Mechanical characterization and fatigue behaviour of carbon/flax/sisal hybrid fiber reinforced epoxy composites

: This study investigates the fatigue behaviour and mechanical performance of hybrid epoxy composites reinforced with carbon fiber (CF), flax, and sisal fibres. Five laminate configurations with varying carbon fiber content (20%–100%) were fabricated to evaluate the influence of CF proportion on tensile strength, flexural strength, impact resistance, and fatigue life. The hybridization mechanism combines the high stiffness and load-bearing capability of carbon fibers with the crack-bridging, energy absorption, and damage-tolerant characteristics of flax and sisal fibres, thereby improving stress transfer and delaying crack propagation under static and cyclic loading. The results reveal that tensile, flexural, and fatigue properties improve progressively with increasing carbon fiber content, with the 100% CF laminate exhibiting the highest strength and fatigue resistance. Specifically, tensile strength increased from 80 MPa (20% CF) to 398.45 MPa (100% CF), while flexural strength increased from 138 MPa to 584 MPa. Similarly, impact strength improved from 95.59 J/m to 186.55 J/m, and fatigue strength increased from approximately 45.8 MPa to 159.38 MPa at 10 6 cycles with increasing CF content. However, this configuration demonstrated complete brittle fracture under cyclic loading, indicating low damage tolerance. In contrast, hybrid composites incorporating flax and sisal fibres exhibited slower damage propagation and enhanced crack resistance due to their inherent ductility. Although the 100% CF laminate exhibited the highest tensile, flexural, impact, and fatigue properties, the 40% CF hybrid laminate demonstrated a balanced combination of strength, deformation capability, and fatigue resistance, highlighting the effectiveness of carbon–flax–sisal hybridization. Further, Regression modelling was performed for flexural test and obtained results were validated from obtained regression value (R 2 =0.99) for the cubic order. Scanning Electron Microscopy (SEM) analysis reveals the differences in fracture mode of tested samples. Overall, carbon–flax–sisal hybrid composites offer a promising trade-off between strength, fatigue performance, economic viability, and environmental benefits, making them suitable for lightweight structural applications such as aerospace and unmanned aerial vehicle (UAV) secondary structures.