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By developing a thermo-mechanical coupled model, we quantitatively analyzed the effect of thermo-oxidative ageing on the temperature, thermal strain and thermal stress histories of braided composites during high-speed impact. We also considered temperature dependence of elastic modulus during impact in this model. The proposed model was first applied to the unaged composites, then the results were compared to the impact responses captured by the split Hopkinson bar and the high-speed camera. The comparisons of the experimental and numerical results show that the proposed model could capture the adiabatic temperature rise, stress-strain responses and failure process of braided composites during high-speed impact. The validated model was subsequently employed to thermo-oxidative aged composites. Results show that the temperature rise, thermal strain and thermal stress all depend on the mechanical properties of resin, and they decreased with the increase of ageing temperature since the performance of resin is compromised by thermo-oxidation ageing. The change of resin performance after ageing leads to the change in the temperature rise during impact, so the stiffness of composites will decrease in varying degrees after ageing. [ABSTRACT FROM AUTHOR]

The serious damage caused to 3D braided carbon fiber (CF)/epoxy (EP) composites in high-energy irradiation environments is a pressing research topic that has not yet been reported. This study analyzed the γ-irradiation multi-scale damage of the matrix, interfaces, and near-interface regions in 3D braided CF/EP composites with three different braiding angles (18°, 28°, 38°) at atomic, microscopic, and macroscopic scales. The molecular structure and atomic charge information alterations of epoxy matrix were characterized using soft X-ray absorption spectroscopy (sXAS). When the irradiation dose reached 1000 KGy, the compressive strength of composites decreased by 20.88 %, 18.07 %, and 16.60 %, respectively, with an increase in the braiding angle. Micro-CT observation, along with statistical computing, confirmed that irradiation causes interface damage and increases the defect volume of 3D braided composites. Nanoindentation was used to compare the modulus of carbon fiber, interface, near-interface regions and matrix in irradiated composites and the function of CF/resin interface as an irradiation defect capture site in the irradiation resistance of resin-matrix composites has been newly established. In addition, the mechanism behind the effect of braiding angle on macroscopic properties was also analyzed. [Display omitted] • The braided structures exhibited varying responses to irradiation, with 3D-38° showing the highest performance retention. • Soft X-ray absorption spectrometry was developed to characterize the valence changes of oxygen and carbon in irradiated resin. • Interface as an irradiation defect capture site consumes most of the energy and reduces the damage of resin near the interface. [ABSTRACT FROM AUTHOR]

Abstract The understanding of dynamic damage and failure of thermo-oxidative aged 3D carbon fiber/epoxy braided composite is critical to its durability design. Here we aim at a fine description of damage and failure of aged composites under impact compression by means of experiment and finite element analysis (FEA). The composites and epoxy resins were aged in air at 110 °C, 130 °C and 150 °C for prescribed time, respectively. The failure processes were recorded with high-speed camera. We found that with the increase of ageing temperature, the interface damage becomes more serious and the epoxy resins tend to a rubbery behavior. For the composites aged at lower temperature (110 °C), the impact resistance degradation is mainly attributed to the degradation of epoxy resin. While interface degradation is found to be the dominant degradation mechanism when the composites aged at higher temperature (130 °C, 150 °C). FEA results showed that in the process of impact, the plastic deformation induced temperature and thermal stress of the aged composites decrease compared with the unaged composite. For the unaged composites and the composites aged at lower temperature, the damage of reins is the main factor that affects the energy absorption of fiber tows. The interface damage plays a more important role in the energy absorption of fiber tows when the composites aged at higher temperature. [ABSTRACT FROM AUTHOR]