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The performance change and mechanism of polyimide composites with carbon fiber sizing and desizing states during thermal aging at 330 ℃ were compared and analyzed from the aspects of thermal oxidation mass loss, surface morphology and micro morphology, mechanical properties and glass transition temperature. The results show that carbon fiber desizing can reduce the mass loss rate of thermal oxidation of polyimide composites. The interface layer of the composites with sizing carbon fiber contains epoxy sizing agent, which has low temperature resistance. Therefore, the interface is easy to be damaged in the process of thermal aging, resulting in more cracks and rapid separation of edge fibers. Although the composites with sizing carbon fiber has high interlaminar shear strength before aging, it decreases rapidly in the early stage of thermal aging. On the contrary, the interlaminar shear strength of the composites with desizing carbon fiber is higher after aging for 200 h. The bending strength of the two composites before aging is basically the same, but the bending strength of the composites with sizing carbon fiber decreases rapidly in the early stage of aging; however, desizing has little effect on bending modulus. The glass transition temperature of the composites with desizing carbon fiber increases by about 15 ℃. After thermal aging, the glass transition temperature of the composites with sizing carbon fiber increases slightly, while the composites with desizing carbon fiber remain unchanged. Therefore, the high temperature desizing treatment of carbon fiber is conducive to improving the stability of polyimide composites for long-term use at high temperature.

According to the flanged structure of composite casing, a typical sample with laminated structure manufactured by RTM process was designed using T800 grade carbon fiber fabrics reinforced polyimide composite. The strength property under the uniaxial tension condition was simulated by finite element method, and the calculated fracture load was 21.7 kN. The maximum strain was appeared at the corner of flanged structure, indicated that the corner area of flanged structure was weak relatively. Ultrasonic C-scan and optical microscope methods were applied to analyze the internal quality of the corner area. The results show that the overall internal quality of the sample corner area is good without obvious delamination defects. However, some micro-pores enriched at near surface and the fill region are still visible. The measured average tensile fracture load is 19.81 kN, and that is very close to the calculated value. After cyclic loading with 25% of the fracture load for 24000 times, the residual tensile fracture load is still 19.94 kN, this result means fatigue test has no effect to the mechanical property of the samples. Little interlayer defects and cracks happened in several samples at the corner area, but no visual damage. It shows that the sample can still bear the load after the delamination defects appear.

Thermoplastic polyether ether ketone (PEEK) composites are widely used in aerospace field due to their excellent fracture toughness, impact resistance and material versatility. Sizing agent as the core auxiliary product of carbon fiber has an important impact on the interface of composites. Limited by the decomposition temperature, the traditional thermosetting sizing agents are difficult to meet the use of PEEK composites, which restricts the development and application of high-performance PEEK composites. Therefore, it is of great significance to develop a matching carbon fiber sizing agent for PEEK composites. In this paper, the interfacial properties of composites and the action mechanism of sizing agent were analyzed and introduced; the research progress and results of modified PEEK, polyimide precursor and polyetherimide sizing agents were focused, and different systems of sizing agents were analyzed and summarized.Finally, the relevant suggestions on carbon fiber sizing agents for PEEK composites were put forward while the environmental and multi-function developments for sizing agents were prospected.

Aiming at the need of aero-engines, investigation was made into the curing process and thermal stability of a polyimide-matrix structural composite. The curing kinetic equation of EC-380A resin was established. The curing degree of the resin was simulated as a function of the curing temperature and time. Further, combined with the resin rheology, the curing process of the EC-380A composite was established and verified. Large-scaled aero-engine typical structural components were fabricated. Thermal stability of the composite was estimated by mass loss, internal quality of the flaw-embedded laminate, and mechanical property after thermal ageing. By multi-temperature step curing at 330-380℃, the flaw-free composite could be manufactured under the circumstance of an integrated layup. The composite has excellent thermal stability with thermal resistance of 370-400℃. The mass loss rate is around 1.3% after ageing for 1000 h at 370℃ and 285℃.No new flaw or propagation of the embedded flaw occurs for the composite after thermal ageing at 400℃, indicating a high-temperature structural stability.

Resin matrix composites have many advantages such as high specific strength and modulus, good fatigue performance, corrosion resistance, and have become the application and development trend of aero engine components under 400 ℃. Foreign research on resin matrix composites for aero engine started earlier, which have been applied in fan blades, fan casings, outer ducts, nacelles and other components of multi-engine, and developed towards the trend of better structure, higher material performance, lower manufacturing cost and higher automation degree. The development foundation of domestic resin matrix composites is good, but compared with foreign countries the application proportion of resin matrix composites in engines in not high. It is necessary to furthur improve the technical level of design, materials, manufacturing, experiment and engineering ability. In this paper, foreign development status was discussed in the field of structures, materials and processing methods of aero engine composite components, the development trend was analyzed and corresponding suggestions were given, from the aspects of building composites system for aero engine, strengthening application research and design guide, promoting the transformation of pre-research achievements and application of automation technology.

A resin-transfer-molding (RTM) polyimide resin, RTM-PI, was developed, and the matrix-graded polyimide composite along thickness directions was fabricated with RTM processing. The matrix-graded composite was composed of surface layers with polyimide matrix having high thermal stability, and core layers with RTM-PI matrix. The RTM processing parameters were established based on thermogravimetry, rheology and curing data of PI resins. The results show that RTM-PI resin exhibits the properties of low viscosity, super long processable time, and high thermal resistance. The lowest viscosity is below 0.1 Pa·s, processable time is longer than 300 min, and glass-transition-temperature reaches 445 ℃. The internal quality of the matrix-graded polyimide composite along thickness direction is excellent, and the reduction of mass loss after the thermal ageing at 350 ℃ for 100 h is more than 48% lower than that of the pure RTM-PI composite.

Serious shrinkage and warpage are obstacles to the development of ideal isotactic polypropylene (iPP) materials for polymer‐based powder bed fusion (PBF) technique. In this work, the variations of the dimensional accuracy of the PBF‐printed iPP parts were investigated with various printing parameter and nucleating agent. The iPP parts printed at a scanning speed of 700 mm/s exhibit smaller extents of shrinkage and curling than those at lower speeds, due to a lower degree of crystallinity. Interestingly, iPP blended with the α‐ or β‐nucleating agent demonstrates more serious part deformation with respect to neat iPP. Especially, α‐nucleating agent tends to trigger the most severe curling under the investigated printing parameters. Based on X‐ray diffraction and differential scanning calorimetry (DSC) results, both neat iPP and α‐iPP parts crystallize into α‐crystal, while β‐iPP parts display the coexistence of β‐ and α‐crystals. And, the difference of the crystallinity is less than 3% in three specimens. This suggests that both crystallinity and crystalline structure are not the main reasons for the shrinkage and warpage in this case. Instead, the severe part deformation of the α‐iPP parts is assigned to the narrower sintering window as well as the higher onset crystallization temperature of α‐iPP, which hinder relaxation of residual stresses. This work provides insights into the part deformation mechanism for PBF‐processed polymer materials. [ABSTRACT FROM AUTHOR]