Mechanical modeling of an auxetic tubular braided structure: Experimental and numerical analyses

An auxetic braided structure was developed in our previous work for overcoming the slippage problem encountered in the double-helix yarn. However, the influences of the micro-geometric parameters on the macroscopic behavior, which are significant for the structure's design and application, have not been addressed yet. By using a standard tubular braiding technology, several samples of the auxetic braided structure were fabricated in this work and the design concept and manufacture process were described in detail. Then, systematic parameters studies were completed through experimentally validated finite element models. The study showed that the braided structure could achieve a robust auxetic behavior and its Poisson's ratio and stiffness were strongly dependent on their initial micro-geometric parameters, especially initial braiding angle and diameter of component yarns. A maximum negative Poisson's ratio of -9.49 could be achieved by lowering the angle value to 15 degrees. Higher negative Poisson's ratio effect could be obtained with grosser stiff yarn and finer elastic yarns. However, the existence of the elastic wraps would diminish the auxeticity of the braided structure, which should be taken into consideration in design and application.