Fatigue of flax-epoxy and other plant fibre composites: Critical review and analysis

Examining response of natural fibre composites (NFC) under cyclic loading is essential to encourage confidence in their mechanical durability. Considerable variation exists amongst reported studies in fibre architecture (UD-based, twill-fabric, short-fibre), testing parameters (frequency, stress ratio), and fibre content. There is need to conduct a holistic review of these disparate studies in order to establish the state-of-the-art. Testing parameters and physical variables (off-axis plies, moisture, 'out-of-plane' weave) are seen to influence longevity. Stress-life data of various NFCs are analysed and found to be well-modelled by linearised relationships. Specific stress-life (density-normalised) is proposed as a fairer measure of comparing fatigue endurance that minimises the influence of fibre content. Flax fibre offers better fatigue resistance than Hemp, but comparable performance to Sisal and Jute. Several NFC laminate configurations are found to exceed, or be similar to, Glass-laminates in fatigue endurance. Contradictory reports of stiffness evolution is found: fibre-direction elastic modulus may increase or decrease over fatigue life depending on test parameters. Limitations of available fatigue studies are identified. Existing knowledge of fatigue damage evolution is deficient or ambiguous, therefore inadequate for engineering design consideration.