With the properties of high stiffness, high strength, and lightweight, fiber-reinforced composites have been widely applied in engineering applications ranging from micro-scale components, such as vascular stent, to macro-scale structures such as the aerospace and civil infrastructure applications. Within the service life of fiber reinforced polymer composites for several decades, the variety of environmental conditions, including temperature changes, chemical corrosion, and impact on mechanical properties is inevitably for aerospace and civil infrastructure applications. Although the fiber-reinforced composites are understood to be durable, it is still observed though experiment and practical applications that the long-term environmental innervation may result in notable degradation. The structural integrity of the fiber-reinforced composites depends on the properties of both the interface and the constitutive materials. To realize the full potential of the fiber-reinforced composites and guide the invention of innovative engineering materials, this work conducted a comprehensive multi-scale study on the degradation mechanism of fiber-reinforced composites under different environments.

For the glass fiber-reinforced composites which is intensively used in marine applications, the degradation at the glass fiber/matrix interface through molecular dynamics simulations in hygrothermal environment and chloride is investigated. The glass fiber reinforced polymer composite has been modeled using amorphous silica substrate and different polymer matrix including epoxy and vinylester. The degradation mechanism in hygrothermal and chloride environment is indicated through the reduction of decreased adhesion energy, reduced interfacial stress, and the weakened intermolecular interactions with the consideration of hydration bond. Moreover, softened epoxy molecules in hygrothermal conditioning possess a lower density near the fiber surface, which inhabits the stress transfer between fiber and matrix, eventually leading to the deteriorated interfacial adhesion. The performance of different degraded polymer matrices is also compared to provide guidance for designing more durable polymeric composites. Our simulation results echo with the experimental measurements, which can be further calibrated and utilized as inputs in micromechanical models to bridge the gap between the macroscopic and microscopic behavior of civil infrastructures. This study provides fundamental information on interfacial deterioration in glass fiber reinforced composites, which forms the basis for predicting degradation of macroscopic performance.

For the carbon fiber-reinforced composites, the carbon fiber of diamond nanothread, which is a novel carbon-based nanoadditive with outstanding mechanical properties, has attracted intensive attention in polymeric nanocomposite. However, the precise roles of diamond nanothread in the enhancement of the thermomechanical properties are unknown. In this work, we investigate the influences of diamond nanothread with various topological structures on the glass transition temperature of polymeric composites and reveals the glass-rubber transition mechanism through molecular dynamics simulation. We found that diamond nanothread exhibits better improvement than other carbon-based nanoadditive in enhancing the glass transition temperature of epoxy nanocomposite. The glass-to-rubber transition is induced through the increase in intermolecular motions, which is brought by the enlargement of free volume at the interface that ruled by Van der Waals interaction. Our results demonstrate that diamond nanothread with different topological structures processes variability in the enhancement of thermomechanical properties to the polymeric materials, which is related to rigidness, the dihedral angle between diamond nanothread and aromatic ring in the polymer chain, and the different mechanical interlock. These findings reveal the thermal degradation mechanism of polymeric composite, also provide guidance to improve the reinforcing efficiency of nanomaterials in engineering applications. We anticipate our research to be a starting point for the design of new nano-additives that are tailored for different reinforcement needs of polymeric composite.

Molecular dynamic simulation provides a bottom-up approach for the material research, which can simulate the degradation process from the atomistic scale. However, due to the differences in space and time scales, it cannot be directly compared with the macroscopic experiment. The coarse-grained model can not only explore the deterioration mechanism in atomistic scale but also can directly reproduce the macroscopic experimental observation and measurements. The framework of coarse-grained model for fiber-reinforced composites that can be applied in the engineering application is also proposed in our work that contained in the future work. Coarse-grained model builds a bridge between the atomistic scale and the macroscopic scale, which is of great significance in the research on the deterioration of engineering composite materials and the development of new materials. The multiscale modeling used in this research provides a powerful new approach to link nano-level to macro-level for complex material behavior.