Microcracking resistance of 3D printed fibre composites at cryogenic temperatures

Abstract

Thermoplastic composites present considerable promise for 3D printing cryogenic fuel storage tanks, offering enhanced recyclability and repairability compared to thermoset composites. However, a significant knowledge gap remains regarding their ability to withstand cryogenic environments without suffering ply cracking, especially the effects of high thermal residual stresses and voids on the microcracking resistance of 3D printed composites. This study investigates the microcracking behaviour of continuous carbon fibre reinforced thermoplastic (CFRTP) composites fabricated through extrusion-based 3D printing. Three-point bending tests were conducted to assess the microcracking resistance of the as-printed and heat-treated composites at room and cryogenic temperatures. The results reveal that the nylon-based CFRTP composites printed at room temperature exhibit a remarkable ability to withstand an applied strain of 0.60 % without ply cracking at liquid nitrogen temperature. This performance surpasses that of conventional carbon fibre reinforced epoxy composites, which typically experience ply cracking even at zero applied strain. Some of the microcracks were traced back to manufacturing defects, which could be fused by a post-heat treatment at 180℃ for 60 minutes. Unexpectedly, however, the treatment reduced the ply-cracking strain to 0.40 % at the liquid nitrogen temperature. Computational micromechanical modelling revealed that this unexpected decline in ply-cracking resistance resulted from the increased thermal residual stresses induced by the heat treatment. The findings of this study suggest that 3D-printed thermoplastic composites exhibit robust resistance to microcracking at cryogenic temperatures, making them a promising solution in the quest for sustainable lightweight cryogenic fuel storage solutions.