Mechanical behaviour of polypropylene pipes after the multiple-pass flow-forming process

Abstract

Flow-forming, which is known also as tube spinning or shear forming, is a cold forming, rotary-point extrusion process. Recent research publications on polypropylene have shown that the mechanical properties: yield strength, tensile strength, tensile modulus and sustainable hoop stress; are improved with increasing percentage of thickness reduction. The present research investigates the possibility to further upgrade the properties of the material by multiple-pass flow-forming instead of single-pass flow-forming. In the investigation, polypropylene pipes were flow-formed in a single pass, two passes, three passes or four passes on a conventional lathe using a twin-rollers system. To evaluate the mechanical properties of the material, the specimens were subjected to the hydrostatic pressure test, and to hardness and tensile tests. The hydrostatic pressure test shows that both hardness and hoop-stress data increase with the number of passes of flow-forming. Observation on the pipe rupture reveals that there are a few macroscopic layers radially across the pipe wall and the rupture gradually transforms from a ductile to brittle nature when the number of passes increases, which suggests that the internal and external surfaces of the polypropylene are more compressive and anisotropic when compared with the interior of the material. The tensile yield behaviour of the material was studied using a Shimadzu Universal Testing Machine. It was found that generally the yield stress and ultimate stress decrease with the number of passes of flow-forming. A reasonable explanation is that when the number of passes increases, the external pipe surface which was flow-formed, and so anisotropic, will offer greater resistance to successive flow-forming passes. This renders even greater non-uniformity in the material texture in that the outer layers are becoming more orientated, hence the yielding process occurs inhomogeneously and the material fails prematurely. Another factor that causes deterioration in the material property is chain polymer scission.