Investigations of Non-Linear Viscoelastic Properties for Polypropylene/Clay-Nanocomposites through Melt Flow Birefringence and Damping Function

Rheological investigations are reported for pure polypropylene and its clay-nanocomposites to establish viscoelastic properties and filler concentration relationship. Flow birefringence is performed through a slit-die to obtain centerline principal stress difference during extensional flow. The centerline stress profile of clay-nanocomposite revealed additional viscoelastic nature even at low silicate concentrations whereas no exceptional strain hardening was reported. Effects of higher filler concentrations are further examined during the simple shearing flow to consider non-linear viscoelasticity in terms of damping function. The increase in damping coefficient with increasing clay concentration shows polymer-nanocomposites are more strain sensitive. The Wagner's exponential damping function could adequately describe the time-strain separability at all clay concentrations studied. The results of both investigations reveal that the polymers are time-strain separable at all clay concentrations studied during elongational and simple shearing flows, whereas filler orientations are found to be different for different melt flow behavior.

Study on the Size Effect of Auxetic Cellular Materials

The objective of this paper is to investigate the effects of scale of an auxetic cellular material sample on the evaluation of elastic properties. Size and boundary effects are studied in detail. This is achieved by conducting computer simulations of the auxetic structure under the typical loading exerted by the compression and simple shearing test performed by means of ABAQUS FEA. The material microstructure is discretized by the plane network of Timoshenko beam elements. The results of the studies give insight to the scale effects. Structures with designed properties can be potentially used for engineering applications.