Effects of geometrical and processing parameters on mechanical properties of auxetic polyurethane foams

This study aimed to investigate the influence of processing parameters on the mechanical properties of auxetic polyurethane foams including Poisson's ratio and Young's modulus. 12 different processing scenarios were considered using the method of Plackett-Burman in the design of experiments with three replicates for each one. Eventually, 36 foams were prepared with different densities and initial thicknesses, heating temperatures and times, applied compression ratios, and the rest times between two heating steps. The microstructures of the conventional and auxetic samples were observed by scanning electron microscopy (SEM). All samples were subjected to tensile loading in one direction with two different strain values. The strains of the foams in two other directions were recorded using a digital image correlation method. Also, the required force to create each strain value was recorded. The results showed that depending on the changing parameters, Poisson's ratio of about 42% and 58% of the samples reduced at the strains of 10% and 20%, respectively. Heating temperature and time, the initial thickness of the foam, and the applied compression ratio were proved to have significant effects on the variations of both Poisson's ratio and Young's modulus of the foams. It was concluded that the Poisson's ratio of foams was reduced at higher heating temperature, time, and applied compression ratio and also a lower foam initial thickness. On the other hand, these changes increased Young's modulus of the polyurethane foams. The strain energy of the auxetic samples showed higher amounts of energy compared to the other foams.