Mechanical failure and strengthening mechanism of nanoporous phenolic composites reinforced with needle‐punched fiber preforms of different needle‐punched densities

Abstract Needle‐punched fiber preforms are extensively employed as reinforcements for low‐density thermal protection materials (TPMs) due to their high design flexibility and cost‐effectiveness. However, the inherent complexity and randomness of the needle‐punched technique present challenges in elucidating the impact of needle‐punched parameters on the mechanical performance of these materials. Herein, needle‐punched preforms, including needle‐punched fiber felts (NF) and needle‐punched woven fabric/felt (NWF), are deliberately designed with varying needle‐punched densities (0, 10, 30, 50 punches/cm²) to reinforce the nanoporous phenolic composites (NPCs). The mechanical performance and failure mechanism of these composites are comprehensively investigated. Tensile testing results reveal that a small number of needle‐punched fiber bundles along the Z‐axis improve the in‐plane tensile properties of both NF and NWF reinforced NPCs, owing to the rivet‐like structure of needle‐punched fibers that hinders crack propagation. However, as the needle‐punched density increases, the tensile performance of NPCs gradually decreases due to increased damage induced by the needle‐punched fibers. Conversely, the interlaminar shear performance and fracture toughness of NPCs are progressively improved with increasing needle‐punched density, as the effectiveness of the rivet‐like structure is enhanced. The present work will provide valuable insights for the design and optimization of needle‐punched fiber preform reinforced composites. Highlights The fiber preforms with varying needle‐punched densities are designed. Impact of needle‐punched density on mechanical behavior is investigated. Effect of rivet‐like structure of needled fiber is revealed by micro‐CT. Micro‐failure mechanisms of tensile and shear are investigated.