The use of carbon fiber-reinforced plastic (CFRP) foam core sandwich structures in the cryogenic tanks of launch vehicles is expected to aid in weight reduction. The tensile properties of CFRP foam core sandwich composites were investigated under cryogenic conditions. Typically, the coefficient of thermal expansion (CTE) of foam materials is much larger than that of CFRP. Under cryogenic conditions, this results in the generation of significant tensile thermal strain in the foam core, in addition to tensile mechanical strain. To identify composites with improved cryogenic behavior, the CTEs and cryogenic tensile fracture strains of several foam materials were recorded. CFRP sandwich tensile specimens with polyisocyanurate foam were tested in liquid nitrogen and assessed via finite element method analysis. The test results revealed that the polyisocyanurate foam is a potential core material for CFRP sandwich composites used in cryogenic tanks.

An applicable method for fatigue life prediction in the interfacial normal plastic strain (INPS) range is proposed for unidirectional carbon fiber-reinforced thermoplastic (CFRTP) specimens made of PA6. The INPS is defined as the plastic strain of the resin at the midpoint of the segment, which corresponds to the shortest distance in the direction of separation between two carbon fibers. The Drucker-Prager yield criterion is used for the microscopic elasto-plastic analysis of the specimen, which clearly distinguishes the resin and carbon fibers. The INPS range successfully predicts the fatigue of a 90°unidirectional CFRTP specimen by utilizing the relationship between the plastic strain range and number of cycles to failure of a PA6 specimen. In comparison to the von Mises yield criterion, the Drucker-Prager yield criterion for the CFRTP is advantageous in that the isotropic deformation of the resin is strongly constrained by the fibers.

Predicting the temperature- and time-dependent nonlinear material behavior of thermoplastics and fiber-reinforced thermoplastics requires the selection and parameter identification of appropriate constitutive laws. However, many material parameters cannot be predicted in advance, and their actual values can span over ten orders of magnitude. In such cases, it is difficult to find the optimal design parameters within a wide search range using multi-point search methods employing ordinary meta-heuristic algorithms. In this study, we propose a new optimization method that applies the adaptive range search algorithm to ordinary meta-heuristic algorithms, such as PSO and DE and the search area is adjusted based on the optimization results in each trial. The elastic-creep-damage combined model is employed as a material constitutive law for thermoplastic resins, and the effectiveness of the proposed method is demonstrated through the parameter identification of a typical thermoplastic resin. The proposed method exhibited excellent search capability for finding the global optimum over a wide range (10^-9–10⁹) of design parameters.

Recently, we attempted to optimize the microstructures of composite materials, composed of collagen fibers and hydroxyapatite (HAp) particles, by mimicking bone microstructures. This study further investigates these materials to clarify how the tensile strength of the HAp/collagen composite fiber bundles is affected by the electric current used during their preparation via electrophoresis. The collagen fiber preparation and HAp deposition were carried out using the bioinspired method and biomimetic deposition, respectively. The tensile strength was evaluated by micromechanical testing. The tensile strength of collagen fiber bundles and HAp/collagen composite fiber bundles increased and then decreased with increasing electric current. This indicated a trade-off relationship between tensile strength and bundling. The optimal electrophoresis current value was determined to be 4–5 mA. Additionally, the optimal method to introduce the adhesive protein to the bundles was discussed.