TENSILE AND PRESSURE YIELD BEHAVIOR OF FLOWFORMED POLYPROPYLENE PIPE.

Abstract

Flowforming is a cold-forming, rotary point extrusion process. Polypropylene pipes were flowformed using a single roller on a conventional lathe machine to various degrees of percentage radial reduction ranging from 17 percent to 78 percent. In order to study the forces involved in the reduction process, radial, axial and tangential forces were measured. The flowformed pipes were tested under uniaxial tension and uniaxial pressure burst tests. Both the tensile strength and maximum hoop stresses of the pipes increased with percentage radial reduction. At high percentage radial reduction, the tensile strength increases more than 4-fold over the original tensile strength value. The extension at yield increases to a maximum at about 60 percent radial reduction after which the extension at yield decreases. Results from the pressure tests show that flowforming increases the ductility of the pipes. At large percentage reductions the pipes yield in a balloon-like manner. Various fracture phenomena are also presented.