Time-dependent poisson's ratio of polypropylene compounds for various strain histories

Due to the viscoelastic behavior of polymers mechanical properties are strongly affected by the loading history. To obtain the time-dependent Poisson's ratio without further data manipulation, stress relaxation tests have to be carried out. Only few results for viscoelastic materials have been published to date, but the theory of Poisson's ratio in the framework of linear viscoelasticity has received some attention with respect to loading histories other than relaxation, i.e. creep and constant rate of strain tests. The main objective of this work is to compare the potential of different testing methods to determine Poisson's ratio. Transverse and axial strain have been measured in relaxation tests, creep experiments and displacement rate controlled tensile tests. Relaxation tests are evaluated accounting for the finite loading time and the results are compared with those of tensile creep and displacement rate controlled tensile tests. An optimization based method to determine linear viscoelastic material functions developed previously is applied to calculate Poisson's ratio.