Macroscopic yielding of oriented polypropylene (OPP) tubes at elevated temperatures

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. In addition, the ductility of these materials is improved with heating. If the elevated temperature forming of oriented thermoplastic tubes is to be assessed, an accurate representation of the yield surface for the material at a range of temperatures is required. In this study, an experimental representation of the yield surface for oriented polypropylene (OPP) tube at various elevated temperatures is presented, by means of uniaxial mechanical testing of the tube as well as via tube forming tests. The fit of three theoretical yield criteria, the Hill anisotropic yield criterion, the pressure-modified Mises criterion and the pressure-modified Hill criterion, were compared with the experimentally obtained yield surface at a given temperature. It was found that the pressure-modified Hill criterion accurately captured the uniaxial and biaxial yielding of the OPP tube.