Characterization and Modeling of Natural-Fibres-Reinforced composites (Moisture Absorption Kinetics, Monotonic Behaviour and Cyclic Behaviour)

  • Author / Creator
    Fotouh, Ahmed
  • Natural fibres have been shown to offer a good potential in replacing or supplementing synthetic fibres in composite material applications. To fully utilize these new materials in design, however, engineering models of the mechanical behaviour need to be developed and validated. In this research, the moisture absorption and mechanical behaviour of hemp-fibre-reinforced polyethylene composites at various fibre volume fractions were investigated and modelled. In terms of environmental exposure, the effects of fibre volume fraction (vf) and matrix crystallinity along with matrix stiffness and contraction on the mechanisms of moisture sorption were investigated. The maximum amount of absorbed moisture (Mtmax) was determined for each fibre volume fraction. The composite diffusion coefficient (D) was measured to distinguish the ability of water molecules to diffuse into the biocomposite. The increase in the matrix crystallinity level in addition vf of the tested composites increased the moisture absorption rate. Fickian diffusion was found to be the dominant moisture diffusion behaviour. The stress-strain behaviour of the hemp fibre composites were analyzed and modelled for both monotonic (rate dependent) and cyclic loading conditions. An exponential model was developed to simulate the monotonic stress-strain uniaxial behaviour. A strain rate hardening detected and a model was developed by applying the non-linear form of Norton-Hoff rheology model for viscoplastic material to simulate the relationship between the strain rate ( ε ) and each mechanical property of the tested composites. The strain rate hardening model was later incorporated with an exponential model to develop a new general stress-strain model to simulate the monotonic tensile behaviour of the tested natural-fiber-reinforced composites. The developed new model took into account the effect of ε and vf of the composite as well as the effect of moisture absorption. Fatigue tests were also performed at two fibre volume fractions as well as the reinforced polymer under both wet and dry conditions. The fatigue strength of the polymer was slightly improved by addition of hemp fibers; though, the sensitivity of the developed fatigue life curves did not change. A generalized model was developed using the normalized fatigue life diagrams. These diagrams were normalized by a new developed modified stress level (Sm). The previously developed strain rate hardening model was then incorporated into the fatigue model to capture the effect of the changes in the loading rate. The new fatigue model was capable of predicting the fatigue life at different frequencies (f), fatigue stress ratios (R), fatigue stress amplitudes (Δσ) and vf. Additionally, the fatigue model succeeded to simulate the degradation effect of moisture absorption on the fatigue strength. The new developed models provide essential tools for designers to incorporate this new material into a new generation of reliable products.

  • Subjects / Keywords
  • Graduation date
    Fall 2014
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.