Fibers and Polymers 0000, Vol.0, No.0, 1-8 1 On Energy Absorption Capacity, Flexural and Dynamic Properties of Flax/Epoxy Composite Tubes Libo Yan * , Nawawi Chouw, and Krishnan Jayaraman 1 Department of Civil and Environmental Engineering, The University of Auckland, Auckland Mail Centre, Private Bag 92019, Auckland 1142, New Zealand 1 Department of Mechanical Engineering, The University of Auckland, Auckland Mail Centre, Private Bag 92019, Auckland 1142, New Zealand (Received October 2, 2013; Revised December 23, 2013; Accepted December 31, 2013) Abstract: In this study, energy absorption capacity, flexural and dynamic properties of flax fibre reinforced epoxy polymer composite (FFRP) tubes are investigated. The energy absorption capacity of the tubes is investigated under uniaxial compression. Flexural behaviour of the tubes is studied under four-point bending and the dynamic properties (i.e., natural frequency and damping characteristics) are evaluated by impact hammer vibration testing of the tube specimens. The damping characteristics of the tubes are determined by using both a logarithmic decrement curve and the half-peak bandwidth method. The influence of tube laminate thickness and specimen size on the mechanical properties of FFRP tubes is determined. Compressive testing indicates that the FFRP tube provides a specific absorbed energy of 22 J/g, which is close to the conventional metal energy absorption materials, i.e. stainless steel and aluminium tubes. Flexural study shows that the FFRP tube exhibits a brittle failure as similar to that of the FFRP composites in a flat-coupon tension. The load carrying capacity and deflection of the tube increase with an increase in the tube thickness. Impact loading test concludes that an increase in tube thickness leads to a reduction in natural frequency and damping ratio of the tubes. The FFRP tubes have size- dependent dynamic properties, i.e. an increase in tube size increased the natural frequency but reduced the damping ratio of the specimens remarkably. However, all FFRP tubes have high damping ratios, thus reducing the effect of dynamic loading on the structural response. Therefore, this study suggests that FFRP tubes could be used in several structural applications, i.e. in automotive as energy absorbers and in civil infrastructure as poles. Keywords: Energy absorber, Flax FRP tube, Vibration, Thickness effect, Damping characteristics Introduction Synthetic fibre-reinforced composite tubes have been widely used in aerospace, automotive and civil engineering due to their high strength and stiffness and high corrosion resistance [1]. In recent years, the use of bio-fibres to replace synthetic fibres as reinforcement in polymer composites has gained popularity in engineering applications due to increasing environmental concern [2]. Polymer matrices reinforced by bio-fibres, e.g. flax, hemp, jute and sisal, are cost-effective, have low density with high specific strength and stiffness, and are readily available [3]. The advent and application of nanotechnology have generated renewed interest in bio- composites which show promising potential as the next generation of structural materials [4,5]. Because of the relative ease of fabrication, fibre reinforced polymer (FRP) composites have been tailored into structures with different geometries, i.e. plates, shell, tube, etc. In the form of tubes, FRP are widely considered in the application areas such as energy absorbers [5,6], pressure vessels [7], pipe lines [8] and civil structural components [9]. For energy absorption application, conventional glass or carbon fibre reinforced composite tubes have been extensively investigated. Elgalai et al. [10] studied the effect of tube length on energy absorption capacity of woven roving glass/epoxy composite tubes. It was concluded that the crushing behaviour and energy absorption capabilities are sensitive to the change in tube length (111, 174, 237 and 300 mm, respectively). In addition, the tested tubes exhibited different failure modes with a change in tube length. Elgalai et al. [11] also studied the energy absorption capacities of carbon/epoxy and glass/ epoxy corrugated tubes. The study indicates that introducing corrugation could significantly enhance energy absorption capability of both carbon/epoxy and glass/epoxy tubes in a uniform manner. The measured specific absorbed energy was 13.49-15.7 J/g and 6.75-9.53 J/g for carbon/epoxy and glass/epoxy tubes, respectively. Abosbaia et al. [12] studied the energy absorption capacity of segmented and non- segmented composites tubes with different fibre materials, i.e. tissue mat glass, cotton fabric and carbon fabric. The study shows that the non-segmented tissue mat glass/epoxy tubes were very brittle and the non-segmented cotton fabric fibre/epoxy tubes had low energy absorption capacity. On the other hand, the segmented composite tubes including carbon fabric and cotton fabric fibres exhibited good energy absorption capability as well as stable load carrying capacity. Considering natural fibre reinforced polymer composites for energy absorption application, Mahdi et al. [13] investigated filament wound cotton/epoxy circular tubes with different diameters. It was found that the load-displacement curves of these tubes are strongly dependent on the geometry configuration and the fibre orientation. The specific absorbed energy for *Corresponding author: lyan118@aucklanduni.ac.nz DOI