fibre debris causes contamination of the water passing along the pipe, whereas natural fibre debris may result in such health hazards being avoided [23]. Additionally, FFRE composites are approximately 16% the density of steel and 50% the density of concrete [24], such that FFRE pipes are expected to be more convenient for transportation and installation when compared to their steel and concrete counterparts. Furthermore, the reduced mass of FFRE results in lower inertia forces being attracted during dynamic loadings such as earthquake excitations when compared to their steel and concrete counterparts.

Although FFRE composites have the potential to be used in pipe manufacture, several challenges for practical application of FFRE pipes still exist. One major concern is the durability of FFRE composites [25], [26], with the shortage of existing data related to the durability of FFRE being one major challenge that needs to be overcome prior to widespread acceptance of FFRE for pipe manufacture. FFRE composites have relatively poor moisture resistance because of the presence of hydroxyl and other polar groups in the fibres when compared to glass or carbon FRP composites [27], [28], and flax fibres with a high moisture uptake have weak fibre/matrix interfacial bonding which may compromise the mechanical properties of the FFRE composites [28], [29]. It is therefore necessary to enhance the hydrophobicity of the natural fibres by treating them with suitable coupling agents or by coating the fibres with an appropriate resin matrix to generate FFRE composites having better mechanical properties and superior durability performance [13], [30]. A more comprehensive analysis should involve potential fracture mechanics and the fatigue behaviour of FFRE pipes. Other issues to be considered are that the void ratio and void behaviour may be different for gravity flow when compared to low, moderate and high-pressure pipe flow, and that the life span of FFRE pipes will merit attention and comparison against the performance of existing pipe materials. Also, the pressure requirements for different pipe applications will vary and the design requirements will be dependent on the application. By properly defining the required specification, the performance of the FFRE composite pipes can be properly benchmarked and assessed.

Lateral compression can arise within buried pipelines during their operation from the weight of the soil above the pipe (dead load) or by vehicles traversing the pipeline on the ground surface [31], [32]. Tubes are also commonly used as energy-absorbing devices that are subjected to lateral compression during impact events. For example, tubes are used to improve the crashworthiness of vehicles such as cars, lifts, aircraft, and ships [33], [34], crash barriers [35], road bridges, offshore structures, and oil tankers [34], or tubes are used to confine concrete columns or soil, with the application of that being associated with reinforcement of concrete columns or soil in infrastructure applications [16], [17]. In this study the term ‘tube’ is used to refer to cylindrical hollow sections used for structural purposes such as energy-absorbing devices or concrete filled steel/composite tubes used in infrastructure applications, whereas the term ‘pipe’ is used to refer to a hollow tubular section that transmits fluids or gasses.

The aim of the study reported herein was to address the feasibility of FFRE pipes to replace pipes manufactured from non-sustainable resources, such as GRP and PVC pipes, from structural performance perspective. If so, FFRE pipes would have a substantial potential to be used especially in developing countries where flax fibres are available in abundance. In this study, the response of FFRE pipes with varying diameters and fabric layers when subjected to lateral compression loading was investigated. A complete explanation of the pipe fabrication and testing methodology is provided, and pipe crown vertical deformation, pipe springline horizontal deformation, strength, circumferential strain at different locations, and pipe failure mechanisms are evaluated. Additionally, energy absorption and specific energy absorbed (SEA: energy absorbed per unit mass) characteristics of FFRE pipes presented herein are compared to energy-absorbing tubes with different materials.

Parallel-plate loading is a standardised test for ensuring that the stiffness and strength of a pipe subjected to lateral compression meets specified levels of performance [36], [37]. There are many studies in the literature which focus on steel pipe/tube lateral compression, with the main concerns of these studies being the effect of different geometric parameters on the energy absorption properties of the tubes [38], [39], [40], [41], [42], [43]. Research on lateral compression of composite pipes/tubes has been previously undertaken by many researchers [44], [45], [46], [47], [48], [49], [50], [51], [52]. Faria [47], Sebaey and Mahdi [48], Park et al. [49], Gupta and Abbas [45], and Abdewi et al. [46] investigated the lateral compression behaviour of composite pipes manufactured from glass-fibre resins, and showed that inter-layer delamination and fibre breakage were the main pipe damage mechanisms. El-Sobkhy and Singace [44] experimentally investigated the deformation of double-skin profiled polyethylene pipes subjected to lateral compression and concluded that increasing the pipe diameter improved the pipe energy absorbed and pipe specific energy absorbed. Niknejad et al. [50], Moeinifard et al. [51], and Rouzegar et al. [52] performed experimental studies to investigate the effect of pipe geometric properties on structural behaviour of E-glass fibre composite tubes when subjected to lateral compression, with the results showing that the total energy absorption capacity of the larger specimens is more than for smaller tubes.