Effects of the strain rate and temperature in stress-strain tests: study of the glass transition of a polyamide-6

In this work new insights are presented on the measurement of the tangent a nd secant moduli from stress-strain curves in polymeric systems. Expression s for the strain-rate and strain dependence of both moduli are derived for systems characterised by a distribution of relaxation times. The equivalent frequency of the stress-strain experiments is shown to be dependent on the strain rate and on the strain at which the measurements are carried out. S uch considerations enable using quasi-static tensile stress-strain tests to study relaxational processes in polymeric materials. The tensile behaviour of a 30% glass fibre reinforced polyamide 6 was characterised at different strain rates and temperatures, covering the glass transition region. A mas ter curve of the tangent modulus as a function of strain rate was successfu lly constructed by simple horizontal shifting of the isothermal data. The t emperature dependence of the shift factors was well described by the WLF eq uation. It was also possible to fit the master curve considering a polymeri c system with a distribution of relaxation times, relevant parameters such as the KWW beta parameter being extracted. The results were found to be con sistent with dynamic mechanical analysis results. (C) 2001 Elsevier Science Ltd. All rights reserved.