Mechanical behaviour of conventional and negative Poisson's ratio thermoplastic polyurethane foams under compressive cyclic loading

This work presents a comparative analysis between the cyclic loading compressive behaviour of conventional, iso-density non-auxetic and auxetic (negative Poisson's ratio) thermoplastic polyurethane foams. While the three types of foam share the same base material (open cell rigid PU), one batch is transformed into auxetic, (i.e., negative Poisson's ratio) using a special manufacturing process involving moulding and exposure to particular temperature profiles to stabilize the microstructure transformation. The specimens have been loaded in cyclic compression with a sinusoidal waveform in displacement control. The static tests show the specific stress-strain compressive mechanical behaviour of these auxetic thermoplastic foams, opposite to conventional ones and other similar data on auxetics available in open literature. The effect of the load loss, stiffness degradation, the evolution of dynamic rigidity and accumulation of energy dissipation versus the number of cycles are discussed for different loading levels. The analysis of the results shows that the fatigue behaviour until failure occurs in two stages, subjected to cyclic loading, depends on the loading level. The hysteresis loop tends to close itself as function as the number of cycles N, while the slope of the dynamic stiffness decreases with increasing N, therefore with decrease of dissipated energy. The energy dissipated by the auxetic foams is significantly higher than the one from conventional parent phase and the iso-density foams at every number of cycles and loading level. (c) 2006 Elsevier Ltd. All rights reserved.