Optimisation of waisted tensile test specimen geometry and determination of tensile energy welding factors for different polyethylene pipe wall thicknesses

Abstract

High-density polyethylene (HDPE) pipes are employed in a wide range of industries such as water, gas, nuclear and energy. HDPE pipes have steadily replaced clay, copper, asbestos-cement, aluminium, iron and concrete pipes in various applications. Butt fusion welding is one of the most commonly used techniques to weld HDPE pipes. There are many test methods available for assessing the short-term performance of butt fusion welded joints in HDPE pipes. Recent research publications have shown that waisted tensile test specimen is the most discriminating short-term examination in a tensile test, such as those described in ISO 13953, EN 12814-2, EN 12814-7 and WIS 4-32-08. The current challenge of using the abovementioned standards is to quantify the quality of welds since the same waisted geometry is used for any pipe size diameter with any thickness. As the thickness of the specimen increases, the degree of ductility reduces significantly in both welded and unwelded specimens. Therefore, a new specimen geometry that can be used for specimens with all thicknesses must be defined to allow a more accurate measurement of weld quality. In order to simplify the aforementioned problem, specimens using an unwelded flat sheet made from HDPE were used to investigate the geometry parameters, which believed to have the most influence on the fracture of the specimen. The effects of parameters such as width and radius of the waisted section, diameter and distance of loading holes, and overall width of the specimen were investigated through various experimental procedures, tensile tests, and Central Composite Design (CCD) optimisation. After understanding the effect of geometry parameters, Finite Element Analysis (FEA) techniques using constitutive equations are used to confirm experimental findings. FEA modelling also covered a wide range of specimen geometries, where the experimental investigation was not feasible due to machining and testing limitations of specimens with large thicknesses (e.g., 20-100mm). The final contribution of this thesis is to propose a modified geometry, which could be used for all pipe sizes and demonstrate its advantages over the standard geometry specimen. This task was carried out on pipes with Outer Diameter (OD) of 140, 160, 250, 280, 500 and 630mm. Improvements on 140, 160 and 250mm with a thickness of less than 20mm, are providing more considerable elongation using parent pipe material and necking starting at earlier stages of the tensile test. Improvements on 280, 500 and 630mm OD with a thickness of over 20mm, are having the specimen fail in a ductile manner with ductility in welded and unwelded specimen whereas, no ductility could be observed when standard geometry was used. X-ray photography was taken to verify the types of failures accrued in specimens. The outcome of the research conducted in this thesis, which proposed a modified geometry for tensile tests, paves the way to enhance reliability in the examination of HDPE weld qualities.