Owing to its superior mechanical properties and recyclability, the carbon fabric/polyetheretherketone (CFF/PEEK) composite has seen increasing application in engineering domains. However, studies examining the impact of hygrothermal aging on its performance remain relatively limited in the existing literature. To investigate its durability in hygrothermal environments, this study fabricated CFF/PEEK composites with a fiber volume fraction of 55 vol% and subjected them to equilibrium hygroscopic treatment at 70 °C. The hygroscopic behavior of polyetheretherketone (PEEK) resin and CFF/PEEK composites, along with their tensile and compressive properties under dry conditions at room temperature (RTD) and wet conditions at 70 °C (ETW), were systematically evaluated. The results indicated that both PEEK resin and CFF/PEEK composites exhibited Fickian diffusion behavior during the initial stages of aging but diverged in later stages. The equilibrium moisture absorption rates were approximately 0.32% for PEEK resin and 0.19% for CFF/PEEK composites. After aging at 70 °C, the strength of both materials decreased significantly, while the modulus showed only minor changes. Under ETW conditions, the tensile strength retention rate of PEEK resin was 74.92%, and the compressive strength retention rate was 81.85%. For the CFF/PEEK composites, the tensile strength retention rate was approximately 85%, and the compressive strength retention rate was about 95%. The typical failure modes of CFF/PEEK composites did not exhibit notable differences between tensile and compressive specimens after hygrothermal aging. Resin debonding was observed in the moisture-absorbed composite specimens, while no microcracks or delamination were detected. The degradation of mechanical properties is predominantly attributed to the deterioration of the resin matrix and interface characteristics, which are caused by water molecule intrusion and the adverse effects of wet strain mismatch between the resin and fibers.

Carbon fabric reinforced polyetherketoneketone (CFF/PEKK) composites have garnered significant attention from researchers due to their superior properties and have been successfully applied in various engineering fields. Environmental conditions are known to directly influence the mechanical properties and service life of composites; however, limited literature exists on the mechanical behavior of CFF/PEKK composites under different environmental conditions. This study elucidates the correlation between the bending and shear behaviors of CFF/PEKK composites and environmental factors, thereby offering robust data support for engineering applications. In this work, CFF/PEKK composite laminates with a fiber volume fraction of 55 vol% were fabricated and subjected to saturated moisture absorption treatments at 70 °C. The moisture absorption characteristics of the material were investigated. The bending and shear properties of CFF/PEKK composites were characterized under three environmental conditions: −55 °C dry state (CTD), room temperature dry state (RTD), and 70 °C wet state (ETW). Failure modes and mechanisms of composite specimens were also analyzed. The equilibrium moisture absorption rate of CFF/PEKK composites is approximately 0.27%. Hygrothermal aging resulted in noticeable fiber pull-out in mechanical specimens, indicating damage to the interfacial performance of the composites. Furthermore, no cracks or delamination were observed. Results indicate that in the CTD condition, the bending strength and shear strength of CFF/PEKK composites are higher compared to those in the RTD condition, while the modulus remains relatively unaffected. In the ETW condition, both bending and shear properties exhibit a significant decline, with the most pronounced reduction observed in interlaminar shear strength. No significant differences in failure modes were noted across different environmental conditions.

To clarify the effect of surface characteristics on the interfacial properties of T1100-grade carbon fiber (CF)/bismaleimide (BMI) composites, three CFs (F1, F2, and F3) with different surface treatments and sizing agents were studied. Surface physicochemical properties and sizing–resin reaction behavior were characterized; nano-infrared spectroscopy was innovatively used to quantify interfacial structure. The correlation among surface features, interfacial structure, and mechanical properties was established. All dry-jet wet-spun T1100 CFs show smooth surfaces with similar roughness, and mechanical interlocking contributes little to interfacial adhesion. F3 possesses the highest active carbon, oxygen content, and epoxy value. Its sizing agent exhibits strong reactivity with BMI, forming a ~200 nm thick interface and the highest interfacial shear strength (IFSS) of 95.9 MPa. Constructing a “thick and strong” interface promotes shear failure from brittle to tough, significantly enhancing 90° tensile and interlaminar shear strength (ILSS). This work provides guidance for interface design and engineering applications of T1100/BMI composites in aerospace primary load-bearing structures.

It has long been desired to achieve mechanical enhancement and structural health monitoring by introducing carbon nanotubes (CNTs) into traditional carbon fiber (CF) composites. Herein, the initiation of micro-damage and crack propagation has been investigated by utilizing in situ electrical resistance changes in interlaminar hybrid CNT network/CF composites during the shear loading process. The results show a clear relationship between the crack propagation and the electrical resistance response particularly when approaching the failure of the single-layer CNT network hybrid composites. Furthermore, the chemically modified CNT network exhibits evident enhancement on main mechanical properties of the CF composites, superior to the thermoplastic toughening method. The characterizations manifest that the multiscale interlayered CNT/CF structure can simultaneously resist the crack propagation along both the in-plane direction and the cross-plane direction, which consequently enhances the flexural and compressive strengths of the composite material. This discovery provides a novel idea for the potential application of CNT network/CF hybrid composites in the integration of mechanical reinforcement and structural health monitoring, namely, that the CNT network acts not only as a reinforcing phase but also as a sensor for the structural health monitoring of the composites.