Nonlinearly Viscoelastic Behavior of Polycarbonate. I. Response under Pure Shear

Abstract

The nonlinearly thermo-mechanical creep behavior of (bisphenol A)polycarbonate was investigated at different temperatures (0 to 140°C) under pure shear loading. The shear creep in the linearlyviscoelastic range was measured with a torsiometer for referencepurposes and a master curve, along with a shift factor curve, wasconstructed. While the master curve is well defined with no detectabledeviation, the shift factor can be represented by essentially twostraight lines and offset at the β transition temperature ofpolycarbonate. The shear creep tests in the nonlinearly viscoelasticrange were conducted on an Arcan specimen geometry at differenttemperatures and under different stress levels, utilizing digital imagecorrelation for recording the creep strains. The difference between thenominal stress and the actual stress distribution in the Arcan specimenwas explored via numerical simulations (ABAQUS) by assuming linearlyquasi-elastic and quasi-plastic analyses in place of the as yetuncertain material characterization. Isochronal plots were created fromthe creep data. Nonlinearly viscoelastic behavior starts to take effectnear 1% strain at the temperatures considered. The applicability of thestress-clock representation for material characterization has beenexplored and is found, at best, to be dubious for this material. The`yield-like' behavior of polycarbonate has been examined in terms of theisochronal stress-strain response and a corresponding `yield-like shearstress' has been determined to be a monotonically decreasing function ofthe temperature, but again with an interruption or `jog' at the βtransition temperature. Time-temperature trade-off as practiced for`time-temperature shifting' at small strains does not apply in thenonlinear domain. The results are generally in agreement with thosefound for Poly(Methyl Methacrylate), thus fostering the idea that thepresent results can be generalized – with additional work – to other amorphous polymers.