Carbon Fiber Reinforced Plastics (CFRP) is widely used in the structural components of aircraft. Upon retirement, aircraft generate a large amount of CFRP waste, typically disposed of in landfills as industrial waste. However, for CFRP to become a sustainable industry staple in the future, methods for recycling and reusing it must be established. Domestic and overseas venture companies are actively working on exploring and establishing technologies to extract carbon fibers. However, despite ongoing research, there remains a scarcity of initiatives aimed at recycled carbon fiber (rCF) utilization. To aim for a complete circular economy in the future, it is desirable to ensure sufficient mechanical properties when turning rCF into CFRP. This paper introduces efforts aimed at improving the mechanical properties of rCFRP.

This work extends the applicability of a recent variational model that has been used for analyzing laminates to determine the stress state in a laminate with alternating material properties in the longitudinal direction subject to tensile and bending loading. For simplicity, a plane stress problem was assumed in this study. The laminate was subdivided in the longitudinal (tensile loading) direction as in the previous analysis. In addition, the laminate was subdivided in the thickness direction by using the ply refinement technique. To introduce the ply refinement technique and to account for continuity boundary conditions between divided regions in the loading direction, the Lagrange multiplier strategy was used. Closed form expressions for the energy function derived from using the strategy led to a very concise solution for determining the approximate stresses in the laminates. As an example, a laminate with ply discontinuity was analyzed, and the resultant stress distributions were then compared to the previous variational analysis model and finite element method (FEM). Compared to the previous analysis, stress concentration at the region interfaces could be captured, and excellent agreement between FEM was demonstrated.

A digital image correlation (DIC)-based crack detection system was developed for composite materials. To avoid incorrect displacement determination, subsets for DIC analysis around cracks were rearranged from their original positions according to their correlation coefficients. The system validity was evaluated through in situ cyclic loading test observations, in which fatigue crack growth in carbon fiber reinforced polymer (CFRP) was captured using a trigger system synchronized with cyclic loading. DIC analysis was carried out in the 90° ply of a cross-ply CFRP laminate. The cross-sections of carbon fibers were utilized as a random speckle pattern for the analysis. The results showed that the correlation coefficients remained higher over the inspected area with subset rearrangements but became lower without rearrangements around transverse cracks. The proposed method successfully evaluated the locations of cracks and crack opening displacements. Consequently, the propagation behavior of the transverse cracks was clearly captured under cyclic loading.

Thin-ply CFRP laminates with ply thickness less than 0.05 mm exhibit superior strength than conventional CFRP laminates. To elucidate the damage behavior of CFRP laminates with different layer thicknesses, we performed progressive damage analyses in open-hole compression (OHC) and filled-hole compression (FHC) tests. Finite element analyses incorporating continuum damage mechanics with 3D-Hashin criterion and cohesive zone model were used to simulate the progressive damage failure. The failure mode and morphology estimated by the numerical simulations are consistent with the experimental results. Furthermore, the orders of open-hole and filled-hole (without fastening torque and with 7.0 Nm fastening torque) compressive strengths obtained from the experiments are in good agreement with the simulation results. The results of progressive damage analyses confirm that thinner plies exhibit higher open-hole and filled-hole compressive strengths than thicker ones.

This study examined lightning strike damage to CF/toughened epoxy and CF/epoxy composites to understand the role of inter-laminar toughened layers. An impulse electric current of 40 kA with negative polarity, in accordance with SAE ARP 5412B, was applied to CF/toughened epoxy, CF/epoxy, and aluminum specimens. Temperature response on the rear surface of these specimens was measured using an IR camera during arc entry tests. Compared to CF/epoxy, CF/toughened epoxy suffered damages such as carbon fiber breakage and delamination. Electrical conductivities through the thickness of CF/toughened epoxy and CF/epoxy were measured to be 0.0809 S/m and 15.1 S/m, respectively. Cross-sectional observation results revealed that the inter-laminar toughened layer affects through-thickness electrical conductivity. These results suggest that the inter-laminar toughened layer affects impulse current conduction paths in these materials, leading to varied damage on the rear surface or heating rate.