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This paper describes the application of the full-notch tensile fatigue (FNTF) test to fusion strength evaluation (butt, socket, saddle and electrofusion).When a full notch is made at the fusion interface in a joint, the fusion strength of the joint can be evaluated in the same manner as that of a substrate. While the conventional stress rupture test is not suitable for fusion interface strength measurement, our proposed method permits measurement of fusion strenth variation by means of the actual time to failure.Besides butt fusion strenth, the FNTF test method is applicable to the fusion strenth evaluation of various types of joints, determination of compatibility and selection of the optimum fusion conditions. The fusion strength correlates strongly with the melt flow rate of resin. The higher the melt flow rate is, the closer the fusion strength becomes to that of the substrate, providing a wider range of optimum fusion conditions. For butt fusion between two different brands of polyethylene, it is preferable to select resins having the same melt flow rate. The FNTF test method has already been specified in the JIS as one of the recommended evaluation methods for fusion joints.