Automated fiber placement can manufacture composite laminates at a low cost and in a short time; however, gaps and overlaps of plies are introduced due to the lack of positional accuracy of the robot arm. To clarify the effects of these processing defects, we observed the damage progression in carbon fiber reinforced plastic laminates with a manually introduced gap or overlap. Specimens were prepared under different forming conditions depending on the number of processing defects and the presence or absence of a cowl plate. In the case of formation without a cowl plate, specimens with a gap exhibited ply waviness in the through-thickness direction, the resin-rich region, and thickness reduction at the gap position. Thickness increase and ply waviness were observed at the overlap position. Sequential edge face observation experiments showed that specimens without cowl plates exhibited early delamination near the gap/overlap defect. The strain distributions at the defect location were measured using the digital image correlation method. Results showed that the thickness change at the defect location caused a shift in the neutral plane position, resulting in point-symmetric shear strain. Therefore, the early delamination was caused by the bending moment near the defect location.

Recently, carbon-fiber-reinforced plastic (CFRP) has been increasingly used in the aerospace industry. To apply CFRP as an aircraft-grade structural material, thermosetting resin is used as a matrix to achieve high mechanical properties and environmental resistance. Additionally, the development of multifunctional CFRPs using matrix resins with functional properties, such as flame retardance, heat resistance, and vibration damping, is ongoing. In this study, molecular dynamics (MD) simulations and experiments were performed to analyze the thermomechanical properties of epoxy/cyanate resins compared with those of conventional epoxy/amine resins, to evaluate the application of this heat resistant resin as an aircraft-grade structural material. This study clarified the properties of epoxy/cyanate resins, which had no cure shrinkage, high glass transition temperature, and low thermal conductivity. These properties are attributed to the characteristic cross-linked structure of epoxy/cyanate resins, which demonstrates their potential as a structural material for next-generation aircrafts.

Recent years have witnessed the development of 3D printing systems of continuous carbon fiber reinforced thermoplastics (CFRTP) for use in various applications. In terms of convenience, 3D printing of CFRTP has attracted considerable attention in various fields and industries. In this study, non- and open-hole compressive (NHC and OHC) properties of 3D-printed CFRTP were evaluated. In an NHC test, the compressive elastic modulus and strength in the fiber alignment (0º) direction were obtained with values of 58.1±4.0 GPa and 368.0±33.3 MPa, respectively. A theoretical study of the relationship between compressive strength and the initial fiber misalignment angle showed that the latter was greater than that of conventional hot-pressed CFRTPs, suggesting that the fiber waviness of 3D printing has a negative effect on the longitudinal compressive strength. In an OHC test, cross-ply laminates with center holes were prepared, where the OHC strength was 165.6±1.2 MPa. Both fracture surface and X-ray CT observations showed that kink bands occurred in the 0º layer at the edge of the center hole and that extensive delamination was observed at the 0º/90º interface around the hole. These observations suggest that the voids attributed to 3D printing merged with the delamination derived from fiber buckling near the center hole, leading to rapid expansion of the delamination.

Polymer materials have a viscoelastic property that is time- and temperature-dependent. The time-temperature superposition principle (TTSP) is usually applied to estimate viscoelasticity. In this study, creep analyses were performed using molecular dynamics (MD) simulations to understand the phenomenon of TTSP. For MD, an amorphous polyethylene model was assumed and generated, and creep analyses were performed at four different temperatures. Arrhenius-type TTSP was then applied to the creep simulation results. To gain insight into the phenomenon of TTSP, the potential energies of the models were analyzed. All potential energy curves at the different elevated temperatures were superimposed, and the shift factors and activation energies of each potential energy were obtained. The bond angle and torsion potential energy were found to have a significant effect on TTSP compared with other factors.