Damaged Response In Natural Fibre Reinforced Composites: Characterisation and Modelling Under Quasi-Static and Fatigue Conditions
Authors: Mahboob Ziauddin 
Published: May 2021

‘Natural’ fibrous material are subjects of accelerated research on account of the non-renewability and environmental costs of traditional ‘synthetic’ engineering fibres like Carbon and Glass. Of all candidates, Flax plant fibres have been found to offer composite reinforcement similar, or even superior, to Glass fibres in specific mechanical properties. Despite repeated evidence of its potential from independent studies, industry adoption of natural fibre reinforcement for load-bearing applications is still negligible, owing to their relatively immature body of research that discourages confidence in their long-term strength, durability, and predictability. This work contributes original findings on the complex damaged-condition response of natural fibre composites (NFC), and proposes modelling approaches to simulate the same. Material properties and mechanical behaviour of several Flax-epoxy composites are determined under tensile and compressive static loading, and correlated to internal damage mechanisms observed by micrography. Stiffness degradation and accumulated permanent strain are quantified along principal in-plane orthrotropic directions, which are used to develop a Continuum Damage Mechanics-based mesoscale model wherein constitutive laws are specifically formulated to reproduce NFC quasi-static response, including their highly nonlinear fibre-direction stiffness loss and inelasticity progression. Current progress of fatigue research is critically and extensively reviewed. Reported fatigue endurance and progressive damage behaviour of several NFC laminates are analysed. Existing knowledge on NFC fatigue damage is found to be insufficient and ambiguous, therefore inadequate for engineering design consideration. The unique fatigue-stiffening phenomenon reported for Flax-epoxy specimens is argued to be a misleading consequence of increasing strain-rate under constant stress-amplitude cycling. To minimise the influence of a varying strain-rate, original constant strain-amplitude fatigue tests are conducted on Flax-epoxy laminates, where no evidence of stiffening is observed. Considering this sensitivity to strain-rate, strain-amplitude controlled fatigue tests may be better suited for NFC investigation. Strain-controlled fatigue lives of Flax-epoxy can be modelled by a linearised strain/log-life relationship. Evolution of several material properties and dissipation phenomena (inelastic strain, peak stress, stiffness, hysteresis energy, superficial temperature) are measured, and correlated with SEM-observed damage mechanisms in the microstructure. An evolution/growth model is proposed to simulate laminate-scale stiffness degradation and cumulative inelastic strain as a function of applied peak strain and fatigue cycles, and is found to well-capture experimental trends for Flax-epoxy. The combined contribution of this work provides much-needed original data on the damaged-condition mechanical behaviour of Flax-epoxy and other NFCs under a variety of loading conditions, clarifies contradictory aspects of critical NFC behaviour, and proposes numerical methods to replicate observed progressive damage and failure in NFCs.