The Evaluation of Mechanical Testing Techniques and the Influence of Defects on MDPE Butt Fusion Welds [PE]

Various test geometries with known defects, such as might occur in practice. were evaluated and compared to the British Gas standard for butt fusion welds. Novel testing techniques are described. A suitable test geometry was established and used to assess the influence of defect size on properties. Small defects were found to be more significant as the pipe wall thickness increased. 

Plastics pipes are increasingly used for gas and water distribution, sewage and effluent handling and in industrial process plant. Of the joining techniques used, the simplest is the butt fusion process, and is the only process that can join large diameters. 

The butt fusion welding parameters for PE80 grade pipe in sizes up to 250mm diameter are well established. For larger sizes, however, there is less practical experience, the correct process parameters are less well defined and are based on broad extrapolation from smaller sizes. A further concern is that the significance of defects in welds in plastics pipes has not been defined and that current non-destructive techniques for d e t e c t i n g d e f e c t s are inadequate. This is of particular concern as plastics pipes are beginning to be used in more critical applications or where the cost of a failure during installation or use would be high. In addition, care is needed with fabricated fittings, such as mitred bends. Stresses on these welds can be much higher than in straight butt joints, so poor quality joints will be more likely to fail. 

Currently, the only way to detect large defects or contamination is by the removal of the bead and small defects are very difficult to detect. The following paper concerns a study of the influence of defects on the mechanical properties of butt fusion welds in medium density polyethylene.