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High-density polyethylene (HDPE) is increasingly used for pipe applications owing to its lower cost and higher resistance to chemical corrosion and biological attack compared with metallic materials. Hence, HDPE with improved mechanical properties for pipe application were developed. In addition, additives are used to improve the long term performance of PE pipes. Carbon black (CB) is one of the most widely used additives in PE pipes due to its low cost and absorption of UV which contributes to the resistance to photo degradation. However, it is difficult to achieve good homogenisation of HDPE and additives by the single-screw extrusion method. Insufficient homogenisation is observed as black (where HDPE and CB are well mixed) with white striations (only HDPE without CB); the white striations are called "windows". It has been shown that windows could lead to reduction of HDPE pipe performance and several methods have been developed to detect them. However, there is lack of study on the effect of windows on the integrity of butt fusion joints and there is only one method reported in open literatures which can quantify the window. However, this method causes material wastage. The aim of this study was to investigate the influence of windows on the mechanical integrity of HDPE pipe butt fusion joints and the failure mechanism at microscale, and to determine the threshold value of window level below which the mechanical integrity is sufficient for industrial applications. The main objectives were: 1) to develop a method for quantifying the window levels that does not waste the pipe materials; 2) to investigate the mechanical performance of butt fusion joints containing different levels of windows under the loads of tension, bending and high-speed tension to establish a correlation of each property with the window level; 3) to determine the threshold window level in HDPE pipe joints; 4) to study the failure mechanisms at the microscopic scale. The study was primarily conducted by mechanical testing and material characterisation using optical microscopy, scanning electron microscopy (SEM). Key findings are as follows. Thin slices extracted from a HDPE pipe containing high level of windows were observed under transmission light and polarised light using an optical microscope. The results revealed that there is a thin and grey interphase, between window area and surrounding black areas, with different crystallite structure. Tensile, bending, and tensile impact tests were conducted to evaluate the performance of HDPE pipe butt weld joints containing different levels of windows, which was quantified as window percentage area by image analysis of ribbon samples generated in the trimming stage of butt fusion welds. The results showed that all the three mechanical properties decreased with the increase of window level. For the tensile tests, tensile ductility and toughness of test specimens reduced with the increase of window level. The "energy to break" parameter was selected to determine 3 the threshold level of windows owing to its sensitivity to the window level and this property is required as a threshold in the water supply pipe industry. The results showed that the threshold window percentage area was 0.5%. Optical microscopy and SEM have revealed that high window level caused brittle fracture. Microtome slice extracted perpendicular to the fracture surface showed that windows were aligned layer by layer parallel to the weld interface in the melt zone. One layer of window with a thin layer of grey material above was observed under all the main crack initiation regions. The crack initiated at the weld region from around the mid-thickness where the highest level of windows was found and the crack grew toward weld beads. Craze remnant was observed from the crack initiation region and stable crack growth region, indicating that the fracture was initiated form the crazing initiated from the interphase (the grey layer) between the windows and the adjacent black HDPE material. For the tensile impact tests, impact ductility and toughness of test specimens reduced with the increase of window level. For each specimen, a shear band out of the weld region was observed before necking occurred. Vertical cracks adjacent to vertical windows were observed during the necking stage for specimens containing high level of windows. The more and larger windows existed in necking area, the faster the vertical cracks appeared. This has further proved that the grey interphase was the source of the fracture. Finite element modelling result showed that stress concentration should appear at the corner of weld bead with parent pipe. This indicated that, under tensile impact load, the weak boundary between heat affect zone and parent pipe is more dangerous than the stress concentration point. For the bending tests, the number of the specimens of each window level that developed cracks before being deformed to a right-angle shape increased dramatically with the increasing of window level. In addition, average crack length increased with the increasing of window level, and average time to failure decreased with the increasing of window level. What is more, from the fracture surface of a further broken specimen, a large window going across a large crack was observed. These indicated that the higher the window level, the more and the weaker interphase exist in weld region, and the interphase result in failure. All the results in this study indicate that insufficient homogenisation during HDPE pipe extrusion, which was observed as windows, result in a thin interphase between window area and surrounding black areas with different crystallite structure. The interphase resulted in the failure appeared in all the three mechanical tests. The worse the homogenisation of the pipes, the more and weaker the interphase in the pipes and weld region, the worse the mechanical properties of their butt fusion joints.