Objectives
The objectives of the proposed project are:
- Predict the effect of thickness, radius and width of the waisted section on the stresses at the weld of waisted tensile specimens using finite element analysis and verify these results by experimental testing on specimens cut from parent pipe
- Propose the most appropriate specimen geometry for the tensile test using a waisted specimen, which provides the most discrimination between different weld qualities
- Determine the effect of specimen thickness, PE material and welding procedure on the value of tensile energy welding factor using the above optimised specimen geometry
- Determine the relationship between weld heat affected zone dimensions/weld bead geometry and tensile energy welding factor
- Produce graphs of tensile energy welding factor against pipe wall thickness for different grades of PE pipe and different standard welding procedures, which can be used by industry to qualify PE pipe butt fusion welding procedures and welding operators
Project Outline
Previous CRP projects (Chipperfield and Troughton, 1999; Brown and Troughton, 2004) have shown that a tensile test using a waisted specimen is the most discriminating mechanical test for assessing the short-term integrity of butt fusion welds in polyethylene (PE) pipes. This type of test is therefore specified in a number of standards relating to the qualification of butt fusion welding procedures and welding operators for PE pipes (EN 13067, AWS B2.4).
In the tensile test using a waisted specimen, Chipperfield and Troughton (1999) showed that the most discriminating test parameter is the energy to break the specimen. There are a number of standards that describe a tensile test on butt fusion welds in PE pipes where the cross-sectional area is at a minimum at the weld interface, including ISO 13953, EN 12814-2, EN 12814-6 and EN 12814-7. Most of these tests specify that the tensile strength of the welded specimen be determined and compared to that of a specimen cut from the parent pipe. However, Chipperfield and Troughton (1999) showed that tensile strength is a poor discriminator of weld quality. ISO 13953 specifies that the fracture surfaces of the tested specimen should be examined and categorised as either ductile (large-scale deformation and yielding of material at the weld interface) or brittle (little or no large-scale deformation of material at the weld interface). However, this categorisation is subjective and qualitative, and the degree of ductility reduces significantly with wall thickness (Murch and Troughton, 1995).
EN 12814-7 does specify measuring the energy to break of the specimen and determining a tensile energy welding factor. The values of energy-to-break have been shown to be dependent on wall thickness (Murch and Troughton, 1995) and specimen geometry (Wilson, 1995), and will probably also be dependent on PE resin and butt fusion welding procedure.
Relevant Industry Sectors
Technical and Economic Benefits
As an example of one industrial need, a government review of the UK’s electrical energy requirements for 2025 has indicated that 60GW of net new capacity will be required in order to meet demand. This will mostly come from new nuclear fission installations which require the joining of PE pipes. The implementation of HDPE pipe in nuclear power plants with all the stringent guidelines, regulations, rules and inspection practices that this industry has developed and followed for decades, certainly is a vote of confidence for the integrity and performance of HDPE pipe and at the same time strengthen the urge to a better and more accurate prediction of these pipes lifetime. Therefore, it is clear that there is a need for optimization of the short-term mechanical tests of plastic pipes and welds to allow a better understanding of their integrity.