MECHANICAL & FRACTURE BEHAVIOUR OF ROLL-FORMED HIGH DENSITY POLYTHYLENE PIPES

Abstract

Structurally altered high density polyethylene (HDPE) pipes were produced by roll-forming, a rotary point 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, energy absorption and hoop stress) with percentage reduction, were studied. Significant increase in tensile strength was observed for reductions above 20%. Improvement (exceeding 200%) of energy absorption was observed in the axial and hoop direction. Pressure rupture tests revealed that hoop stress of the roll-formed pipes increased only after 50% reduction. Both tensile and pressure rupture tests revealed that the ductility of the pipes also increased with percentage reduction. This was well demonstrated by the extensive bulging and volumetric deformation during the pressure rupture tests. The stress-strain behaviour of the roll-formed pipes exhibited less phenomena of yielding and cold drawing as percentage reduction increased. In addition, the fracture behaviour of roll-formed HDPE pipes were evaluated using two fracture mechanics approach i.e. the stress intensity factor, K (LEFM) and the J-Integral approach. It was observed that the fracture resistance of the roll-formed pipes improved over that of the unformed pipe. The improvement was significant in the 50% reduction specimens and more pronounced in the 65% reduction specimens where extensive crack deflection mechanism was also observed. The crack deflection mechanism is akin to the fracture mode observed in polymeric composites i.e. delamination. Both the K and J analysis indicate maximum fracture resistance at crack-to-thickness ratio, a/t between 0.65 - 0.75. This behaviour could be attributed to the behaviour of materials under contact stresses and can be modelled by a simple sphere in contact with a plane.