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

电熔接头广泛应用于聚乙烯管道的相互连接。在接头和管道外周向边缘之间产生的非熔合区域作为三维周向裂纹。实验观察到接头的断裂是从裂纹尖端开始的。因此，应首先根据断裂力学原理对接头进行设计。本文采用三维等参奇异单元法，采用有限元方法计算了周向裂纹尖端的应力强度因子。得到了熔接区长度、接头厚度、裂纹长度等接头尺寸与应力强度因子K_I的关系，以及裂纹尖端K_<II>的关系。在-196 ~ -120℃的温度范围内，对聚乙烯的断裂韧性进行了实验研究。在-196 ~ -120℃范围内，聚乙烯的平面应变断裂韧性随温度的升高而减小。在- 120℃时，K_<IC>的最小值为3.47MPam^<1/2>。由分析结果可知，在接头和管道可能施加的最大内压下，组合应力强度因子K = (K_I^2 +K_<II>^2)^<1/2>的值约为0.45 mpa ^<1/2>。在此基础上，通过蠕变试验确定了该节点的一维蠕变本构方程为符合Bailey-Norton定律的形式。采用米塞斯理论将得到的本构方程扩展为三维形式。采用扩展本构方程，采用改进的拉格朗日有限元法分析了尖缺口处的蠕变应变。结果表明，缺口尖端的蠕变应变随时间的增加而增大。当缺口的尖端变得更尖锐时，蠕变应变的值增加，与经过的时间无关。少

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