Paul A. O'Connell

The Non-Linear Viscoelastic Response of Polycarbonate in Torsion: An Investigation of Time-Temperature and Time-Strain Superposition

Data are presented from torsional stress relaxationexperiments on a commercial polycarbonate. Tests were performedon samples over a range of torsional strains from 0.0025 to 0.08and at temperatures from 30 to 110°C at a fixed aging time of64,800 s (18 h). Following the scaling approach of Penn andKearsley [Trans. Soc. Rheol.20 (1976)] we were able todetermine the stress relaxation response at shear strains to0.07. Then the individual data sets at each strain andtemperature could be described using a stretched exponential formrelaxation function. Over the range of temperatures studied thedata at each strain were superimposed using conventional time-temperature superposition. For strains up to the yield strainthe data at each temperature could also be superimposed to form amaster curve following the principle of time-strainsuperposition. Interestingly, the master curves found from time-strain superposition at each temperature did not have the sameform. Similarly, the master curves found from time-temperaturesuperposition at each strain did not have the same form.

The relationship between slow crack propagation and tensile creep behaviour in polyethylene

The creep characteristics of drawn samples of a number of grades of polyethylene have been studied over a range of applied stresses and strains. In each case, the data allowed a unique and reproducible true stress:strain:strain rate surface to be determined. These data have been used in a new computer-based model for the mechanism of craze growth during the initiation of slow crack propagation. Using the model it has been possible to define the contribution the creep of the drawn material makes to craze growth and ultimately to the failure of the craze.

Measurements of fibre orientation in short-fibre-reinforced thermoplastics

Abstract A technique is described for accurately measuring the spatial variation of fibre-orientation-distribution averages in injection-moulded plaques of short-fibre-reinforced carbon-fibre/PEEK composites. It is based on measurements of the ellipticity and orientation of the elliptical images of circular fibres meeting a polished surface of a single section through the plaque when viewed in reflected light. Automatic image analysis of the approximately 10 000 fibre images per section was carried out on the COSMOS facility at the Royal Observatory, Edinburgh. Details are provided of the estimated uncertainties of these orientation averages in order to establish that the technique has sufficient accuracy and resolution to characterise quantitively the skin-core-skin structures expected for such materials. These and additional data will be used in a subsequent paper to explain, qualitatively and quantitatively, the variation in the fracture toughness of the material as a function of position.