This paper focuses on the transverse tensile and compressive mechanical properties of T800-grade unidirectional (UD) carbon fiber-reinforced polymers (CFRPs). Firstly, transverse tensile and compressive tests were conducted on UD composite laminates, yielding corresponding stress–strain curves. The results indicated that, for tension, the transverse tensile modulus, strength, and failure strain were 8.7 GPa, 64 MPa, and 0.74%, respectively, whereas for compression, these values were 8.4 GPa, 197.1 MPa, and 3.43%, respectively. The experimental curves indicated brittle failure under tensile loadings and significant plastic failure characteristics under compressive loading for the T800-grade composite. Subsequently, fractography experiments were performed to observe the fracture morphologies, revealing that tensile fractures were through-thickness cracks perpendicular to the loading direction, while compressive fractures were at a 52° angle to the loading direction. Finally, a micromechanical finite element method (FEM) was employed to simulate the tensile and compressive failure processes of the unidirectional composite, and the tensile and compressive properties were predicted. The simulation results showed that under both tensile and compressive loadings, interfacial elements failed first, causing stress concentration and damage to nearby resin elements. The damaged resin and interfacial elements expanded and connected, leading to ultimate failure. The predicted tensile stress–strain curve exhibited linear characteristics consistent with the experimental results in most regions but showed more pronounced nonlinearity before ultimate failure. The predicted compressive stress–strain curve aligned well with the experimental results in terms of nonlinearity. The predicted elastic modulus, failure strengths, and failure strains were in good agreement with the experimental results, with differences of 1.1% (tension modulus), 3.2% (tension strength), and 13.5% (tension failure strain), and 3.6% (compression modulus), −8.5% (compression strength), and −3.8% (compression failure strain). The final failure morphologies were in good accordance with the fractography experimental observations.

Modified phenylethynyl terminated polyimides (PIs) were successfully prepared by using neodymium oxide (Nd₂O₃) via high-speed stirring and ultrasonic dispersion methods. In addition, the structure and properties of the Nd₂O₃-modified imide oligomers as well as the thermo-oxidative stability of the modified polyimides (PI/Nd₂O₃ hybrid) and its modification mechanism were investigated in detail. The thermogravimetric analysis (TGA) results indicated that the 5% decomposition temperature (Td5%) of the PI/Nd₂O₃ hybrids improved from 557 °C to 575 °C, which was also verified by the TGA-IR tests. Meanwhile, the weight loss rate of the PI/Nd₂O₃ hybrids significantly decreased by 28% to 31% compared to that of pure PI under isothermal aging at 350 °C for 450 h when the added content of Nd₂O₃ was between 0.4 wt% and 1 wt%, showing outstanding thermo-oxidative stability. Moreover, the mechanism of the enhanced thermo-oxidative stability for the modified PIs was analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD).

The nanocomposites with reversible cross-linking covalent bonds were prepared by reacting furfurylamine (FA)-modified diglycidyl ether of bisphenol A (DGEBA) and furfuryl-functionalized aniline trimer-modified graphene (TFAT-G) with bismaleimide (BMI) via the Diels-Alder (DA) reaction. The successful synthesis of the TFAT modifier is confirmed by nuclear magnetic resonance (NMR) hydrogen spectroscopy and IR spectroscopy tests. The structure and properties of TFAT-G epoxy nanocomposites are characterized by scanning electron microscopy (SEM), differential scanning calorimeter (DSC), tensile, and resistivity. The results show that TFAT-G was uniformly dispersed in the resin, and 1 wt% TFAT-G composites increased to 233% for tensile strength, 63% for elongation at break, 66% for modulus, and 7.8 °C for Tg. In addition, the addition of unmodified graphene degrades the mechanical properties of the composite. Overall, the graphene/self-healing resin nanocomposites have both good self-healing function and electrical conductivity by adding 1 wt% modified graphene; this allows for the maintenance of the original 83% strength and 89% electrical conductivity after one cycle of heating repair.

A polyimide (PI) molecular model was successfully constructed to compare the performance of PIs with different structures. In detail, the structure of the cross-linked PI resin, the prepolymer melt viscosity, and the glass-transition temperature (T_g) were investigated using molecular simulations. The results indicate that benzene ring and polyene-type cross-linked structures dominate the properties of the PIs. Moreover, the prepolymer melt viscosity simulations show that the 6FDA-APB and the ODPA-APB systems have a low viscosity. The results for the T_g and the distribution dihedral angle reveal that the key factor affecting bond flexibility may be the formation of a new dihedral angle after cross-linking, which affects the T_g. The above results provide an important reference for the design of PIs and have important value from the perspective of improving the efficiency of new product development.

In the face of the difficulty in achieving high-quality integrated molding of longitudinally and transversely stiffened panels for helicopters by resin-matrix composite materials, we combine the prepreg process and the resin transfer molding (RTM) process to propose a hybrid resin transfer molding (HRTM) for composite stiffened panel structures. The HRTM process uses a mixture of prepreg and dry fabric to lay up a hybrid fiber preform, and involves injecting liquid resin technology. Using this process, a longitudinally and transversely stiffened panel structure is prepared, and the failure modes under compressive load are explored. The results show that at the injection temperature of the RTM resin, the prepreg resin dissolves slightly and has little effect on the viscosity of the RTM resin. Both resins have good miscibility at the curing temperature, which allows for the overall curing of the resin. A removable box core mold for the HRTM molding is designed, which makes it convenient for the mold to be removed after molding and is suitable for the overall molding of the composite stiffened panel. Ultrasonic C-scan results show that the internal quality of the composite laminates prepared using the HRTM process is good. A compression test proves that the composite stiffened panel undergoes sequential buckling deformation in different areas under compressive load, followed by localized debonding and delamination of the skin, and finally failure due to the fracture of the longitudinal reinforcement ribs on both sides. The compressive performance of the test specimen is in good agreement with the finite element simulation results. The verification results show that the HRTM process can achieve high-quality integrated molding of the composite longitudinally and transversely stiffened panel structure.

The crystallization behavior of poly(aryletherketone) (PAEK) determines its applicable molding process and profoundly affects its mechanical properties. However, research on the crystallization behavior of new PAEKs and their impact on performance is still insufficient. In this work, the isothermal crystallization behavior of a novel PAEK was studied and compared with that of standard poly(etheretherketone) (PEEK). The influence of molding temperatures on the mechanical properties of thermoplastics was revealed by controlling the crystallization temperatures and analyzing the crystallization behavior. The results indicate that due to the disruption of the molecular structure regularity of PAEK, its melting temperature for primary crystallization is generally about 30 °C lower than that of PEEK, which is beneficial for its molding at lower temperatures. At the same undercooling level, the crystallization rate of PAEK is lower than that of PEEK, making it easier to control the crystallinity of PAEK through process parameters. The crystallinity of the thermoplastics increases with the increase in soaking time, thereby improving their tensile strength and modulus. The maximum crystallinity of PAEK is approximately 20.5%, which is lower than PEEK’s value of 31.8%. Therefore, under the same undercooling condition, the tensile strength and modulus of PEEK increase by up to 29.5% and 17.1%, respectively, compared to PAEK. Therefore, by precisely controlling the molding process parameters of PAEK, their crystallization behavior can be managed, enabling the achievement of various properties as needed.