Tunable microstructure transformations and auxetic behavior in 3D-printed multiphase composites: The role of inclusion distribution

We experimentally and numerically investigate instability-induced pattern transformations and switchable auxetic behavior in multiphase composites consisted of circular voids and stiff inclusions periodically distributed in a soft elastomer. We specifically focus on the role of inclusion distribution on the behavior of the soft transformative composites. The inclusions are distributed in either square or triangular periodic configurations, while the voids are distributed in the triangular periodic array - the configurations enabling cooperative buckling induced transformations of the unit cells. Through the survey of microstructure parameter, we show that tailored positioning of the stiff inclusions can be exploited to expand the set of admissible switchable patterns in multiphase composites. Thus, extreme values of negative Poisson's ratio can be attained through applied strains; moreover, the onset of instabilities, and the corresponding switches to extremely soft behavior are shown to be controlled by the inclusion arrangements and volume fractions. Furthermore, the dependence of the microstructure buckling and post-buckling behavior on loading direction is investigated, and the composite anisotropic properties depending on the microstructure parameters are discussed.