Fabrication and Modeling of Functionally Graded Porous Polymer Materials
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Functionally graded porous materials (FGPMs) are porous materials with porosity gradient distributed over the volume. They have many applications in aerospace, biomedical, and other engineering fields. Despite a lot of effort being made to fabricate FGPMs, the manufacturing processes are either complex, expensive, unable to control exact porosity distribution, or unable to create closed cell structures. This work presents a new approach to manufacturing polymer FGPMs with both closed cell and open cell structures using the thermo-bonding lamination process. Under applied compressive load, controlled heating temperature, and appropriate holding time, it was shown that this thermally-induced bonding technique can bond layers of polymer sheets to create porous three-dimensional objects. An investigation on effects of different factors on the bonding shear strength was performed. It was found that the bonding shear strength can be controlled by properly setting the pressure, temperature, and time. The fabricated FGPMs specimens with different porosity configurations were characterized using the compression test in the normal direction and transverse direction, and the effective moduli were obtained. An analytical model for predicting the elastic properties of the FGPMs was derived. The model is based on Mori-Tanaka’s approach while extended to graded and porous cases. A more generalized case – the polynomial varying strain field – was assumed and the Eshelby’s tensor for polynomial varying eigenstrain was obtained to facilitate the derivation. In addition, an analytical model for FGPMs with open cell structure was also developed. A solution to the overall eigenstrain of the interconnected voids was provided by considering the disturbed stress field outside of the voids. It was shown that the models can accurately predict the mechanical response of closed cell and open cell FGPMs. Numerical model based on representative volume element (RVE) with the periodic boundary condition applied was also developed for FGPMs to investigate the mechanical response of the material and to obtain the corresponding effective modulus. It was shown that the results from the numerical model agree well with experimental and analytical results, indicating that the developed model can predict the material response accurately. In summary, the developed fabrication technique is an effective method to produce 3-D graded porous objects for practical applications. The proposed analytical and numerical models can be adopted directly by researchers in the fields of micromechanics and mechanics of composite.
Zhang, Ying (2016). Fabrication and Modeling of Functionally Graded Porous Polymer Materials. Doctoral dissertation, Texas A & M University. Available electronically from