Recycling of polymers is of great interest to society with industry and government focusing on new applications to reduce waste generation. Most polymers have low modulus and strength compared to metals, and based on the application, various reinforcing fibers such as carbon, Kevlar, and glass are used to improve their properties. Glass fiber reinforced composites have gained popularity based on their low cost, high modulus, and strength. This research focuses on the use of recycled post-industrial/post-consumer polymers such as High-Density Polyethylene, and Polypropylene with glass fibers as reinforcement. To identify variables and contaminants of interest that impact the final part performance, an extensive thermal analysis is presented. Large format composite structures are manufactured using extrusion molding with blowing agents enabling the formulation of a dense outer core and a foamed inner core with cells of varying sizes and locations. This research presents a methodology to predict the macroscopic part behavior as a function of variations in microstructural components. The methodology allows for variations in the fiber aspect ratio, constitutive properties of polymer blend, and cell size and density. This research presents the model development and validation to predict the structural performance of the composite structure under flexural loading using constitutive material properties. Full scale finite element simulations are performed using the proposed multiscale nonlinear modeling approach, and the results are within the statistical spread of the experimentally observed data sets of four-point bending on fullscale composite crossties. This work also provides a methodology to predict the response of spatially varying foamed composite structures using a novel method to characterize the cells with digital image analysis. It was found that variations in cell size and distribution did not affect the load-deflection curve through the loading history, but the stresses within the core were found to be ~11 % greater than the model causing an premature onset of failure in the processed product. This work concludes with a series of investigations of damage and viscoelastic recovery of the surface of the composite structure due to impact, where the recovery profile is experimentally characterized using 3D microscopy.