Fatigue behaviour of flax fibre composites: Influence of manufacturing-induced microstructural parameters

This study investigates the effects of manufacturing-induced variations, specifically fibre orientation, fibre volume fraction, and void content, on the tensile and fatigue behaviour of natural fibre composites (NFCs). Fibre misalignment was quantified using X-ray computed tomography coupled with VoxTex analysis, while fatigue damage evolution was monitored via acoustic emission (AE). The results show that improved fibre alignment significantly enhances static mechanical performance. The composite with a 2° standard deviation in fibre orientation exhibits a modulus of ∼26 GPa and a strength of ∼259 MPa, representing increases of approximately 13% and 12%, respectively, compared with a composite containing an 8° misalignment. Fatigue tests further reveal that composites with better fibre alignment demonstrate higher modulus retention, lower residual strain, and longer fatigue life. Specifically, samples with improved alignment show ∼17% higher modulus during fatigue loading and reduced plastic deformation compared with their more misaligned counterparts. AE analysis confirms increased damage initiation and propagation in composites with greater misalignment and higher void content. Moreover, when fibre orientation is held constant, void content is shown to exert a stronger influence on fatigue behaviour than fibre volume fraction. Overall, the findings demonstrate that fibre orientation is the most influential parameter governing fatigue performance, with even minor degrees of misalignment or void content causing substantial reductions in durability. These results provide valuable insight for refining manufacturing processes to ensure improved performance of NFC structural components under fatigue loading.