Auxetic or negative Poisson’s ratio materials are of great research interest as they exhibit improved mechanical properties such as impact resistance, fracture toughness, hardness and shear modulus over conventional materials with a positive Poisson’s ratio and same stiffness. Most auxetic materials manufactured to date are porous, and the major hindrance of using these materials in structural applications is their low strength. In this research a new approach for manufacturing auxetic fiber-reinforced composites by embedding an auxetic fibrous network in a conventional polymer matrix is investigated using numerical and experimental methods.
In this project, a finite element model was developed to investigate the proposed hypothesis of manufacturing an auxetic composite. Compressed mats of sintered stainless steel fibers are known to have negative Poisson’s ratio out-of-plane. It is hypothesized that their auxetic behavior is from the arrangement of fibers within the matrix, relative stiffness of the matrix and network of reinforcement. First, tension tests were performed on the compressed sintered mats and the experimental results have shown that Poisson’s ratio for all metal mat specimens is negative in transverse direction. Next, mechanical tests were performed and microscopic images were taken of the composite samples made by embedding the stainless steel mats with silanol terminated polydimethoxysiloxane (PDMS) polymer. Finally, polymer nanocomposites were made through the inclusion of high loading carbon nanofibers in the polymer matrix (PDMS). The experimental results of both the composite mats and nanocomposites confirm the feasibility of manufacturing an auxetic composite by embedding an auxetic network in a conventional polymer. The auxetic behavior in the composite mats is due to the arrangement of fibers in the network and compression of stainless steel mats during fabrication process. It was assumed that the auxetic behavior of the nanocomposites is due to effective network formation of nanofibers and the compression of the sample during preparation process.