A micromechanical investigation of biomimetic composites: experimental characterization and three-dimensional computational modeling
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Biological and natural composites have been naturally optimized over millions of years. These materials benefit from high-performance responses under various loading conditions. Mimicking these materials offers the opportunity of understanding materials-design key features; and hence, the chance of developing such a high-performance material with synthetic constituents. The main objectives of this research are summarized as follows: (i) Develop a computational tool for assessing the elastic responses of biomimetic composites using 3D finite element micromechanical modeling. (ii) Make a 3D-printable nanocomposite ink comprised of a plant oil-based polymer and nanoparticles for bone-mimetic applications. (iii) 3D printing nanocomposite filaments having staggered nanostructures and testing in order to validate 3D micromechanical models using mechanical properties. Two 3D finite element micromechanical models were developed to study biomimetic composites with non-uniformly dispersed staggered hexagonal platelets and cylindrical inclusions. A novel algorithm termed staggered hardcore algorithm (SHCA) was used to rapidly generate 3D periodic representative volume elements (RVE) for these types of microstructures. The spatial dispersions of inclusions in these generated 3D RVEs were assessed using autocorrelation analysis, demonstrating the effectiveness of the SHCA algorithm. A new technique was developed within the commercial finite element software ABAQUS to produce required matching mesh patterns on opposite surfaces of the 3D RVE, and to apply the corresponding periodic boundary conditions (PBCs) using custom PYTHON scripts. To verify the developed 3D RVEs, orthotropic elastic properties were computed and compared with available experimental data from the literature for nacre-mimetic and short-fiber composites. Also, these data were compared with established analytical models, namely modified shear-lag, Mori-Tanaka, and Halpin-Tsai. These comparisons showed that 3D RVE predictions had excellent correlations with experimental data. The capabilities of the computational model were further demonstrated through a comparative study of orthotropic elastic constants for the cylindrical and hexagonal inclusion composites. The study revealed the necessity to use 3D micromechanical models with realistic inclusion dispersions for accurately assessing the response of high inclusion volume fraction biomimetic composites. These 3D RVE models were also validated and compared with experimental data obtained in this study. Three-dimensional printable nanocomposite inks consisting of a plant oil-based polymer (epoxidized soybean oil acrylate (SOEA)), and nanohydroxyapatite (nHA) particles were made for different nHA volume fractions. Silanization process was implemented on nHA particles to enhance bonding between nHA and biopolymeric resins. A second ink was made by adding an additional monomer 2-hydroxyethyl acrylate (HEA) to SOEA for improving the rheology of the ink. Also, ethanol (EtOH) was employed during ink preparation to improve nHA particles dispersions. Using these two inks, bone-mimetic filaments with staggered nanostructures were fabricated with direct ink writing (DIW) technique. Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) were performed to characterize the material microstructure. These analyses revealed actual nHA volume fractions, the effective value of Si on nHA, as well as, nHA dispersions and alignments in different regions of 3D-printed nanocomposite inks. A number of uniaxial tensile tests using a very small universal machine and digital image correlation (DIC) measurements were conducted to determine the mechanical properties of biopolymeric resins and 3D-printed nanocomposite filaments. 17%Si-nHA/SOEA+HEA and 20% Si-nHA/SOEA ink had perfectly dispersed and aligned nanoparticles. Thus, the strength and toughness of SOEA+HEA and SOEA had been remarkably improved. The extracted experimental data for both biopolymeric resins were used to run 3D finite element micromechanical models. While the experimental data for the nanocomposite filaments were employed to validate the 3D FE micromechanical models. Eventually, the results of 3D RVEs were compared with measured experimental data and Mori-Tanaka prediction. According to the notable difference between the stiffness of biopolymeric resins and nanohydroxyapatite inclusions, the predictions of 3D RVEs were correlated well with experimental data, particularly for Si-nHA/SOEA+HEA ink. These comparisons showed the influences of inclusion misalignments and agglomerations as well as limitations of generating staggered nanostructures. The 3D RVEs had relatively good and acceptable predictions for nano-scale inclusions; while their predictions for micro-scale inclusions were more reliable. In future work, developed 3D FE micromechanical models may be used to predict the onset and evolution of local damage and cracking in different inclusion-reinforced biomimetic composites as well as local nonlinear or time-dependent behavior. Furthermore, these micromechanical models can be an applicable and efficacious tool in designing a variety of new composite material systems and optimizing their microstructures.
Cite this version of the work
Aram Bahmani (2018). A micromechanical investigation of biomimetic composites: experimental characterization and three-dimensional computational modeling. UWSpace. http://hdl.handle.net/10012/14204