Static and fatigue compression behaviour of conventional and auxetic open-cell foam

This study aims to modify and characterize an open-cell polyurethane foam with a density of similar to 28.7 kg/m(3) to study the mechanical behavior in static compression and cyclic fatigue. A technique is presented for transforming this type of foam from a conventional state with a positive Poisson's ratio to an auxetic transformed state with a negative Poisson's ratio (density of similar to 119.59 kg/m(3)) via a specially designed device. The results obtained show the degree of influence of the transformation on the mechanical behavior of this type of foam. The cyclic fatigue tests of auxetic foam were carried out under control-displacement for different loading (r) levels (0.725, 0.75, 0.80, 0.85, 0.90 and 0.95). The monitoring of cyclic fatigue damage of auxetic samples allowed tracing the evolution of the stiffness degradation (F/F-0) as a function of the number of cycles N taking place in two different phases. The hysteresis loops of the different r levels were identified as a function of the number of cycles. The evaluation of the dissipated energy (E-d) and damping (eta) as a function of N was also carried out. The results show that the maximum forces involved between 28 N and 580 N are considerably higher than those previously found in the literature. This leads to a significant dependence of the hysteresis loops, E-d and eta of the auxetic foam as a function of the number of cycles and the loading rate, with a remarkable stiffness performance compared to other auxetic foams.