Improvement of the Reduced Section Tensile Test Specimen Geometry for Assessing the Integrity of Butt Fusion Welds in Polyethylene Pipes
TWI Industrial Member Report 1154-2021 [pdf / 3231KB]
By M Taghipourfard, S Willis, C Yu and M Troughton
Previous Core Research Programme projects have shown that a tensile test using a reduced section 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, such as EN 13067 and AWS B2.4.
In the tensile test using a reduced section specimen, previous Core Research Programme projects have also shown 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 12814-7 and WIS 4-32-08. Most of these tests specify 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 TWI has shown that the degree of ductility reduces significantly with wall thickness. Others specify that the tensile strength of the welded specimen be determined and compared to that of a specimen cut from the parent pipe. However, TWI has shown previously that tensile strength is a poor discriminator of weld quality.
Although the energy to break a specimen is a quantitative indication of the ductility of the weld, the values of energy to break have been shown to be dependent on wall thickness and specimen geometry. As the thickness of the specimen increases, greater directional stresses (stress triaxiality) are generated, which decrease the ductility of the sample, even for the parent material. For this reason, the specimen geometry that is specified for joints in thin walled pipe will not be as discriminating for joints in thick walled pipe, because both good and bad joints will fail in a brittle or mixed manner.
Therefore, there is a need for optimising the reduced section tensile test specimen geometry to obtain a better understanding of the integrity of butt fusion joints in PE pipes, no matter what the wall thickness is.
Photograph of a tensile test on a reduced section specimen according to EN 12814-7
- A modified geometry and a recommended maximum thickness of 30mm for the reduced section tensile test specimen is proposed.
- For butt fusion joints with a pipe wall thickness greater than 20mm, the modified specimen geometry significantly increases the ductility and energy to break per unit cross-sectional area.
- Although the modified geometry for specimens from butt fusion joints with pipe wall thickness below 20mm does not improve the ductility, it significantly reduces the elongation in the loading holes and therefore results in a more representative value of energy to break per unit cross-sectional area, without the need for an extensometer.
Finite element analysis boundary conditions and meshing size used for determining the effect of width of the waisted section and loading hole diameter on the elongation in the loading hole
Examples of nominal stress vs displacement curves for standard specimens (current geometry defined in EN 12814-7) and modified specimens (proposed new geometry, for specimens cut from the outer half of the joint [top] and inner half of the joint [bottom]) from butt fusion joints in 500mm SDR11 PE100 pipes