Microstructure-based Computational Modeling of the Mechanical Behavior of Polymer Micro/Nano-composites

This dissertation is devoted to the virtual investigation of the mechanical behavior of micro/nano polymer composites (MNPCs). Advanced composite materials are favored by the automotive industry and army departments for their customizable tailored properties, especially for strength and ductility compared to pure polymer matrices. Their light weight and low finished cost are additional advantages of these composite materials.

Many experimental and numerical studies have been performed to achieve the optimized behavior of MNPCs by controlling the microstructure. Experiments are costly and time consuming for micro scale. Hence, recently numerical tools are utilized to help the material scientists to customize and optimize their experiments.

Most of such numerical studies are based on characterizing the MNPCs through simple microstructures, as circular particles or straight fibers embedded in a specific polymer matrix. Although these geometries are effective in virtual modeling some types of composite material behavior, they fail to address some critical key micro-structural features, which are important for our goals. Firstly, they fail to properly address the randomness of particles. Secondly, 2D analyses have limitations and they can provide qualitative insight, rather than evaluate the quantitative response of the material behavior. Thus, in order to fill this gap, a user friendly software program, REV_Maker, is developed in this project for generating 2D and 3D RVEs (representative volume elements) to precisely represent the morphology of material in microstructural level.

In models, polymers are usually considered as viscoelastic-viscoplastic or hyperelastic-viscoplastic materials without taking into account viscodamage models. Therefore, in this work rate- and time-dependent damage (viscodamage) is separately considered to fully investigate the initiation and growth of damage inside polymer composites. Besides, most of the common viscoelastic and viscoplastic models assumes small deformation; therefore, in this dissertation a procedure is established, which incorporates all required modifications to generalize a small strain constitutive model to its identical large deformation range. Thus, here a straightforward generalization and implementation method based on classical continuum mechanics is proposed, which due to its simplicity, can be applied to a wide range of elastoplastic constitutive models. Then, the available viscoelastic and viscoplastic models are extended to large strain framework. By applying the generalized viscous models, one may address and measure the large deformation response of MNPCs. Numerous simulations were conducted to predict the overall responses of micro/nano composites with different morphologies (particles volume fractions, orientations, and combinations). The effect of each particle, and the combination of particles on the composite responses are compared and presented.