Understanding the Mechanical Behavior of Polymer Composites Across Stress States, Length and Time Scales Via Size Effect, Multi-axial Testing and Computational Modeling

Abstract

Advanced composite materials have been developed for several decades whereas the current rising demand for lightweight and high-performance materials across many engineering fields is still boosting the global market of these composite materials. A quintessential condition for the efficient, safe, and durable applications of composite materials is the attainment of high-fidelity computational models that can capture all the possible effects such as curing process, manufacturing defects, stacking sequence, structural geometries and sizes, nanomodification, statistical behavior, multi-axiality ratio, loading type, etc. However, many aspects are still poorly understood in the community of composite materials in spite of tremendous efforts into these subjects thus weakening the full exploration of these materials. Towards this direction, the entire work here is expected to make contributions to the proper understanding of these aspects and the further development of composite materials. The initial investigation focused on the effects of local stress state and size scaling on the plastic deformation and fracturing behavior of thermoset polymers and related fiber-reinforced composites. It was concluded that the entire local tensorial stress components and the multi-scale behavior of the materials must be considered into the computational micro-mechanics otherwise the determination of the damage initiation and the morphologies of the damage evolution in these materials cannot be computationally reproduced. The latter aspect further leads to the inspiration of leveraging micro-scale behavior of the materials to improve the structural capacity via engineered porosity. This approach was shown to make thermoset polymers as tough as conventional metals. Further attention was moved to explore the fracturing behavior and size scaling of polymer nanocomposites. It was found that the investigated graphene nanocomposites and most of generic nanocomposites in the literature exhibit significant quasi-brittleness both in quasi-static and fatigue loading conditions due to the non-negligible Fracture Process Zone (FPZ) in the materials and this important feature cannot be described through the Linear Elastic Fracture Mechanics (LEFM) which was extensively used in the current literature. The correct analysis on the polymer composites must leverage quasi-brittle mechanics and high-fidelity computational models otherwise the characterization of the materials and related structures by means of LEFM can lead to unacceptable errors. In addition to the forgoing studies, the mechanical behavior of fiber-reinforced composites due to the effects of stress multi-axiality ratio, loading type, stacking sequence, and the structural geometry were also investigated and the detailed damage mechanisms triggering the foregoing behavior were also clarified. It was most interestingly found that the loading multi-axiality ratio can significantly affect the fracturing behavior and morphology of fiber-reinforced composites whereas the loading type can lead to a remarkable difference in the damage progression of fiber-reinforced composites. These studies are utmost of importance for the calibration and validation of high-fidelity computational models which enable the description of all the foregoing aspects with respect to the structural size.