Material and Process Models for the Forming of Oriented Polymer Tubes

Abstract

<p>The lightweight and good strength of oriented thermoplastic tubes, produced through the ram extrusion process, renders this class of material advantageous from an automotive structural part forming perspective. An analytical model, capable of predicting the forming limits for these materials at elevated temperatures, would avoid the need for difficult and time-consuming tube forming experiments.</p> <p> The uniaxial tensile stress-strain properties of the oriented polypropylene (OPP) tubes along the axial and hoop direction wene determined while methods were developed for obtaining the uniaxial compressive properties of the tube. The uniaxial and biaxial testing of the OPP tubes enabled representation of the experimental plastic work contours for the material at a range of temperatures. The use of the pressure-modified Hill criterion, the concept of plastic work contours in stress space, and the stress strain data from uniaxial testing of the tube allowed accurate prediction of the plastic work contours for the OPP tubes as a function of temperature. The analytical prediction was able to capture the anisotropy, pressure dependency and anisotropic hardening exhibited by the material at elevated temperatures. In addition, the use of a localized necking criterion, which involved the point at which the maximum force along the hoop direction of the tube was reached, allowed the temperature dependent forming limits for the material to be predicted. The forming limits increased with temperature for all of the strain paths considered. In addition, at a given temperature, the forming limits decreased with increasing strain ratio. The limiting strains achieved with axial end feeding were markedly higher than those in which no axial end feeding was used. </p> <p> Good agreement was observed between the predicted forming limits and the results from tube forming tests of the material, along a range of strain paths and at various temperatures.</p>