Improving porosity distribution and mechanical performance of multilayer sandwich composites using a new strategy of gradient curing cycles during internal thermal expansion molding process

This work introduces a novel gradient curing cycle strategy for optimizing resin impregnation flow in the internal thermal expansion molding process, aimed at significantly improving porosity distribution and enhancing mechanical performance. The proposed strategy meticulously aligns the foaming characteristics of thermal expansion foam with the curing behavior of prepregs, effectively surpassing conventional curing temperature limitations. By allowing a controlled temperature hold above the resin gelation point, this approach leverages continuous and stable foam expansion to achieve superior results. A key innovation of this work lies in demonstrating the capability of the gradient curing cycle to produce CFRP with ultra-low porosity levels as low as 0.1 % sandwich composite structures. This advancement enables a systematic investigation into the spatial evolution of inter-bundle voids and elucidates the underlying mechanism for void suppression. Furthermore, the strategy enhances the bending fracture toughness and shear strength of CFRP by 22.6 % and 6.5 %, respectively, marking a significant leap in performance compared to traditional internal thermal expansion molding methods. The findings of this work establish a solid foundation for extending the application of complex components fabricated via the internal thermal expansion molding process to more demanding operational environments and increasingly complex working conditions.