Strain localization and cyclic damage of polyurethane foam cylinders: experimental tests and theoretical model

In uniaxial compression tests performed by the authors on polyurethane foam cylinders it has been observed the typical three-regime response curve of cellular material. An initial almost linear stress-strain response, followed by a large increase in displacements at almost constant load (plateau regime), and a final regime characterized by large stress increase under relatively moderate deformation. At unloading the response follows a different path where this three regimes are less evident. There is experimental evidence that in the plateau regime a localization of the deformation occurs. Moreover, it has been observed a progressive stepping down of the loading curve (\textit{cyclic damage}) under cyclic deformation. These two phenomena are described in a rheological model, where the foam is represented as chain of springs with non-convex strain energy connected in parallel with a series of surface elements undergoing contact with recoverable adhesion. This brings the theoretical response curves close to those obtained in the experiments.