Molecular degradation process and damage distribution for low speed impact properties weakening of three-dimensional braided carbon fiber/epoxy resin composites under thermo-oxidative environment

The investigation into the change mechanism of low-speed impact performance of three-dimensional braided carbon fiber composites in thermo-oxidative environment holds significant guiding implications for the durability design of structural components. Utilizing ReaxFF reaction molecular dynamics and Micro-CT techniques, this study analyzed the changes of molecular structure and the spatial distribution characteristics of damage within the matrix after thermos-oxidative treatment. It revealed the effects of thermo-oxidative environment on the low-speed impact properties of braided composites. The findings indicate that following 384 hours of treatment at 180°C, the molecular structure of matrix undergoes fragmentation and recombination, leading to an increase in molecular fragments, natural volume expansion, and reduction in free volume, which induced the whole volume degradation. The primary gaseous products, in descending order, are H₂O, CO₂, and H₂, which induced the whole mass degradation. Thermo-oxidative-treatment induced damage exhibits a pronounced directional effect, predominantly distributed and propagated along the in-plane direction, encompassing interface cracks and matrix cracks. The maximum crack depth was measured at 0.91 mm, with a volume fraction of 0.23%. These changes in molecular structure and matrix damage along in-plane direction result in diminished low-speed impact performance of the braided composites. Specifically, the decrease in impact strength and stiffness is more pronounced in the in-plane direction (23.8% and 28.7%, respectively) compared to the out-of-plane direction (5.6% and 8.6%).