The use of non-destructive examination (NDE) for assessing the quality of butt fusion joints in polyethylene (PE) pipes has been included in the draft Mandatory Appendix XXVI to Section III of the ASME Boiler and Pressure Vessel Code (Rules for construction of Class 3 buried polyethylene pressure piping). However, currently, there are no acceptance criteria for flaws in butt fusion joints in PE pipes. There is an ASME Task Group on flaw evaluation for PE pipe, which is developing a code case using linear elastic fracture mechanics (LEFM) to determine critical flaw sizes. However, the initial experimental crack growth data generated suggests that linear elastic fracture mechanics is not able to adequately describe slow crack growth in PE materials. In addition, this work is only considering planar lack of fusion flaws in the joint; it is not considering other critical flaw types that can occur in butt fusion joints, such as particulate contamination and cold fusion. TWI has developed procedures using mechanical testing to develop flaw acceptance criteria for butt fusion joints in PE pipes. This is based on inserting lack of fusion flaws of known size and particulate contamination flaws of known concentrations into butt fusion joints and determining the effect of these flaws on both the short-term and long-term integrity of the joints. An important aspect of this work is to determine which of the wide array of mechanical tests available for assessing the integrity of butt fusion joints in PE pipes are the most discriminating. This paper describes the procedures developed for inserting simulated flaws into butt fusion joints in PE pipes, the experimental work to compare the results from different standard short-term and long-term tests on flawed and unflawed joints and the procedures developed to determine flaw acceptance criteria. Results have shown that the most discriminating short-term test for butt fusion joints in PE pipes is a tensile test using a waisted specimen, such as those defined in EN 12814-2, EN 12814-7 and ISO 13953, and the most discriminating property is the energy to break the specimen. The most appropriate long-term test for butt fusion joints in PE pipes is the whole pipe tensile creep rupture test, as defined in EN 12814-3; this is the only long-term whole pipe test that consistently generates slow crack growth in the fused joint, even if it contains no flaws.

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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 longterm 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.

Tensile and flexural strengths of single and double gated polybutylene terephthalate and polycarbonate (PBT/PC) reinforced with 0-30 wt-% short glass fibres were investigated. Results showed that tensile and flexural strengths of single gated mouldings conform to modified rule of mixtures with overall fibre efficiency parameter g s of 0?128 for tensile strengths and 0?132 for flexural strengths. The presence of weldlines in double gated mouldings significantly reduced strength of PBT/PC composites but had a minor effect on the strength of the unreinforced PBT/PC matrix. Results indicated that the flexural strengths of both single and double gated mouldings were always greater than tensile strengths. The weldline integrity factor for both strengths decreased with increasing fibre concentration in a linear manner and was independent of the test method. The strength data for both specimen types and test methods were found to fit a two parameter Weibull distribution reasonably well. The Weibull modulus m for single gated composites was found to be independent of the test method and fibre concentration (m<48) and was higher than the value for the unreinforced matrix (m<30). The presence of weldline in double gated composites reduced the value of m quite significantly. The m value for double gated mouldings decreased with increasing fibre concentration. Analysis showed that the variation in strength values with test method cannot be explained solely by the two parameter Weibull distribution model.