Rate-dependent function in the correlation between hardness and yield stress of polyethylene composites

A study of the validity of a correlation between compressive yield stress and Vickers hardness of CaCO3/high density polyethylene composites was carried out. Composites with calcium carbonate in the range of 0-0.40 were produced by twin screw extrusion followed by compression molding. Hardness was implemented by indenting the samples with Vickers indenter and compression test were carried out at various crosshead speeds in the range of 0.2-50 mm min(-1). It was found that increasing calcium carbonate content increased both hardness and yield stress. In addition, compressive yield stress increased with increasing crosshead speed. The correlation between hardness and yield stress was observed to be independent on filler content and valid when the deformation rate employed in both tests were within the similar ranges which was in the order of 10(-1) mm min(-1). Tabor's equation was modified to take into account for the rate-dependent behavior of composites and was found to be applicable and yield relatively accurate results.