Shape memory behaviour in auxetic foams: Mechanical properties

Shape memory is the property of a material to remember its original shape despite subsequent plastic deformation. The exposure to specific temperature profiles induces a shape memory effect on auxetic (negative Poisson’s ratio) foam specimens, returning them to their initial dimensions. This behaviour is a one-way effect, and it is a property of the polyurethane (PU) constituent of the foam. The foam specimens were transformed from conventional Poisson’s ratio to auxetic, returned to conventional and once again to auxetic under multiple mechanical and thermal loading. At each stage the foams were mechanically characterized under cyclic tensile and compressive loading. The manufacturing route adopted for the auxetic PU specimens involved a multiaxial compression of the native foam, heating of the compressed specimens above the Tg of the foam polymer, and cooling under running water. Twenty specimens, divided equally between two varieties of PU-based foam, were fabricated. Two different compression ratios were used during auxetic conversion for both kinds of foam, and the same final temperature of 135 °C was adopted. Tensile and compressive cyclic tests were performed in order to measure the tangent modulus (i.e. stiffness), Poisson’s ratios and energy dissipated per unit volume. Remarkable differences before and after the memory effect took place were found, showing mechanical behaviour unique to each of these phases. The shape memory effect plays an important role in the mechanical behaviour of the auxetic foams.