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Time and temperature effects on Poisson’s ratio of poly(butylene terephthalate)

“…On the other hand, in the case of Poisson's ratio, a time dependent correlation has to be established between two strains and a stress generating one of them. As it was underlined by theoretical studies [1,[3][4], when investigating Poisson's ratio time dependence, it is of primary importance to provide a proper The importance of this aspect is often neglected [1], although the obtained results are shown to depend on testing conditions [3]. In spite of the time dependent behaviour of polymers, and due to considerable experimental difficulties, Poisson's ratio of polymeric materials is often assumed as a constant, or, occasionally, defined for values corresponding to glassy and rubbery equilibrium conditions.…”

“…Poisson's ratio of polymeric materials can thus be thought in the framework of linear viscoelasticity as one of the various viscoelastic response functions, but its determination can involve more complexity [1][2][3]. In fact, relaxation moduli and creep compliances are usually determined in conditions that allow an easy description of a time dependent correlation between stress and strain (i.e.…”

“…In spite of the time dependent behaviour of polymers, and due to considerable experimental difficulties, Poisson's ratio of polymeric materials is often assumed as a constant, or, occasionally, defined for values corresponding to glassy and rubbery equilibrium conditions. In both cases its viscoelastic nature is completely neglected [1]. Nevertheless, it was shown that only in few cases the assumption of a constant Poisson's ratio can be reasonably considered valid [2,4], and that neglecting its time and temperature dependence may lead to predicting models markedly deviating from the experimental observations [5][6][7].…”

“…More detailed information on the viscoelastic behaviour of Poisson's ratio is therefore important not only in principle [8][9][10], but also for practical reasons. In fact, it would allow us to interconvert material functions [1], to refine computational methods [2], to reduce the number of required experimental parameters to fit analytical models [8], to better estimate the effects of the so called Poisson's strains, originating from misfits in Poisson's ratio of joined materials [1]. So far, only few trials have been attempted to comprehensively measure and describe the viscoelastic nature of Poisson's ratio.…”

“…So far, only few trials have been attempted to comprehensively measure and describe the viscoelastic nature of Poisson's ratio. In a review article, Tschoegl et al [1] summarize the main results concerning the measurement and the prediction of viscoelastic Poisson's ratio, and also give a critical perspective on the problems regarding its determination. A viscoelastic Poisson's ratio can be experimentally evaluated following two procedures; a so called 'direct' measurement, which involves the measurement of the transverse and axial deformations of the tested specimen; and a so called 'indirect' measurement, where, on the basis of the linear viscoelasticity, Poisson's ratio is calculated from the measurement of two other material's time dependent parameters.…”