Barreling at cut ends of PE pipe: numerical and experimental investigation

Session 9B
12:00 pm

Andrea Guevara-Morales, Imperial College London, UK

When a thermoplastic pipe is extruded, non-uniform cooling sets up residual stresses. These stresses can, when the pipe wall is cut, be released and cause local bending. For a thick, externally-cooled pipe the internal surface bends concave so that when it is cut to length the end decreases in diameter. A less well known observation is that moving back inwards from the cut end, the measured pipe diameter does not simply increase to its initial value but locally overshoots to a new maximum. These changes in diameter at cut ends are often referred to as pipe ‘barreling’.

Barreling can have a significant effect on butt fusion and electrofusion welding. In butt fusion the reduction in the pipe diameter at the cut end will reduce the hydraulic capacity of the pipe at the weld. In electrofusion joining the barreling gives an axial variation in fit between pipe and socket and this may generate undesirable axial variations in temperature and pressure during welding. This paper studies the development of the barreled shape in terms of residual stresses frozen in during post-extrusion pipe cooling. The objective here is to improve the installation procedures for electrofusion joints and ultimately, perhaps, their design.

Barreling has been simulated for PE pipe of various dimensions and processing conditions. A non-linear heat transfer analysis determines the thermal history of the material during solidification, considering temperature dependant material properties and the kinetics and effects of crystallisation. Residual stresses in the cooled pipe are determined using a thermoelastic model and compared with experimental data obtained using the layer removal method. A shell-theory solution for barreling is coupled to the numerical analysis to determine the profile of diameter versus distance from the cut pipe end. Finally, the model is validated by comparing the results to data provided by a pipe supplier.

Andrea Guevara Morales, Research Student
Patrick Leevers, Reader in Polymer Engineering

Department of Mechanical Engineering
Imperial College London