The effect of strain rate on the die-drawing of polyoxymethylene at elevated temperatures

Abstract
In this paper we describe the intrinsic deformation behaviour of polyoxymethylene in uniaxial tension, shear and plane strain compression at 160C for strain rates from 10 ^{- 3} to 1 s ^{- 1}. In tension and shear, the deformation was determined by a novel video-controlled testing system. There was a significant evolution of volumetric strain in tension, indicating that damage mechanisms play a key role in the plastic deformation behaviour. Post-mortem analysis on the shear and compressed samples showed no evidence of dilatation. In uniaxial tension, significant strain hardening was observed at high strains while only a small increase of stress was observed in plane strain compression. In shear, the plastic deformation occurred at constant stress. For the specific case of die-drawing of polyoxymethylene, it has been shown with the aid of small angle X-ray scattering that voids are nucleated at the die-entrance by inhomogeneous shear deformation under negative (compressive) hydrostatic stress. The growth of the voids is then aided by the tensile stresses that are predominant in this process at die-exit. Extensive voiding occurred in the oriented sample produced at high strain rates, as revealed by the scanning electron microscopy, which affects the fracture strength and hence the viable production speeds of the die-drawing process.