The structure of phenolic resins depends on the pH under which the resin intermediate is produced. Resins intermediates produced under acidic conditions are called novolacs, and those obtained under alkaline conditions―resols. In this study, the activation energy and heat of the reaction that produces novolac and resol, followed by a thermosetting reaction, were obtained using quantum chemical calculations and used in molecular dynamics simulations of the reaction. The calculated resin structures were evaluated for each degree of curing, and the results showed that the structures were generally reasonable. The glass transition temperature increased with the degree of curing and tended to be higher for resol resins than for novolac resins. This trend is qualitatively consistent with the experimental data. The higher glass transition temperatures of resol resins may reflect the two-dimensional spreading of bonds owing to ortho- and para-orientation of chemical bonds during the formation of resins from resols. The reason for the higher glass transition temperatures of novolac resins at 20% conversion was analyzed, and it was attributed to the intermolecular forces being greater than in resol resins.

The development of spread-tow thin-ply technology and automated fiber placement has allowed for improved freedom concerning the design of composite laminates. This allows the previously inapplicable innovative stacking sequence to be applied to structural components. A disadvantage to this application is that numerous candidate stacking patterns must be reviewed in the design process. Furthermore, it was reported that the superior transverse strength and longitudinal shear strength of thin-ply CFRP do not always positively affect the notched laminate strength. To optimize the future design of CFRP laminated structures by tailoring the ply thickness and orientation, this study develops a nonlinear extended finite element method (XFEM) that considers both geometric and material nonlinearity, including plasticity, damage, and non-Hookean behavior. Using the developed scheme, the open-hole tensile strength of the CFRP laminates is evaluated in cases concerning various ply thicknesses and layups; the effects of nonlinearity on the laminate strength are subsequently examined.

Thin-ply composites, owing to their excellent damage resistance and mechanical strength, can significantly contribute to structural weight reduction. Additionally, they have the advantage of improving the design freedom in composite structures. In this study, the effects of the ply thickness and laminate layup on properties such as the damage resistance and mechanical strength in CFRP laminates with toughening interlayers were experimentally evaluated. Non-hole tensile (NHT), open-hole tensile (OHT), and filled-hole tensile (FHT) tests were conducted on CFRP laminates with various ply thicknesses and 0° layer ratios. The damage characteristics of laminates with toughening interlayers were investigated via damage observation using an optical microscope and soft X-ray inspection. The obtained damage resistance and mechanical strength were compared to those reported in previous studies conducted on specimens without a toughening interlayer. The correlation between the damage resistance obtained from NHT tests and mechanical strength obtained from OHT and FHT tests was discussed. The results showed that the toughening interlayer increased the OHT and FHT strengths, although the structure was more susceptible to cracks.

A new algorithm, force-based fully partitioned method (FFP), is proposed to analyze the fluid-structure interaction in the static aeroelastic problem of an aircraft wing. The FFP evaluates internal and external forces independently, assuming deformed shapes and corresponding deformation parameters. Consequently, the deformed shape that minimizes the residual force is defined as the equilibrium solution. The FFP is different from the conventional staggered method in which an iterative simulation is not required in the direction of fluid-structure interaction. The present study performs the structural and compressible inviscid flow simulations using the modal analysis coupled with a greedy method, referred to as the greedy modal FFP (GM-FFP), to represent high-order geometries. Applicability of the GM-FFP was examined for a transonic aircraft wing made of a carbon-fiber-reinforced plastic. The method exhibited reasonable convergence results compared to the conventional nonlinear block Gauss-Seidel method (BGS).