New strategy based on crystallinity control in enhancing the flexural strength of drilled <scp>CF</scp> / <scp>PEEK</scp> composite laminate

Abstract For the first time, this paper proposes a new strategy based on crystallinity control in enhancing the flexural strength of drilled carbon fiber reinforced polyetheretherketone (CF/PEEK) composite laminate. The high‐positive correlation between the unique crystalline characteristic of the CF/PEEK composite and its flexural strength is introduced into the strategy, and the superposition of pre‐heat treatment, intermediate‐heat treatment, and post‐heat treatment is carried out to increase the crystallinity. The optimal heat treatment temperature of 320 °C is identified through in situ X‐ray Diffraction (XRD) patterns. The results show that the proposed strategy effectively increased the crystallinity by 21.70%, leading in turn to an increase the flexural strength of the CF/PEEK by 19.56%. The heat treatment compacts the molecular chains, enhancing the intermolecular interactions, which in turn result in increased thrust force in the drilled laminate. Four‐point flexural loading experiments of the open‐hole laminate reveal that CF/PEEK generates compression and tension failures at the macroscopic level and interlaminar and intralaminar failure at the microscopic level. Finally, the enhancement mechanism is elucidated, in which the transcrystallization of the matrix PEEK caused by heat treatment results in stronger bonding strength between the fiber and matrix, ultimately leading to an improved flexural strength of the drilled CF/PEEK laminate. Highlights Novel strategy enhances CF/PEEK flexural strength via crystallinity control. 21.70% crystallinity increase boosts CF/PEEK flexural strength by 19.56%. Optimal heat treatment at 320 °C is identified for CF/PEEK composites. Flexural tests reveal compression, tension, and interlaminar failures in CF/PEEK. Heat treatment fosters crystallization, improving CF/PEEK flexural strength.