Fracture Mechanical Studies on Polyethylene Erectro-Fusion Joint Containing Three-Dimensional Circumferential Crack

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The electro-fusion joint is widely used to connect Polyethylene pipes each other. The non-fused region which is produced between the joint and the outer circumferential edge of the pipe is served as a three-dimensional circumferential crack. The fracture of the joint is observed experimentally to start from the tip of the crack. Therefore, the joint should be designed first of all based on the fracture mechanics. In this study, the stress intensity factors at the tip of the circumferential crack were calculated by FEM analysis using a three-dimensional isoparametric singular element. The dependence of the sizes of the joint, such as the length of fused region, the thickness of the joint, the crack length, etc., on stress intensity factor K_I, and K_<II> at the tip of the crack can be obtained. The fracture toughness of a Polyethylene was evaluated experlimentally over the temperature range from -196ﾟC to -120ﾟC.The value of the plane strain fracture toughness of Polyethylene decreases … More with increasing temperature within the range from -196ﾟC to -120ﾟC.The minimum value of K_<IC> is 3.47MPam^<1/2> at - 120ﾟC.From the analytical results, the value of the combined stress intensity factor K = (K_I^2 +K_<II>^2 )^<1/2> is about O.45MPa^<1/2> under the maximum internal pressure which may be applied to the joint and pipes. The value of the safety factor of the joint can be concluded to be about 7 from these results, In addition to these studies, the one-dimensional creep constitutive equation was evaluated as a form obeying the Bailey-Norton's law by performing the creep tests. The constitutive equation obtained was extended to the three-dimensional form by adopting the Mieses theory. The creep strain at the tip of sharp notch was analyzed by the updated lagrangian FEM analysis using the extended constitutive equation. As a result, the creep strain at the tip of notch increases with increasing the elapsed time. As the tip of the notch becomes more sharp, the value of the creep strain increases irrespective of the elapsed time. Less