The mechanical properties of carbon fiber-reinforced plastic (CFRP) foam core sandwich structures were investigated under cryogenic conditions. The use of CFRP in the manufacture of the cryogenic tanks of launch vehicles has been expected to help achieve weight reduction. The tank structure is required to withstand not only the tension load due to the internal pressure but also the axial external compressive load. Therefore, the CFRP foam core sandwich was considered suitable for use as a cryotank material. In this study, the basic applicability of the sandwich in composite cryotanks was evaluated. Polymethacrylimide foam was utilized to fabricate the sandwich core as it has a closed cell that could eliminate the risks associated with the cryopump. Compression and tension tests were carried out on the foam. Flatwise tension and edgewise compression tests were conducted on the CFRP sandwich. All the tests were conducted under room-temperature and cryogenic conditions. For cryogenic tests, the specimens were immersed in liquid nitrogen. The compressive and tensile strengths of the foam were observed to be lower under cryogenic conditions than those at room temperature. The flatwise tensile strength of the sandwich was lower than that at room temperature and than the tensile strength of the foam. On the other hand, the edgewise compressive strength under cryogenic conditions was higher than that at room temperature; this may be attributed to an increase in the foam-core stiffness under cryogenic conditions.
