Fatigue behaviour and damage mechanisms under strain controlled cycling: Comparison of Flax-epoxy and Glass-epoxy composites

Recent research showed that strain-controlled (constant strain-amplitude) cycling may be ideal for natural fibre-composite (NFC) fatigue testing, since enforcing constant strain-rates eliminate the apparent modulus 'improvement' reported by several stress-controlled studies. This article re-examines Flax-reinforcement as an alternative to Glass by fatigue testing under constant strain-amplitude. Two Glass-epoxy crossply configurations ([0/90]3S, [& PLUSMN;45]3S) are tested alongside four commonly-studied Flax-epoxy layups ([0]16, [0/90]4S, [& PLUSMN;45]4S, quasi-isotropic [0/45/90/-45]2S). Fatigue endurance plots for all layups are modelled by linearised logarithmic strain-life relationships, for which unique parameters are identified. Evolution of several mechanical phenomena (stiffness damage, inelasticity, peak-stress) are evaluated as potential indicators of progressive damage, correlated with SEM-observed microstructure cracking. Flax-epoxy specimens have superior resistance to stiffness loss, lower inelastic strains , and withstand higher stresses per-unit-density, when compared to similar Glass-specimens. Flax remains a promising alternative to Glass for mechanical reinforcement against fatigue degradation in engineering structures.