Characterization and Model Validation for Large Format Chopped Fiber, Foamed, Composite Structures Made from Recycled Olefin Based Polymers

Abstract

The purpose of this research is to predict the material performance of large format foamed core composite structures, such as crossties or structural timbers, using only constitutive properties. These structures are fabricated from recycled post-consumer/post-industrial waste composed of High-Density Polyethylene (HDPE) and Glass Filled Polypropylene (GFPP). A technical challenge in predicting the final part performance is the mathematical correlation between the microstructural variations and the macroscopic responses as a function of fiber aspect ratio, cell density, and constitutive properties of the polymer blend. The structures investigated have a dense and consolidated outer shell and a closed cell foamed core. The non-linear shell and the foamed core material properties are analyzed with micromechanics models, and the reference stress of the shell and core is predicted using a modified Rule of Mixtures model. The predicted properties are used as the inputs for a Finite Element Analysis (FEA) model, and the computational results are compared to experimental four-point bend test results for sixteen samples performed on a 120-kip compression stage. The results show that the mean of the characterized deflections from the four-point bend tests did not show any variations for an isotropic and transversely isotropic model using a linear analysis. This model was then extended to a non-linear analysis using the Ramberg–Osgood model to predict the full crosstie four-point bend test behavior. The FEA model results show a deviation of 2.45 kN compared to the experimental variation of 3.58 kN between the samples measured.

Strength Prediction Sensitivity of Foamed Recycled Polymer Composite Structures due to the Localized Variability of the Cell Density Distribution

“…For the overall sample size of 7.38 × 5.38 inches, the image is approximately 3000 × 2300 pixels, and the size of each pixel is 63.5 µm. For the composite structure discussed, the radius of the fibers was measured to be 8.56 µm ± 0.57 µm [39] and thus the fibers are not visible within the image. Optical microscopy was performed on the surface of the cells, and the fibers did not protrude from the cell wall.…”

“…Based on the previous research [39], material properties were predicted using only constitutive properties. This was accomplished using the inputs of raw materials, including as elastic moduli and Poisson's ratios of HDPE and GFPP, glass fiber aspect ratio, and foamed core density.…”

“…The underlying unidirectional stiffness tensor of the composite was calculated using the method given by Tandon and Weng [29], which uses the matrix and fiber material properties and the fiber aspect ratio as shown in Table 1. Within the fiber aspect ratio calculations, the weighted average and number average fiber lengths were calculated and the weight average fiber length was used for the aspect ratio based on the discussion as shown in [39]. From the calculated unidirectional stiffness tensor C ijkl , the effective anisotropic stiffness tensor < C ijkl > was computed using the equation,…”

“…In a previous work [39], the aforementioned micromechanical modeling on coupon samples was experimentally validated for the shell and the core region. The earlier work was extended to include a full non-linear finite element solution, with inputs of the constitutive properties of the neat polymers and fibers.…”

“…In a recent work by the authors [39], the composite structures made from recycled materials were studied by integrating the fiber aspect ratio, uniform cell density, orientation state of the fibers, and the constitutive properties of the raw materials. These properties were used as inputs into micromechanics models, and the effective elastic moduli of the solid shell outer core of the composite as well as the foamed core were predicted.…”