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.

In thermoset-based carbon-fiber-reinforced plastics, the viscosity of the matrix resin governs their formability. The present study analyzes the relationship between the molecular structure and viscosity of the thermoset resin during curing by employing two types of all-atom molecular dynamics (MD) simulations. One is a crosslinking simulation that considers the reaction dynamics for determining the resultant curing structures. The other is a non-equilibrium MD simulation utilizing the Lees–Edwards boundary condition for viscosity evaluation. The results of this study clarified that the viscosity of the thermoset resin is changed by its composition, and can be described by the Doolittle equation, which presents the relationship with the free volume ratio. Furthermore, the increasing rate of viscosity during curing can be correlated with the increasing weight-averaged molecular weight.

A reliable numerical model for composite structures in vehicle crash simulations is needed because passive safety requirements are stricter than ever. The objective of the present study is to evaluate the prediction capability of a finite element (FE) model for bending and axial compression deformations, which are the main modes of vehicle structures in a crash scenario. First, we performed four-point bending simulations of a carbon fiber reinforced polymer (CFRP) laminated component using a multi-layered shell model, where the laminate consists of homogenized plies and interfaces between plies. The continuum damage mechanics (CDM) model is introduced to the intra-ply, and the damage parameters in the longitudinal direction are identified from a crack growth resistance curve (R-curve). The predicted fracture behavior and the force–displacement relationship are in good agreement with the test results. Next, we propose a discrete FE modeling scheme where cohesive elements (CEs) are inserted at all possible crack locations in axial progressive crushing. Because there is no mass loss due to element deletion, it is possible to accurately simulate the load transmission effect. Numerical studies with different laminate configurations reveal that the proposed model can capture the crushing mode under different circumstances.

During typical compression resin transfer molding (CRTM), resin is first injected into a gap between the mold and fiber preform, and the preform is then impregnated in the out-of-plane direction. CRTM thus has an advantage of reduced molding time because of the short impregnation distance. This study predicts resin flow during CRTM of composite structures by performing a resin impregnation simulation using the finite-element method. In the case of the CRTM of a thin curved plate, resin penetrated the preform beneath the gate by the resin pressure, when the injection of the resin into the gap was finished. Reduction of the molding time cannot be achieved in such a case, because impregnation proceeds over a long distance in the in-plane directions. Furthermore, in CRTM with multiple gates and multi-axial compression for an L-shaped component connecting two plates, impregnation was concentrated near the connecting part, and a non-impregnated area remained after finishing compression of the preform. The molding time by CRTM was always longer than that of vacuum-assisted resin transfer molding even at an increased compression speed. These results indicated that optimization of the molding conditions is necessary to achieve the benefits of CRTM.

Pitch-based low modulus carbon fiber increases the flexural impact properties of PAN-based carbon fiber/epoxy composites when it is used as an outer reinforcing layer. This is due to its larger compressive failure strain, which delays the fiber microbuckling failure of the PAN-based fiber layer on the compression side. In this paper, low modulus fiber was applied to the tip segment of a golf shaft as an outer reinforcing layer to provide higher impact performance. In a model circular composite tube for a golf shaft, a PAN-based fiber layer with a fiber modulus of 300 GPa was reinforced with a low modulus outer layer having a fiber modulus of 55 GPa. A flexural impact test showed that the low modulus fiber increased the flexural impact strength and the energy absorption of the composite tube. A finite element analysis based on damage mechanics indicated that the compressive strength of the PAN-based fiber layer increased from 1, 570 MPa in a monolithic unidirectional laminate to 2, 450 MPa in the hybrid tube. This analytical result suggests that the hydrostatic stress state around the loading point restrained the microbuckling failure of the PAN-based fiber layer and increased its compressive strength.

In the processes of liquid molding of fiber composites, such as resin transfer molding, fabrics are often formed in deep drawing, which gives rise to large shear deformation. In this paper, we simulate the fabric shaping process using the commercially available FEM program, LS-DYNA, in order to predict the properties of the resulting fiber composites. Plain woven fabric and power-net are modeled using beam and spring elements, which simulate yarn tensile property and fabric bending and shearing property, respectively. Inter-yarn sliding and yarn waviness are neglected. Coulomb law is applied on contact surfaces. The numerical results on yarn deformation are in good agreement with experiments. The deep drawing processes of plain woven fabrics and power-nets have been examined. The good formability of power-net structure has been confirmed numerically.

Extensive experimental research was conducted to pursue mechanical behavior occurred in the actual Compression-after-Impact (CAI) tests for conventional (brittle) CF/Epoxy composite plates. Two test methods, SACMA and NASA were used and examined for future possible improvements. Comparison of experimental results obtained by the two methods is another key issue in the present paper. Focuses are placed on the pursuit of mechanism of behavior during the tests. By examining the mechanics, an appropriate parameter was discovered for unified data reduction obtained by the two methods. It is based on a ratio of a virtual cylinder with an effective diameter of delamination excluding the “brim” region to a total sectional area of the specimen. CAI strengths and initial local buckling in delaminated area were universally explained by this parameter.

The present paper shows a reliable and efficient optimization approach on a minimum weight design of laminated composites under a single in-plane loading. Layer orientation angles as well as layer thicknesses are used as design variables, and the optimization technique is based upon a mathematical programming method. Transformed design variables with respect to the layer orientation angles in the principal loading direction are introduced to reduce the nonlinearity between strength constraints and design variables. A technique is also proposed to delete the strength constraints of almost zero thickness layers.