Plastic Pipes Conference Association # 1985 York
Parmar, Bowman
The performance of aligned and controlled misaligned butt welds in 63 and 90mm polyethylene pipe systems was determined under fatigue loading conditions designed to induce brittle failure. The results showed a significant reduction in fatigue lifetime for butt welds having fractional misalignments greater than 10% of the pipe wall thickness when compared to aligned butt welds. The fatigue performance of the 90mm pipe was at least the equal of the 63mm pipe. The performance of both the 63 and 90mm pipe samples could be explained in terms of an amplified axial stress.
Examination of the crack initiation and propagation path of the circumferential failures of butt welds showed the crack invariably initiated from the notch located at inner weld bead, and initial propagation was outside the MFZ. Final rupture took place at the centre of the outer bead. The micromorphology of the weld bead did not appear to influence the crack propagation path.
The strength of a polyethylene pipe system depends on the strength of the pipe, the associated injection moulded fittings and also on the integrity of any joints within the pipe system.
Mirror plate butt fusion jointing (1) is widely used for jointing polyethylene pipe systems, and previous studies have examined how its strength compares to the base resin (2) and how welding conditions influence performance (3)(4). It is concluded from these works that the strength of the butt fusion joint is good with, for the welding conditions examined, the fusion joint strength exhibiting little sensitivity to welding conditions. However, a number of research workers have acknowledged that the long term strength of a fusion joint may be influenced or controlled by misalignment (2)(5)(6)(7). There is a paucity of published literature on this important topic; the objectives of this programme of work were therefore to derive such data for joints between polyethylene pipes, and to attempt to gain an insight into the factors controlling the strength of butt fusion joints.
The programme of work has used off-the-shelf Water grade polyethylene pipes of three diameters. The pipe has been fusion jointed and tested under fatigue. The results demonstrate clearly how lifetimes can be reduced if significant misalignments are present.
In addition to the experimental laboratory work, a limited field survey was undertaken to assess the level of the problem in practice. The results of this work are presented and show that, generally, the standard of welding is high but, on occasions, significant misalignment can be introduced. Attention to this problem is therefore required.
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