Effect of Lay-up Structures on Mechanical Properties of Yarn-Level Wrapped Flax/Basalt Woven Hybrid PLA Thermoplastic Composites

Abstract This study established a systematic experimental research method to investigate the effect of lay-up structure (i.e., fiber hybridization mode) on the mechanical properties of natural-inorganic fiber hybrid polylactic acid (PLA)-based thermoplastic composites. Using flax fibers (natural reinforcement phase) and basalt fibers (inorganic reinforcement phase) as reinforcements and environmentally benign PLA as the matrix, wrapped yarns (with flax/basalt as the core and PLA as the wrapping fibers) were fabricated via a yarn-level wrapping process, which were subsequently woven into flax and basalt plain-woven preforms, and finally six laminated composite specimens with different lay-up structures were prepared by hot-pressing. To clarify the performance discrepancies among composites with different lay-up structures, their flexural, tensile, and low-velocity impact properties were systematically characterized and evaluated, coupled with fracture morphology analysis for mechanism investigation. The results showed that the lay-up structure exerts a influence on the mechanical properties of thec omposites: the pure basalt lay-up composites exhibit the optimalstrength but inferior toughness, while the pure flax lay-up composites display outstanding toughness but relatively lower strength. Among the hybrid lay-up structures, the alternating lay-ups show superior comprehensive mechanical properties compared to the concentrated lay-ups-the former can achieve uniform stress transfer, improve the fiber-matrix interfacial bonding state, and mitigate basalt fiber buckling, while the latter exhibits limited performance enhancement due to stress concentration. Fracture morphology analysis further confirmed that flax fibers can effectively retard the crack propagation process of the composites. This study confirms that through rational fiber hybridization design (especially the alternating lay-up strategy), the strength, stiffness, and toughness of the composites can be effectively balanced, providing important theoretical support and technical reference for the development of high-performance and environmentally benign PLA-based composites.