TENSILE AND PRESSURE RUPTURE BEHAVIOR OF FLOW-FORMED HIGH-DENSITY POLYETHYLENE PIPES

Structurally altered high density polyethylene pipes were produced by now-forming, a point rotary cold rolling process. The process was performed by using three rollers on a conventional lathe machine. The variation of mechanical and pressure rupture properties (tensile strength, yield strain, toughness and hoop stress) with percentage reduction was studied. The stress-strain behaviour of the flow-formed pipe exhibited less yielding and cold drawing as percentage reduction increased. Significant increase in the tensile strength was observed for reductions above 20%. Improvement (exceeding 300%) of toughness was observed in the axial and hoop direction. Pressure rupture tests revealed that the hoop stress of the flow-formed pipes increased only after 50% reduction. Both tensile and pressure rupture tests revealed that the ductility also increased with percentage reduction. This was well demonstrated by the extensive bulging during pressure rupture tests. Scanning electron micrographs revealed a significant amount of structually altered macrofibrils in the flow-formed pipe.