The effect of irregularity, residual convex units and stresses on the effective mechanical properties of 2D auxetic cellular structure

The purpose of the present study is to investigate the influences of changes in the microstructure of the re-entrant auxetic foam during the conversion-based fabrication on its elastic and plastic behaviors. The 2D re-entrant honeycomb is used for the finite element (FE) study. The FE method is firstly validated with theoretical calculations. Then, for the first time, the irregularity of the re-entrant honeycomb is considered in the study of irregularity-mechanical relationship. Compared to the regular re-entrant honeycomb, the irregular structures tend to have lower magnitudes of the effective elastic modulus, yield strength and Poisson׳s ratio. It is also highlighted that the irregularity has indirect effects on the effective mechanical properties of the re-entrant honeycomb by changing its relative density. Moreover, the residual convex units (the uncompressed convex corners of the conventional foams during the conversion-based fabrication) make the Poisson׳s ratio of the re-entrant honeycomb less negative. Therefore, the superior abilities brought by the negative Poisson׳s ratio such as the shear modulus and indentation hardness can be impaired by the residual convex units. Besides, the role of the residual stress is also explored with FE methods. The present study is helpful to the understanding of the structure-mechanical relationship in the auxetic foams.